Modelling of thermal rock mass properties at the potential sites of a Swedish nuclear waste repository

A repository for spent nuclear fuel will generate heat. For the SKB KBS-3 repository with some 6000 waste canisters positioned at 500 m depth in crystalline rock, the layout together with the thermal properties of the host rock will control the temperature evolution in the near-field and in the far-field. The canister spacing will be determined by thermal design criteria, which place limits on the maximum temperature that can be allowed in the bentonite buffer surrounding the canisters. Since the rock volume available for deposition may be limited, there may also be a need for optimizing the layout of the repository.In this paper the simulation model used to define canister spacing guidelines for SKB's repository layout is presented and discussed. Heat transport properties of the rock are shown to be key factors for the repository layout. It is shown how results from measurements of temperatures at different points in the interior of KBS-3 type deposition holes in the Prototype Repository in Äspö HRL, southern Sweden, are used to find values of buffer and canister heat transport parameters to be used as input in the model. A brief summary of the data and model uncertainties is given.The methodology employed for thermal modelling in the SKB site investigations is presented. Thermal properties are mainly measured in the laboratory or calculated from mineral composition. Both determination methods are applied to small volumes (cm scale), but thermal conductivity values are required at a scale relevant for the canisters. A methodology for upscaling has been established. In order to describe the thermal properties of large volumes of rock the site is divided into a number of rock domains, each with a characteristic geology.