Temperature Criterion Related to Clay Based Backfill Materials in the Framework of a Geological Repository of Heat Producing Radioactive Waste (HLW)

Abstract

Abstract The objective of this paper is to define a criterion for the maximum temperature allowed within clay based backfill materials, used as engineered barriers, in the framework of a geological repository of heat emitting radioactive waste. This criterion applies to all candidate geological formations in Belgium (non-indurated clay). According to the current Belgian reference repository for vitrified High Level Waste (HLW), waste packages will be disposed in deep geological clay formations after a cooling period of about 50 years in surface storage facilities. The heat release of this waste has a important impact on the design of the different components of the disposal system. The temperature increase and the possible effects of this increase will have to be considered on different locations (e.g. disposal galleries, backfill, lining, host rock, aquifer, and biosphere). In this paper, we will focus mainly on the temperature increase in the backfill material of the galleries. In the past the temperature in the host rock (Boom Clay) was limited to 100°C. Consequently, higher temperatures (above 100°C) were allowed in the components between the waste and the host rock. During the preparation of the in situ demonstration project PRACLAY, a new more stringent criterion for the limitation of the temperature in the backfill has been proposed, namely the limitation of the temperature in the backfill to 100°C. The different scientific and technical reasons for the use of this lower temperature design criterion are described in this paper. Phenomena like steam generation, mineral transformations and thermally induced, coupled effects can be reduced or avoided at lower temperatures. Another important advantage is the reduction of the corrosion rate of metal components (e.g. overpack). Finally, problems of a more technical nature (difficulty of characterization of materials, uncertainty in modeling and problems with instrumentations) can also be avoided by the use of this new criterion. Thermal calculations point out that this new criterion can be met by respecting a cooling time, for the present repository design of vitrified HLW, of 60 years instead of 50 years. We can conclude that the application of this lower temperature criterion provides better predictability of coupled effects and increases the performance, lifetime and robustness of the different barriers of the repository and of the disposal system as a whole.