Composition and structure of marls from the Sumer Formation as an environment for geological disposal of radioactive waste

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.