OFFICIAL. OFFICIAL Page 1 of 10. HALES Evaporator C. Reduction in MOP reservation held on Evaporator C & reallocation to Evaporator A & B

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1 Title of document HALES Evaporator C Reduction in MOP reservation held on Evaporator C & reallocation to Evaporator A & B Project Assessment Revision 0 15 September 2014 Page 1 of 10

2 Office for Nuclear Regulation, 2014 If you wish to reuse this information visit for details. Published 10/14 For published documents, the electronic copy on the ONR website remains the most current publicly available version and copying or printing renders this document uncontrolled. Office for Nuclear Regulation Page 2 of 10

3 EXECUTIVE SUMMARY This report summarises ONR s review of Sellafield Ltd s proposal to reduce the Magnox Operating Plan (MOP) reservation held on Evaporator C by 50% and to reallocate the duty to Evaporators A & B within the High Level Waste Plants (HLWP) complex. [Background] There are 3 evaporators (A, B & C) within the HLWP complex that concentrate (evaporate) the Highly Active Aqueous Raffinates (HAAR) from reprocessing (Magnox and THORP fuels) and highly active effluents from the waste vitrification process to produce Highly Active Liquor (HAL) prior to conversion to a vitrified waste that is suitable for long term storage. Each of the evaporators is capable of processing Magnox liquors but Evaporator C is the only evaporator capable of handling oxide (THORP) liquors. There are two principal corrosion mechanisms that challenge the integrity of the evaporators heating/cooling components (coils and base) that provide primary containment of the highly active material being processed. The first of these is generalised attack by the process liquors on the outer surface of the coils and inner surface of the vessel, producing a generalised thinning of the steel. The second is waterside pitting corrosion, which occurs on the nonprocess side of the vessel and coils and results in pinhole failure. The operating life, and hence future throughput, of the evaporators will be determined and limited by the progress of corrosion to the heating/cooling components To date Evaporator C, unlike the other two evaporators, has not suffered any pinhole failures of the coils but these have incurred significant thinning due to generalised corrosion; Oxide liquors are significantly more corrosive than Magnox Liquors Some years ago, due to uncertainty regarding the availability of Evaporators A & B to meet the MOP requirements, Sellafield Ltd decided to reserve sufficient capacity, equivalent to that necessary to process all the remaining Magnox fuel, on Evaporator C to ensure completion of the MOP by Any remaining remnant life (capacity) would be used for Oxide fuel processing. When allocating remnant life (projected total future throughput) for evaporator C, priority has always been given to ensuring that sufficient corrosion allowance, i.e. metal thickness, has been given to completion of the MOP as this fuel degrades during pond storage and there is currently no other option available to provide its long-term safe storage. The consequence of this approach is that it limited the amount of oxide fuel that could be processed and Sellafield Ltd currently predict that oxide reprocessing would need to cease by late 2014 to ensure sufficient capacity is reserved on Evaporator C to meet MOP requirements. An improved understanding of the corrosion process has led to the use of an operational regime that is less corrosive and has increased levels of monitoring and oversight. This, together with the return to service of evaporator B, has led Sellafield Ltd to propose a reduction in the MOP reservation held on Evaporator C to that necessary to process 50% (down from 100%) of the outstanding MOP with the remainder being assigned to evaporators A and B. [Assessment and inspection work carried out by ONR in consideration of this request] ONR has reviewed Sellafield Ltd s proposal as articulated within their Plant Modification Proposal (PMP) and associated Justification Paper. ONR considers that Sellafield Ltd s proposal is supportable on the basis that the estimates regarding the corrosion based impact from the Magnox reprocessing are suitably conservative, the measures to monitor usage and Office for Nuclear Regulation Page 3 of 10

4 predict and manage the impact of the ensuing corrosion are adequate and we have appropriate confidence in the continuing availability of sufficient evaporative capacity in evaporators A and B to complete their assigned Magnox duty. Furthermore, Sellafield Ltd s operating philosophy for Evaporator C continues to ensure MOP processing receives appropriate priority [Conclusions] It is ONR s opinion that Sellafield Ltd s proposal to reduce the corrosion allowance on Evaporator C coils down from that equivalent to the whole of the remaining MOP to that equivalent to half the MOP is underpinned by appropriate conservatisms associated with the MOP processing capabilities (remnant life) of evaporators A and B and that Sellafield Ltd have appropriate management controls in place to monitor evaporative capacity and take appropriate action to ensure completion of the MOP by 2020 is not compromised. The MOP corrosion allowance on Evaporator C released under this proposal would be used to support continued oxide reprocessing. [Recommendation] ONR confirms that it has no objections to Sellafield Ltd s proposal to reduce the corrosion allowance on the Evaporator C coils from that equivalent to 100% of the outstanding MOP down to 50% and reallocate the duty to Evaporators A & B. Office for Nuclear Regulation Page 4 of 10

5 LIST OF ABBREVIATIONS HAAR HAL HALES HLWP MOP MSC ONR PAR PMP WVP Highly Active Aqueous Raffinate Highly Active Liquor Highly Active Liquor Evaporation and Storage High Level Waste Plants Magnox Operating Plan Management Safety Committee Office for Nuclear Regulation Project Assessment Report Plant Modification Proposal Waste Vitrification Plant Office for Nuclear Regulation Page 5 of 10

6 TABLE OF CONTENTS 1 PERMISSION REQUESTED BACKGROUND ASSESSMENT AND INSPECTION WORK CARRIED OUT BY ONR IN CONSIDERATION OF THIS REQUEST CONCLUSIONS RECOMMENDATIONS REFERENCES Office for Nuclear Regulation Page 6 of 10

7 1 PERMISSION REQUESTED 1. This report summarises ONR s review of Sellafield Ltd s proposal (Ref 1) to reduce the Magnox Operating Plan (MOP) reservation held on Evaporator C by 50% and reallocate the duty to Evaporators A & B. ONR chose to make a regulatory judgement on the adequacy of Sellafield Ltd s proposal, given the importance of evaporator capacity in supporting hazard and risk reduction from spent fuel reprocessing activities at the Sellafield site. 2. Sellafield Ltd s proposal modifies assumptions made under a previous Plant Modification Proposal (PMP) (Ref 2) and Licence Instrument (LI 836) (Ref 3), which required sufficient remnant life to be reserved on Evaporator C to support magnox liquor arisings to ensure completion of the MOP by This report covers Sellafield Ltd s proposal to change the management of the MOP reservation from Evaporator C to across the evaporator fleet. 2 BACKGROUND 3. Highly Active Aqueous Raffinates (HAAR) are generated from reprocessing activities (Magnox & THORP) on the Sellafield site. The raffinates go through an evaporation process within the High Level Waste Plants (HLWP s) to produce Highly Active Liquor (HAL) that is stored in Highly Active Storage Tanks (HASTs), prior to conversion into a vitrified glass waste within the Waste Vitrification Plant (WVP) that is suitable for longterm storage. The evaporators within the Highly Active Liquor Evaporation and Storage (HALES) facility therefore form a fundamental part in supporting hazard and risk reduction from spent fuel reprocessing activities at the Sellafield site. 4. There are three evaporators (A,B & C) within the HLWP s that concentrate the raffinates from Magnox reprocessing and highly active effluents from the waste vitrification process. Evaporator C is the only evaporator capable of handling THORP (Oxide) as well as Magnox liquors. 5. All of the evaporators have incurred significant corrosion attack and there are two principal corrosion mechanisms that challenge the integrity of the evaporators. The first of these is generalised attack by the process liquors on the outer surface of the coils and inner surface of the vessel, producing a generalised thinning of the steel. The second is waterside pitting corrosion, which occurs on the non-process side of the vessel and coils and results in pinhole failure. 6. It is widely recognised that the operating life, and hence throughput of the evaporators will be determined and limited by the progress of corrosion to the heating / cooling components (coils and base), which provide primary containment for highly active material being processed. Sellafield Ltd therefore monitor the progress of corrosion on the evaporators to assure themselves that there is sufficient metal remaining within the coils and the base of the vessel to ensure end of life safety case limits and hence integrity of the evaporators is not breached. 7. To date Evaporator C has not suffered any pinhole failures but is subject to significant generalised corrosion as Oxide liquors are more corrosive than Magnox Liquors. However, Evaporators A & B have suffered a number of pin-hole failures on their coils in the mid 2000 s as a result of waterside pitting. Due to concerns regarding the availability of Evaporators A & B to process Magnox (as a result of a number of pin hole failures on the coils) Sellafield Ltd decided to reserve sufficient capacity on Evaporator C to ensure completion of the MOP. When allocating remnant life (projected total future throughput) for evaporator C, priority has always been given to ensuring that sufficient corrosion allowance, i.e. metal thickness, has been given to Office for Nuclear Regulation Page 7 of 10

8 completion of the MOP as this fuel degrades during pond storage and there is currently no other option available to provide its long-term safe storage. The consequence of this approach is that it limited the amount of oxide fuel that could be processed. 8. Sellafield Ltd currently predict that oxide reprocessing would need to cease 2014/2015 (Ref 1) to ensure sufficient capacity is reserved on Evaporator C to complete the MOP. Sellafield Ltd claim that there are strong drivers to maximise oxide reprocessing as they consider: the storage of vitrified product containers is safer than the long term storage of irradiated fuel there is some irradiated oxide fuel that is not compatible with long term storage in the fuel pond so needs to be reprocessed so it does not create a long term nuclear legacy issue. space will be created within the oxide fuel ponds, which can be used to support an extension to the Advanced Gas Reactor station life, thereby safeguarding power generation 9. An improved understanding of the corrosion process has led to the use of an operational regime that is less corrosive and has increased levels of monitoring and oversight. This, together with the return to service of evaporator B, has led Sellafield Ltd to propose a reduction in the MOP reservation held on Evaporator C to that necessary to process 50% (down from 100%) of the outstanding MOP with the remainder being assigned to evaporators A and B. This PAR supports ONR s regulatory judgement on the adequacy of Sellafield Ltd s proposal to manage the MOP reservation across the evaporator fleet. The MOP corrosion allowance on Evaporator C released under this proposal would be used to support continued oxide reprocessing. 3 ASSESSMENT AND INSPECTION WORK CARRIED OUT BY ONR IN CONSIDERATION OF THIS REQUEST 10. ONR has carried out a review of Sellafield Ltd s proposal, as articulated within the PMP (Ref 1) and the associated justification paper (Ref 4). Both these documents have been endorsed by the HALES Management Safety Committee (MSC). 11. Although Evaporators A & B have been subject to a number of pinhole failures on their coils, Sellafield Ltd estimate that Evaporators A & B currently have the capacity to support completion of the MOP. Sellafield Ltd claim that since 2009 the availability of Evaporator A has been high and has demonstrated that it is capable of supporting 67% of the daily MOP arisings on jacket only, leaving the remaining 33% to be processed via Evaporator B and/or Evaporator C. In addition, Sellafield estimate Evaporator B will be capable of supporting 100% of the MOP arising on jacket and its one remaining operational coil and 67% of arisings on base only (if last remaining coil fails). This would leave the remaining 33% of the MOP to be processed via Evaporator A and/or Evaporator C. 12. ONR requested Sellafield Ltd to consider the extended unavailability of both Evaporators A & B and the impact this may have on the completion of the MOP by Sellafield Ltd have confirmed (Ref 1) that in such an event, a review of operating arrangements would be undertaken and necessary decisions made to ensure that sufficient capacity remains across the evaporator fleet to meet the MOP requirements. In addition, Sellafield Ltd will be monitoring evaporative capacity on a quarterly basis to assure themselves in the ability of the entire fleet to meet the MOP requirements. ONR is content with this approach, which aligns with the requirements agreed under LI 836. Office for Nuclear Regulation Page 8 of 10

9 13. The reallocation of 50% of the MOP reservation held on Evaporator C will support approximately 12 months of oxide processing. However, ONR notes that Sellafield Ltd have initiated a number of programmes of work (Ref 1) aimed to extend the remnant life of Evaporator C to meet the remaining oxide capacity gap until Evaporator D becomes available in ONR considers that Sellafield Ltd have a good understanding of the corrosion mechanisms that are prevalent in the evaporators, which has led to the use of an operational regime that is less corrosive and has increased levels of monitoring and oversight. ONR also considers Sellafield Ltd have appropriate arrangements (as agreed under LI 836) in place to control, monitor and report the impact of ongoing processing (corrosion) to ensure the integrity of the evaporators and completion of the MOP by 2020 is not compromised. Furthermore, ONR considers that the proposal is underpinned by appropriate conservatisms associated with MOP processing capabilities (remnant life) and we have appropriate confidence in the continuing availability of sufficient evaporative capacity to meet the MOP. The availability of Evaporator D in 2017 provides further confidence to ensure MOP requirements will be met. 15. Overall, ONR considers that Sellafield Ltd s proposal for the 50% reduction in MOP reservation held on Evaporator C is supportable on the basis that it continues to ensure that processing of the MOP receives appropriate operational priority. The MOP corrosion allowance on Evaporator C released under this proposal would be used to support continued oxide reprocessing. 4 CONCLUSIONS 16. This report presents the findings of ONR s regulatory judgement on the adequacy of Sellafield Ltd s proposal to reduce the MOP reservation held on Evaporator C by 50%. In summary ONR is content that: Sellafield Ltd s proposal (PMP (Ref 1)) and the associated justification paper (Ref 4) for the reduction of MOP reservation held on Evaporator C has been endorsed by the HALES MSC thus giving increased regulatory confidence. The proposal is underpinned by appropriate conservatisms to ensure the reallocation of 50% of the MOP is unlikely to present an unacceptable challenge to the completion of the MOP by Sellafield Ltd have appropriate management controls in place to monitor the evaporative capacity and take appropriate action to ensure completion of the MOP by 2020 is not compromised. The proposal continues to ensure that the MOP receives appropriate operational priority, with any remaining remnant life being managed to support continued oxide reprocessing. 17. To conclude, ONR is broadly satisfied that Sellafield Ltd have made an adequate case justifying the management of the MOP reservation across the evaporator fleet, which will enable further oxide fuel raffinates to be processed. 5 RECOMMENDATIONS 18. ONR confirms that it has no objections to Sellafield Ltd s proposal to reduce the corrosion allowance on Evaporator C coils from that equivalent to 100% of the outstanding MOP down to 50% and reallocate the duty to Evaporators A & B. Office for Nuclear Regulation Page 9 of 10

10 6 REFERENCES 1. Plant Modification Proposal. Reduction in MOP reservation on Evaporator C and reallocatin to Evaporators A and B - HALES/B***/1291, 9th July TRIM 2014/ Plant Modification Proposal. Implementation of revised managerial controls to monitor and report Evaporator C heating/cooling component usage within current safety case limits - PMP HALES/B***/1099, April TRIM 2012/ Licence Instrument 836. Agreement to Commence Modification to an Existing Plant Implementation of Revised Managerial Controls to Monitor and Report Evaporator C Heating/Cooling Component Usage, Within Current Safety Case Limits (PMP HALES/B***/1099), July TRIM 2012/ Sellafield Ltd. Justification for the partial re-allocation of MOP reservation from Evaporator C to Evaporators A & B - HALES MSC P(14)039, 19th June TRIM 2014/ Office for Nuclear Regulation Page 10 of 10