Resolution of Generic Safety Issues: Item A-46: Seismic Qualification of Equipment in Operating Plants (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–34 )
The design criteria and methods for the seismic qualification of mechanical and electrical equipment in nuclear power plants underwent significant changes during the course of the licensing of commercial nuclear power plants. Consequently, it was believed that the margins of safety provided in equipment to resist seismically-induced loads and to perform their intended safety functions could vary considerably. Therefore, to ensure the ability of plants to achieve a safe shutdown condition when subject to a seismic event, the seismic qualification of equipment in operating plants had to be reassessed.
The objective of this issue was to establish an explicit set of guidelines that could be used to judge the adequacy of the seismic qualification of mechanical and electrical equipment at all operating plants, in lieu of attempting to backfit existing design criteria to new plants. This guidance was to address equipment required to safely shut down a plant as well as equipment whose function is not required for safe shutdown but whose failure could result in adverse conditions that might impair shutdown functions. Also, explicit guidelines were to be established for use in requalifying equipment whose qualification was found to be inadequate. The issue was declared a USI in February 1981 and published in NUREG-0705.44 A detailed action plan for resolving the issue was published in NUREG-0649,1061 Revision 1.
Resolution of the issue was based mainly on work completed by the Seismic Qualification Utility Group (SQUG) and EPRI using the seismic and test experience data approach1751 that was reviewed and endorsed by the Senior Seismic Review and Advisory Panel (SSRAP) and the NRC staff. The scope of the review was narrowed down to equipment required to bring each affected plant to hot shutdown and maintain it there for a minimum of 72 hours. A walk-through of each plant was deemed necessary to inspect equipment in the scope. Evaluation of the equipment was to include: (a) adequacy of equipment anchorage; (b) functional capability of essential relays; (c) outliers and deficiencies (i.e., equipment with non-standard configurations); and (d) seismic systems interaction. Work completed on the issue resulted in the publication of NUREG/CR-3017,1063 NUREG/CR-3875,1064 NUREG/CR-3357,1065 NUREG/CR-3266,1066 NUREG-1030,919 and NUREG-1211.1067 The issue was RESOLVED when requirements were issued in Generic Letter 87-02.1069 Verification of licensee actions was pursued with Generic Letter 87-03.1387