Surface Mediated Processes in the Interaction of Spent Fuel or alpha-doped UO 2 with H 2
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1104-NN03-05
The Interaction of Dissolved Hydrogen with Spent Fuel or UO2 Doped with Alpha Emitters. Daqing Cui1, Ella Ekeroth1, Patrik Fors2, and Kastriot Spahiu2 1 Studsvik Nuclear AB, Nyköping, 611 82, Sweden 2 Nuclear Chemistry, Chalmers University of Technology, Göteborg, 412 96, Sweden ABSTRACT In most deep disposal concepts, large amounts of hydrogen are expected to be produced by the anoxic corrosion of massive iron containers. At repository temperatures, hydrogen is quite inert and is not expected to contribute to the redox capacity of the deep groundwaters. In several recent works, a large impact of dissolved hydrogen on the dissolution of the LWR or MOX fuel and UO2(s) doped with 233U or 238Pu has been observed. For hydrogen concentrations above a certain limit, the dissolution rates of these highly radioactive materials drop to very low values. A discussion of the results obtained with spent fuel or α-doped UO2 in the presence of a range of hydrogen concentrations is presented. Typical for all measurements under such conditions are the very low long term concentrations of uranium and other redoxsensitive radionuclides, such as Tc and the minor actinides. The concentrations of U are systematically lower than the values measured during UO2(s) solubility measurements carried out in the presence of strong reducing agents. Measurements of the radiolytic oxygen after long leaching periods result in values below detection limit. The investigation of the surface of spent fuel or UO2(s) pellets doped with 233U by XPS after long periods of testing shows absence of oxidation. The kinetics of the release of non-redox sensitive elements such as Sr and Cs, used to estimate fuel matrix dissolution rates, is also discussed. An attempt is made to propose potential mechanisms responsible for the observed behaviour, based mainly on data from studies on the interaction of water adsorbed on the surfaces of metal oxides or actinide oxides with radiation. Another important effect observed in recent studies is the existence of a threshold for the specific alpha activity below which no measurable influence of the alpha radiolysis on the uranium release from UO2 is observed. The importance of such a threshold for the behaviour of spent fuel under repository conditions encompassing very long time scales will be discussed, as well as the necessity to better investigate the mechanisms of recombination reactions in a thin water layer on the surface of actinide oxides affected by α- radiolysis. INTRODUCTION The direct disposal of spent nuclear fuel as a waste form is currently under consideration in several countries. In most disposal concepts for high level waste, spent nuclear fuel will be encapsulated in canisters made of or containing large amounts of iron. The canisters will then be placed in deep repositories built at several hundred meters depth in granite, clay or salt geologic media. In most concepts, each canister will be surrounded by compacted bentonite clay. This arrangement constitutes a multiple barrier system, contrib
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