Use of Margin in Seismic Analysis

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Use of Margin in Seismic Analysis

” Currently operating reactors (licensed prior to 1997) had their seismic regulatory basis in the General Design Criteria 2 in Appendix A to Part 50. That standard required that the design bases include “sufficient margin.” This was an undefined term that varied across facilities.

* Proposed new reactors (submitted after 1997) have to demonstrate a seismic margin of 1.67 times the site-specific 10,000 year ground motion that defines the Safe-Shutdown earthquake ground motion (SSE). That is, the high confidence of low probability of failure for a plant (where failure is defined by some type of core damage) should be at least 1.67 times the ground motion acceleration of the design basis safe-shutdown earthquake (SSE). (HCLPF=the high confidence (more than 95% confidence) of low
probability(less than 5%))

” For the ongoing and planned GI 199 analyses we anticipate evaluating the seismic margin in the existing facilities using the same techniques as we are using for new
facilities.

Other helpful info: The seismic margin in existing plants arises from several factors including;

(1) the use of a relatively infrequent design basis earthquake scenario,
(2) the use of conservative design spectral shape,
(3) the application of conservative design philosophy in the codes and standards used by the nuclear industry. The “design spectral shape” is the relative strength of different frequencies of vibration in the ground motion. )

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