Evolution of Hazard Understanding
As discussed in Section 8.2, seismic design basis for a nuclear power plant (NPP) involves determination of the vibratory ground motion or SSE at the site for which the Seismic Category I structures, systems, and components, including the containment structure and systems are to be designed. The SSE is defined in terms of response spectra. For most operating plants, these spectra were derived using estimates of peak ground acceleration (PGA) at the site and anchoring a standard spectral shape to the PGA; for example, RG 1.60 . Figure 8.1 shows a comparison between SSE spectrum anchored to PGA of 0.2 g and a ground motion response spectrum (GMRS) derived for a recent combined license (COL) application using probabilistic seismic hazard analysis (PSHA) and risk-informed/performance-based approach (RG 1.208  and ASCE/SEI Standard 43-05-).
There are several things to be noted here. The shape of the SSE spectrum in the Fig. 8.1 has been derived from using the western US recorded earthquakes that were available at the time of development of the Regulatory Guide 1.60 in early seventies. This spectrum is rich in the lower frequencies when compared to the GMRS. The GMRS reflects the recent understanding of seismic sources and ground motion attenuations in the Central and Eastern US (east of the Rockies). The NPP SSCs, in general, have their fundamental
COMPARISON OF DESIGN SPECTRA: PAST APPROACH VS. CURRENT APPROACH
EXAMPLES OF SEISMIC HAZARD CURVES FOR A SITE IN TERMS OF SPECTRAL ACCELERATIONS AT DIFFERENT RESPONSE PERIODS
frequencies below 10 Hz, and therefore, the higher ground motion estimated in the high frequency range is of less concern, except for chatter prone equipment. Modern solid state control instruments and components are generally not chatter prone. In fact, because of the richness in the low frequency ground motions, the existing NPP SSCs have significant margin beyond the design basis ground motions.
The advent of PSHA came with the seminal publication by Prof. Allin Cornell in 1968 . This publication has allowed explicit consideration of the uncertainties and frequencies of exceedance in estimates of ground motions of various levels at a given site. Figure 8.2 of typical seismic hazard curves show likelihood of occurrence of ground motion of various levels at a site, from the application of a PSHA. NUREG/CR- 6372, “Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts,” U.S. NRC (1997) , NUREG-2117, “Practical Implementation Guidelines for SSHAC Level 3 and 4 Hazard Studies, USNRC (2012) , Reiter , and Regulatory Guide 1.208  are some of the references that discuss concepts and guidance for conducting a PSHA.
Advances such as paleo-liquefaction studies, more accurate dating techniques, and additional strong motion data from recent events have significantly enhanced our ability to estimate parameters, such as timing and size of pre-historical events and recurrence frequencies and magnitude distance variability of ground motion that are input to both deterministic and probabilistic hazard assessments.
By using the results from the PSHA, studies have shown that the mean probability of exceeding the deterministic seismic design basis (SSE) of the currently operating plants roughly ranges from 10-3/year to 10-6/ year. Thus, the deterministic design basis is neither hazard nor risk consistent based on current knowledge. The use of PSHA in conjunction with the risk-informed/performance-based considerations allows establishment of risk-consistent seismic design basis, [6, 12, 17]. The GMRS shown in Fig. 8.1 is derived using such concepts.