ABSTRACT:

Two rock mechanics issues of importance to the safety analysis of the KBS-3 repository for spent nuclear fuel are identified: 1) the risk of canister damage from seismically induced reactivations of rock fractures that intersect the canisters and 2) that of thermally induced spalling in deposition holes containing the canisters. In the safety analysis the reactivation issue is controlled by establishing respect distances between canister positions and potential earthquake faults, and is described briefly here. The handling of the spalling issue is described in more detail, using the Forsmark candidate site in Sweden as an example. The contents of the site reports and the rules for specifying the spacing between the heat-generating canisters are described. Numerical and analytical methods are used to predict the thermo-mechanical evolution of a repository at the Forsmark site at different scales. The spalling strength is found to be exceeded for a large majority of the deposition holes. The possibility of controlling the spalling and assessing its actual impact on the long term safety is discussed.

1. INTRODUCTION

1.1. General

The Swedish Nuclear Fuel and Waste Management Co (SKB) are now concluding site investigations at the Forsmark and Laxemar sites, southeast Sweden. The upcoming safety analysis SR-Site will evaluate the longterm safety consequences of constructing and taking into operation a KBS-3 repository for geological disposal of spent nuclear fuel at these sites. In the KBS-3 concept, copper canisters with a cast iron insert containing the fuel are surrounded by bentonite clay for isolation and mechanical protection [1]. The canisters are deposited in vertical deposition holes in the floor of horizontal tunnels at between 400 and 700 m depth in crystalline rock (Fig. 1).

Fig. 1. Schematics of KBS-3 repository at depth H below ground surface and with tunnel and canister spacing py and px, respectively.(available in full paper)

The primary safety function of the barriers of the KBS-3 multibarrier system is to isolate the waste. Should the isolation be breached, for instance because of corrosion or direct mechanical canister damage, the secondary function is to retard a potential release of radionuclides from the repository [1]. The only rock mechanics process that could cause a direct breach of the isolation would be a large shear displacement along a fracture that intersects a deposition hole, whereas there are numerous processes that potentially could influence the retardation safety function.

1.2. Isolation Safety Function

If a fracture shear displacement is fast and the fracture intersects the deposition hole at an unfavorable angle, the load transferred across the buffer may damage the canister. At present a 0.1 m fracture shear displacement across a deposition hole counts as canister damage, regardless of the shear velocity and the intersection geometry. For all time-continuous mechanical, hydromechanical and thermo-mechanical load scenarios forecasted in the 1 million year SKB safety assessment timeframe, a fracture shear displacement of that magnitude would require rock fractures with dimensions of about 500 m or more [2,3]. Fractures of such dimensions will be reliably detected during repository construction [4].

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