LOW LEVEL WASTE CONCEPTUAL DESIGN ADAPTATION TO POOR GEOLOGICAL CONDITIONS

摘要

Since the early eighties, several studies have been carried out in Belgium with respect to arnrepository for the final disposal of low-level radioactive waste (LLW).rnIn 1998, the Belgian Government decided to restrict future investigations to the four existingrnnuclear sites in Belgium or sites that might show interest.rnSo far, only two existing nuclear sites have been thoroughly investigated from a geological andrnhydrogeological point of view. These sites are located in the North-East (Mol-Dessel) and in thernmid part (Fleurus-Farciennes) of the country.rnBoth sites have the disadvantage of presenting poor geological and hydrogeological conditions,rnwhich are rather unfavourable to accommodate a surface disposal facility for LLW.rnThe underground of the Mol-Dessel site consists of neogene sand layers of about 180 m thickrnwhich cover a 100 meters thick clay layer. These neogene sands contain, at 20 m depth, a thinrnclayey layer. The groundwater level is quite close to the surface (0-2m) and finally, the topographyrnis almost totally flat.rnThe upper layer of the Fleurus-Farciennes site consists of 10 m silt with poor geomechanicalrncharacteristics, overlying sands (only a few meters thick) and Westphalian shales between 15 andrn20 m depth. The Westphalian shales are tectonized and strongly weathered.rnIn the past, coal seams were mined out. This activity induced locally important surface subsidence.rnFor both nuclear sites that were investigated, a conceptual design was made that could allow anyrnunfavourable geological or hydrogeological conditions of the site to be overcome.rnIn Fleurus-Farciennes, for instance, the proposed conceptual design of the repository is quiternoriginal. It is composed of a shallow, buried concrete cylinder, surrounded by an accessiblernconcrete ring, which allows permanent inspection and control during the whole lifetime of thernrepository. Stability and drainage systems should be independent of potential differentialrnsettlements and subsidences. Potential radionuclides releases are controlled and have a singlerndischarge point to the biosphere.