Application of a novel approach to assess the thermal evolution processes associated with the disposal of high-heat-generating waste in a geological disposal facility

Abstract

The widely accepted solution for the long-term management of higher activity radioactive waste is disposal in a suitably engineered facility, located deep underground. A geological disposal facility (GDF) consists of a series of engineered and natural barriers preventing or inhibiting the release of radioactivity. These barriers include: the radioactive wasteform, the waste disposal container, the buffer material to protect the container and the natural barrier provided by the rocks in which the facility is constructed. This multi-barrier system aims to isolate the waste and contain the harmful effects of the radioactivity on humans and biota in the surface environment.The engineered barrier system (EBS) used in a GDF can include buffers based on cement and clay-based materials. The choice of buffer can have significant implications for the disposal system; the heat must be managed such that the properties of the buffer are not compromised to the extent that it cannot deliver the required level of safety. One of these materials is bentonite, usually rich in sodium montmorillonite, selected for its swelling properties and low hydraulic conductivity when saturated. In the presence of significantly elevated temperatures sodium montmorillonite can undergo mineral alteration, reducing the swelling properties of this material. This paper describes an efficient approach to assess strategies for meeting a maximum temperature constraint placed on either the buffer or geosphere surrounding the waste container. In preparation for designing and building a GDF, it is important, for the purpose of robust planning, to understand the important factors, and uncertainties, affecting the maximum temperature. The objective of this work is to inform the future emplacement strategy to enable: appropriate decay storage times; acceptable waste package loading and spatial configurations of the packages to be determined, to thus enable high-heat-generating waste to be safely disposed in a GDF.