98 Effects of Recent U.S. Seismic Hazard Studies on New Nuclear Power Plant Design Criteria (PSAM-0087)

The understanding and characterization of earthquake occurrences and the expected resulting seismic ground motions is constantly evolving, particularly in areas of the world where earthquakes are not that frequent. As a result, the seismic hazard has not been researched in as much detail. These areas include the Central and Eastern United States (CEUS), and most of Northern and Central Europe. New data is continuously being collected, and analysis methods are being improved. The earthquake characterization in the CEUS is oriented toward analytical procedures because of the low rate of occurrence of moderate-to-large seismic events in the region. This low rate means that relatively little empirical data is available, and the hazard from earthquake shaking must be estimated from analysis and engineering judgment from experts in the field.

Within the nuclear power industry, new information on seismic sources has been compiled in conjunction with three Early Site Permit (ESP) applications for new nuclear plants submitted by three separate utility groups. The new information leads to new seismic sources in the Central United States and in the Charleston, South Carolina area, and to revised parameters (principally, estimates of maximum magnitude) for other sources. Also, new models of earthquake ground motion have been derived in a major study by Electric Power Research Institute (EPRI) in 2004, which better defines the ground motion and the associated uncertainties as a function of magnitude and distance.

The overall effect that has been demonstrated by use of both the new seismic source information and the new models for ground motion has been to increase the seismic accelerations expected for a given return period at some locations in the CEUS. For these locations where an increase in seismic hazard has been calculated, the increased accelerations would normally be expected to directly translate into a corresponding increase in the required seismic design spectra for new nuclear power plants. However, the increase in the accelerations exhibited from these new seismic hazard studies is primarily in the high-frequency range of the response spectrum, i.e., at frequencies greater than 10 Hz, and much of the seismic hazard is caused by earthquakes of small magnitude that are expected to be non-damaging.

EPRI and the U.S. Department of Energy (DOE) are currently conducting a major research effort focused on quantifying the effects of the increased CEUS seismic hazard on new nuclear power plant seismic design. The seismic research tasks encompass:

• Assessment of performance-based approach for determining seismic ground motions

• Effect of small magnitude earthquakes on seismic hazard analyses

• Effect of ground motion variability and truncation of lognormal distribution for ground motion

• Effect of seismic wave incoherence on foundation and building response

• Effect of negligible inelastic behavior on high-frequency response

• Potential for regulatory changes associated with seismic design of new plants

The purpose of this paper is to summarize the status of this ongoing research program as well as to provide preliminary results and conclusions related to the effect of the latest seismic hazard studies to the seismic design spectra for a new nuclear power plant.