Resolution of Generic Safety Issues: Item A-45: Shutdown Decay Heat Removal Requirements (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
In March 1981, this issue was identified as a USI in NUREG-0705.44 A program was initiated to evaluate the safety adequacy of the decay heat removal (DHR) function in operating LWRs and to assess the value and impact (i.e., the benefit and cost) of alternative measures to improve the overall reliability of the DHR function.
The program employed PRAs and deterministic evaluations of those DHR systems and support systems required to achieve hot shutdown and cold shutdown conditions in both PWRs and BWRs. Systems analysis techniques were used to assess the vulnerability of DHR systems to various internal and external events. The analyses were limited to transients, small-break LOCAs, and special emergency challenges such as fires, floods, earthquakes, and sabotage. Cost-benefit analysis techniques were used to assess the net safety benefit and cost of alternative measures to improve the overall reliability of the DHR function.
Six plants were analyzed after an initial selection process which considered vendor, product line, other issues in which each particular plant might be involved, operational status, and utility willingness to participate. In terms of expected core damage frequency caused by DHR failure/RY, the range for the six plants was 7 x 10-5 to 4 x 10-4 with an average value of 2 x 10-4 if credit is allowed for feed-and-bleed operation on the PWRs and containment venting on the BWRs.. Neither the above ranges nor the averages were found to be significantly changed when several other existing, reliable PRA results were also included.
Based on its analysis, the staff found that the six plants met the health effects quantitative objectives in the Commission's Safety Goal (i.e., 0.1% of the expected accident or cancer fatality risks from causes not related to nuclear plants). Guidance for an acceptable core damage frequency was not explicitly provided. However, in order to provide assurance that (1) core damage due to a DHR failure-related event will not occur in the lifetime of the present population of plants, (2) consistency is maintained with the 10-5/RY contribution to core damage frequency from station blackout expected after resolution of USI A-44, and (3) the frequency of a severe release will be less than the Commission's safety goal guidance of 10-6/RY, the staff selected a goal that core damage due to failure of DHR function should be less than 10-5/RY. This staff-selected goal was intended only for application to the resolution of USI A-45. The results indicated that the DHR-related frequency of core damage at certain plants may be considerably above this goal. To address the question of whether corrective actions could be cost-effective, six possible alternatives addressing potential DHR vulnerabilities were identified and then evaluated.
In presenting its proposed for resolution of USI A-45 in SECY-88-260,1143 the staff recognized the ongoing actions in implementing the Commission's Severe Accident Policy, one of which was a generic letter to require all plants in operation or under construction to undergo a systematic examination termed the Individual Plant Examination (IPE) to identify any plant-specific vulnerabilities to severe accidents. The IPE analysis is intended to examine and understand the plant emergency procedures, design, operations, maintenance, and surveillance to identify vulnerabilities. The analysis will examine both the DHR systems and those systems used for other functions. It is anticipated that a future extension of the IPE program will require examination of externally-initiated events, some of which significantly contribute to DHR failure-related core damage frequency.
To resolve USI A-45, one of the alternatives proposed by the staff was to have each licensee perform a risk assessment for its plant. This assessment would be done in conjunction with the IPE program. Available options for acceptable risk assessments include performing a Level-1 PRA (enhanced) or performing an analysis using the IDCOR IPEM. Thus, USI A-45 was RESOLVED with the requirement for plant-specific analyses to be conducted under the IPE program.