Snapshot Review of the Regulation of Gas Voids in Nuclear Steam Supply Systems in the United States

Abstract

The recent NRC Generic Letter (GL) 2008-01 titled “Managing Gas Intrusion in Emergency Core Cooling, Decay Heat Removal, and Containment Spray Systems” to nuclear power plant licensees in the United States requires demonstration of suitable design, operational testing and control measures in order to maintain licensing commitments [1]. The generic letter outlines a number of actions that are detailed in nature; such as establishing pump void tolerance limits, limits on pump suction void fractions, etc. Each addressee was requested to evaluate their Emergency Core Cooling System (ECCS), Decay Heat Removal (DHR) system, and Containment Spray (CS) system. For each of these systems, design, operation, and test procedures were evaluated to assure that gas intrusion is minimized and monitored in order to maintain system operability and compliance with the requirements of 10 Code of Federal Regulation (CFR) 50 Appendix B [2]. In the GL 2008-01, licensees were requested to evaluate the ECCS, DHR and CS systems along four principal areas to ensure that gas accumulation is maintained less than the amount that challenges the operability of the systems, and that licensees shall take appropriate actions when the conditions are identified. The four principal areas are licensing basis, design, testing, and corrective actions. Each addressee was requested to provide a summary description of how the “REQUESTED ACTIONS” in the generic letter were addressed within nine months of the generic letter issue date. If an addressee determined that system or procedure modifications were necessary based on the review of the requested actions but cannot be accomplished within nine months of the date of the generic letter, then the addressee should provide a plan and schedule for completion of the actions. Many plants used their corrective action programs to accomplish this task. In its response, the licensee addressed any alternative course of action that it proposed to take, including the basis for the acceptability of the proposed alternative course of action. The nuclear industry, under the auspices of the Nuclear Energy Institute, has worked collaboratively with the industry to develop solutions and responses to the nine month NRC request, and these responses were submitted in October of 2008. Since that time, the NRC has been reviewing the plant submittals and issuing requests for additional information (RAIs) to the plants for clarification of their respective programs. This paper provides a snapshot review of the regulation of gas voids in the United States by focusing not only on industry actions to address the generic letter but also on the nature of the NRC requests to the nuclear plants for clarification of plant gas mitigation programs. The goal of the paper is to explore if the RAIs will provide some insights on NRC expectations of the industry as plants address gas intrusion in safety related Nuclear Steam Supply Systems (NSSS).