Resolution of Generic Safety Issues: Issue 173: Spent Fuel Storage Pool (Rev. 4) ( NUREG-0933, Main Report with Supplements 1–34 )
In November 1992, two engineers who had previously worked under contract for the Pennsylvania Power and Light Company (PP&L) filed a report contending that the design of the Susquehanna station failed to meet regulatory requirements with respect to sustained loss of the cooling function to the SFP that mechanistically results from a LOCA or a LOOP. PP&L and the engineers each made a series of additional submittals to the NRC and participated in public meetings with the NRC to describe their respective positions on a number of technical and licensing issues. In order to inform the nuclear power industry of the issues, NRC issued IN 93-83 on October 7, 1993. The staff evaluated the issues as they related to Susquehanna, using a probabilistic safety assessment, a deterministic engineering assessment and a licensing basis analysis, and issued an SER on June 19, 1995.
A generic action plan1623 was developed with two parts: (1) Part A, which encompassed the staff's review of generic issues relating to the SFP at operating reactor facilities; and (2) Part B, which included applicable issues from the Part A review and concerns from the Dresden-1 special inspection,1601 particular to permanently shutdown facilities with stored, irradiated fuel to establish evaluation criteria for spent fuel pools at permanently shutdown facilities. Part B was included after the special inspection at Dresden-1 determined that problems in implementing the facility's decommissioning plan combined with certain SFP design features created the potential for a substantial loss of SFP water inventory. Dresden-1, which is permanently shutdown, experienced containment flooding due to freeze damage to the service water system on January 25, 1994, and the licensee for Dresden-1 reported a similar threat to SFP integrity. This licensee report resulted in the special inspections1601 of La Crosse, Humboldt Bay, Rancho Seco, Trojan, San Onofre-1, Yankee Rowe, and Indian Point-1. The two parts of this issue were evaluated separately.
ISSUE 173.A: OPERATING FACILITIES
The principal concerns included in Part A of the generic action plan1623 involved the potential for a sustained loss of SFP cooling capability, which was identified through the report filed with the NRC relating to Susquehanna, and the potential for a substantial loss of SFP coolant inventory, which was given renewed emphasis following the Dresden-1 special inspection. Postulated adverse conditions that may develop following a LOCA or a sustained loss of power to SFP cooling system components could prevent restoration of SFP decay heat removal. The heat and water vapor added to the building atmosphere by subsequent SFP boiling could cause failure of accident mitigation or other safety equipment and an associated increase in the consequences of the initiating event. Incomplete administrative controls combined with certain design features, particularly at the oldest facilities, may create the potential for a substantial loss of SFP coolant inventory and the associated consequences, which include high local radiation levels due to loss of shielding, unmonitored release of radiologically contaminated coolant, and inadequate cooling of stored fuel.
The action plan was intended to encompass SFP issues identified through a 1994 special inspection at Dresden-1, the staff's review of loss of SFP cooling concerns at Susquehanna, and other SFP concerns identified as part of this plan. Specific review areas identified through implementation of this action plan include plant design features and administrative controls that affect the probability of spent fuel pool boiling, adverse environmental effects on essential equipment due to boiling, significant loss of spent fuel pool coolant inventory, adverse radiological conditions, unplanned spent fuel pool reactivity changes, undetected spent fuel pool events, and adverse effects of control system actuations. This issue was identified in an NRR memorandum1601 to RES in February 1996.
The postulated events do not pose an undue risk to the public based on the availability of design features that help protect stored irradiated fuel, protect essential reactor safety systems, and prevent development of adverse radiological conditions. These design features include the provision of diverse means of cooling, the strong structural design of the spent fuel pool, the absence of drainage paths from the pool, the anti-syphon protection on piping within the spent fuel pool, the availability of multiple sources of make-up water, spent fuel pool instrumentation with control room annunciation, the maintenance of a substantial shutdown reactivity margin in the pool, radiation shielding provided by coolant inventory, and spent fuel pool water purification systems. Additionally, the relatively slow evolution of these events in the spent fuel pool resulting from the initial large cooling water inventory creates significant opportunity for operator recovery prior to experiencing adverse conditions or consequences.
Specific actions include: (1) determination of the safety significance of identified concerns; (2) determination of the facilities where the concerns may be applicable; (3) evaluation of the adequacy of present SFP designs; (4) evaluation of the adequacy of current NRC guidance for SFP designs; and (5) evaluation of the need for generic actions to address significant issues at operating and permanently shutdown facilities. Based on findings from these review areas and their risk significance, the staff will develop criteria for specific spent fuel pool operations for potential use in formulating generic communications, revisions of regulatory guidance, and other appropriate regulatory actions.
This issue was considered nearly-resolved1731 since a solution had been identified and resolution was in progress with an approved Action Plan. It was later given a HIGH priority ranking in SECY-98-166.1718
In pursuing a resolution to this issue, the staff performed a comprehensive study of the Susquehanna SFP. The results of the special inspection of Dresden-1, after rupture of the SWS occurred inside containment, were transmitted to licensees in IN 94-38.1624 The identification of concerns for evaluation and review of existing guidance were completed along with on-site safety assessments of spent fuel storage at Brunswick, Monticello, Comanche Peak, and Ginna. The assessment team concluded that the potential for a sustained loss of SFP cooling or a significant loss of SFP coolant inventory at the sites visited was remote, based on certain design features and operational controls. The team found that other concerns within the scope of the action plan review were much less significant in terms of risk at the plants visited. An FSAR-based review was undertaken to identify facilities whose design was not well represented by any of the facilities reviewed through on-site assessments. As a result, approximately 26 concerns were identified in the major review areas; additional concerns associated with the Millstone-1 SFP (adequacy of SFP cooling during refueling with a full core off-load) were included. Each concern was to be addressed on the basis of a qualitative safety assessment. The concern for SFP criticality control (Boraflex degradation) was pursued through issuance of an information notice and a planned generic letter.
Following reports1693,1694 to the Commission on its findings, the staff committed to complete regulatory analyses associated with plant-specific backfits, implement plant-specific backfits, and complete revisions to Regulatory Guide 1.131697 and SRP11 Sections 9.1.1 and 9.1.3. The regulatory analyses were pursued by NRR under the proposed rulemaking on shutdown and fuel storage pool operation. In July 1997, the staff’s proposed rule was presented to the Commission in SECY-97-1681695 following which, the Commission directed1696 the staff not to issue the proposed rule. After performing plant-specific evaluations and considering a license renewal period of 20 years, the issue was RESOLVED with no new or revised requirements.1807
ISSUE 173.B: PERMANENTLY SHUTDOWN FACILITIES
The staff issued Bulletin 94-011625 requesting all holders of licenses for nuclear power reactors that were permanently shut down with spent fuel in the spent fuel pool to take actions to ensure the quality of the SFP coolant, the ability to maintain an adequate coolant inventory for cooling and shielding, and the necessary support systems were not degraded. In order to evaluate the management controls and SFP activities at permanently shutdown reactors, the NRC initiated a series of special team inspections at permanently shutdown facilities with stored, irradiated fuel in the SFP. This Part B effort was expected to use the results of Part A activities to establish evaluation criteria for SFPs at permanently shutdown plants to support rulemaking and other generic activities initiated by NRR. This issue was identified in an NRR memorandum1601 to RES in February 1996.
The postulated events involving a loss of cooling do not pose undue risk to the public because of the low residual decay heat in the spent fuel at permanently shutdown reactors and the associated long period of time available for recovery. Concerns involving maintenance of the coolant quality and ability to control coolant inventory were addressed through the special inspection activities. Therefore, continued facility operation was justified.
Specific actions included in Part B of the generic action plan1623 were: (1) the determination of significant identified concerns from Part A applicable to permanently shutdown facilities; and (2) the evaluation and implementation of additional requirements specifically applicable to permanently shutdown facilities with stored, irradiated fuel.