Treatment of kappa in Recent Western US Seismic Nuclear Plant Probabilistic Seismic Hazard Studies
Abstract:The three operating nuclear plants (Diablo Canyon, Palo Verde, and Columbia Generating Station) in the western United States recently performed SSHAC Level 3 seismic hazard studies in response to a Request for Information by the Nuclear Regulatory Commission, following the accident at the Fukushima Dai-ichi nuclear facility. The treatment of zero-distance kappa, referred to as kappa_0 and commonly attributed to material damping and scattering in the shallow crust, was given extensive consideration in these studies. Available ground motion prediction equations (GMPEs) do not typically include kappa_0 as a prediction parameter and are developed for an average kappa_0 of the host region. Kappa scaling is routinely applied to adjust for the differences in average kappa between the GMPEs host regions and the target regions. The impact of kappa scaling on the results of probabilistic seismic hazard analyses is significant for nuclear and other facilities that are sensitive to high frequency ground motions (frequencies greater than about 10 Hz). There are several available approaches for deriving kappa scaling factors to GMPEs, which all require estimating kappa_0 at the target site. It is difficult to constrain the target kappa_0 empirically due to the scarcity of ground-motion data from hard-rock sites in ground-motion databases.
The hazard studies for the three nuclear power plants had different data, faced different challenges in the estimation of kappa_0, and used different methods for the estimation of the effect of kappa_0 on the site-specific ground motions. This presentation summarizes the approaches used for the evaluation of kappa_0 and for their incorporation in the probabilistic seismic hazard analysis. Emphasis is given to the quantification of the kappa_0 uncertainty, and on the evaluation of its impact to the resulting seismic hazard at the different sites.