A seismic margin assessment procedure一种抗震裕度评估方法

The primary impetus for having seismic margin reviews performed has come from the Advisory Committee on Reactor Safeguards (ACRS), which has expressed interest in the capability of nuclear power plants to withstand earthquakes greater than the SSE for a number of years, and from seismic probabilistic risk assessment studies (SPRAs) which have shown that, although the seismic risk is small, it is generally not negligible when compared to other sources of risk. For U.S. plants in lower seismic zones for which SSE levels are commonly set between 0.12g and 0.25g, these SPRA studies have indicated that the dominant seismic risk comes from earthquake ground motion that is 2 to 5 times greater than the design SSE level. This statement implies large margin over the SSE level. However, SPRA studies have large uncertainties, both in the seismic hazard and seismic fragility (capability) aspects. For this reason, interest has been expressed in establishing more direct, simpler, and less controversial methods to evaluate the seismic margins of nuclear power plants that have been indicated by SPRAs. E P R I / N R C held a workshop on the subject of seismic margin for nuclear power plants in October 1984 [1]. The consensus was that large seismic margin generally exists over the SSE. It was concluded that seismic margin reviews should be treated as safety reevaluations and not as design evaluations. New ap-
propriate criteria and approaches should be established for conducting practical and economical seismic margin reviews. The purpose of margin review should be to demonstrate adequate margin over the SSE for plant safety and to find any "weak links" which might potentially have less than adequate margin to safely withstand some SME bigger than the SSE.
0 0 2 9 - 5 4 9 3 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
62
R.P. Kennedy et al. / A seismic margin assessment procedure
Nuclear Engineering and Design 107 (1988) 61-75 North-Holland, Amsterdam
61
A SEISMIC MARGIN ASSESSMENT R.P. K E N N E D Y
PROCEDURE
1, R.D.
C A M P B E L L 2 a n d R.P. K A S S A W A R A 3
1 RPK/Structural Mechanics Consulting, 18971 Villa Terrace, Yorba Linda, CA 9"2696, USA 2 NTS Engineering, 6695 East Pacific Coast Highway, Long Beach, CA 90803, USA 3 Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, CA 94303, USA
1. Introduction
Nuclear power plant structures and safety-related systems have been generally designed conservatively for a safe shutdown earthquake (SSE) and more conservatively for a smaller operating basis earthquake (OBE). Depending upon the relative conservatism of the design criteria, either the SSE or the OBE will control the design. For plants with SSE levels less than 0.2g, often non-seismic loadings control the design. In recent years, increasing knowledge in the geoscience field has led to a better understanding that, although highly unlikely, it is possible for the nuclear power plant to be subjected to earthquake ground motion greater than that for which the plant was designed. For this reason, interest has developed in demonstrating that nuclear plant structures and safety-related systems can safely withstand earthquake ground motion larger than their design earthquake ground motions (SSE and OBE). Within this paper, this larger-than-design earthquake ground motion will be called the seismic margin earthquake (SME) to distinguish it from the design earthquakes. The plant has already been designed. Therefore, for the SME, the goal is not to design the plant, but to determine the performance of already-designed structures, components, and systems when subjected to the SME.
Received 1 June 1987
This paper presents recommendations for a seismic margin assessment program for operating nuclear power plants. The goals of such a program are stated, and an overview of two alternate approaches is presented. Brief guidance for performing a seismic capability walkdown (an integral part of such a margin program) is provided. One of the prime purposes of such a walkdown is to screen out from review those components which will not be significant contributors to seismic risk. As guidance, the dominant contributors to seismic risk as found from existing seismic probabilistic risk assessments are presented. Seismic capacity screening guidelines are also provided. Lastly, a conservative deterministic failure margin (CDFM) for estimating a high-confidence-low-probability-of-failure seismic margin leveOverview of margin assessment approaches
In mid-1984, the NRC formed an "Expert Panel on the Quantification of Seismic Margins" to make recommendations of an approach for seismic margin review. This panel has produced two reports [2,3] on an approach for seismic margin reviews. This panel recommended an approach which they considered to be very cost-effective for demonstrating high confidence of a low probability of seismic-induced core damage for earthquake ground motion levels up to about 0.3g and for determining "weaker links" which might have highconfidence-of-a-low-probability-of-failure ( H C L P F ) capacity less than this level. The recommended approach relies heavily on earthquake experience data, generic qualification and fragility test data, extensive use of expert judgment and experience to reduce the level of reevaluation work required, and substantial levels of seismic margin plant walkdowns to discover "weaker links" and to determine where more detailed evaluations needed to be performed. The "Expert Panel" recognized that this approach was most cost-effective for SME levels less than about 0.3g, where an extensive data base exists and where expert judgment and experience is most easily applied. As the SME level is increased above 0.3g, the approach becomes significantly more difficult as less data exist in the "data base" at higher ground motion levels and expert judgment becomes less well-founded. At levels above about 0.5g, the approach breaks down into essentially a complete reevaluation of all required safety systems which might be vulnerable to seismic loading and no longer could be considered cost-effective. At the same time, the panel recognized that it is highly unlikely that the vast majority of nuclear power plants in the U.S. would be asked to demonstrate high confidence of low probability of seismic-induced core damage for ground motion levels above 0.25g because of the extremely low annual probability of exceedance of such ground motion levels for these plants. For this reason, the panel concentrated its efforts on SME levels of 0.3g and less, but also provided a lesser level of information for SME levels of 0.5g and less.