Leaching Behavior of Unirradiated High Temperature Reactor (HTR) UO 2 -ThO 2 Mixed Oxides Fuel Particles
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Catherine Landesman, Sandra Delaunay and Bernd Grambow Laboratoire Subatech, UMR 6457, F-44307 Nantes Cedex 03, France $%675$&7
The dissolution rate of small UO2, ThO2 and mixed UO2-ThO2 particles, representative of unirradiated HTR fuel, are investigated in the pH range 1.7 to 5.5 under oxic conditions. For UO2 particles, the dissolution is kinetically control while for ThO2 particles, a solubility control mechanism seems to prevail for pH > 2. In the mixed oxides UO2-ThO2 particles, a selective release of uranium is observed, which could be either the result of a dissolution/precipitation mechanism in which thorium reprecipitate as oxide or hydroxide phase, or a result of a higher accessible surface area. Under oxic conditions, uranium may be considered as a tracer for the other soluble elements. So, assuming constant conditions with time, one can estimate from these preliminary results that unirradiated thorium based fuel kernels seem to present intrinsic confinement properties for 15000 years relative to soluble elements. ,1752'8&7,21 Recent High Temperature Reactor (HTR) designs are considered as promising medium-term alternatives to present light water reactors. The specific advantages that contribute to the interest of the HTR result mainly from their basic concept, which distinguishes them from other types of reactors: an all-ceramic fuel in the form of sealed coated particles embedded in a graphite matrix. The expected benefits of this core configuration are related to i) a high thermal efficiency (and its inherent safety advantages), ii) a minimization of wastes and iii) an enhanced retention capability of the intact coated particles towards the release of radionuclides (RN) [1]. The behavior of disposed spent HTR fuel is based on the integrity of the coated particles. Indeed, it has been shown that no radionuclide can be released significantly from the fuel particles as long as the coating layers are intact [2,3]. But, if the coatings fail due to mechanical or chemical interactions, the long-term behavior depends mainly on the leaching stability of the fuel kernels in the deep disposal aquatic phases. The aims of our research project are: i) to assess the rate of dissolution of fuel kernels under experimental conditions relevant to deep disposal conditions and ii) to model experimental data with a geochemical model in order to obtain a source term for the long term safety calculations. In the present study, we focus on the leaching behavior of HTR unirradiated particles (based on UO2 and ThO2) in oxidative media. Actually, for uranium, the dissolution rates, obtained in this media, are maximal values because they are not limited by the solubility of the U(VI) species. So, uranium release data may be used to test the retention capacity of those ThO2 based matrixes for soluble elements.
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(;3(5,0(17 )XHOSDUWLFOHVFKDUDFWHUL]DWLRQ Four different types of HTR fuel coated particles were
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