Abstract:
Rock salt provides the world's best barrier, and has thus long been considered as a host rock for nuclear waste repositories. Salt domes have vertical extensions of several kilometres and are consequently well-suited for deep borehole disposal, as an alternative to a mined repository.
The isolation capacity of undisturbed salt rocks is based on the creep behaviour which tends to close any access paths. A number of natural analogues where fluids have been contained in cavities under high pressure for millions of years support this picture. For waste disposal, one has to show that the tightness of the geological barrier is not compromised by the repository excavation and the thermal loading due to the waste, and that suitable geotechnical barriers can be constructed.
We discuss the option of deep borehole disposal in rock salt. In contrast to the mined repositories which are usually considered, this option employs boreholes of several kilometres, drilled into a salt dome. The waste canisters are emplaced in the lower part of the borehole. As backfill material, we propose molten salt mixtures, similar to the ones used in solarthermal plants as heat exchange and storage fluids. The waste-generated heat will keep the salt liquid for a long time, ensuring complete containment without the possibility of ground water reaching the waste. The upper part of the filled boreholes, still inside the salt dome, converges under lithostatic pressure due to salt creep, and the top part can be sealed with asphalt, bentonite and concrete. We present some experimental results on the closure of boreholes under pressure and the properties of liquid salt to support our proposal.
Introduction
The favourable barrier properties of rock salt have long been exploited for cavern storage of gas and oil, and make it a prime candidate for as waste repository host rock. Usual concepts, both in bedded and domal salt, assume a repository mine in a salt formation in a depth of several hundred to a thousand metres where heat-generating waste can be emplaced in drifts or in boreholes below drifts. After emplacement, drifts can be backfilled, and the repository is closed using suitable shaft seal systems. Over time, salt creep will cause convergence in the mine and enclose the waste, completely isolating it from the biosphere.
Meanwhile, in the context of other host rocks such as claystone or crystalline rocks, the deep borehole disposal concept has been suggested. In this paradigm, boreholes are drilled to a depth of several kilometres, and the waste canisters are emplaced directly from the surface. Details of casing, seals etc. depend on specific concepts and host formations. In the crystalline basement, the waste will be presumably be in contact to ground water, however, due to the large depth and hydrogeological factors, the waste can be effectively decoupled from the biosphere. Technically, this approach can be much simpler than a mined repository.
Here we propose to combine the advantages of these concepts: Salt domes can have thicknesses of several kilometres, and can correspondingly accommodate deep boreholes. In large depth, convergence is much faster, and waste can be completely contained rather quickly, without any groundwater access. A novel backfill material – eutectic molten salt mixtures – can further enhance this idea, offering immediate complete containment of the waste as well as retrievability.
