Dissolution Behaviour of UO 2 in Anoxic Conditions. Comparison of Ca-Bentonite and Boom Clay
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0985-NN01-07
Dissolution Behaviour of UO2 in Anoxic Conditions. Comparison of Ca-Bentonite and Boom Clay Thierry Mennecart, Christelle Cachoir, and Karel Lemmens SCK-CEN, Boeretang 200, MOL, 2400, Belgium ABSTRACT In order to determine in how far the clay properties influence the dissolution of spent fuel, experiments were carried out with depleted UO2 in the presence of either compacted dry Cabentonite with Boom Clay groundwater (KB-BCW) or compacted dry Boom Clay with Boom Clay groundwater (BC-BCW). The leach tests were performed at 25°C in anoxic atmosphere for 2 years. The U concentrations in the clay water were followed during these 2 years, and the amount of U in the clay was determined after 2 years in order to determine the UO2 dissolution rate. The uranium concentration after 0.45 µm filtration was 50 times higher in the Boom Clay with Boom Clay water (2.0x10-7 mol.L-1) than in Ca-bentonite with Boom Clay water (6.5x10-9 mol.L-1), probably due to colloid formation in the Boom Clay system. Most released uranium was found in the clay. The fraction of uranium, dissolved from the UO2 pellet and found on the clay represents about 42 % of total uranium release in the system BC-BCW and more than 76 % in the system KB-BCW. The higher uranium retention of Boom Clay goes together with a higher dissolution rate. Global dissolution rates were estimated at about 2.0x10-2 µg.cm-2.d-1 for the BCBCW system and 3.4x10-3 µg.cm-2.d-1 for the KB-BCW system. This is not much lower than for similar tests with spent fuel, reported in literature.
INTRODUCTION The current Belgian nuclear waste management strategy considers the direct disposal of spent fuel as an alternative for reprocessing and vitrification of High Level Waste. Studies are focused on disposal in the Boom Clay layer. A general concept for the disposal of spent fuel is based on the multibarrier principle. In such a concept, the barriers for radionuclides released into the environment are the host rock, the backfill (in general bentonite), the canister overpack and the fuel itself. The use of a bentonite backfill is studied by countries such as the Sweden and Korea [1-3]. Bentonite is proposed as a backfill material a.o. because it prevents water flow and thus decreases the fuel matrix dissolution in the case of a rupture of the overpack. Although many experimental studies have been reported on the spent fuel or UO2 dissolution behavior under various conditions, only few were carried out in bentonite or clay [4-5]. However it was demonstrated that the dissolution of UO2 upon groundwater contact depends strongly on the (geo)chemical constraints prevailing in the repository [3]. The aim of this work, performed at SCK•CEN in collaboration with KAERI, was to have a better phenomenological understanding of the processes occurring during the dissolution of UO2 in close contact with slightly compacted clays and a real clay water. Because the test conditions are still very different from the in-situ conditions, the results cannot be used to define which clay would be bette
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