Stereological Method for Reducing Probability of Earthquake-Induced Damage in a Nuclear Waste Repository

Abstract

In Sweden, spent nuclear fuel is planned to be placed in copper/iron canisters and deposited at a depth of approximately 500 m in granitic rock. Earthquakes may induce secondary shear movements in fractures intersecting canister deposition holes, thereby threatening the integrity of the canisters. The extent of a secondary movement is related to earthquake distance and magnitude and to the size of the intersecting fracture. A probability of a canister being intersected by a critically large fracture can be calculated for given fracture size and orientation distributions, assuming that no measures are taken to identify and avoid such fractures. This paper analyses a stereological method of reducing this probability through observations of fractures fully intersecting the drift tunnels overlying the deposition holes. Deposition positions located in the planar extension of such full intersections are rejected. Both exact, numerical solutions and approximate solutions to this stereological problem are derived and the correctness of the solutions is verified by simulations. Also, the cost in terms of unutilised deposition positions is calculated. The probability of critical canister/fracture intersections is a few percent for typical fracture populations determined from field observations at a candidate site for a spent nuclear fuel repository in Sweden. By applying the suggested method, it is demonstrated that this probability can be reduced by a factor of about 35 in a typical case. The expense in terms of unutilised tunnel length is around 10 percent, which is seen as reasonable.