Geochemical Processes at the Carbon Steel/Bentonite Interface in Repository Conditions

The Deep Geological Repository (DGR) is currently the most accepted management option for the isolation of high level radioactive wastes. The DGR is based on a multi-barrier system, which will limit releases of mobile radionuclides to the biosphere. In the Spanish design of the repository the spent fuel is encapsulated in canisters of carbon steel. The space between the canister and the host geological formation will be filled with bentonite buffer clay. The effects of the reactions occurring in the canister/compacted bentonite interface should be understood for assessing the waste isolation. If canister failure due to corrosion occurs [1] the iron will be in contact with the bentonite affecting its properties, both in terms of the chemical evolution of the pore water and the properties of the bentonite. Iron precipitates can significantly change the properties of bentonite crucial for the migration of radionuclides such as porosity or sorption capacity. Ferrous ions can also pass through bentonite and precipitate as iron oxy-hydroxides that can form pseudocolloids with radionuclides and quickly migrate in the host rock without sorption. But, the major effect of corrosion products will probably be the change of pH and Eh affecting the stability of the barrier and the release rate of radionuclides. There are a number of studies on the corrosion of metals that could be used as canister [2], also studies on the iron-bentonite interaction [3], but not as many studies are focused to the iron-compacted bentonite interaction [4,5] and the associated mineralogical, chemical and physicochemical transformations of the bentonite [6]. The experimental studies conducted by CIEMAT are focused on the iron canister corrosion products interaction with the bentonite system and are based on a series of short term and medium term experiments conceived at different scales, from conventional laboratory experiments and experiments in cylindrical cells, to those specifically designed 3D mock up experiments, the so called 'GAME (Geochemical Mock up experiments) scale'. The experiments proposed in the context of the NF-PRO project (which is focused on understanding physical and numerical modelling of the key processes in the Near-Field, and their coupling, for different host rocks and repository strategies) have two main objectives: the study of the corrosion products generated in the canister/bentonite interface at the repository conditions, and to determine how the corrosion affects the properties of the bentonite. This paper describes the first tests performed to reach the objectives proposed and presents the results on the analyses of the corrosion products from carbon steel in contact with bentonite for a period of six months and one year subjected to heating and hydration. (authors)