Contrary effects of the water radiolysis product H₂O₂ upon th dissolution of nuclear fuel in natural ground water and deionized water

Summary

Radiolysis of water is a phenomenon which alters the prevailing conditions in the nearfield of a final geological repository for high-level nuclear waste (HLW), because the nominally anoxic conditions in the repository may change due to the production of oxidants close to the solid-liquid interface of the fuel, caused by the alpha radiolysis of water.

The influence of water chemistry and oxidant concentration in the liquid phase was tested by experiments which simulate chemical radiolysis effects. Leaching experiments with solutions of de-ionized water (DI) and natural groundwater (GW) containing the water radiolysis product H₂O₂ in various concentrations were performed contacting pellets of depleted UO₂ as a solid phase. After the experiment, U concentration was measured and the solid surfaces was examined with scanning electron microscopy and electron-dispersive X-ray analysis (SEM-EDX). Uranium concentrations showed an unexpected behaviour; at low H₂O₂ concentrations in solution, U concentrations were highest. U concentrations in the groundwater leachates were always lower than in pure water. The SEM-EDX examination showed the presence of different alteration phases on the sample. The sample surfaces treated in pure water showed optically a yellow discolouration and were covered with a thick layer of oxidation products. In the case of the treatment in groundwater, the samples remained black, and no dense layer was formed. Instead, crystals of octahedral shape were found on the surface, while the matrix grain structure remained intact. All alteration phases were found to contain only U (and possibly O, H, and C also). A reaction mechanism is proposed for the oxidation reactions taking place under the various conditions. Comparisons with natural analogues and previous work are made, and geochemical solubility limits of possible newly formed substances are examined. The results suggest that different polymorphs of uranium(IV) oxyhydroxides (para-, meta-schoepite, hemihydrate) and of uranium(VI) peroxide (Studtite, meta-studtite) have formed, and that in the case of the samples treated with groundwater, a scavenging mechanism, possibly controlled by an ion present in groundwater, leads to the deactivation of H₂O₂ in the solution.