The heat output from high level radioactive waste buried in hard rock can give rise to groundwater convection currents. These flows can change the natural groundwater flows for thousands of years after the decommissioning and sealing of a depository. This paper presents the results of some recent calculations of this effect and discusses the possible consequences for water-borne leakage of radionuclides back to the biosphere.
One of the proposed options for disposing of radioactive waste from the nuclear power industry is to bury it in depositories deep in hard rock (KBS. 1977; Roberts. 1979). The heat generated by the decaying radionuclides in high level waste would be dissipated largely by thermal conduction using the rock mass as a heat sink. For a depository' containing a three-dimensional array of waste canisters, the resulting temperature field would extend several hundred meters into the rock, and rise and fall on a timescale of centuries (Beale, Bourke and Hodgkinson. 1979).
The most likely way in which radioactivity could reach Man from such a depository is by groundwater leaching radionuclides from the waste and transporting them to the biosphere. This leakage path is inhibited by a number of in-series barriers including the high leach resistance of the waste form. its containment in a corrosion resistant canister and the impermeability and absorptivity of the rock mass (Hill and Grimwood. 1978). This paper addresses one aspect of this problem namely the perturbation of the existing groundwater flow paths by buoyancy flows driven by the above temperatures.
A vertical section through a hypothetical region, is used to illustrate the hydrogeological effects of a high level waste depository on its surroundings. The rock mass is treated as a porous medium with constant values of permeability (10−16m2) and flowing porosity (10−4) chosen to be representative of those measured in fractured hard rock masses (Axelsson and Carlsson. 1979; Davison. 1979; Lundstrom and Stille. 1978). It is assumed that precipitation far exceeds the infiltration into this low permeability rock mass so that the water table is always coincident with the land surface which therefore acts as a constant pressure boundary.
The temperature field around a depository will cause water in the fractures of the rock to rise because of buoyancy effects. Such upward flows are of concern because they could shorten the time taken by radionuclides, leached from the waste, to reach the surface.
Temperature profiles along the vertical centreline of a depository are shown. The temperature rise near the centre of the depository reaches a maximum of 6l °C after about a century and then slowly decays as heat is distributed over an ever increasing volume of rock. However, the total amount of heat contained in the rock mass has not started to decay at the times shown. This heat energy has the potential to cause buoyancy flows long after the temperature rise at the centre of the depository has fallen to a fraction of its maximum value.
