The interaction of dissolved hydrogen with α-radiolytic oxidants during nuclear fuel dissolution

Abstract: Used nuclear fuel contains radiotoxic elements that will need to be kept isolated from the biosphere over a time period of 100 000 years. The KBS-3 geological storage concept aims to prevent the migration of radionuclides into the environment by different engineered barriers. As a complement to these engineered barriers, the low solubility of UO2 in water will limit the release of radiotoxic elements from the fuel in the event of groundwater intrusion. However, the strong radiation field from the used fuel will result in radiolysis of any intruding water, creating H2O2 and other oxidative species. Although radiolytic oxidants have the ability to alter UO2 dissolution behaviour, hydrogen gas formed in the anoxic corrosion of iron in the canisters has been shown to protect the fuel from radiation-induced dissolution.

In this research, the interaction of dissolved hydrogen with oxidants produced in the radiolysis of water subjected to α-radiation was studied. The studies were performed experimentally by means of homogeneous α-radiolysis, as well as leaching of SIMFUEL and a high activity MOX fuel. The possible inhibition of the hydrogen effect by the presence of bromide in groundwater was also investigated. Preparations for the studies included oxidation and neutralization of a 238Pu solution, as well as method development for low concentration oxygen measurements by gas mass spectrometry.

The results suggest that the presence of dissolved bromide does not affect the production of α-radiolytic oxidants. It was further shown that the consumption of α-radiolytic oxidants by hydrogen does not occur in the bulk solution, which confirms the role of fuel surfaces in the activation of hydrogen. In the leaching of SIMFUEL in water with added H2O2, only 0.02% of the H2O2 consumption could be related to fuel dissolution by oxidation of uranium, while the main part was shown to decompose on the fuel surface. Formation of semi-heavy water (HDO) during leaching in a deuterium atmosphere indicated that a reaction may occur between dissolved hydrogen and hydroxyl radicals from the hydrogen peroxide decomposition. It was also shown that the protective effect of hydrogen on the radiation-induced dissolution of the fuel is maintained even at α-activities comparable to those of fresh spent fuel.