Geological storage involves the placement of the canisters containing spent fuel elements in a stable stratum typically 1 km below the surface. Such rocks can be assumed to contain water, since the depth would be well below the water table. However, the water is not expected to play a large role in the heat trans­fer from the blocks, and the store would be designed to maintain the surface temperature of the canisters at no more than 100°C or so. However, the pres­ence of groundwater means that material that is leached from the storage blocks may be transported through the stratum in the water. and this is an im­portant consideration in the design of such systems. Circulation of water through the rock as a result of density differences induced hv temperature gr:i-

dients over long periods (the thermal buoyancy or thermal circulation effect) is important in determining the migration of the fission products. This is a very slow process and is not expected to present a serious hazard, but it must be very carefully taken into account for long-term disposal systems. We discuss such systems further in considering the disposal of fission products from repro­cessing plants in the next section.

The choice among the various methods of disposal will be dictated by the availability of suitable storage sites. More geological data will be required be­fore optimum choices can be made. However, studies in many countries indi­cate that spent fuel can be managed and disposed of without undue risk to humans or the environment.