Geological settings are of major importance in selecting appropriate environment for disposal of high-level radioactive waste from nuclear power plants. Clay rocks are preferred in a number of countries as a suitable host rocks due to their insulating properties and ability to self- seal cracks and other disturbances. In Bulgaria, on the basis of multi-criterion comparative analysis, several prospective sites have been selected for further consideration. One of them is the Varbitsa site located in the Fore-Balkan Unit. The marls of the Sumer Formation are the host rock. Similar clay host rocks for high-level radioactive waste disposal have been extensively studied in France, Switzerland and Belgium. In this context, the experience gained in these countries is particularly valuable for the further development of the geological disposal in Bulgaria. The current paper presents a comparison between the geological settings of the Varbitsa site (NW Bulgaria) and the Bure site (SE France), whose argillites are similar to the Bulgarian marls. The outcomes of this comparison would be useful for planning and conducting future research activities, as well as for the public acceptance of the geological disposal.
In Bulgaria, from the preliminary analyses performed for site selection of deep geological disposal of high-level waste (HLW) and spent fuel (SF), it was concluded that the most promising host rocks are the argillaceous sediments of the Sumer Formation (Lower Cretaceous), situated in the Western Fore-Balkan Mts. The present paper aims to compare the transport of three major radionuclides from a hypothetical radioactive waste disposal facility, which incorporates an engineering barrier of bentonite into the argillaceous (marl) medium. The simulations were performed by using HYDRUS-1D computer programme. The results are used for a preliminary estimation of argillaceous sediments as a host rock for geological disposal of HLW.
When selecting a site for the construction of a repository for high-level radioactive waste (HLW), the mineral composition of the geological environment is of primary importance. The content and type of the clay component and its structure have important implications for the insulating and sorption properties of the host rock and migration of radionuclides. In such previous studies in Bulgaria for site selection of the geological disposal of HLW the Lower Cretaceous marls (argillites) of the Sumer Formation (Western Pre-Balkans) were considered as ones of the prospective formations. This paper presents data about grain size distribution and mineral composition of the marls (argillites) at depths of 50 m and 400 m. A study was conducted on the structure of samples taken at a depth of 800 m from the Golyamo Pestene borehole. The obtained results are compared in order to clarify the homogeneity of the formation in depth. An increase in the clay fraction and the illite contained in it are observed in depth – from 22% at 18 m to 29% at 400 m, additionally, there is a slight increase in the carbonate content from 13.9% in samples up to 50 m to 26.9% on samples of 400 m. Data are presented on the volume, surface area, size and distribution of pores in the marls from 200 m depth (P-5 “Golyamo Peshtene”), which are necessary in assessing the migration of radionuclides from the repository. The majority of pores (82.4%) have a radius below 0,1 μm. A comparison is made of the mineral composition and structure of the argillites (marls) with similar clay rocks in Western Europe, where underground research laboratories have been constructed for in-situ study of the host rock.
The construction of an underground research laboratory (URL) precedes the construction of a repository for geological disposal of а radioactive waste. The article considers the types of laboratories, their main objectives and tasks. Based on a comparative analysis between the sites selected as prospective for geological disposal in Bulgaria, the Sumer site in the Sumer Formation is proposed for further investigation as an alternative for URL construction.
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