Site-Specific Seismic Site Response Model for the Waste Treatment Plant, Hanford, Washington
- Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 (United States)
The seismic design basis for the Waste Treatment Plant (WTP) at the Department of Energy's Hanford Site near Richland, Washington, was established in 1999 based on an extensive probabilistic seismic hazard analysis completed in 1996 by Geomatrix Consultants, Inc. In subsequent years, the Defense Nuclear Facilities Safety Board (DNFSB) staff questioned the some of the assumptions used in developing the seismic design basis, particularly the adequacy of the site geotechnical surveys. Existing site-specific shear wave velocity data were considered insufficient to reliably use California earthquake response data to directly predict ground motions at the Hanford Site. To address this concern, the Department of Energy's Office of River Protection (ORP) and Pacific Northwest National Laboratory (PNNL) developed and executed a plan for acquiring site-specific soil data down to approximately 500 feet, and for reanalyzing the effects of deeper layers of sediments interbedded with basalt. New geophysical data were acquired, analyzed, and interpreted with respect to existing geologic information gathered from other Hanford-related projects in the WTP area. Existing data from deep boreholes were assembled and interpreted to produce a model of the deeper rock layers consisting of inter-layered basalts and sedimentary interbeds. These data were analyzed statistically to determine the variability of seismic velocities. The earthquake ground motion response was simulated on a large number of models resulting from a weighted logic tree approach that addressed the geologic and geophysical uncertainties. Weights in the logic tree were chosen by a working group based on the strength or weakness of the available data for each combination of logic tree parameters. Finally, interim design ground motion spectra were developed to envelope the remaining uncertainties. The results of this study demonstrate that the site-specific soil structure (Hanford and Ringold formations) beneath the WTP is thinner than was assumed in the 1996 Hanford Site-wide model. This thinness produces peaks in the response spectra (relative to those in 1996) near 2 Hz and 5 Hz. The soil geophysical properties, shear wave velocity, and nonlinear response to the earthquake ground motions are known sufficiently, and alternative interpretations consistent with this data did not have a strong influence on the results. The structure of the upper four basalt flows (Saddle Mountains Basalt), which are inter-layered with sedimentary interbeds (Ellensburg Formation), produces strong reductions in the earthquake ground motions that propagate through them to reach the surface. Uncertainty in the strength of velocity contrasts between these basalts and interbeds resulted from an absence of measured shear wave velocities (Vs) in the interbeds. For this study, Vs in the interbeds was estimated from older, limited compressional wave (Vp) data using estimated ranges for the ratio of the two velocities (Vp/Vs) based on analogues in similar materials. The Vs for the basalts, where Vp/Vs is well defined, still is limited by the quality and quantity of the Vp data. A range of possible Vs for the interbeds and basalts was included in the logic trees that produced additional uncertainty in the resulting response spectra. The uncertainties in these response spectra were enveloped at approximately the 84. percentile (based on the logic tree) to produce conservative design spectra. This conservatism increased the seismic design basis by up to 40% compared to the 1999 values. Because of the sensitivity of the calculated response spectra to the velocity contrasts between the basalts and interbedded sediments, additional boreholes and direct Vs measurements through these layers are now being planned. The new measurements are expected to reduce the uncertainty in the site response that is caused by the lack of direct knowledge of the Vs contrasts within these layers. (authors)
- Research Organization:
- WM Symposia, Inc., PO Box 13023, Tucson, AZ, 85732-3023 (United States)
- OSTI ID:
- 21208764
- Report Number(s):
- INIS-US-09-WM-06321; TRN: US09V1075079551
- Resource Relation:
- Conference: Waste Management 2006 Symposium - WM'06 - Global Accomplishments in Environmental and Radioactive Waste Management: Education and Opportunity for the Next Generation of Waste Management Professionals, Tucson, AZ (United States), 26 Feb - 2 Mar 2006; Other Information: Country of input: France; 3 refs
- Country of Publication:
- United States
- Language:
- English
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