The Central and Eastern U.S. (CEUS) is generally not a highly seismic region, and information regarding the characteristics of earthquake sources is sparse in comparison to the west coast. This makes quantifying the seismic hazard in the CEUS more difficult, and estimates of earthquake magnitudes and return periods continues to evolve as new information becomes available. As a result, estimated seismic hazards in the CEUS have been creeping upward, and the Nuclear Regulatory Commission (NRC) has instituted a multi-year study (Generic Issue 199) to address the implications on existing plants. In September 2011, the NRC issues a draft letter (for public comment) that describes, in detail, the peer-reviewed analysis that they intend to request from existing plant operators. Exponent is pleased to offer our Structural and Geotechnical Engineering expertise to assist the nuclear industry to reevaluate seismic risks, and formulate technically sound responses to issues raised in the draft letter.
The draft letter requests two stages of response. The first phase is intended to be a short-term, pass/fail evaluation of seismic risk based on comparing the design basis earthquake to new hazard curves derived from the latest seismic source characterization. This preliminary evaluation requires three fundamental tasks:
- Rather than providing fragility curves for critical structures and equipment, the capacity of the plant to withstand ground shaking will be estimated by the design basis earthquake. Exponent can help you define this level of shaking based on IPEEE results and plant improvements since the original design.
- Plants are being asked to provide site-specific ground motion response spectra. These curves must be derived from the latest source information, accounting for potential site soil amplification. Exponent can help you define the regional hazard for your plant, and perform the calculations required to show how the site soils can affect the hazard curves.
- Exponent can help you compare the hazard and design basis, formulating a defensible response to the NRC showing whether or not a comprehensive SPRA or SMA is warranted.
Should the results of the Phase I response indicate that a full reevaluation of the seismic risk is warranted, Exponent is pleased to offer a full range of seismic expertise including development of fragility curves, adaptation of internal event fault trees, and ultimately calculation of core damage frequency or other risk measures. We can also provide peer reviews of seismic risk assessments as required by the NRC.