Caesium retention and migration behaviour in Boom Clay: Topical report – status 2015

The Belgian radioactive waste management organisation, ONDRAF/NIRAS, is responsible for the long-term management of radioactive waste in Belgium. At present, the long-term solution proposed by ONDRAF/NIRAS is a deep disposal facility for low-level and intermediate-level long-lived radioactive waste (LILW-LL) (Category B waste), vitrified high-level radioactive waste (HLW) and spent nuclear fuel (SNF, or SF-spent fuel), together comprising Category C waste. The primary aim of the ONDRAF/NIRAS programme for Category B and C wastes is to establish the feasibility of a deep disposal facility, without any presumption about precise repository location. This report is part of the ONDRAF/NIRAS RD&D plan in support of the establishment of a Safety and Feasibility Case (ONDRAF/NIRAS, 2013) The current topical report deals with defining a phenomenological description and parametrisation of the retention and migration behaviour of caesium in Boom Clay, which is considered as a representative element for elements characterized by cation exchange sorption. The report will provide both qualitative (understanding of the underlying mechanisms) and quantitative (migration parameters) justifications for the safety-relevant arguments that are made, and document the associated uncertainties. It deals with conditions referring to the Boom Clay formation, as it is situated nowadays at the Mol site, with present-day compositions of pore water and mineral matrix.