CONTROLLING RISKS AROUND EXPLOSIVES STORES Review of the requirements on separation distances

HSE
Health & Safety
Executive
CONTROLLING RISKS
AROUND EXPLOSIVES STORES
Review of the requirements on
separation distances
© Crown copyright 2002
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First published 2002
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Prepared by MBTB Limited
for the Health and Safety Executive
Peter Moreton
28 Hazelborough Close
Gorse Covert
Warrington WA3 6UL
United Kingdom
This report and the work it describes were funded by the
Health and Safety Executive (HSE). Its contents, including
any opinions and/or conclusions expressed, are those of
the author alone and do not necessarily reflect HSE policy.
Controlling risks around explosives stores
Introduction and background
Anyone wishing to store more than 30 kilograms of high explosives must hold a
licence. Local authorities currently can licence stores to hold up to 1800 kg of
explosives, while the HSE is the licensing authority for quantities in excess of this
amount. The main requirement for storeholders with licensed stores is to maintain
minimum distances between the store and neighbouring ‘protected works’ - inhabited
buildings and other ‘places of public resort’. These distances are related to the
quantity of explosive held and are listed in tables which have come to be known as
‘quantity-distance’ tables.
While the system has worked well, there are two main reasons why a review was
necessary. Firstly, the results of recent trials carried out by the MoD (Ministry of
Defence)(see Appendix 3) suggested that the quantity of debris generated in an
explosion and the distance to which it would be thrown could be considerably greater
than had previously been thought. This was particularly true for smaller stores and
stores built of brick and concrete. This suggested the possibility that, in certain cases,
distances set solely to protect against the effects of blast might not offer sufficient
protection against flying debris. Secondly, the distances do not take into account the
numbers of people at risk – the same distances would apply whether the ‘protected
works’ were a single house or a high-density housing estate.
The review of the distances for stores holding high explosives has had three main
parts:
·
·
·
developing models to estimate the risks to an individual living near an
explosives store and the risks of an explosion involving multiple fatalities;
(Chapter 2)
using the models to test the existing separation distance requirements. These
case studies involved hypothetical situations which could be permitted under
the existing rules; (Chapter 4)
considering recommendations for new quantity-distance tables.
The Working Group has also considered issues concerned with:
·
·
the distances for stores holding fireworks and propellants;
the distances which HSE requires, at the sites which it licences, between explosives
stores (‘inter-magazine distances’) and between process buildings and other buildings on
the site (‘process building distances’).
The Working Group’s approach to the review has been informed by a number of general
principles:
·
the models used for estimating risks and for deriving new recommendations should be
documented and transparent;
·
where the existing distances needed to be replaced, the revised separation distances
should, were feasible, reflect explicit risk criteria;
1
·
the approach to the regulation of explosives stores should be consistent with the approach
to the regulation of the storage of other hazardous substances;
·
as far as possible the tables used for setting separation distances should be consistent,
whether the store is licensed by HSE or by a local authority.
Predicting lethality
The first task of the Working Group was to construct a model for predicting lethality
at various ranges from an explosion and the risks both to an individual living near an
explosives store, and the risk of a multiple-fatality explosion.
There were two main ‘building blocks’ used in constructing the model. The first was
an estimate of the probability of an explosion. The second was a method for
estimating the risks to people in the event of an explosion, which would in turn
depend on models of blast and fragmentation effects including assumptions on issues
such as the trajectory of flying debris, and assumptions about the proportion of people
present in the risk area and proportions indoors and outdoors.
Historical accident records were used to derive an estimate for accident likelihood[9].
It was agreed to use an accident rate of 10-4 (one chance in ten-thousand) per
storehouse-year.
In developing this model of lethality, the major issues the group needed to consider
were:
·
the assumptions about trajectory of the debris;
·
the minimum kinetic energy a piece of debris must possess to be considered
potentially lethal;
·
the likely target area presented by individuals in the zone where debris is falling.
Assessing individual risk
This model was then used to assess the individual risk to a person located at a given
distance from an explosives store. This in turn involved assumptions about the
amounts of time an individual would spend in the area of the store and the relative,
amounts of time indoors and outdoors.
2
Case studies
A number of case studies were carried out with the aim of establishing:
·
the maximum level of risk which could, in theory, exist around local authority
stores under the present licensing arrangements, and
·
what levels of risk exist around these stores in more typical situations.
The results of the case studies has shown that the present rules might permit the
building of an explosives store in a location where, although members of the public
would not be exposed to intolerably high levels of individual risk, they could face
levels of individual risk higher than that which HSE would normally regard as
‘broadly acceptable’.
The present rules would also permit stores near high-density housing where, if there
were an explosion, there could be a large number of fatalities. (It should be stressed
that this was the result of a study of a hypothetical situation and HSE has not
identified ‘real-life’ instances where this is the case). Thus there appeared to be good
grounds for revising these rules
Revised separation distances
The next step in the Working Group’s work was to consider what the criteria should
be in setting revised distances. There were two sets of issues to consider here, the first
concerned with individual risk and the second concerned with group risk.
Individual risk, as the name implies is the risk to an identifiable individual, for
example, someone living or working near an explosives store. In the present context
it is measured as the annual probability that that person will be killed as a result of an
accident leading to an explosion inside the store. Group risk measures the number of
fatalities that could be expected in an accident and so can be thought of as a measure
of “disaster potential”. It is normally expressed in the form of a graph showing the
annual probability of an accident leading to N or more fatalities.
The Working Group’s view was that explosives should be regulated on a basis that is
consistent with other hazardous substances. This led the Working Group to take the
view that the criterion should be that members of the public living near a store should
not face a risk of death greater than one in a million per year.
The case studies referred to in chapter 4 show that situations can occur where the risk
to any one person is very low but where an accident could still cause many fatalities.
The Working Group took the view that there should be an additional set of distances
which should apply at stores in areas of high population density (taken as more than
4210 inhabitants per square kilometre).
This in turn raised the issue of what risk criterion should be used in setting these
distances. Whilst there are well-established criteria for evaluating individual risk,
there are no equivalent widely-accepted criteria for the evaluation of group risk.
3
There was much discussion on this issue within the Working Group. The starting
point was the recommendation contained in the first report of the Advisory
Committee on Major Hazards: that the chance of a serious accident (involving the
death of 10 or more people) at any one major non-nuclear plant should be less than
one in one thousand (10-4) per year[12].
Guidance was also provided by the HSE discussion document ‘Reducing Risks,
Protecting People’, which proposes that for a single major industrial activity the risk
of an accident causing the death of 50 or more people should be less than one in five
thousand (2.10-4) per annum[11]. The Working Group considered that this might be
taken as an anchor point for an FN line of slope minus one to delineate a level of
group risk at the upper limit of what is tolerable. This approach would give a figure
of 1 in one thousand as the limit of tolerability for an event resulting in the death of
ten people.
This approach would define the upper limit of tolerability. The Working Group took
the view that it would not be appropriate for the tables to reflect a level of risk which
was only just on the border of tolerable and intolerable and that they should be set at a
level of risk significantly below this level.
It is not clear how such a broadly acceptable definition of negligible level of group
risk might also be defined. There were differing views within the Working Group
about whether multiple-fatality risks were acceptable at all, but there was a general
consensus that the aim of the controls should be to limit the number of fatalities to
less than ten. It was therefore agreed that the distances for high population density
areas should be set to ensure, with a ninety per cent confidence level, that the number
of fatalities would be less than ten (this equates to a risk of 10-5 per year of an
accident involving more than 10 fatalities).
Mounding and other safety measures
Analysis of the MoD trials data has highlighted the impact of two safety measures:
mounding and the removal of the detonator annex from steel stores. In the light of
these findings the Working Group agreed that the tables for separation distances
should take account of these safety measures. There are therefore separate tables for
mounded stores and for steel stores where the annex has been removed.
Storage of HT1 explosives in registered premises
The Working Group noted that the present requirements on separation distances did
not apply to registered premises. However the data from the MoD trials clearly
demonstrated that even small quantities of HT1 explosive presented a considerable
hazard. The present exclusion of registered premises from the separation distances did
not appear to be justifiable on safety grounds. The Working Group therefore
recommended that the new tables should also cover registered premises.
Internal separation distances
In addition to the requirement to maintain separation distances between explosives
stores (and other explosives buildings) and inhabited buildings off-site, HSE also
requires operators of explosives factories to maintain separation distances between
production buildings and explosives stores (‘process building distances’). The
requirements for these distances reflect the fact that production activities have the
4
highest risk and there is a need to limit the severity of the consequences of an
explosion in a production area by a) limiting the amounts of explosive that may be
kept in them and b) separating these buildings from the those holding the bulk stores
of explosive.
In addition to these distances, where there is more than one explosive store on site the
operator is required to maintain separation distances between them – these are
referred to as ‘inter-magazine distances’.
The Working Group’s view was that, in the absence of more complete information,
the existing process building distances should stand. However, it felt that HSE should
consider further research on this topic as part of its research programme.
The Working Group agreed that although a brick magazine could be destroyed in the
event of the detonation of an adjacent magazine, the present distances would ensure
that this building collapse would almost certainly not initiate an explosion in most, if
not all, explosives in commercial or military use, and therefore that the present
distances did not need to be changed.
Vulnerable buildings
At present factories and magazines with HSE licences are required to maintain a
greater separation distance between explosives buildings and buildings of vulnerable
construction – for example, offices or flats using ‘curtain wall’ construction. These
distances (known as ‘vulnerable building distances’) are calculated using the formula
44Q1/3– ie double the normal distances based on blast hazard. There are no similar
distances for local-authority licensed stores.
The Working Group recommended that the vulnerable building distances should be
retained and there appeared to be a strong case for including similar requirements in
the tables for distances to be maintained around local authority stores.
The Working Group’s view was that, because the hazard was primarily blast related,
the distances should continue to use the formula 44Q1/3.
Vulnerable populations
The Working Group considered the issue of whether the distance requirements should
be increased for stores near schools, hospitals or sheltered accommodation for old
people. The analogy would be with the process for assessing applications for planning
consent for other hazardous installations where, in assessing the tolerability of the
level of risk, greater weighting is given to vulnerable populations because they would
be more difficult to evacuate in the event of an emergency and more vulnerable to the
effects of toxic substances. In the case of explosives the Working Group did not
believe that given the instantaneous effects of explosives hazards the greater
weightings for vulnerable populations applied.
The Working Group did recognize that there might be societal concerns over the
siting of explosives stores where these groups would be at risk. This would therefore
be an issue which licensing authorities might wish to take into account in deciding
whether a store should be sited in a given location.
5
Public traffic routes
At the moment the law requires a separation distance between stores and public traffic
routes and 'places of public resort’ of half the distance which would apply to an
inhabited building. This applies whether the traffic route is an occasionally-used
footpath or a busy motorway.
The Working Group agreed that the revised distances should take account of the
traffic densities, with the full separation distances applying at very busy roads. On the
other hand where the traffic route is lightly used the Working Group felt that the
requirements should be relaxed and indeed, where the route was only rarely used, the
separation distance requirement should be disregarded.
Fireworks
The Working Group noted that the present tables for local authority stores impose the
same distances for manufactured fireworks as for high explosives (when explosive
content is compared, rather than the gross weight). The distances required by HSE for
Hazard Type 1.3 and 1.4 fireworks are significantly less - reflecting the fact that the
primary hazard is fire/heat radiation. The Working Group’s view was that, for Hazard
Type 1.3 and 1.4 fireworks, the distances required by the present local authority tables
are not justified by the risk; the distances used by HSE are more than sufficient to
maintain a high standard of safety.
At the same time the Working Group noted that up to 1 tonne (gross – 250kg net) of
‘shop goods’ fireworks could be kept in registered premises without any external
separation distance. Again the Working Group did not believe that this anomaly was
justified and recommended that the distances applied in the local authority sector
should be brought into line with those required by HSE.
Storage of black powder
At various points in the course of this work the Working Group considered issues
concerned with the storage of black powder in domestic premises. While there were
good grounds for believing that even relatively small quantities could present a
significant hazard there was very little data either on the number of incidents
involving the storage of black powder in domestic premises, or the risks to individuals
in the event of an explosion. In the light of this the Working Group agreed with HSE
that it needed to conduct further research to enable the present requirements to be
considered and to enable HSE to publish soundly-based practical guidance on the
safety measures to be taken when storing black powder.
6
Chapter 1
Introduction and Background
Introduction
1. This report reviews the requirements for separation distances around explosives stores.
The review was carried out on behalf of HSE by Dr Peter Moreton of MBTB consultants
under the auspices of a Working Group set up by the Health and Safety Commission’s
Advisory Committee on Dangerous Substances (ACDS). The Working Group drew together
experts from the industry, MoD and HSE as well as the local authority associations
(Appendix 1).
Background
2. Anyone wishing to store more than 30 kilograms of high explosives must hold a licence.
Under current legislation, local authorities can license the storage of up to 1800 kg of
explosives or 7200 kg (gross weight) of ‘shop goods’ fireworks. Anyone wishing to store
more than these quantities must obtain a licence from the HSE. The main requirement for
licensed storeholders is to maintain minimum distances between the store and neighbouring
‘protected works’ - inhabited buildings and other ‘places of public resort’. These distances
are related to the quantity of explosive held and are listed in tables which have come to be
known as ‘quantity-distance’ tables.
3. The tables used by local authorities are set out in the 1951 Stores for Explosives Order[1].
HSE uses a similar set of tables (referred to as ‘Appendix K’) but covering a greater range of
types and quantities of explosives1. The formula for the curve is 22Q1/3. Similar systems are
used in other European countries although the values used in the formula may differ
somewhat. The distances used for Hazard Type 1 explosives derive largely from data on
bomb blast damage collected during the war. The system of separation distances has been
effective in protecting the safety of the public – in the half century since the introduction of
the current rules there have been no off-site fatalities from explosions at licensed explosives
sites. A more detailed explanation of the basis for the current rules is given in Appendix 2.
4. While the system has worked well, there are two main reasons why a review was
necessary. Firstly, the results of recent trials carried out by the MoD (see Appendix 3)
suggested that the quantity of debris generated in an explosion and the distance to which it
1
Distances for Hazard Type 1 explosives are calculated according to the following formula:
22.4.Q1/3
IBD =
[1 + (3175/Q) 2 ]1/6
where IBD is the inhabited building distance (m) and Q is the net explosives quantity (kg).
Similar systems are used in other European countries although the values used in the formula
may differ somewhat).
7
would be thrown could be considerably greater than had previously been thought. This was
particularly true for smaller stores and stores built of brick and concrete. This suggested the
possibility that, in certain cases, distances set solely to protect against the effects of blast
might not offer sufficient protection against flying debris. Secondly, the distances do not take
into account the numbers of people at risk – the same distances would apply whether the
‘protected works’ were a single house or a high-density housing estate.
5. The review of the distances for stores holding high explosives has had three main parts:
·
developing models to estimate the risks to an individual living near an explosives store
and the risks of an explosion involving multiple fatalities; (Chapter 2)
·
using the models to test the existing separation distance requirements. These case studies
involved hypothetical situations which could be permitted under the existing rules;
(Chapter 4)
·
considering recommendations for new quantity-distance tables.
6. The Working Group has also considered issues concerned with:
·
the distances for stores holding fireworks and propellants;
·
the distances which HSE requires, at the sites which it licences, between explosives
stores (‘inter-magazine distances’) and between process buildings and other buildings on
the site (‘process building distances’).
General principles
7. The Working Group’s approach to the review has been informed by a number of general
principles:
·
the models used for estimating risks and for deriving new recommendations should be
documented and transparent;
·
where the existing distances needed to be replaced, the revised separation distances
should, where feasible, reflect explicit risk criteria;
·
the approach to the regulation of explosives stores should be consistent with the approach
to the regulation of the storage of other hazardous substances;
·
as far as possible the tables used for setting separation distances should be consistent
whether the store is licensed by HSE or by a local authority.
8
‘Fixed rules’ or individual safety cases?
8. Under the present system local authorities have no discretion over whether to refuse a
licence or to vary the distances required2. In contrast HSE has the ability to vary separation
distances to take into account other safety measures, and it is open to the applicant to apply to
the HSE for such a licence.
9. There was some debate in the Working Group about whether the present ‘fixed rules’
system should be replaced by a system based on individual site safety cases. The
representatives of the Institute of Explosives Engineers were strongly of this view. This issue
was included in the Discussion Document published by the Health and Safety Commission in
1999. The view of the overwhelming majority of respondents was that they wished to retain
the present system because it was inexpensive, transparent, and consistent, while maintaining
a satisfactory level of public safety. The recommendations of the report were informed by the
outcomes of this consultation.
Sources of uncertainty
10. Finally it should be borne in mind that the models used can only provide estimates
and not precise predictions of lethality at most ranges of practical interest. The output
of these models will inevitably be subject to some degree of uncertainty. This
uncertainty arises from many sources, but crucial amongst these are:
·
limited data. While the tests carried out by the MoD provide the most extensive
data on debris effects available for the types of store commonly used for
commercial storage of explosives, the high cost of these trials inevitably meant
that only a limited number could be carried out and so the data set is small
·
the use of generic values for factors which are inherently variable – such as
climatic and topographical effects and variation in the vulnerability of the exposed
population to explosion effects. It should be appreciated that such models cannot
provide precise predictions of lethality - the exact outcome will depend on a
number of diverse factors (including the state of repair of the exploding building,
the stacking arrangements within the building, climatic conditions, topography,
the state of repair of the exposed buildings, and the age and state of health of the
exposed population) not all of which are amenable to modelling.
·
the use of accident rates based on historical experience. There is also a valid
argument that the use of such data implicitly assumes the continuance of errors
and oversights which give rise to accidents. In general, it might be expected that
processes would become safer over time as a better understanding of risk is gained
with experience and corresponding safety improvements made; in particular, it
might be expected that lessons learnt from accidents and “near misses” would
result in the necessary corrective action to prevent recurrences. From this point of
2
The intention is that the new regulations should give local licensing authorities greater
discretion to refuse a licence where they regard the site as unsuitable for the safe storage of
explosives – for example if there are bulk stores of flammable substances nearby.
9
view, accident rates derived from historical data might be thought to give a
pessimistic indication of current and future accident likelihood.
·
the use of national average accident rates, (which had to be derived from an
estimate of the numbers of stores operational during the period), as opposed to a
synthetic approach identifying all the potential causes of explosions, taking into
account site-specific factors might possibly justify the use of a lower rate than that
suggested here.
11. In the light of these uncertainties, the Working Group considered that the
underlying assumptions built into the models should err on the side of caution. The
models should likely produce slight overestimates of risk for the typical situation but
not produce results which are patently exaggerated. This approach was in keeping
with the ‘conservative best estimate’ philosophy advocated by the HSE[2] in regard to
land-use planning.
10
Chapter 2
Risk Models
12. The first task of the Working Group was to construct a model to estimate the risk
to persons indoors and outdoors at various ranges from an explosives store and the
chance that an explosion would cause multiple fatalities
13. There were two main ‘building blocks’ used in constructing the model. The first
provided an estimate of the probability of an explosion. The second was a method for
estimating the risks to people in the event of an explosion, which would in turn
depend on models of blast and fragmentation effects including assumptions on issues
such as the trajectory of flying debris, and assumptions about the proportion of people
present in the risk area and proportions indoors and outdoors.
Probability of an explosion
14. There are several factors which could influence the likelihood of accidental
explosion, these include:
·
the inherent sensitivity of the explosives substances and articles stored; the
types of handling processes employed (which may include a number of builtin engineered safeguards);
·
managerial and procedural safeguards, of which safety culture and training
and supervision of staff are important aspects;
·
security measures.
15. Historical accident records were used to derive an estimate for accident
likelihood[10]. These showed that there had been nine major explosions over the period
1950 –1999. It is estimated that 27,000 storehouse-years had accrued over this period,
giving:
9
= 3.10-4 per storehouse-year.
27,000
16. If the data set is restricted to incidents involving local-authority-type stores and
the post-HSWA period, i.e. 1974 to the present date, three incidents are of relevance
and the corresponding number of storehouse–years is estimated at 15,000. This gives
an accident rate of:
3
= 2.10-4 per storehouse-year
15000
It must be noted that there are considerable uncertainties regarding the numbers of
stores that were operational over this period. There was some debate within the
11
Working Group about both these numbers and the appropriate reference period. It was
agreed to use an accident rate of 10-4 (one chance in ten-thousand) per storehouseyear.
Risks to an individual in the event of an explosion
Modelling blast and fragmentation effects
17. Two blast models were selected by the Working Group: the MoD (Explosives
Storage and Transport Committee - ESTC) Outdoor Blast Model[3] (for population
located in the open) and the MoD (ESTC) Indoor Blast Model[4] (for population
located inside buildings of conventional construction). These models were previously
used by the ACDS for assessing risks from the handling of explosives in ports[5] and
have been used extensively by the MoD for assessing risks around military sites.
Both the Outdoor Blast Model and the Indoor Blast Model predict lethality as a
function of scaled distance. Both models have recently been categorized by the ACDS
‘Consequence Modelling for Explosives Working Party’ as models which “we are
generally content with … have a measure of confidence in”[6].
18. The models have been designed specifically for use in risk assessment studies and
are designed to err on the side of caution, the priority being to avoid any underprediction of risk. In the view of some members of the Working Group these models
give results which are overly pessimistic. However, the issue was not crucial for the
important reason that the blast hazard is minor in comparison with that from flying
debris.
19. While it was possible to predict blast-induced lethality using existing models, the
Working Group had to develop a lethality model for debris effects based on the MoD
trials data. In developing this model the major issues the Working Group needed to
consider were:
·
the assumptions about trajectory of the debris;
·
the minimum kinetic energy a piece of debris must possess to be considered
potentially lethal;
·
the likely target area presented by individuals in the zone where debris is falling.
(Please see Appendix 3 for a full detailed discussion of the development of the
model).
12
Trajectory
20. The data from the MoD trials described the directions and distances from the store
where the pieces of debris had come to rest. Estimating the risk to people in the event
of an explosion involves making assumptions about the trajectory of the debris – if a
piece of debris is thrown outwards horizontally and flies at head height or below there
is a risk that it could strike someone at any point along its path of flight, on the other
hand if a piece of debris is thrown high into the air and lands at a sharp angle there is
only a risk to people in the area where it falls.
21. The Working Group took the view that stores built of brick and concrete would be
likely to behave differently from steel stores, and mounded stores would be likely to
behave differently from unmounded stores.
22. The Working Group also took the view that, in the case of brick and concrete
stores, the debris produced from the break up of the concrete roof would be propelled
upwards while the brick-wall debris would be propelled mostly horizontally outwards.
23. The video evidence collated by the MoD from various trials led the Working
Group to distinguish between brick-built stores holding less than 50 kg of explosives,
where all of the debris produced from the break up of the walls is assumed to be
projected out horizontally, and brick-built stores holding larger quantities of
explosives where up to two-thirds of wall debris will be projected horizontally.
24. Where a mound is erected around a store, horizontally projected debris would
either be trapped/slowed by the mound or else ricochet in an upward direction. Roof
debris would similarly be projected high into the air and it too would land at a sharp
angle. For mounded brick stores it was assumed that all of the debris had been
launched upwards and thus only posed a hazard within the area in which it had fallen.
25. The Working Group’s view was that steel stores would behave somewhat
differently. In the ‘ideal’ case, a steel store would balloon before fragmenting, leading
to an even distribution of debris launch angles over the range 0° - 90° with respect to
the horizontal. Ballistic calculations show that in this case very little of the debris
passing over a sector would fly past at head height or below.
26. All debris produced from mounded steel stores was assumed to pose a risk only to
persons in the area where it landed. For unmounded steel stores, one-third of the
debris found in any sector was assumed to have passed at below head height through
the inner sectors. The debris densities calculated on the basis of the latter assumption
are known as modified pseudo trajectory normal (MPTN)[7]. The procedure is not as
conservative as it might first appear, as the MPTN algorithm computes debris
densities in the horizontal rather than the smaller vertical plane; as such it can be
shown that it equates to an assumption that only about 1 in 30 fragments passing a
given range will be travelling at head height or below.
13
Kinetic energy
27. Not all falling debris will be potentially lethal. The traditional criterion in the US
and Western Europe [8] is that debris possessing a kinetic energy of 80 joules or more
should be considered potentially lethal3. An example of such a missile would be a
cricket ball (mass approximately 160g) thrown hard. In the view of some members of
the Working Group the criterion is pessimistic. Much will depend on the part of the
body stuck; a missile possessing a kinetic energy of 80J might well prove fatal should
it strike a person on the skull but is unlikely to do so should it strike a limb. After
discussion the Working Group agreed to retain the 80 J criterion in the spirit of the
conservative best estimate approach[2].
Target area
28. The probability that a person at a given range from an exploding building will be
struck by debris is the product of the density of the debris at that range and the
effective target area presented by the person, i.e.:
E=D%A
where:
E is the expected number of hits,
D is the debris density (m-2) and
A is the effective target area (m2)
3
th
The origins of the 80 J criterion date back to the early 19 century but more recently it has
been shown to envelope the many more sophisticated debris mass/velocity/fatality models
[13]
that have been developed
14
29. The effective target area will depend on both the size and shape of the person and
the trajectory of the incoming debris. If, for ease of calculation, a cuboid shape is
assumed for the person facing the explosion, then the effective target area will be as
illustrated in the following diagram.
Figure 1:
Effective target area measured in horizontal plane
W
B
H
a
The target area is then given by:
A = WB + HWCot(a)
where a is the angle of descent at impact.
30. It is clear that the target area diminishes as a increases. Ballistic calculations
show that debris landing in the mid to far field (where the IBD will be located) will
mostly impact the ground at angles between 49° and 76°. Taking commonly used
values for the dimensions of the cuboid, W = 0.2 m, B = 0.4 m, H = 1.14 m, the target
area is found to range between 0.31 m2 and 0.14 m2, with 0.22 m2 being the average.
31. With regard to horizontally projected debris, the target area is taken as 0.56 m2
(this is slightly conservative given the above model but has been retained as a value
used by a number of authorities in the field).
32. In general, people indoors are at less risk from flying debris because of the
protection offered by the walls and roof of the building Clearly the degree of
protection will increase the smaller the area of glazing and the greater the thickness
and strength of the walls and the roof.
15
33. The approach adopted was the simple one of factoring down the predictions of the
outdoor fatality model to take account of the sheltering effect of the building
structure. A reduction factor was calculated based on the assumption that building
occupants will only be at risk from those pieces of debris which strike an area of
glazing. Taking account of typical debris descent angles and dimensions for modern
housing a value of 1/12 was derived.
Deriving lethality functions
34. Fatality probabilities were calculated from the expected number of hits by
potentially lethal debris, according to the Poisson formula:
Lo = 1 - e - D´ A
where D
A
is the lethal debris density, and
is the effective target area of the exposed person
This formula takes account of the fact that even when the expected number of hits is
one or greater, there is still a chance that the person may escape being hit and the
fatality probability is consequently less than unity. For instance, when the expected
number of hits is exactly one, the fatality probability is: 1-e-1 = 0.6.
35. Finally, a lethality function is derived by plotting the lethality against range data
points and performing a regression analysis to find the best-fit polynomial curve.
This is discussed in greater detail in Appendix 3.
Fractional exposure and time spent indoors/outdoors
36. Clearly people are only at risk from explosives stores while they are within range
of the harmful effects that would be produced in the event of an explosion. This time
of exposure will vary for persons living, working or travelling by the store. A passing
motorist, for example, will be exposed to risk for only a very short period of time;
indeed this time may be so short as to make that person’s individual risk negligible.
At the other extreme, a housebound person living in close proximity to the store may
be constantly exposed to risk.
37. Residents are assumed to have a fractional exposure of unity (i.e. they are exposed
to the hazard 100% of the time). This is very much a worst-case assumption based on
the possibility that there may be a housebound individual, perhaps an elderly or infirm
person, living close to the inhabited building distance.
38. The assumption that FE = 1 for residents has become standard practice in risk
assessment. This assumption has been adopted by HSE in various studies of the risks
arising from industrial activities[13].
39. Residents are further assumed to spend 89% of their time indoors. Again, this is a
figure typically used by HSE in risk studies of hazardous industrial plants.
16
40. Workers are assumed to work a maximum 48-hour week, giving a fractional
exposure of:
48
= 0.29
7 ´ 24
The model for estimating individual risk
41. The individual risk to a person located at a given distance from an explosives store
is given by the following formula:
IR = P ´ FE ´ (TO ´ LO + TI ´ LI )
where
P is the likelihood of accidental explosion, expressed as an annual probability;
FE is the person’s fractional exposure, i.e. the fraction of time per year that the person
is present at the specified distance;
TO is the fraction of time the person spends outdoors at the location;
LO is the conditional probability that the person would be killed in the event of an explosion,
given that he or she is outdoors;
TI is the fraction of time the person spends indoors at the location;
and LI is the conditional probability that the person would be killed in the event of an
explosion, given that he or she is indoors.
17
Chapter 3
Estimating numbers of fatalities
42. The number of fatalities that could be expected in the event of an accident leading
to an explosion will depend on a number of factors: the ranges out to which the blast
and debris effects remain lethal, the population density within those ranges, and the
degree of protection afforded to any exposed persons – in particular whether they are
indoors or outdoors. The procedure for estimating total numbers of fatalities is
illustrated in the following diagram.
Figure 2:
Hazard zones surrounding an explosives store
43. For any given store, the IBD is first determined. It is assumed that members of the
public would not be present within the IBD (inhabited building distance) (except in
those cases where there is a public traffic route (PTR) or open place of resort at the
PTR distance – under previous rules, the PTR distance was set at half the IBD.
44. Next, the areas of successive 20 metre-wide annuli surrounding the store are
determined from the IBD out to a range where the effects of the explosion could to all
intents and purposes be considered sub lethal. The numbers of persons within each of
these annuli are then estimated as the product of area and population density.
18
An estimate for the total number of fatalities, NF, is then given by the following
formula:
n
NF = å Ai ´ Di ´ ( LOi ´ TOi + LIi ´ TIi )
i
Where Ai is the area of annulus i (A1, A2, etc.)
Di is the population density of annulus i
LOi is the lethality for outdoor population in annulus i
TOi is the fraction of time persons in annulus i are outdoors
LIi is the lethality for indoor population in annulus i
TIi is the fraction of time persons in annulus i are indoors
n is the total number of annuli considered.
45. It turns out that that the spread of the debris from the detonator annex produces a
directional effect. Population within the 90º arc drawn from that side of the store to
which the detonator annex is attached would be exposed to the highest risk; this is due
to the additional flying debris produced by the break up of the detonator annex. The
Working Group agreed that tables with two sets of distances (one for the annex side
and another for the other three sides) would be very difficult to administer in practice.
Instead the Working Group has proposed that there should be two sets of tables, one
for stores where the annex had been removed, and one for those stores where the
storeholder had chosen to keep the annex in place. The distances for the latter group
would be derived to reflect the additional hazard to those facing the annex side. The
Working Group felt that while arguably these distances are more than are necessary
for the remaining three sides this approach was the most practical.
19
Chapter 4
Case studies
46. A number of case studies were carried out with the aim of establishing:
·
the maximum level of risk which could, in theory, exist around local authority
stores under the present licensing arrangements, and
·
what levels of risk exist around these stores in more typical situations.
47. Brief details of the sites studied are presented below:
450 kg steel store within the grounds of a quarry: This is a real-life case. The
nearest building to the store is a small office/weighbridge located
approximately 70 metres away. This building is occupied by a maximum of
seven staff during normal office hours (06:00 –18:00 Monday to Friday) and
by four staff on a Saturday between 06:00 and 12:00. There is also a crusher
plant located some 335 metres away and this is occupied by a single member
of staff during the same hours as the office/weighbridge. The nearest building
beyond this range is a workshop located some 425 metres away.
450 kg steel store located near to a housing estate: This is a hypothetical case,
designed to give an indication of the maximum level of individual and group
risk that could arise under the present rules. The estate comprises terraced
housing, the nearest houses forming a ring around the store at a distance of 89
metres (the minimum distance currently permitted).
1800 kg brick store located near to a housing estate: This is similar to the
previous case. The estate comprises terraced housing, the nearest houses
forming a ring around the store at 215 metres – again the minimum distance
allowed under the present rules.
48. Details of the calculations are presented in Appendix 4. The results of the
analyses are tabulated below
Table 2:
Results of case studies
Scenario
Maximum
individual risk
Unmounded 450 steel store (detonator annex
attached) in remote quarry
Unmounded 450 kg steel store (detonator
annex attached) near housing estate
Unmounded 1800 kg brick store near
housing estate
8.10-7
20
Potential
number of
fatalities
2
2.10-6
9
2.10-5
50
49. In none of the cases examined were members of the public found to be exposed to
intolerably high levels of individual risk. The most striking result was that obtained
for the hypothetical brick store located near to a housing estate, which indicates that
the existing rules could allow storage of explosives in locations where an accident
could potentially cause many fatalities.
21
Chapter 5
Developing recommendations for new
requirements on separation distances
50. The work outlined in Chapter 2 provided models for estimating the individual risk
to persons living near an explosives store and the number of fatalities expected in the
event of an explosion.
51. Applying these models to real life and hypothetical case studies has shown that
the present rules might permit the building of an explosives store in a location where
it would present a level of individual risk higher than that which HSE would normally
regard as ‘broadly acceptable’. The present rules would also permit stores near highdensity housing where, if there were an explosion, there could be a large number of
fatalities. Thus there appeared to be good grounds for revising these rules
52. The next step in the Working Group’s work was to consider what the criteria
should be in setting revised distances. There were two sets of issues to consider here,
the first concerned with individual risk and the second concerned with group risk
53. Individual risk, as the name implies is the risk to an identifiable individual, for
example, someone living or working near an explosives store. In the present context
it is measured as the annual probability that that person will be killed as a result of an
accident leading to an explosion inside the store. Group risk measures the number of
fatalities that could be expected in an accident and so can be thought of as a measure
of “disaster potential”. It is normally expressed in the form of a graph showing the
annual probability of an accident leading to N or more fatalities.
Individual risk
54. In recent years individual risk and group risk have become important parameters
in safety assessments of hazardous industrial activities. Indeed modern safety
standards are based on a philosophy of ensuring that these risks are below certain
minimum levels and have been reduced to a level as low as reasonably practicable
(ALARP). This philosophy forms the basis of the Tolerability of Risk (TOR)
framework, developed by the HSE in the 1980s[11].
55. The TOR framework has been adopted by the Working Group both for evaluating
the individual risk from existing local authority stores and for deriving a revised set of
QD prescriptions for new stores.
22
56. The Working Group’s view was that explosives should be regulated on a basis
that is consistent with other hazardous substances. This led the Working Group to take
the view that the criterion should be 10-6 – it would be difficult to justify setting
distances which represented a level of risk where, in other circumstances, HSE would
advise further consideration.
Distances based on an individual risk criterion of 10-6
57. It will be recalled from the discussions presented in Chapter 2 that individual risk
is calculated according to the following formula:
IR = P ´ FE ´ ( LO ´ TO + LI ´ TI )
The values assigned to the parameters in the above formula are listed in the following
table
Parameter
The likelihood of accidental
explosion (P)
The fractional exposure of
the population at risk (FE)
Residents
The fraction of time those at
risk are indoors (TI)
The fraction of time those at
risk are outdoors (TO)
The lethality for population
indoors/outdoors (LI, LO)
Blast
Value assigned
10-4 per storehouse-year
1
0.89
0.11
Lo =
e
æ
ö
æ
ö
ç - 5.785´ç R
÷
÷
1 ÷ +19.047 ÷
ç
çç
÷
3
è Q ø
è
ø
100
2
æ æ R öö
æ æ R öö
æ R ö
ç ç
ç ç
÷÷
÷
÷÷
ç Q1/ 3 ÷ - 0.853.ç Log ç Q1/ 3 ÷ ÷ + 0.356.ç Log ç Q1/ 3 ÷ ÷
øø
è
ø
øø
è è
è è
3
Log ( LI ) = 1.827 - 3.433.Log ç
Debris
Values derived from models specially constructed for
this study
58. Since the explosion consequence models relate lethality to explosives quantity and
distance, back calculations can be performed to obtain the minimum separation
distance that must be set to ensure conformance with the individual risk criterion of
10-6.
Group risk
59. The case studies reported in the previous chapter show that situations can occur
where the risk to any one person is very low but where an accident could still cause
many fatalities.
23
60. Whilst there are well-established criteria for evaluating individual risk, there are
no equivalent widely-accepted criteria for the evaluation of group risk.
61. There was much discussion on this issue within the Working Group. The starting
point was the recommendation contained in the first report of the Advisory
Committee on Major Hazards (ACDS): that the chance of a serious accident
(involving the death of 10 or more people) at any one major non-nuclear plant should
be less than 10-4 per year[12].
62. Guidance was also provided by the HSE discussion document ‘Reducing Risks,
Protecting People’, which proposes that for a single major industrial activity the risk
of an accident causing the death of 50 or more people should be less than one in five
thousand (2.10-4) per annum[11]. The Working Group considered that this might be
taken as an anchor point for an FN line of slope minus one to delineate a level of
group risk at the upper limit of what is tolerable. This approach would give a figure
of 1 in one thousand as the limit of tolerability for an event resulting in the death of
ten people.
63. This approach would define the upper limit of tolerability. The Working Group
took the view that it would not be appropriate for the tables to reflect a level of risk
which was only just on the border of tolerable and intolerable and that they should be
set at a level of risk significantly below this level.
64. It is not clear how such a broadly acceptable or negligible level of group risk
might also be defined. There were differing views within the Working Group about
whether multiple-fatality risks were acceptable at all, but there was a general
consensus that the aim of the controls should be to limit the number of fatalities to
less than ten. It was therefore agreed to propose a criterion which would ensure, with
a ninety per cent confidence level, that the number of fatalities would be less than ten
(this equates to an assumed average number of fatalities of 6.225 and a risk of 10-5 per
year of an accident involving more than 10 fatalities).
Distances based on the group risk criterion
65. The number of casualties produced in an accident will vary with the density of
population surrounding the store. Thus, if the group risk criterion is to be met, it may
be necessary to have separate quantity-distance tables for areas of different population
density.
66. The Working Group initially considered four areas of population density, taking
as a starting point previous work by the ACDS concerning the risks arising from the
transport of dangerous goods[13]. These areas were defined as:
4210 persons per km2
1310 persons per km2
210 persons per km2
20 persons per km2
Urban
Suburban
Built-up Rural
Rural
24
67. However, it soon became apparent that the group risk criterion would only take
effect in the case of stores located near to areas of urban population density4. Thus
only two types of area needed to be considered in the further stages of study: urban
(or “high density”) and non-urban (or “low density”).
68. For the first of these areas, the IBD (inhabited building distance) needs to be set to
ensure that the chance of an accident causing 10 or more fatalities would be less than
10-5 per storehouse-year. Given that the likelihood of accidental explosion is assessed
as 10-4 per storehouse-year, it can be shown that the group risk criterion is met when
the average number of fatalities expected in the event of an accident does not exceed
6.2255.
69. From this it follows that the minimum IBD conforming to the group risk criterion
can be obtained from the following equation:
6.225 = A × D × (LO × TO + LI × TI)
where
A is the area of the danger zone
D is the population density in the danger zone, and
LO, TO, LI and TI are defined as before
70. The danger zone is defined as that area between the IBD and the range where the
effects of any potential explosion would decay to a level which could be considered,
for all practical purposes, sub lethal. The latter range is defined as the distance at
which lethality falls to 0.01%, as predicted by the explosion consequence models.
This range corresponds to an individual risk of 10-8, a value generally regarded as
negligible.
71. For a mounded steel store, without detonator annex, holding 450 kg of high
explosives, the range to 0.01% lethality is found to be 292 metres. Further
calculations show that when a store of this type is located in an urban area, the group
risk criterion will be met when the IBD is set to 88 metres.
Deriving new tables from a limited data set
72. Similar calculations were performed for all other configurations of stores and for
each value of NEQ (Net Explosives Quantity) used in the magazine trials. Clearly if
4
And also, in a few exceptional cases, stores located near to areas of suburban population
density.
5
Assuming a Poisson probability distribution, a value can be calculated for the average
number of fatalities per accident, m, that would meet the criterion. This value is found by
solving the following equation:
0.1 =
m N ´ e-m
N!
N =10
¥
å
It turns out that m = 6.225.
25
the model is to be more generally useful it must be possible to use it to estimate risks
for stores holding quantities of explosives other than those used in the magazine trials.
73. It must be borne in mind that the MoD programme provided results for seven
different quantities of explosives detonated in brick-built stores and only two
quantities of explosives in steel stores. Inevitably, a major issue considered by the
Working Group was how to draw the appropriate function curves given the limited
number of data points. There is a point at which blast becomes the primary hazard and
at which the existing ‘Appendix K’ tables are appropriate. However, it is not clear
where this point lies. Inevitably the Working Group needed to consider what method
to use to extrapolate from the test results and how to integrate the resultant functions
with the ‘Appendix K’ curves.
Steel stores
74. Such an extrapolation is far from being an exact science and indeed is largely a
matter of judgement. This is particularly true in the case of the steel stores for which
two data points only are available, spanning a very small range of NEQ. The
difficulty is illustrated in the following diagram, which shows a plot of the data points
for the configuration of store without mounding and with detonator annex attached.
Figure 5:
QD data points for a steel store without mounding and with
detonator annex attached.
450
400
350
Range (m)
300
250
200
150
100
50
0
0
1000
2000
3000
4000
5000
6000
NEQ (kg)
Appendix K
Urban
Non-urban
75. The previous standard is also shown in this diagram as the solid line (‘Appendix
K’ is the industry terminology for the curve, the formula is 4RB – see Appendix 2). In
the absence of any further information, the Working Group took the view that the new
prescriptions should be derived by (1) drawing straight lines between the two data
26
7000
points and (2) drawing tangents from the curve representing the existing standard to
join with the 450 kg data points. This is shown in the following diagram.
Figure 6:
Distance and Quantity for a steel store without mounding and with
detonator annex attached.
450
400
350
Range (m)
300
250
200
150
100
50
0
0
1000
2000
3000
4000
5000
NEQ (kg)
Appendix K
Urban
Non-urban
Fn(Urban)
Fn(Non-urban)
76. It is acknowledged that this procedure is largely subjective, and that were further
trials to be carried out, a very different relationship between IBD and NEQ could
emerge. It is indeed possible that the tangents may understate the risk for stores
holding NEQ in excess of 450 kg; but in the absence of any hard evidence, the
Working Group was of the opinion that more onerous prescriptions would not be
justified.
27
6000
Brick stores
77. The relationship between IBD and NEQ is more easily discerned in the case of
brick stores. The following diagram shows a plot of the data points for brick stores
without mounding.
Figure 7:
QD data points for brick stores without mounding
600
500
400
300
200
100
0
0
1000
2000
3000
Appendix K
4000
Urban
5000
6000
Non-urban
78. It is apparent that the minimum IBD necessary to achieve conformance with the
risk criteria gradually levels off as NEQ increases. As noted previously, this reflects
the fact that as the power of the explosion increases (a) more and more of the building
material is pulverized into dust and (b) those lethal fragments which are produced are
dispersed more thinly over a wider area.
79. There was some debate within the Working Group about the best procedure for
extrapolating between the data points so as to allow IBD values to be read off for any
given value of NEQ. Two options were considered: (1) connect the data points by
straight lines to produce a series of linear functions which would effectively
“envelope” the points, (2) fit a curve through the points. The majority view favoured
the fitting of a curve so as to produce a continuous function over the range. This is
shown in the following diagram.
28
7000
Figure 8:
Distance and quantity for brick stores without mounding
600
500
Range (m)
400
300
200
100
0
0
1000
2000
3000
4000
5000
NEQ (kg)
Appendix K
Urban
Suburban/BUR/Rural
Fn(Urban)
Fn(Suburban/BUR/Rural)
80. A curve-fitting software package was used to obtain the best-fit curves for the
data. One important feature of this relationship is the sensitivity of the explosives
limit to changes in the IBD over the range 400 – 500 metres. This sensitivity arises
regardless of whether the data points are joined by straight lines or curves.
81. Finally, a connection was made with the existing standard by drawing tangents
from the Appendix K curve through the 5600 kg data points.
29
6000
Chapter 6
Other issues
Other risk reduction measures
82. Analysis of the MoD trials data has highlighted two other important findings:
·
a mound constructed from a double row of 0.6 m3 Hesco Bastion earth-filled units
and with the row stacked three units high proved to be very effective in reducing
the debris hazard from steel stores. However, a mound constructed from a single
row of Hesco Bastion units was found to be largely ineffective;
·
the steel detonator annex attached to the back of the proprietary steel magazine
was found to have a marked effect on the debris pattern. Over 50% of the total
amount of lethal debris produced was projected in a direction normal to the back
of the store. This result is explained by the additional secondary fragmentation
produced by the break up of the detonator annex. Clearly the removal of the
detonator annex would constitute a significant risk reduction measure.
In the light of these findings the working group agreed that the tables for separation
distances should take account of these safety measures. There are therefore separate
tables for mounded stores and for steel stores where the annex has been removed.
Storage of HT1 explosives in registered premises
83. The Working Group noted that the present requirements on separation distances
did not apply to registered premises. However the data from the MoD trials clearly
demonstrated that even small quantities of HT1 explosive presented a considerable
hazard. The present exclusion of registered premises from the separation distances did
not appear to be justifiable on safety grounds. The Working Group therefore
recommended that the new tables should also cover registered premises.
Internal separation distances
84. In addition to the requirement to maintain separation distances between explosives
stores (and other explosives buildings) and inhabited buildings off-site, HSE also
requires operators of explosives factories to maintain separation distances between
production buildings and explosives stores (process building distances). The
requirements for these distances reflect the fact that production activities have the
highest risk and there is a need to limit the severity of the consequences of an
explosion in a production area by: a) limiting the amounts of explosive that may be
kept in them and b) separating these buildings from the those holding the bulk stores
of explosive.
30
85. In addition to these distances, where there is more than one explosive store on site
the operator is required to maintain separation distances between them – these are
referred to as ‘inter-magazine distances’.
Process building distances
86. Process distances are those observed from any explosives building or stack to
buildings in which an explosives process or work in connection with the processing of
explosives is being carried out. These distances are designed to provide a reasonable
degree of immunity from severe injury for the operators in the receptor building.
In summary the findings are as follows:
·
there have been 77 fatal explosives accidents on licensed UK manufacturing sites
during the last 50 years.
·
there is no evidence to show that any of these accidents resulted in fatal injury to
persons at the process building distance.
·
there is ample evidence to show that some of these accidents did cause injury to
persons in other buildings on the site. However, details are sketchy; in none of the
reports examined were the exact distances at which persons were injured recorded,
and nor were details provided of the extent of these injuries.
·
a number of these accidents resulted in significant damage to process buildings
and other buildings on site.
87. The Working Group’s view was that, in the absence of more complete
information, the existing distances should stand. However, it felt that HSE should
consider further research on this topic as part of its research programme.
Propagation of explosions
88. The inter-magazine distances are intended to prevent the instantaneous
communication of an explosion - ie to ensure that an accidental initiation of a quantity
of explosives in one storehouse does not propagate immediately to the explosive
material in a neighbouring storehouse. The Working Group agreed that although a
brick magazine could be destroyed in the event of the detonation of an adjacent
magazine, the present distances would ensure that this building collapse would almost
certainly not initiate an explosion in most, if not all, explosives in commercial or
military use.
Vulnerable buildings
89. At present factories and magazines with HSE licences are required to maintain a
greater separation distance between explosives buildings and buildings of vulnerable
construction – for example, offices or flats using ‘curtain wall’ construction. These
distances (known as ‘vulnerable building distances’) are calculated using the formula
31
8RB – i.e. double the normal distances based on blast hazard. There are no similar
distances for local authority licensed stores.
90. The Working Group recommended that the vulnerable building distances should
be retained and there appeared to be a strong case for including similar requirements
in the tables for distances to be maintained around local authority stores.
91. The Working Group’s view was that, because the hazard was primarily blast
related, the distances should continue to use the formula 8RB.
Vulnerable populations
92. The Working Group considered the issue of whether the distance requirements
should be increased for stores near schools, hospitals or sheltered accommodation for
old people. The analogy would be with the process for assessing applications for
planning consent for other hazardous installations where, in assessing the tolerability
of the level of risk, greater weighting is given to vulnerable populations because they
would be more difficult to evacuate in the event of an emergency and more vulnerable
to the effects of toxic substances. In the case of explosives the Working Group did not
believe that, given the instantaneous effects of explosives hazards, the greater
weightings for vulnerable populations applied.
93. The Working Group did recognize that there might be societal concerns over the
siting of explosives stores where these groups would be at risk. This would therefore
be an issue which licensing authorities might wish to take into account in deciding
whether a store should be sited in a given location.
Public traffic routes
94. At the moment the law requires a separation distance between stores and public
traffic routes and 'places of public resort’ of half the distance which would apply to an
inhabited building. This applies whether the traffic route is an occasionally used
footpath or a busy motorway.
95. The major difficulty the Working Group faced was that, because the fractional
exposure is so low, the individual risk from an explosives store to passersby (whether
in vehicles or on foot) would be likely to be extremely low. Also, given that drivers
and passengers of vehicles will be relatively well protected from blast and fragment
effects, the primary cause of loss of life would be likely to be that of vehicle crashes
caused by drivers being startled etc. The consequences of an explosion near to a busy
motorway could be extremely grave however it would be extremely difficult to model
these effects.
96. The Working Group’s view was that, in view of these difficulties, it was best to
err on the side of caution and to retain the present system with modifications to take
greater account of traffic density. The Working Group therefore endorsed proposals
for revising these requirements.
97. The proposals are that
32
Ÿ where there were more than 500 but less than 5,000 person/vehicle movements in
any 24 hour period a separation distance of half the full distance should be required.
This distance should also apply to all passenger railway lines;
Ÿ where there were more than 5,000 person/vehicle movements in any 24 hour
period the full separation distance should apply;
Ÿ where there are less than 500 person/vehicle movements in any 24 hour period the
required separation distance would be one quarter the inhabited building distance;
Ÿ where there were less than 50 vehicle/person movements in any 24 hour period
then there no separation distance would be necessary.
98. Temporary variations in traffic levels (due to diversions etc) would be
disregarded.
Storage of HT4 and HT3 explosives
99. The Working Group noted that the present tables for local authority stores impose
the same distances for manufactured fireworks as for high explosives (when explosive
content is compared, rather than the gross weight). The distances required by HSE for
Hazard Type 1.3 and 1.4 fireworks are significantly less - reflecting the fact that the
primary hazard is fire/heat radiation. The Working Group’s view was that, for Hazard
Type 1.3 and 1.4 fireworks, the distances required by the present local authority tables
are not justified by the risk; the distances used by HSE are more than sufficient to
maintain a high standard of safety.
100. At the same time the Working Group noted that up to 1 tonne (gross – 250kg
net) of ‘shop goods’ fireworks could be kept in registered premises without any
external separation distance. Again the Working Group did not believe that this
anomaly was justified and recommended that the distances applied in the local
authority sector should be brought into line with those required by HSE.
101. There was some discussion within the Working Group about the treatment of
small arms ammunition and similar explosives articles which shared the HT4 category
with fireworks. Recent explosions at firework stores suggest that there are
uncertainties about the behaviour of fireworks stored in bulk. HSE has recently
carried out research on this issue and submitted proposals to the European
Commission for collaborative research on this issue. To a degree the distances set by
HSE reflect this uncertainty. Some members of the Working Group believed that
while there might be uncertainties over the behaviour of fireworks, there was good
evidence to support the conclusion that a fire in a store of certain types of nonfireworks HT 4 would be contained within the storage building and there was
therefore no need to require more than a nominal separation distance. The Working
Group agreed this issue should be considered further.
Storage of black powder
102. At various points in the course of this work the Working Group considered
issues concerned with the storage of black powder in domestic premises. While there
33
were good grounds for believing that even relatively small quantities could present a
significant hazard there was very little data either on the number of incidents
involving the storage of black powder in domestic premises, or the risks to individuals
in the event of an explosion. In the light of this the Working Group agreed with HSE
that it needed to conduct further research to enable the present requirements to be
considered and to enable HSE to publish soundly-based practical guidance on the
safety measures to be taken when storing black powder.
34
Chapter 7
Proposed tables
103. The following tables set out the recommendations for new separation
distances. The distances are the distances to be maintained between the explosives
building and inhabited buildings which are not occupied by the storeholder/site
operator. Separation distances must also be maintained between explosives buildings
and public traffic routes and public spaces. The distance to public traffic routes will
depend on traffic densities – see paragraph 97.
HT1 brick-built (mounded)
Quantity of
explosives
(kg)
Low
Density
Distance
(m)
0.1 - 25
101
25 - 50
50 - 75
75 - 100
100 - 150
150 - 200
200 - 300
107
112
118
128
139
161
300 - 400
183
400 - 450
450 - 500
500 - 600
600 - 700
700 - 800
800 - 900
900 - 1000
1000 - 1100
1100 - 1200
1200 - 1300
1300 - 1400
1400 - 1500
1500 - 1600
1600 - 1700
1700 - 1800
1800 - 1900
1900 - 2000
2000 - 3000
193
204
204
204
204
204
204
204
204
204
204
204
204
208
215
222
229
285
3000 - 4000
328
4000 - 5000
362
5000 -10000
475
Maximum
number of
dwellings in Reference
Reference zone radius
(m)
Zone
High
Density
Distance
(m)
-
-
-
-
-
-
81
96
128
257
278
322
-
-
-
-
-
206
206
206
206
206
206
206
206
206
206
214
229
244
259
408
408
408
408
408
408
408
408
408
408
416
431
444
458
-
-
-
-
-
107
112
118
128
139
161
183
231
238
245
250
255
259
263
266
269
272
274
277
279
281
-
35
101
142
156
180
-
Vulnerable
Building
Distance (m)
193
204
216
238
260
280
300
319
337
354
370
386
402
416
431
444
458
570
656
724
950
Quantity of
explosives
(kg)
Low
Density
Distance
(m)
10000 - 15000
548
15000 - 20000
606
20000 - 25000
653
25000 - 30000
695
Maximum
number of
dwellings in Reference
Reference zone radius
Zone
(m)
-
-
36
High
Density
Distance
(m)
-
Vulnerable
Building
Distance (m)
1097
1211
1306
1389
HT1 brick-built (unmounded)
Low
Quantity of
Density
Explosives
Distance
(kg)
(m)
0.1 - 25
141
25 - 50
160
50 - 75
180
75 - 100
199
100 - 150
230
150 - 200
256
200 - 300
293
300 - 400
320
400 - 450
331
450 - 500
340
500 - 600
355
600 - 700
367
700 - 800
377
800 - 900
385
900 - 1000
392
1000 - 1100
398
1100 - 1200
403
1200 - 1300
408
1300 - 1400
412
1400 - 1500
415
1500 - 1600
418
1600 - 1700
421
1700 - 1800
424
1800 - 1900
426
1900 - 2000
428
2000 - 3000
442
3000 - 4000
449
4000 - 5000
454
5000 -10000
495
550
10000 - 15000
606
15000 - 20000
653
20000 - 25000
695
25000 - 30000
Vulnerable
Building
Distance (m)
141
160
180
199
230
256
293
320
331
340
355
367
377
385
392
398
403
408
412
415
418
421
431
444
458
570
656
724
950
1097
1211
1306
1389
37
HT1 steel (mounded), detonator annex on or off
Quantity of
Explosives
(kg)
0.1 - 25
25 - 50
50 - 75
75 - 100
100 - 150
150 - 200
200 - 300
300 - 400
400 - 450
450 - 500
500 - 600
600 - 700
700 - 800
Maximum
Low
High
Number of
Density Dwellings in Reference
Density
Distance Reference zone radius Distance
(m)
(m)
(m)
Zone
34
6
68
45
37
7
74
45
40
8
80
45
43
9
86
48
49
12
97
55
54
15
109
62
68
23
136
76
83
89
96
108
119
130
-
800 - 900
140
900 - 1000
150
1000 - 1100
159
1100 - 1200
168
1200 - 1300
177
1300 - 1400
185
1400 - 1500
193
1500 - 1600
201
1600 - 1700
208
1700 - 1800
215
1800 - 1900
222
1900 - 2000
229
2000 - 3000
285
3000 - 4000
328
4000 - 5000
362
5000 -10000
475
10000 - 15000
548
15000 - 20000
606
20000 - 25000
653
25000 - 30000
695
-
-
38
-
Vulnerable
Building
Distance (m)
40
48
54
66
86
104
136
165
178
191
216
238
260
280
300
319
337
354
370
386
402
416
431
444
458
570
656
724
950
1097
1211
1306
1389
HT1 steel (unmounded), detonator annex removed
Quantity of
Explosives
(kg)
0.1 - 25
25 - 50
50 - 75
75 - 100
100 - 150
150 - 200
200 - 300
300 - 400
400 - 450
450 - 500
500 - 600
600 - 700
700 - 800
800 - 900
900 - 1000
1000 - 1100
1100 - 1200
1200 - 1300
1300 - 1400
1400 - 1500
1500 - 1600
1600 - 1700
1700 - 1800
1800 - 1900
Maximum
Low
High
Number of
Density Dwellings in Reference
Density
Distance Reference zone radius Distance
(m)
(m)
(m)
Zone
38
9
74
43
41
9
82
46
43
9
86
49
45
10
91
55
50
12
100
66
55
15
110
78
68
23
136
101
83
34
165
124
89
39
178
135
96
45
191
138
108
57
216
144
119
70
238
150
130
83
260
156
140
97
280
162
150
111
300
168
159
168
177
185
193
201
208
215
222
-
1900 - 2000
229
2000 - 3000
285
3000 - 4000
328
4000 - 5000
362
5000 -10000
475
10000 - 15000
548
15000 - 20000
606
20000 - 25000
653
25000 - 30000
695
-
-
39
-
Vulnerable
Building
Distance (m)
40
48
54
66
86
104
136
165
178
191
216
238
260
280
300
319
337
354
370
386
402
416
431
444
458
570
656
724
950
1097
1211
1306
1389
HT1 steel (unmounded), detonator annex on
Quantity of
Explosives
(kg)
0.1 - 25
25 - 50
50 - 75
75 - 100
100 - 150
150 - 200
200 - 300
300 - 400
400 - 450
450 - 500
500 - 600
600 - 700
700 - 800
800 - 900
900 - 1000
1000 - 1100
1100 - 1200
1200 - 1300
1300 - 1400
1400 - 1500
1500 - 1600
1600 - 1700
1700 - 1800
1800 - 1900
1900 - 2000
2000 - 3000
3000 - 4000
4000 - 5000
Maximum
Low
High
number
Density of dwellings Reference
Density
Distance in Reference zone radius Distance
(m)
(m)
(m)
Zone
38
11
74
53
43
11
86
53
48
11
96
60
53
14
106
77
63
20
127
110
74
27
147
143
94
44
188
209
115
65
229
275
125
77
250
308
128
81
257
309
135
90
270
311
142
99
283
312
148
109
297
314
155
119
310
316
162
129
324
318
169
140
337
319
175
152
350
321
182
163
364
323
189
176
377
325
195
188
391
326
202
202
404
328
209
215
417
330
215
229
431
332
222
244
444
333
229
259
458
335
285
401
570
353
328
531
656
370
362
-
5000 -10000
475
10000 - 15000
548
15000 - 20000
606
20000 - 25000
653
25000 - 30000
695
-
-
40
-
Vulnerable
Building
Distance (m)
54
54
54
66
86
104
136
165
178
191
216
238
260
280
300
319
337
354
370
386
402
416
431
444
458
570
656
724
950
1097
1211
1306
1389
HT2 (0.7 kg net mass per item or more)
Quantity of explosives
(kg)
0.1 - 25 kg
26-50 kg
51-75 kg
76-100 kg
101-150 kg
151-200 kg
201-300 kg
301-400 kg
401-450 kg
451-500 kg
501-600 kg
601-700 kg
701-800 kg
801-900 kg
901-1000 kg
1001-1100 kg
1101-1200 kg
1201-1300 kg
1301-1400 kg
1401-1500 kg
1500-1600 kg
1601-1700 kg
1701kg-1800 kg
1801-1900 kg
1901-2000 kg
2001-3000kg
30001-4000kg
4001-5000 kg
5001-10000 kg
10001-15000kg
15001-20000kg
20001-25000kg
25001-30000kg
Low Density
Distance
45m
88m
108m
129m
148m
168m
191m
207m
213m
219m
226m
233m
240m
248m
256m
259m
262m
266m
270m
274m
278m
282m
286m
288m
292m
312m
326m
337m
370m
388m
401m
411m
419m
Vulnerable building
distance (m)
90m
176m
216m
238m
296m
336m
382m
414m
426m
438m
452m
466m
480m
496m
512m
518m
524m
532m
540m
548m
556m
564m
572m
576m
592m
624m
652m
674m
740m
776m
802m
822m
838m
41
HT 2 (0.7kg net mass per item or less)
Quantity of explosives
(kg)
0.1 - 25 kg
26-50 kg
51-75 kg
76-100 kg
101-150 kg
151-200 kg
201-300 kg
301-400 kg
401-450 kg
451-500 kg
501-600 kg
601-700 kg
701-800 kg
801-900 kg
901-1000 kg
1001-1100 kg
1101-1200 kg
1201-1300 kg
1301-1400 kg
1401-1500 kg
1500-1600 kg
1601-1700 kg
1701kg-1800kg
1801-1900 kg
1901-2000 kg
2001-3000kg
30001-4000kg
4001-5000 kg
5001-10000 kg
10001-15000kg
15001-20000kg
20001-25000kg
25001-30000kg
Low Density
Distance
37
43
47
51
56
60
66
71
73
74
76
78
81
84
87
88
89
90
91
92
94
95
97
99
101
110
117
122
140
151
159
166
171
Vulnerable building
distance (m)
76
86
94
102
112
120
132
142
146
148
152
158
162
164
174
176
178
180
182
184
188
190
194
198
202
220
234
244
280
302
318
332
342
42
HT3
Quantity of Explosives (kg)
Low Density Distance (m)
0.1 - 25 kg
26-50 kg
51-75 kg
76-100 kg
101-150 kg
151-200 kg
201-300 kg
301-400 kg
401-450 kg
451-500 kg
501-600 kg
601-700 kg
701-800 kg
801-900 kg
901-1000 kg
1001-1100 kg
1101-1200 kg
1201-1300 kg
1301-1400 kg
1401-1500 kg
1500-1600 kg
1601-1700 kg
1701kg-1800 kg
1801-1900 kg
1901-2000 kg
2001-3000kg
30001-4000kg
4001-5000 kg
5001-10000 kg
10001-15000kg
15001-20000kg
20001-25000kg
25001-30000kg
23m
25m
29m
33m
37m
42m
47m
47m
50m
51m
53m
54m
55m
63m
70m
71m
72m
73m
74m
75m
76m
78
79m
80m
91m
100m
107m
136m
156m
172m
185m
199m
43
HT4
Quantity of Explosives (kg)
200 – 400
400 – 500
500 – 900
900 – 1000
1000 - 1100
1100 - 1300
1300 - 1500
1500 - 1700
1700 - 1900
1900 - 2000
2000 - 3000
3000 - 4000
4000 - 5000
5000 - 10000
10000 - 15000
15000 - 20000
20000 - 25000
25000 - 30000
Low Density Distance (m)
5
10
15
20
21
22
23
24
25
30
35
40
45
51
54
55
58
60
44
References
1. The Stores for Explosives Order 1951 (SI 1951/1163)
2. Health and Safety Executive, Risk criteria for land-use planning in the vicinity of
major industrial hazards, HMSO, 1989.
3. Edmondson J N, Fatality Probabilities for People in the Open when Exposed to
Blast, SRD Report RANN/2/49/00082/90 Issue 1, March 1992.
4. Hewkin D J, Consequences of pressure blast: the probability of fatality inside
buildings, Minutes of the Twenty-fifth Explosives Safety Seminar, Anaheim,
California, US Department of Defense Explosives Safety Board, 1992.
5. Health and Safety Commission, Advisory Committee on Dangerous Substances,
Risks from handling explosives in ports, HMSO, 1995, ISBN 0 7176 0917 0.
6. Health and Safety Commission, Advisory Committee on Dangerous Substances,
Selection and use of explosion effects and consequence models for explosives,
HMSO, 2000, ISBN 0 7176 17912.
7. Gould M J A and Swisdak M M, Procedures for the collection, analysis and
interpretation of explosion-produced debris, NATO AC/258(ST)WP/209.
8. McCleskey F et al., A comparison of two personal injury criteria based on
fragmentation, Minutes of the 24th DOD Explosives Safety Seminar, August 1990.
9. Carter D A and Hirst I L, Worst case methodology for the initial assessment of
societal risk from proposed major accident installations, J Haz Mat, 71 (2000), 117128, 2000.
10. Merrifield R and Moreton P A, An examination of the major-accident record for
explosives manufacturing and storage in the UK, J Haz Mat, A:63, 107-118, 1998.
11. Le Guen J et al., Reducing Risks, Protecting People, Health and Safety Executive,
1999.
12. Health and Safety Commission, First Report of the Advisory Committee on Major
Hazards, 1976.
13. Health and Safety Commission, Advisory Committee on Dangerous Substances,
Major Hazard Aspects of the Transport of Dangerous Substances, Report and
Appendices, HMSO, 1991.
14. Gould M J A and Cuthbertson K, UK/Australian Small Quantity Explosion Effects
Tests and their Analysis, Minutes of the Australian Explosives Ordnance
Symposium PARARI 97, 1997.
45
Appendix 1
Membership of the ACDS QuantityDistances Working Group
HSE would like to thank the past and present members of the ACDS QuantityDistance Working Group including
Chairman
Alan Duckworth
HSE Chief Inspector of Explosives
HSE
Dr Roy Merrifield
HSE Methodology and Standards
Development Unit
Paul Rushton
MoD
Lt Col Des Townsend
Mr Jon Henderson
ESTC
EIG
Ian McIntosh
Ron Rapley
Dr Tom Smith
Royal Ordnance plc
Black Cat Fireworks
Davas
IExE
Bill Fowler
Peter McGoff
Bob Wilcox
Demex Explosives Technology
Rocklift Ltd.
ExploSafety
COSLA
Superintendent Jim Moulson Strathclyde Police
TUC
John Wraige
Solar Pyrotechnics
Independent
Dr Peter Moreton
MBTB Ltd
Secretary
Andy Miller
HSE Explosives Policy
Minutes}
Secretary}
Cherry Knight
Cherone Ashdown
HSE Explosives Policy
46
Appendix 2
The basis for the previous Quantity
Distances
The quantity-distance prescriptions previously applied to stores containing HT 1
explosives were not directly aimed at limiting the risk of fatal injury but rather were
aimed at limiting housing damage in the event of an accident. The formula by which
the prescriptions were obtained was derived in the late 1940s from an analysis of
numerous incidents of wartime bombing and accidental explosion. This work led to
the definition of five categories of housing damage. Starting with the most severe,
these categories are:
Category A:
Houses completely demolished.
Houses so badly damaged that they are beyond repair
Category B:
and must be demolished when the opportunity arises. Property is included in
this category if 50 - 75% of external brickwork is destroyed, or in the case of
less severe damage if the remaining walls have gaping cracks rendering them
unsafe.
Houses rendered uninhabitable but which can be
Category C(b):
repaired with extensive work. Examples of damage resulting in such
conditions include partial or total collapse of roof structure, partial demolition
of one or two external walls up to 25% of the whole, and severe damage to
load bearing partitions necessitating demolition and replacement.
Houses rendered uninhabitable but which can be
Category C(a):
repaired reasonably quickly under wartime conditions. The damage sustained
does not exceed minor structural damage, for example partitions and joinery
wrenched from fittings.
Houses requiring repairs to remedy serious
Category D:
inconveniences but remaining inhabitable. Houses in this category may have
sustained damage to ceilings and tiling, batons and roof coverings and minor
fragmentation effects on walls and window glazing. Cases in which the only
damage amounts to broken glass in less than 10% of the cases are not included
Figure A2 shows an example of category B damage. Examples of Category A, C and
- D damage are shown in Annex 1.
47
Figure A2:
Category B Damage
The work carried out in the late 1940s produced the following equation for the
average-circle-radius of Category B housing damage.
RB =
1/3
5.6Q
2 1/6
[1 + (3175/Q) ]
where RB is the average-circle-radius of Category B damage (m)
Q is the explosives quantity (kg)
It should be appreciated that the “average-circle-radius” is a statistical concept and the
above formula does not give a precise estimate of the distance from an explosion at
which Category B housing damage will occur. The equation is empirically derived
and defines a radius of a circle such that the number of houses inside the circle which
sustain a less severe degree of damage than Category B is matched by the number
outside which sustain a degree of damage equal to or more severe than Category B.
For example, the value of the average-circle-radius (RB) corresponding to a 10,000 kg
quantity of explosives is predicted by the above formula to be 119 metres. Were this
quantity of explosives to detonate in a built up area, it might be expected that there
would be as many houses with Category C and D damage within 119 metres of the
explosion as there would be houses with Category A and B damage beyond this
range.
As noted in Chapter 1, the inhabited building distance (IBD) is in fact set at four times
the average-circle-radius of Category B housing damage (hence the term 4RB) i.e.
48
IBD =
22.4.Q 1/3
[1 + (3175 / Q) 2 ]1/6
Subsequent work has shown that the risk of blast-induced fatal injury at the 4RB
distance is very low. This is shown by the data presented in the following table,
which has been complied using the indoor and outdoor blast fatality models
developed by the ESTC during the 1990s. The predictions are for a range of net
explosives quantity (NEQ), from the Appendix K lower limit of 50 kg to 100,000 kg,
Table A2.1:
Fatality probability predicted by the MoD (ESTC) Indoor and Outdoor
Blast Models
NEQ
(kg)
IBD (4RB)
(m)
50
250
500
1000
5000
10,000
100,000
21
60
96
150
362
475
1040
Fatality Probability
Persons
Persons
Outdoors Indoors
0
0.08
0
0.01
0
0.004
0
0.002
0
0.0004
0
0.0003
0
0.0003
It is seen that for persons in the open the blast hazard is negligible. Persons indoors
are at slightly greater risk of fatal injury due to: (a) a potential flying glass hazard
from shattered windows and; (b) a small possibility of building collapse.
However, 4RB is essentially a blast criterion which does not take account of the debris
hazard resulting from the break up of the roof and walls of the storage building.
Magazine trials carried out in recent years by the ESTC, the results of which are
reported here, show that there could be a considerable risk of fatal injury from flying
debris at the 4RB distance. The risk is most acute in case of brick stores containing
relatively small NEQ, as shown by the data presented in the following table.
Table A2.2:
NEQ
(kg)
50
100
250
500
1800
5600
Probability of fatal injury due to flying debris
IBD (4RB)
(m)
21
33
60
96
215
380
Fatality Probability
Mounded
Unmounded
brick store
brick store
0.8
1.0
0.5
1.0
0.1
1.0
0.1
1.0
0.01
0.9
0.005
0.1
It is seen that as the NEQ increases the risk of fatal injury diminishes. This result is
explained by two facts: (1) as the power of the explosion increases more and more of
49
the building material is pulverized into dust6; (2) the range to which the debris is
projected does not increase proportionately with NEQ.
The first of these facts is demonstrated by the data shown in the following table,
compiled from the results of three of the brick magazine trials carried out by the
ESTC. This table shows the ratio of the number of pieces of potentially lethal debris
collected in the four 10° arcs drawn normal from the centre of the walls of the
magazine to the total volume of wall and roofing material in the building.
Table A2.3: Ratio of number of pieces of potentially lethal debris collected to total
volume of brick and concrete
Ratio (m-3)
292
80
40
NEQ (kg)
500
1800
5600
It is seen that the number of potentially lethal fragments produced per cubic metre of
wall and roofing material decreases as NEQ increases.
The second of these facts is illustrated by the data presented in Table A2.4, which
again is compiled from the results of three of the ESTC brick magazine trials, and
which shows the percentage of lethal debris projected beyond the current IBD.
Table A2.4: Percentage of potentially lethal debris projected beyond current IBD –
two walls mounded, two walls unmounded
NEQ
(kg)
250
1800
5600
IBD (4RB) Percentage of debris
(m)
projected beyond IBD
60
88%
215
73%
380
20%
It is seen that as the NEQ increases the proportion of the potentially lethal debris
projected beyond the current IBD decreases.
6
Strictly speaking it is not the NEQ that is the critical factor but the loading density: the debris
hazard will not necessarily diminish if the size of the building increases with the NEQ.
50
Appendix 3
Development of debris models
Models have been developed to predict the chance that people at various distances
from an exploding building would be struck by lethal debris. The models are based on
the results of a number magazine trials undertaken by the ESTC over a period
spanning the early 1980s to 1998.
Most of these trials involved buildings of brick and concrete construction and the
results have been reported elsewhere[14] (or can be downloaded from:
http://www.hse.gov.uk/research/content/misc/parari97.pdf ). In 1998, trials were
undertaken with two sizes of proprietary steel magazine typically used in the UK for
storage of blasting explosives. This appendix describes the analysis of the results.
Magazine details
The trials were undertaken with the two sizes of store. The smaller of these (Division
A) has dimensions of 3 ft (length) % 2 ft 6 in (width) % 2 ft 9 in (height) and is used to
store up to 75 kg of explosives. The larger (Division C) has dimensions of 5 ft 6 in
(length) % 5 ft 6 in (width) % 5 ft (height) and may be used to store up to 450 kg of
explosives.
Both magazines are welded structures made out of 6 mm mild steel plate. Both
comprise two compartments, the larger of which is used for storage of bulk explosives
and the smaller for storage of detonators. The bulk explosives compartment is lined
with pine boards and the detonator annex is lined with compressed chipboard. The
larger magazine is shown in Figure A3.1.
51
Figure A3.1: Steel Magazine with Detonator Annex
Explosives details
The explosives used in both trials was Dynoprime, a commercial product
manufactured by Dyno-Westfarmers Ltd. Dynoprime is normally used as a booster
for the initiation of non-cap sensitive explosives used in the mining, quarrying and
construction industries. Essential technical details are as follows:
Density:
Velocity of detonation:
Detonator pressure:
1.78 g cm-3
6700 m s-1
2.1010 Pa
The first trial was carried out with 75 kg of Dynoprime packed into the smaller size
magazine; the second trial was performed with 450 kg of the explosives placed in the
larger magazine.
52
Mound details
In both trials, a mound was constructed to the back and one side of the magazine
using Hesco Bastion units (see Figure A3.1). These units are cubic in shape and
comprise a plastic-coated wire mesh lined with a waterproof membrane which may be
filled with soil or sand.
In the case of the 75 kg trial, the mound comprised a single row of 1 m3 soil-filled
units. The top of the units were level with the top of the magazine. A higher standard
of barricading was employed in the second trial and comprised a double row of 0.216
m3 (i.e. 0.6 m % 0.6 m % 0.6 m) soil-filled units, each row stacked three high. In this
case the top of the Hesco Bastion wall was 0.15 m higher than the roof of the
magazine.
Collection of debris
For the purpose of recording the location of the debris thrown out in the explosion, the
grid shown in Figure A3.2 was employed. The grid was constructed in the following
manner:
1. The middle point of the steel magazine was established.
2. A line was extended from this point through, and at right angles to, the
front door-wall of the magazine.
3. A circle was drawn around the magazine and this was surveyed into 36 x
10° arcs. Each arc was labelled alphabetically in a clockwise direction
from A to JJ. ‘A’ being normal to the centre of the front of the magazine.
4. All zones were pegged out to a 600 radius from ground zero (GZ)
5. Each arc was then divided into 20 m deep sectors as shown in Figure A3.3.
All of the sectors in the first 20 m from GZ were joined to form a single
sector. All the remaining sectors were then labelled according to their
alphabetical arc and the radius of the mid point of the sector from GZ. For
example, the sector starting at 20 m and finishing at 40 m from GZ in arc
R is denoted R30.
53
Figure A3.2: Search area grid
Figure A3.3: 20 m sector divisions within each zone
54
The pieces of debris recovered in each sector were put through a 5 cm sieve. Those
that were retained on the sieve, as well as those which passed yet weighed more than
0.1 kg, were considered to have possessed a minimum kinetic energy of 80 J on
impact, a generally accepted criterion for defining debris that is “potentially lethal”[7].
The total number of such pieces of debris collected in each sector was then recorded.
The results for the 75 kg and 450 kg trials are shown in Tables A3.2 and A3.3
respectively.
55
Table A3.2: Summary of lethal fragments collected in Trial 1 (Detonation of 75 kg of Dynoprime)
ARC FF G
G
SECTOR
30
50
70
90
110
130
150
170
190
210
230
250
270
290
310
330
350
370
390
410
430
450
470
490
510
530
550
570
590
SUM/
ARC
0
0
0
0
0
0
0
0
0
0
0
0
0
1
HH II
JJ A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W X
Y
Z
AA BB CC DD EE
SUM/
SEG
0
0
0
1
0
0
0
0
0
0
0
0
1
1
0
0
0
0
0
1
0
0
0
0
0
0
0
1
0
1
0
0
0
1
0
0
0
1
0
2
0
1
0
0
0
0
1
0
1
1
0
0
0
2
2
1
0
1
0
0
2
1
0
0
2
1
0
1
1
1
1
0
0
0
0
3
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
1
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
1
0
0
0
0
0
0
0
0
0
1
1
1
0
0
1
0
1
0
0
0
0
0
0
0
1
0
0
0
0
1
1
0
1
0
0
3
1
2
1
1
0
0
0
0
1
0
1
0
0
0
1
2
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
2
0
2
0
2
1
0
0
1
0
0
0
0
0
1
0
0
1
0
0
0
2
0
0
0
0
0
0
1
0
0
1
0
0
1
0
1
0
0
0
0
0
0
0
1
0
1
0
0
0
1
0
1
0
0
0
0
0
1
1
0
1
0
0
0
0
0
0
0
0
0
5
9
1
0
1
0
0
0
1
0
0
0
0
1
6 0
1 13
1 6
0 1
0 2
2 2
3 0
0 1
1 2
0 0
2 1
3 2
1 1
3 0
0
0
0
1
2
2
0
0
0
0
1
0
0
0
1
0
0
0
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
1
1
1
1
0
0
0
1
0
2
0
0
0
1
0
0
0
1
0
0
0
0
0
0
1
1
0
3
0
0
0
0
0
0
0
0
0
1
0
0
0
2
0
0
0
1
0
1
0
2
0
0
0
0
1
0
0
1
0
0
0
0
0
3
0
0
1
0
0
0
0
0
0
0
1
0
0
0
0
1
0
2
0
0
0
0
1
0
0
1
1
0
0
0
0
0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
1
0
1
0
0
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
1
0
6
4
5
5
5
4
5
4
5
3
5
0
2
1
1
3
2
6
8 10
5
1
2
1
4
3 11
5
3
7
4
3
4
3 18 23 32
TOTAL No OF
LETHAL FRAGS
56
16
32
14
16
11
21
9
8
13
14
12
15
11
19
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
212
Table A3.3: Summary of lethal fragments collected in Trial 2 (Detonation of 450 kg of Dynoprime)
ARC FF G
G
SECTOR
30
50
70
90
110
130
150
170
190
210
230
250
270
290
310
330
350
370
390
410
430
450
470
490
510
530
550
570
590
SUM/
ARC
0
0
1
1
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
HH II
JJ A
1 0 1 0 2
0 0 1 4 2
0 0 3 0 1
0 0 1 2 0
0 2 0 0 2
0 1 4 1 0
0 0 1 4 0
0 0 0 0 2
0 0 0 0 1
0 2 2 1 1
0 0
0 2
0 0 0 0 0
0 0 0 0 3
0 0 0 3 0
0 1 5 2 1
0 0 0 1 0
0 3 1 1 0
0 0 2 0 2
0 0 1 2 0
0 3 0 1 0
0 0 2 2 0
0 0 2 0 0
0 0 1 2 0
0 0 2 0 0
0 0 0 1 1
0 0 1 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
1 12 30 27 20
B
C
0
0
0
2
0
0
2
0
0
0
0
0
1
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
2
8
D
1
1
0
0
1
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
6
E
0
1
0
2
0
0
1
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
6
F
1
0
1
0
0
0
0
1
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
5
G
0
1
0
0
0
1
0
0
0
0
0
1
2
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
6
H
I
J
K
L
0 1 1 0 2 4
1 0 0 0 2 2
0 0 0 0 1 1
0 1 0 0 0 1
0 1 0 0 1 1
0 1 0 0 1 1
2 1 0 2 1 2
1 0 0 2 1 2
0 0 0 1 1 1
0 1 0 0 0 2
0 2 0 3 0 0
0 0 0 1 1 0
0 0 0 2 0 1
0 0 1 1 0 0
3 2 1 3 0 0
0 3 1 1 2 1
0 0 0 1 0 0
0 0 4 0 0 0
0 0 1 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 1 0 0 0
1 0 0 0 0 0
1 0 0 0 0 0
0 0 1 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 1 0 0
9 13 11 18 13 19
M
N
O
P
Q
R S
2 3 1 17 61 32 19
0 1 0 3 50 1 1
0 0 1 3 57 30 0
0 0 1 0 24 6 0
0 1 0 0
5 2 0
0 1 1 1
1 0 0
0 0 1 0
1 0 0
0 0 0 1
1 0 0
1 0 1 0
1 2 0
1 1 1 0
1 1 2
0 0 1 0
4 2 1
0 0 0 0
0 0 1
0 0 1 2
1 0 1
1 2 1 2
0 2 0
0 0 0 0
0 0 1
0 0 2 0
1 0 0
0 0 1 0
0 0 1
0 1 0 0
0 0 1
0 0 0 0
0 0 1
0 0 0 0
1 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
0 0 0 0
0 0 0
5 10 13 29 209 78 29
T
U
1
0
0
0
0
3
1
0
4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
9
V
0
0
0
2
0
0
0
0
0
0
0
0
1
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
4
W X
Y
Z
AA BB CC DD EE
4 6 7 4 17 24 6
2 5 3 2 5 11 14
0 0 0 2 1 2 3
0 0 0 1 1 0 1
0 1 1 0 1 4 0
0 1 0 2 3 0 0
0 0 0 0
1 0
0 0 1 0 1 0 0
0 1 0 1 0 0 0
1 0 1 1 0 0 0
0 0 2 0 0 0 1
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 3 1
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 2
0 0 0 0 0 0 0
0 0 0 0 0 0 0
1 0 0 0 0 0 0
0 0 0 0 1 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
8 14 15 13 30 45 28
5
3
0
1
0
0
0
1
0
1
0
0
0
0
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
13
2
0
1
0
0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
5
TOTAL No OF
LETHAL FRAGS
57
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
2
SUM/
SEG
225
116
108
47
23
23
23
14
18
21
18
4
15
18
23
13
8
10
5
5
6
2
4
4
4
2
0
3
4
766
Analysis of Results
Trial 1 (75 kg)
The walls and roof of the magazine were fragmented by the explosion. Most of the Hesco
Bastion units were torn apart by the flying debris and the remnants were projected out to
a maximum distance of 8 metres by the pressure wave. A total of 212 pieces of
potentially lethal debris were collected within the search area, the furthest being found at
a distance of 410 metres from GZ.
Figure A3.4 shows the percentage of debris collected in the direction of the four walls.
By far the highest percentage of debris was projected in the direction of the back of the
magazine. This result is explained by the presence of the detonator annex attached to the
back of the building - this provides additional material for conversion into debris.
Figure A3.4: Trial 1 (75 kg)
Percentage of debris collected in different directions
14
18
21
Front (unbarricaded)
Side (unbarricaded)
Back (barricaded)
Side (barricaded)
47
This result suggests an obvious risk-reduction measure: remove the detonator annex from
the building.
Figure A3.5 shows the cumulative percentage of debris collected at various ranges from
GZ.
58
Figure A3.5: Trial 1 (75 kg)
Cumulative percentage of debris collected vs. range
Cumulative percentage of debris collected
120
100
80
60
40
20
0
30
50
70
90
110
130
150
170
190
210
230
250
270
290
310
330
350
370
390
410
Range (m)
Front (unbarricaded)
Side (unbarricaded)
Back (barricaded)
Side (barricaded)
The chart shows that the Hesco Bastion barricading was not particularly effective in this
trial. There was a slight mitigating effect at the rear of the magazine where 45% of the
debris fell within 80 metres - against just 20% on the unbarricaded side. However, the
effect at the other barricaded side was minimal. A possible explanation for this
disappointing result lies in the positioning of the Hesco Bastion units: these were placed
at a distance somewhat greater than one metre from the side of the magazine.
Trial 2 (450 kg)
The walls and roof of the magazine were fragmented by the explosion. All but six of the
90 Hesco Bastion units were destroyed by blast and flying debris. A total of 766 pieces
of potentially lethal debris were collected within the search area, the furthest being found
at a distance of 590 metres from GZ.
Figure A3.6 shows the percentage of debris collected in the directions of the four walls.
Once again, the highest percentage of debris was projected in the direction of the back of
the magazine - a consequence of the additional secondary fragmentation produced by the
break-up of the empty detonator annex attached to the back of the building.
59
Figure A3.6: Trial 2 (450 kg)
Percentage of debris collected in different directions
15
20
14
Front (unbarricaded)
Side (unbarricaded)
Back (barricaded)
Side (barricaded)
51
Figure A3.7 shows the cumulative percentage of debris collected at various ranges from
GZ.
Figure A3.7: Trial 2 (450 kg)
Cumulative percentage of debris collected vs. range
Cumulative percentage of debris collected
120
100
80
60
40
20
0
30
50
70
90 110 130 150 170 190 210 230 250 270 290 310 330 350 370 390 410 430 450 470 490 510 530 550 570 590
Range (m)
Front (unbarricaded)
Side (unbarricaded)
Back (barricaded)
Side (barricaded)
In this case it is seen that barricading did have a significant effect. Whereas 80% of the
lethal debris projected from the barricaded sides of the store fell within 90 metres (the
current IBD), only 23% of that projected from the unbarricaded sides fell within this
range.
60
In determining the density of the lethal debris posing a hazard at various ranges from the
explosion, allowance is often made for the possibility that debris passing over a zone may
be travelling below head height. Thus some workers have proposed that a Modified
Pseudo Trajectory Normal (MPTN) density should be calculated, based on all relevant
debris collected within a zone plus one-third of the debris which had to pass through the
zone to reach a greater range[7].
In the present case the Working Group considered that density measurements on the
barricaded sides of the magazine should be based on the raw debris pick-up data without
accumulation, as any fragments travelling out horizontally from the explosion would
most likely be trapped by the barricade or else ricochet in an upward direction. With
regard to the unbarricaded sides, the amount of debris which could be expected to pass
any given range at head height or below will be determined by the distribution of the
launch angles. Debris launch angles were not measured in the trials and their distribution
is a matter of conjecture. In the ideal case, the steel magazine would “balloon” before
fragmenting, leading to an even distribution over the range 0° - 90° with respect to the
horizontal. Ballistic calculations show that in this case very little of the debris passing
over a sector would fly past at head height or below. However, in the interests of caution,
the MPTN algorithm was retained.
Calculation of fatality probability
The expected number of hits by lethal debris on a person at a given range from the
explosion is simply computed by multiplying the lethal fragment density at that range by
the effective target area for a human.
Where debris densities are measured in the horizontal plane, the effective target area must
also be measured in the horizontal plane. This area varies with both the size and shape of
the person and the angle of descent of the incoming debris. If, for ease of calculation, a
cuboid shape is assumed for the person facing the explosion, then the effective target area
will be as illustrated in the following diagram.
61
Figure A3.8: Effective target area measured in horizontal plane
W
B
H
a
The target area is then given by:
A = WB + HWCot(a)
where a is the angle of descent at impact.
It is clear that the target area diminishes as a increases. Ballistic calculations show that
debris landing in the mid to far field (where the IBD will be located) will mostly impact
the ground at angles between 49° and 76°. Taking commonly used values for the
dimensions of the cuboid, W = 0.2 m, B = 0.4 m, H = 1.14 m, the target area is found to
range between 0.31 m2 and 0.14 m2 , with 0.22 m2 being the average.
With regard to horizontally projected debris, the target area is taken as 0.56 m2 (this is
slightly conservative given the above model but has been retained as a value used by a
number of authorities in the field, including the ESTC).
62
Fatality probabilities are calculated from the expected number of hits by potentially lethal
debris, according to the Poisson formula:
Lo = 1 - e - D´ A
where D
A
is the lethal debris density, and
is the effective target area of the exposed person
This formula takes account of the fact that even when the expected number of hits is
greater than one, there is still a chance that the person may escape being hit and the
fatality probability is consequently less than the value of unity. For instance, when the
expected number of hits is exactly one, the fatality probability is: 1-e-1 = 0.6.
Finally, a lethality function is derived by plotting the lethality vs. range data points and
performing a regression analysis to find the best fit polynomial curve. An example is
shown in the following figure, which is for population located to the front of an
unmounded Division C steel store (maximum NEQ = 450 kg).
Figure A3.9: Example of a lethality function
Lethality vs. Range: front of unmounded steel magazine holding 450 kg of high explosives
1
0.1
Lo
0.01
0.001
0.0001
0.00001
30
50
70
90
110 130 150 170 190 210 230 250 270 290 310 330 350 370 390 410 430 450 470 490 510 530
Range (m)
Lethality
Function
In this case the resulting curve fits the data points very well, due in large part to the data
smoothing effect of applying the MPTN algorithm. However, in the case of the mounded
side of the store, where the debris density is based on raw pick-up data without
accumulation, the fit is not so good. In this and all similar cases it was considered
prudent to derive functions which effectively “enveloped” all the data points. This was
achieved by performing the regression analysis on the outlying points, as shown in Figure
A3.10.
63
Figure A3.10: Example of a lethality function which “envelopes” the data points
Lethality vs. Range: side of mounded magazine holding 450 kg high explosives
1
0.1
Lo
0.01
0.001
0.0001
0.00001
30
50
70
90
110 130 150 170 190 210 230 250 270 290 310 330 350 370 390 410 430 450 470 490 510
Range (m)
Lethality
Function
The regression analysis of the data collected on the unmounded side of the store produced
the following 6th order polynomial:
Log(Lo) = -1.63533146398322E-15×R6 + 2.24571323575187E-12×R5 1.09690422982978E-09×R4 + 1.71332161532993E-07×R3 +
0.0000240091430011881×R2 - 0.0126796875618444×R - 0.896776499011298
where Lo is the fatality probability for persons in the open
R is the range (m) within the limits 30 –530 metres
There is, of course, no underlying physical reason why lethality should be related to the
6th power of the range; the regression analysis is simply a convenient way of providing a
continuous function.
The following table presents a complete set of lethality functions for steel and brick
stores.
Type of store
Steel store, mounded,
Without detonator annex,
NEQ = 75 kg
Steel store, mounded,
With detonator annex,
NEQ = 75 kg
Function
Lo = 0.001
Log(Lo) = -0.0053002 × R - 2.5018
for 30 [ R [ 90
for 90 < R [ 290
Log(Lo) = 5.0059352E-08×R^3 - 3.551112E-05×R^2 +
for 30 [ R [ 410
1.05199E-03×R - 2.02635
64
Type of store
Steel store, not mounded,
Without detonator annex,
NEQ = 75 kg
Steel store, not mounded,
With detonator annex,
NEQ = 75 kg
Steel store, mounded,
Without detonator annex,
NEQ = 450 kg
Steel store, mounded,
With detonator annex,
NEQ = 450 kg
Steel store, not mounded,
Without detonator annex,
NEQ = 450 kg
Steel store, not mounded,
With detonator annex,
NEQ = 450 kg
Brick store, mounded,
NEQ = 25 kg
Brick store, mounded,
NEQ = 50 kg
Brick store, mounded
NEQ = 100 kg
Function
Log(Lo) = 3.71244849E-14×R^6 - 4.14038534E-11×R^5 +
1.701814216E-08×R^4 - 3.43078814E-06×R^3 +
3.7650171E-04×R^2 - 2.809647E-02×R - 1.02546
for 30 [ R [ 290
for 290 < R [ 310
Log(Lo)= -0.0190016 × R + 1.85185
Log(Lo) = -3.74313822E-14×R^6 + 2.57436621E-11×R^5 6.32851206E-09×R^4 + 5.0115983E-07×R^3 +
4.537444E-05×R^2 - 0.01.49686812×R - 0.92397
for 30 [ R [ 290
for 290 < R [ 330
Log(Lo)= -0.02301422× R + 3.225798
Log(Lo) = 1.008125E-05×R^2 - 1.134789E-02×R - 1.04381
for 30 [ R [ 410
Log(Lo) = -2.8268088E-10×R^4 + 2.5657725E-07×R^3 6.511279E-05×R^2 - 4.96115561E-03×R - 0.83046
for 30 [ R [ 410
Log(Lo) = -1.6353315E-15×R^6 + 2.2457132E-12×R^5 1.09690423E-09×R^4 + 1.7133216E-07×R^3 +
2.400914E-05×R^2 – 1.267969E-02×R - 0.89678
for 30 [ R [ 530
Log(Lo) = -1.8763579E-15×R^6 + 2.4490436E-12×R^5 1.00319061E-09×R^4 + 4.522662E-08×R^3 +
5.866865E-05×R^2 - 1.512566E-02×R - 0.24504
for 30 [ R [ 530
for 530 < R [ 590
Log(Lo)= -0.01443453 × R + 3.225798
Log(Lo) = -7.03560716E-12×R^6 + 3.2636940244E-09×R^5 5.0606926289E-07×R^4 + 2.434833657×R^3 +
6.8872975E-04×R^2 - 9.064205E-02×R + 1.54617
for 30 [ R [ 170
for 170 < R [ 210
Log(Lo)= -0.02775742 × R + 1.28476
Log(Lo) = -8.9280713521E-12×R^6 + 5.6236437047E-09×R^5 1.39270231334E-06×R^4 + 1.7259474561E-04×R^3 –
1.126268982E-02×R^2 + 0.35104181×R - 4.17570
for 30 [ R [ 190
Log(Lo)= -2.600075E-02 × R + 2.16393773
for 190 < R [ 250
Log(Lo) = 3.0500071663E-12×R^6 - 2.0219316952E-09×R^5 +
5.2453474819E-07×R^4 - 6.764846236×R^3 + 4.49010343×R^2 0.15219780×R + 1.63836
for 30 [ R [ 210
Log(Lo)= -1.686278E-02 × R + 0.66954
65
for 210 < R [ 290
Type of store
Brick store, mounded
NEQ = 250 kg
Function
for 30 [ R [ 110
Lo = 0.1
Log(Lo) = 1.84444211194E-11×R^5 - 2.41504974307E-08×R^4 +
1.183274528E-05×R^3 - 2.71189311E-03×R ^2 + 0.28034269×R 11.45174
for 110 < R [ 350
Log(Lo) = -2.456481E-02 ×R + 4.85309
Brick store, mounded
NEQ = 500 kg
for 350 < R [ 370
Log(Lo) = -2.47232038E-11×R ^5 + 2.773722834E-08×R ^4 1.180070955E-05×R ^3 + 2.33913743E-03×R ^2 - 0.21880900×R
+ 6.90615
for 110 < R [ 370
Log(Lo) = -2.297814E-02×R + 5.33514
Brick store, mounded
NEQ = 1800 kg
for 370 < R [ 410
Log(Lo) = -2.65170504E-14×R^6 + 5.20678226E-11×R^5 4.074215343E-08×R^4 + 1.614172969E-05×R^3 –
3.38701165E-03×R^2 + 0.35093024×R - 15.57608
for 130 < R [ 570
Log(Lo) = -3.600691E-02 ×R + 16.57014
Brick store, mounded
NEQ = 5600 kg
for 570 < R [ 590
for 110 [ R [ 230
Lo = 0.01
Log(Lo) = 2.598262E-11×R^4 - 7.719161E-08×R^3 +
5.047137E-05×R ^2 - 1.407920E-02×R - 0.52859
for 230 < R [ 590
Log(Lo) = -1.315779E-02 ×R + 3.791735213
Brick store, not mounded,
NEQ = 25 kg
for 590 < R [ 610
for 30 [ R [ 70
Lo = 1
Log(Lo) = 1.72382636E-06×R^3 - 1.00055437×R^2 +
0.13230061×R - 5.06961
for 90 < R [ 170
Log(Lo) = -0.05531454×R + 6.37810
Brick store, not mounded,
NEQ = 50 kg
Lo = 1
for 170 < R [ 190
for 30 [ R [ 90
Log(Lo) = 1.2592569067E-12×R^6 + 8.822937321E-10×R^5 2.43909651322983E-07*A43^4 + 3.260314663E-05×R ^3 2.20192078E-03×R^2 + 7.107020E-02×R - 0.85381
for 90 < R [ 210
Log(Lo) = -6.29679E-02 × R + 10.10538
66
for 170 < R [ 190
Type of store
Brick store, not mounded
NEQ = 100 kg
Function
for 30 [ R [ 90
Lo = 1
Log(Lo) = -6.927860160E-10×R^5 + 6.0361547137E-07×R^4 2.0576418457E-4×R^3 + 3.410252575E-02×R ^2 –
2.74982818×R + 86.49297
for 90 < R [ 250
Log(Lo) = -5.417520E-02 × R + 10.24630
Brick store, not mounded
NEQ = 250 kg
Lo = 1
for 250 < R [ 270
for 30 [ R [ 110
Log(Lo) = -3.62739562E-11×R^5 + 3.736924097E-08×R^4 1.440276615E-05×R^3 + 2.48797683E-03×R^2 - 0.19003888×R +
4.95430
for 110 < R [ 350
Brick store, not mounded
NEQ = 500 kg
for 110 [ R [ 190
Lo = 1
Log(Lo) = -7.104069E-13×R ^5 + 1.27710259E-09×R ^4 8.4767268E-07×R ^3 + 2.3237647E-04×R ^2 - 2.756911E-02×R +
1.17560
for 190 < R [ 570
Log(Lo) = -1.702548E-02 × R + 5.74814
Brick store, not mounded
NEQ = 1800 kg
for 570 < R [ 590
Log(Lo) = -1.227036221E-13×R^6 + 1.726150109E-10×R^5 9.844252555E-08×R^4 + 2.889342422E-05×R^3 - 4.58386168E03×R^2 + 0.37167205×R - 12.03842
for 110 < R [ 390
Log(Lo) = -4.017647E-02 ×R + 13.53893
Brick store, not mounded
NEQ = 5600 kg
for 390 < R [ 450
Log(Lo) = -2.11599649E-14×R^6 + 3.79289371E-11×R^5 2.706435705E-08×R^4 + 9.74005486E-06×R^3 –
1.84649876E-03×R^2 + 0.17148932×R - 6.48261
for 110 < R [ 550
Log(Lo) = -5.320868E-02 ×R + 25.66198
for 550 < R [ 570
67
Appendix 4
Case Studies
A number of case studies were carried out with the aim of establishing:
1. the maximum level of risk which could, in theory, exist around local authority stores
under the then existing licensing arrangements, and
2. what levels of risk existed around these stores in more typical situations.
This appendix illustrates the procedure by describing in detail the analysis carried out for
one particular case, namely, a Division C steel store located near to a housing estate.
Unmounded Division C steel store in a built-up area comprising
terraced housing.
This is a hypothetical case, the circumstances of which have been chosen to give an
indication of the maximum level of individual and group risk that could arise from the
storage of high explosives in Division C steel stores that do not have protective
mounding.
Assumptions
The assumptions are as follows:
·
The store holds controlled explosives and is compliant with statutory security
requirements
·
The store is not mounded.
·
The store is surrounded by terraced housing.
·
There are 30 terraced houses per 0.8 hectares (this is believed to be a typical
figure and has been taken from data supplied by a borough council). In metric
units this is equal to a housing density of 37.5 houses per 10,000 m2.
·
The first row of houses forms a circle around the store the radius of which is the
IBD (89 m - or 90 m when measured from the centre of the store).
·
There are, on average, 2.5 persons occupying each house.
68
·
Any accident occurs without warning, leaving no time for evacuation or
mitigating action.
·
There are no topographical features in the area that would result in focusing of the
blast wave.
·
The effects of the explosion are essentially negligible at distances further than the
range to 0.01% lethality, as predicted by the blast and debris models.
Number of buildings at risk
Given the above set of assumptions, the numbers of houses in each of the 20 metre-wide
annuli surrounding the store from the IBD out to a range of 600 m are as shown in Table
A4.1.
Table A4.1: Division C Steel Store – Number of houses in annuli surrounding store
Range (m) Area of annulus (m-2) No of houses
90-110
12566
47
110-130
15080
57
130-150
17593
66
150-170
20106
75
170-190
22619
85
190-210
25133
94
210-230
27646
104
230-250
30159
113
250-270
32673
123
270-290
35186
132
290-310
37699
141
310-330
40212
151
330-350
42726
160
350-370
45239
170
370-390
47752
179
390-410
50265
188
410-430
52779
198
430-450
55292
207
450-470
57805
217
470-490
60319
226
490-510
62832
236
510-530
65345
245
530-550
67858
254
550-570
70372
264
570-590
72885
273
590-610
75398
283
69
Lethality estimates
Three models have been used to estimate lethality for population within the zones listed
in the above table: the ESTC Outdoor Blast Model, the ESTC Indoor Blast Model and the
HSE/ESTC Steel Fragment Model. The results obtained for outdoor and indoor
population are listed in Tables A4.2 and A4.3 respectively. It may be noted that a cut-off
has been applied at the range above which lethality drops below a value of 0.0001
(0.01%); this range is found to be 590 m for population outdoors and 490 m for
population indoors.
Table A4.2: Division C Unmounded Steel Store – Lethality as a function of range
for population in the open
Range
(m)
90
190
290
390
490
590
Lethality in directions normal
to walls of store
(a)
Blast
Debris(b)
All
Side
Back
directions
0
2.E-02
7.E-02
0
6.E-03
3.E-02
0
3.E-03
2.E-02
0
1.E-03
6.E-03
0
2.E-04
1.E-03
0
6.E-05
(a) The ESTC Outdoor Blast Model
(b) The HSE/ESTC Steel Fragment Model
Table A4.3: Division C Unmounded Steel Store – Lethality as a function of range
for population inside conventional buildings
Range
(m)
90
190
290
390
490
Lethality in directions normal to walls of store
Blast
Debris only(b)
Debris (b) and
only(c)
Blast(c)
All
Side
Back
Side
Back
directions
4.E-03
1.E-03
6.E-03
5.E-03
1.E-02
2.E-04
5.E-04
3.E-03
7.E-04
3.E-03
5.E-05
3.E-04
1.E-03
3.E-04
1.E-03
2.E-05
1.E-04
5.E-04
1.E-04
5.E-04
2.E-05
1.E-04
2.E-05
1.E-04
(c) The ESTC Indoor Blast Model
70
Individual risk
Values for individual risk (IR) can now be calculated according to the formula:
IR = P ´ FE ´ ( LO ´ TO + LI ´ TI )
where, P is the likelihood of the event, FE is the fractional exposure, LO is the lethality for
population outdoors, TO is the fraction of time the individual spends outdoors, LI is the
lethality for population indoors and TI is the fraction of time the individual spends
indoors at the location.
Likelihood of the event (P)
The probability of accidental explosion is taken to be 10-4 per year. This value is deemed
appropriate for stores which hold finished and packaged blasting explosives on secure
sites (see Chapter 2).
Fractional Exposure (FE) and time spent outdoors (TO)/indoors (TI)
It is conservatively assumed that the individual is permanently present at the target
location, being outdoors 11% of the time and indoors for the remaining 89% of the time.
These figures are typically used by MSDU (Methodology and Standards Development
Unit) in assessments of risks around major hazard sites.
The values of individual risk obtained are listed in Table A4.4
Table A4.4: Division C Unmounded Steel Store – Individual risk as a function of
range
Range
(m)
90
190
290
390
490
Individual Risk
In directions
normal to walls of
store
Side
Back
7.E-07
2.E-06
1.E-07
6.E-07
6.E-08
3.E-07
3.E-08
1.E-07
4.E-09
2.E-08
It is seen that the maximum level of individual risk is estimated to be 2.10-6 – this is for a
person located on the IBD in a direction directly facing the annex of the magazine. A
risk of this level is considered to lie in the “low ALARP” region, i.e. the risk is tolerable
provided all reasonably practicable risk-reduction measures have been taken and the cost
of any further improvements would exceed the benefit gained.
71
Group Risk
Finally the number fatalities to be expected in each of the annuli listed in Table A4.1 is
given by:
N = H ´ O ´ (0.25 ´ LA + 0.75 ´ LS )
Where N is the expected number of fatalities, H is the number of houses in the annulus, O
is the occupancy of those houses (assumed to be 2.5 persons per house), LA is the
lethality for population located in the quarter of the annulus normal to the annex side of
the store and LS is the lethality for population located in the rest of the annulus – i.e.
normal to the other three sides of the magazine. The results are presented in Table A4.5.
Table A4.5: Division C Unmounded Steel Store – expected number of fatalities in
annuli surrounding store
Range
(m)
90
190
290
390
490
590
Number of
fatalities in
directions normal
to walls of store
Side
Back
0.59
0.49
1.52
1.94
0.96
1.62
0.60
0.89
0.22
0.34
0.01
0.02
By summing the figures listed in the second and third columns of the table, the total
number of fatalities is found to be 9. The likelihood of the accident is estimated as 10-4
per annum (see Chapter 2). It is worth repeating that this fatality estimate is based on a
number of conservative assumptions and fewer people may be killed in the event of an
accident.
72
Annex 1: Examples of the Different Categories of Bomb
Damage
Figure A2.1: Category A (totally demolished) houses in foreground
Figure A2.2: Category B Damage, with gaping cracks in external walls
73
Figure A2.3: Typical Category C(b) damage in foreground
Figure A2.4: Typical Category C(a) Damage to right-hand house
74
Figure A2.5: Example of severe Category D damage
75
Printed and published by the Health and Safety Executive
C30 1/98
Printed and published by the Health and Safety Executive
C1
3/02
Printed and published by the Health and Safety Executive
C30 1/98