The U.S. Department of Energy (DOE) is constructing a Waste Treatment and Immobilization Plant (WTP) to treat and vitrify legacy nuclear wastes stored in underground tanks at the Hanford Site in southeastern Washington State. The geology underlying both the Hanford Site and the WTP is a sequence of sedimentary units overlying a series of flood basalt flows. The original seismic design basis for the WTP was established in 1999 based on a probabilistic seismic hazard analysis completed in 1996 (Geomatrix Consultants 1996). The Defense Nuclear Facilities Safety Board (DNFSB) subsequently initiated a review of the seismic design basis of the WTP. In March 2002, DNFSB staff questioned the assumptions used in developing the seismic design basis, particularly the adequacy of the site geotechnical surveys, and subsequently raised additional questions about the probability of earthquakes, adequacy of the attenuation relationships that describe how ground motion changes as it moves from its source in the earth to the site, and the large uncertainty in the extrapolation of California attenuation models to the Hanford Site. Between 2002 and 2004, the DOE Office of River Protection (ORP) responded, resolving many of the questions raised, and developed a plan to acquire additional site data and analysis to address remaining questions. The key features of this plan were (1) acquiring new soil data down to about 500 ft, (2) reanalyzing the effects of deeper layers of sediments interbedded with basalt down to about 2,000 ft that may affect the attenuation of earthquake ground motions more than previously understood, and (3) applying new models for ground motions as a function of magnitude and distance at the Hanford Site. In 2004 and 2005, the Pacific Northwest National Laboratory (PNNL) led efforts for DOE-ORP to address features 1 and 2 of the plan by collecting site-specific geologic and geophysical data at the WTP site and conducting modeling of the WTP site-specific ground motion response. New geophysical data were acquired, analyzed, and interpreted with respect to existing geologic information gathered from other Hanford-related projects in the WTP area. Information from deep boreholes was collected and interpreted to produce a realistic model of the deeper rock layers consisting of the interlayered basalts and sedimentary interbeds. The earthquake ground motion response was modeled, and a series of sensitivity studies was conducted to address areas in which the geologic and geophysical information has significant remaining uncertainties. This effort culminated in 2005 with issuance of an updated seismic response analysis for the WTP site (Rohay and Reidel 2005). The updated seismic response analysis used existing and newly acquired seismic velocity data, statistical analysis, expert elicitation, and ground motion simulation to develop interim design ground motion response spectra that enveloped the remaining uncertainties. The uncertainties in these response spectra were enveloped at approximately the 84th percentile to produce conservative design spectra, which contributed significantly to the increase in the seismic design basis.
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