Sellafield Fuel Handling Plant Pondwater update

16 September 2004
Sellafield Fuel Handling Plant Pondwater update
NuSAC(04)P17. (Update of NuSAC(03)P10)
A Richardson, P Maher, August 2004
Introduction
The increased level of beta activity in the Sellafield Fuel Handling Plant (FHP) pondwater has
been an issue of interest to NuSAC since 2001, when a sub group was set up to look into the
issues. A BNFL paper was presented to the November 2002 meeting (NuSAC[2002]24), and
an update presented in late 2003 (NuSAC(03)P10).
This paper provides a summary of the two previous papers and provides an update to August
2004.
Background
Sellafield has historically had the capability to reprocess 1600+ tonnes of Magnox fuel per year
through the Magnox Reprocessing Plant. Because of the need to assemble reasonable batch
sizes of fuel from each station for decanning, the typical fuel stock in FHP has varied between
300 and 1000+ tonnes.
In the late 1990’s, the normal pattern was impacted by a number of factors, specifically:(a) (b) (c) (d) (e) (f) (g) (h) The amalgamation of BNFL and Magnox Electric put additional pressure on BNFL to
reduce costs, to underpin the strategy to operate reactors for an extended period.
A conscious decision was taken to reduce personnel numbers as the pond was empty of
fuel and future reprocessing was anticipated to be at a lower level.
Some equipment was effectively put on standby, believing that the remainder had the
capacity to deal with the demands.
FHP was approaching an age of 20 years, and some equipment was starting to become
unreliable, particularly software.
The reprocessing plant had an extended outage for installation of the new fuel dissolver.
Fuel of increasingly high burn up was being received, causing difficulties at decanning
due to brittleness or bowed shapes. This was particularly experienced with fuel from
Dungeness and Wylfa.
There had been some problems with maintaining pondwater chemistry at reactor, that
led to some deterioration of fuel condition.
Reprocessing was interrupted due to problems with other plants in an integrated site
network.
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The consequence was that reprocessing capacity was severely reduced for several years, to the
extent that only 368 tonnes was decanned in 2000/2001. The pond stocks rose to around 1000
tonnes for an extended period.
The above conditions led to a spiral of deteriorating conditions that had the following features:(a) (b) (c) (d) (e) The longer fuel residence time in the ponds caused fuel cladding to corrode and start to
leak.
The longer residence time caused some of the fuel containers to lose the gas ullage, thus
allowing contamination to spread into the FHP pond.
Leaking fuel at station ponds resulted in low payload for some flask movements. From
an essentially fixed number of flask movements, fuel could not be transported at the
desired rate or stored at sufficient density, causing increased station pond residence
times, corrosion and leakage.
Movement of containers with leaking fuel through the FHP pond and subsequent
preparation of fuel for decanning caused some activity to spread throughout the pond.
Activity levels in the FHP pond had risen from 200 MBq/m3 (1990) to 1000 MBq/m3
(1995) to 4000 MBq/m3 (2001) and 6000 MBq/m3 (2003).
Activity was absorbed into skips, which required time in clean conditions to desorb.
Hence, some activity was transferred from FHP to cleaner ponds at stations.
Sellafield discharges were not significantly impacted as pondwater is cleaned up in the Site Ion
Exchange Plant (SIXEP) prior to release. The radiation dose to workers remained low, at less
than 1 mSv per annum. However, it was decided to impose restrictions on the operations
conducted in FHP. Sellafield had a history of experiencing very high pond activity levels in
B30 in the 1970’s (106 MBq/M3) with the consequential impact on environmental discharge
and personnel dose (50 mSv pa). Consequently there was concern that the new situation might
be the start of a similar chain of events, even though the contamination levels were 250 times
higher in the 1970’s.
Initial of Corrective Actions, 2001/02
It was apparent that there was no ‘quick fix’ to the situation. However, a series of actions were
established which would allow a gradual return to previous conditions. These actions were:(a) (b) (c) (d) Installation of local ion exchange units at some station ponds, particularly to absorb
caesium from the pondwater. Dungeness, Hinkley Point, Bradwell, Sizewell and
Oldbury responded in this way.
Provision of new equipment at FHP to flush through containers in a more thorough
manner prior to them being processed.
Provision of refurbished equipment to allow containers which had lost their ullage in
FHP to be re-ullaged.
The FHP pond purge volume was increased from 1000m3 per day to over 2000m3 per
day to facilitate activity reduction.
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(e) (f) (g) (h) Recruitment of additional personnel for both operations and maintenance was
undertaken.
Reinstatement, refurbishment and improvement of equipment was achieved.
Minor improvements were made to the decanning process itself to assist the handling of
damaged or brittle fuel.
Technical programmes were initiated to further develop understanding.
Progress during 2001/03
The corrective actions outlined above were developed and implemented during 2001/03.
In early 2003, it was discovered that a change to the method of providing sodium hydroxide to
the storage containers that had been introduced in December 1998 had compounded the basic
issues. To reduce environmental spillage risk, sodium hydroxide (which is added directly to
the fuel containers) started to be purchased at 23% w/w instead of 47%, but also at up to 2%
NaCl instead of 0.01%. Chloride is known to add to the risk of Magnox corrosion. The
chloride content was reduced again at April 2003, but there remained an inventory of
600 tes of fuel in contact with high chloride.
At the time of writing the 2003 paper, the following improvements had been delivered:
•
•
•
•
Station pondwater activity had been stabilised by use of ion-exchange units.
Corroded fuel preparation for decanning had been improved.
630 of the 860 fuel containers requiring reullaging had been completed.
An additional 100+ personnel had been recruited into Magnox Reprocessing (operators,
maintainers, technical, engineering).
• Equipment asset care processes had been improved.
• Early process developments had been deployed.
These improvements enabled reprocessing improvements, that in turn allowed the
implementation of a strategy to address the root cause of the problem; ie remove the corroded
fuel. This strategy was:
• Reprocess new deliveries of fuel with much shorter FHP storage times, and avoid any fuel
becoming longer stored.
• Process the corroded fuel as quickly as practicable within the constraints of acceptable FHP
pondwater activity and SIXEP discharges.
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Other developments at Site and Magnox business level had been implemented to consolidate
the progress and prevent regression:
• Implementation of a management of change process requiring safety assessment of any
proposed structural and manning level changes.
• Development of a detailed integrated Magnox Operating Plan to optimally deliver the
closure programme of the Magnox Stations and reprocessing plant, under coordinated
strategic leadership.
Position during 2003 – 4
Fuel Handling Plant
The period 2003/4 was very successful for FHP and Magnox Reprocessing with a series of
notable achievements:
•
•
•
•
•
•
1038 tes of Magnox fuel was decanned, the highest since 1995/6 (Attachment 1).
125 tes of corroded fuel was decanned, enabling the completion of a comprehensive fuel
quality characterisation programme. The stock of corroded fuel was reduced from a peak
of 610tes to 485tes, and now to 425 tes in August 2004. (See attachment 2).
All shorter stored fuel stored in sodium hydroxide containing chloride was decanned.
The stock of insecurely ullaged fuel containers was reduced 50.
FHP pondwater activity level was reduced from 6000 Bq/ml to between 3 – 4000 Bq/ml
while maintaining corroded fuel processing. (See attachment 3).
A comprehensive BPEO/BPM assessment of FHP and SIXEP operations has been
completed and discussed with the Environment Agency. This study confirmed early
processing of corroded fuel as the BPM approach.
Opportunities were also taken during this period to further develop the understanding of
technical issues influencing FHP pondwater activity:
•
•
•
A comprehensive review of Magnox fuel corrosion chemistry was completed. This
confirmed the suitability of high pH, low chloride, low temperature conditions for long
term storage of un-corroded Magnox fuel. Improved procedures to demonstrate these asdesign conditions were in place within FHP from April 2003 for new fuel deliveries.
Fuel received since this date has shown no signs of corrosion.
A close link was identified between bulk pondwater temperature and activity content. A
significant programme of FHP pondwater chiller plant refurbishment was undertaken,
enabling pondwater temperature to be reduced from a peak of 270C in summer 2003 to
achieve steady control at 150C during summer 2004.
“Best practice” protocols targeted at minimising activity release to pondwater during fuel
processing operations were developed and implemented.
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Other business issues were addressed successfully during the same period. An increased
frequency of flask and flatrol contamination was identified, addressed via a coordinated
industry wide team, and improved to a ‘best ever’ standard.
SIXEP and Discharges
The increased throughput of corroded fuel presented a significantly raised level of beta and
alpha activity challenge to SIXEP during 2003/4, leading to a continuation of the previously
observed SIXEP alpha and beta discharges. (See attachment 4).
Improvements were made to the SIXEP deep bed sand filtration and pH control processes to
improve the alpha species discharge performance.
Beta-discharges, primarily Cs137 and Sr90, were managed principally through reduced ionexchange bed lives, with bed changes typically taking place every 75 days compared to every
90 days pre-2001. The enhanced activity challenge and the improvements made resulted in
discharge levels similar to those of 2002. These discharges remain well within the current
discharge authorisation, but significantly closer to the new authorisation coming into force in
October 2004. Discussions with the Environment Agency have led to increases in some of the
new limits, but there is a remaining concern that the proposed SIXEP Sr90 plant limit has the
potential to constrain operations. The new authorisation places greater emphasis on the
demonstrable application of Best Practice Means (BPM) to limit discharges but also contains a
mechanism to apply for a discharge limit variation should a strong case exist.
Technical assessments are being progressed that may lead to further improvements to the
SIXEP process. These include:
•
•
•
•
Potential antimony 125 abatement processes
Potential to pre-treat the ion exchange medium to further improve decontamination
factors, particularly for Sr90
Potential to run 3 ion exchange columns in series (current process uses 2 columns)
Alpha speciation studies and further optimisation of the sand filter process.
The key remaining issues are seen as:
•
•
•
•
•
Management within the new authorisation limits and requirements.
Delivery of a project to extend SIXEP sludge storage capacity.
Recommissioning of standby tanks for the storage of sand/clinoptilolite.
Maintain SIXEP chiller availability and efficiency and deliver a replacement chiller
project.
Competition for SIXEP capability from Site Remediation will require careful
coordination.
These issues are within industry control to manage.
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SIXEP Operating Strategy was delivered in March 2004. This strategy addressed the full
scope of the current and future site requirement of SIXEP, key aspects being the Magnox
reprocessing requirements and future demands emanating from site Legacy Ponds and Silos
retrieval and clean up programmes
Look Ahead
The 2003 NuSAC paper identified that the processing of corroded fuel must be tackled in an
optimised way, taking care to restrict discharges to acceptable levels. Considerable progress
has been made in the past 12 months against this objective. The forward strategy is focussed
on continued removal of the corroded fuel at throughputs of 100+tes/year, within the
constraints of acceptable discharge levels, pondwater activity and plant capability. The
identified improvement tasks will continue to be vigorously progressed, with the increased
technical, engineering and manufacturing resources maintained.
This strategy is likely to lead to discharges similar to current levels for several years to come.
Conclusions
1
Magnox Reprocessing throughput has recovered from the low levels of the late 1990’s,
and new deliveries of fuel are being processed without delay.
2
Considerable progress has been made in demonstrating the required capability to
remove the FHP corroded fuel stock at a rate of 100+ tes/year within the constraints of
the discharge authorisation.
3
FHP pondwater activity has been controlled while addressing the root cause of the
activity source.
4
SIXEP discharges are predicted to continue at the current level for several years. The
activity challenge to SIXEP from Magnox Reprocessing and Site Remediation will
need to be kept under review.
5
The improvement programme presented previously to NuSAC has contributed
significantly to this improved situation, and further potential improvements are being
assessed.
Recommendations
1
It is recommended that NuSAC note the progress made, the strategy to process the
remaining corroded fuel as quickly as practicable within the plant and discharge limit
constraints, the commitment to strive for further improvements, and the likelihood of
the continued discharge pattern.
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Attachments
1
Reprocessing annual throughput chart
2
FHP Magnox fuel stocks
3
FHP pondwater activity chart
4
SIXEP discharge chart
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Attachment 1 Reprocessing annual throughput chart
Magnox Reprocessing Throughput
1200
Tonnes U
1000
800
600
400
200
0
1996-97
1997-98
1998-99
1999-00
2000-01
Financial Year
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2001-02
2002-03
2003-04
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Attachment 2
FHP Magnox fuel stocks
1200
1000
800
To
nn
es
Good Fuel (New deliveries, <300 day stored in FHP)
600
Corroded Fuel (>500 day stored in FHP)
400
Fuel at risk of corrosion (300 – 500 day stored in FHP)
200
0
Apr-02
Jul-02
Oct-02
Jan-03
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Apr-03
Jul-03
Oct-03
Jan-04
Apr-04
Jul-04
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Attachment 3
FHP pondwater activity
FHP Pondwater Activity
7000
6000
Beta Activity (Bq/ml)
5000
4000
3000
2000
1000
0
Jul-99
Feb-00
Aug-00
Mar-01
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Sep-01
Apr-02
Oct-02
May-03
Dec-03
Jun-04
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Attachment 4
GB
q
SIXEP Discharges
Pu-alpha SIXEP Liquid
discharge
GB
q
4.50E
1.00E
4.00E
9.00E
8.00E
3.50E
3.00E
7.00E
October 2004 SIXEP 12 month limit
6.00E
2.50E
Beta-5 SIXEP Liquid
discharge
Rolling 12 month discharge
October 2004 SIXEP 12 month limit
Rolling 12 month
5.00E
2.00E
4.00E
1.50E
3.00E
1.00E
2.00E
5.00E
1.00E
0.00E
0.00E
Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan-
GB
q
Cs-137 SIXEP
Liquid discharge
Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan-
GB
q
Sr-90 SIXEP
Liquid discharge
1.80E
1.00E
1.60E
9.00E
Rolling 12 month
8.00E
October 2004 SIXEP 12 month limit
1.40E
1.20E
Rolling 12 month
October 2004 SIXEP 12 month limit
7.00E
6.00E
1.00E
5.00E
8.00E
4.00E
6.00E
3.00E
4.00E
2.00E
2.00E
1.00E
0.00E
0.00E
Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan-
Sellafield Pondwater - Paper 17(for internet).doc
Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan-