The US Nuclear Regulatory Commission's Containment Performance Improvement Program

Abstract

The Containment Performance Improvement (CPI) program has been one of the main elements in the US Nuclear Regulatory Commission 's (NRC's) integrated approach to closure of severe accident issues for US nuclear power plants. During the course of the program, results from various probabilistic risk assessment (PRA) studies and from severe accident research programs for the five US containment types have been examined to identify significant containment challenges and to evaluate potential improvements. The five containment types considered are: the boiling water reactor (BWR) Mark I containment, the BWR Mark II containment, the BWR Mark III containment, the pressurized water reactor (PWR) ice condenser containment, and the PWR dry containments (including both subatmospheric and large subtypes). The focus of the CPI program has been containment performance and accident mitigation, however, insights are also being obtained in the areas of accident prevention and accident management. Recommendations relative to BWR plants with Mark I containments were made in January 1989. One, hardening of the wetwell vent, is being implemented either voluntarily by the licensees or by invoking the backfit rule (10 CFR 50.109). Other recommended changes are being explicitly reviewed within the Individual Plant Examination (IPE) program to examine individual plants for vulnerabilities to severe accidents. These other changes include: (a) alternate water supply for drywell sprays and vessel injection, (b) enhanced reactor vessel depressurization system reliability, and (c) improved emergency procedures and training. Recommendations on the other containment types were presented to the Commission in March 1990. In general, the same containment challenges and potential improvements were examined for the BWR Mark II and Mark III plants as in the Mark I program, with the addition of improvements related to the hydrogen igniters for Mark III plants. For the PWR ice condenser and dry containments, containment by-pass and direct containment heating are issues. In addition, improvements related to hydrogen igniters have been considered for ice condenser plants. Primarily because the benefits of proposed changes are perceived to be less or because of large design differences among plants, the case for generic recommendations is not so clear cut as for the BWR Mark I pl ants. Therefore, the NRC staff has not identified any recommended generic improvements that would be applicable to all containments of a given type, but has identified improvements to be considered further on a plant-specific basis as part of the IPE program. Improvements to be included in the accident management program and areas requiring additional research have also been identified.