The chapter describes a combined theoretical and experimental investigation on the effects of heat transfer In saturated clay surrounding a projectile containing heat-generating radioactive waste

INTRODUCTION

Solidified high-level radioactive wastes emit substantial amounts of heat due to radioactive decay Temperature levels of the canisters containing the waste at the time of ultimate disposal could exceed 100 °C For any conceptual disposal methods it is necessary to ensure that the emplacement technique itself and the heat generated by the waste would not seriously disrupt the geological barrier. In relation to disposal options in geologic formations the effect of heat in the soil surrounding the buried waste is a very important aspect of the investigation of feasibility. Temperature rises generate excess pore water pressures and the subsequent reduction in effective stresses might lead to fracturing of a ‘disturbed’ clay formation.

The ocean disposal concept investigated by a team of the Cambridge University Soil Mechanics group and summarized m this chapter is the one using freefalling Penetrators. The prototype penetrator could be of the order of 0 65 m in diameter and 8 5 m long The waste contained could have a heat power output of about 2 kW at the time of emplacement and this would gradually decay The method of disposal would be simply to drop the penetrators from a ship through 5–6 km of seawater. A terminal velocity of about 80 m/s would be reached after a distance of about 1000 m. It is anticipated that the penetrators would bury themselves at depths of at least 30 m below the seabed (Ove Arup and Partners, 1988)

MODELLING OF THE BEHAVIOUR OF SOIL SURROUNDING HEAT-GENERATING WASTE PLACED DEEP IN CLAY

In the first few decades after the emplacement of a penetrator containing heat-generating waste into a soft marine deposit, the waste would act as a heat source within th soil. The presence of such a heat source would cause the temperature to rise in the soil surrounding the canister. This temperature rise would cause both the soil pore water and the soil grains to expand. In general the volume increase of the pore water is greater than that of the soil grains and hence the differential volume change between the constituents of the soil would lead to an increase in pore water pressure. The permeable deposit would them consolidate as the excess pore water pressures dissipate. If the excess pore water pressures generated during heat transfer reduce the effective stresses severely, the soil may fracture and the resulting cracks may affect the ability of the soil to act as a barrier to nuclide migration at later stages. It is therefore very important to make an assessment of the effect of heat on the soil surrounding the canister.

The work described here was undertaken largely at the Cambridge University Engineering Department over a period of five years and has involved up to nine people The research programme included both theoretical and experimental investigations.

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