Leaching Behavior of Unirradiated High Temperature Reactor (HTR) UO_2-ThO_2 Mixed Oxides Fuel Particles

摘要

The dissolution rate of small UO_2, ThO_2 and mixed UO_2-ThO_2 particles, representative of unirradiated HTR fuel, are investigated in the pH range 1.7 to 5.5 under oxic conditions. For UO_2 particles, the dissolution is kineticaily control while for ThCh particles, a solubility control mechanism seems to prevail for pH > 2. In the mixed oxides UO_2-ThO_2 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.