Addition versus radiolytic production effects of hydrogen peroxide on aqueous corrosion of UO$_2$
Résumé
The effects of hydrogen peroxide, H2O2, on UO2 corrosion is investigated in aerated deionized water in two types of situations. The H2O2 species is either added to water or produced by radiolysis at UO2/H2O interfaces. The concentrations vary in the range 10−5–10−1 mol l−1. The radiolysis is induced by irradiating the UO2/H2O interfaces with a He2+-beam emerging from the UO2 discs into the solutions. Both the evolution of the aqueous solutions and the UO2 surfaces are characterised. In both types of experiments, the alteration of UO2 results in the formation of the same secondary phase, an hydrated uranium peroxide called studtite (UO2(O)2 · 4H2O). However, the uranium release at the interface differs strikingly. It is much higher when H2O2 is produced by irradiation than when it is simply added. Furthermore, it varies in opposite direction as a function of the H2O2 concentration. This gives evidence that the chemistry at the UO2 interface under irradiation differs significantly from the chemistry induced by simply adding H2O2 to the solution. Rutherford backscattering spectrometry is used to determine the growth rate of the corrosion layer. For H2O2 addition, the layer thickness increases with increasing leaching time, although as time increases, the U release tends towards zero. It is possible to establish the first empirical equation relating the corrosion rates to the added H2O2 concentrations. For H2O2 radiolytic production, the growth is continuous as irradiation time increases but the growth rate seems to decrease as the layer grows and to reach a limit.