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Development, Application, and Evaluation of a Methodology to Estimate Distributed Slip on Fractures due to Future Earthquakes for Nuclear Waste Repository Performance Assessment

Paul R. La Pointe, Trenton Cladouhos^* and Sven Follin

Golder Associates Inc.
18300 NE Union Hill Road
Redmond, Washington 98052
(P.R.L.P., T.C.)
Golder Grundteknik KB
Tullgårdsgatan 12
SE-104 60 Stockholm, Sweden
(S.F.)

Manuscript received 31 January 2000.

The design and licensing of underground repositories to safely store high-level nuclear waste requires the conservative estimation of the risks that earthquakes might pose. In order to meet objectives set forth by the Swedish government for the safe disposal of spent fuel, the Swedish program has postulated a scenario in which the energy released by future earthquakes over the next 100,000 yr may produce slip on fractures intersecting canisters, and they have begun a probabilistic performance assessment of this scenario. Conventional methods for the assessment of seismic impacts to population centers or to relatively short-lived surface structures may not be useful for this purpose. Moreover, there are difficulties in using probabilistic functions calibrated from field data on distributed slip in the Swedish case, such as has been proposed for the U.S. program. To overcome these inadequacies, a new methodology has been developed and applied to three sites in Sweden that serve as laboratories for the design and development of underground nuclear waste repositories.

The methodology is stochastic and incorporates geological information at scales ranging over several orders of magnitude, the earthquake catalog from Fennoscandia, the tectonics and geomechanics of Sweden, and conservative numerical simulation of the impacts of earthquakes in Sweden over the next 100,000 yr at the three study sites. The slip predicted by the simulations is consistent with or conservative with respect to field data on distributed slip. Different measures of canister failure predicted at the three sites show how both differences in future seismicity and in the fracture geology impact canister failure. The calculations also point out areas of uncertainty in the conceptual modeling of future seismicity and input data that may require additional work prior to using the method for licensing support. We also describe applications of this methodology to other earthquake engineering problems.