Spent fuel UO2 matrix alteration in aqueous media under oxidizing conditions

Abstract

The effects of αβγ radiolysis of water on oxidation and dissolution of the UO2 matrix of spent fuel were investigated in aerated media. The prospect of long-term interim storage of spent fuel assemblies in pools requires prior examination of an incident scenario in which a cladding failure could eventually lead to contact between water and the fuel. Radiolytic oxidation of the matrix together with the formation of secondary phases subject to volume expansion could then no longer be disregarded, and could enhance the degradation of the failed rod. To determine the amplitude of the alteration various spent fuel leaching experiments conducted at different degrees of oxidizing conditions and of accessibility of species at the reaction interface are discussed. This work is based on modeling the radiolysis processes and on various possible interpretations of leaching test results. Properly interpreting these results implies full knowledge of the fuel sample history, and the development of a rigorous experimental approach to eliminate any artifacts (problems due to the residual inventory at the grain boundaries, or to inaccurate estimates of the actual surface area). The results indicate that despite highly variable oxidizing conditions (presence of an external γ irradiation source, addition of H2O2, aerated water) the spent fuel matrix alteration determined from the release of tracer elements such as Cs and Sr is always the same. Conversely, allowing for the alteration conditions (no solution renewal and no complexants), the uranium concentrations in solution do indeed depend on the oxidizing conditions as does the nature of the secondary phases liable to precipitate.