Effect of alpha irradiation on UO2 surface reactivity in aqueous media

The option of direct disposal of spent nuclear fuel in a deep geological formation raises the need to investigate the long-term behavior of the UO2 matrix in aqueous media subjected to alpha-beta-gamma radiation. The beta-gamma emitters account for most of the activity of spent fuel at the moment it is removed from the reactor, but diminish within a millennial time frame by over three orders of magnitude to less than the long-term activity. The latter persists over much longer time periods and must therefore be taken into account over a geological disposal time scale. Leaching experiments with solution renewal were carried out on UO2 pellets doped with alpha emitters (Pu-238 and Pu-239) to quantify the impact of alpha irradiation on UO2 matrix alteration. Three batches of doped UO2 pellets with different alpha flux levels (3.30 x 10(4), 3.30 x 10(5), and 3.2 x 10(6) alphacm(-2) s(-1)) were studied. The results obtained in aerated and deaerated media immediately after sample annealing or interim storage in air provide a better understanding of the UO2 matrix alteration mechanisms under alpha irradiation. Interim storage in air Of UO2 pellets doped with alpha emitters results in variations of the UO2 surface reactivity, which depends on the alpha particle flux at the interface and on the interim storage duration. The variation in the surface reactivity and the greater uranium release following interim storage cannot be attributed to the effect of alpha radiolysis in aerated media since the uranium release tends toward the same value after several leaching cycles for the doped UO2 pellet batches and spent fuel. Oxygen diffusion enhanced by alpha irradiation of the extreme surface layer and/or radiolysis of the air could account for the oxidation of the surface UO2 to UO2+x. However, leaching experiments performed in deaerated media after annealing the samples and preleaching the surface suggest that alpha radiolysis does indeed affect the dissolution, which varies with the flux at the UO2/water interface.