Dissolution of unirradiated MOX fuel in the presence of magnetite and chukanovite

In all European concepts for deep geological repositories of spent nuclear fuel, massive canisters with cast iron inserts are designed to prevent early interaction between the spent nuclear fuel and groundwater. Should a canister breach occur, corrosion of the fuel would take place at repository depths of several hundred meters where the surrounding groundwater is generally anoxic. The radiation field surrounding the fuel alters local redox conditions through water radiolysis. Under such conditions, fuel corrosion is anticipated to proceed simultaneously with anoxic iron corrosion. When all iron is corroded, iron corrosion products will be present for very long times in the near field of a repository and thus buffer the redox conditions in the near field. By using a MOX fuel pellet with an extremely high alpha field, we can test the effects of iron corrosion products on fuel dissolution considering that the very old fuel they will contact in the repository has orders of magnitude lower alpha dose rates. In this study, two leaching experiments were conducted on unirradiated MOX fuel pellets containing 10 wt% plutonium (specific alpha activity: 1.79 GBq/g) under an argon atmosphere in simulated granitic groundwater representative of the Forsmark site in Sweden. The experiments were performed in the presence of magnetite and synthesized chukanovite to assess their influence on the corrosion and leaching behavior of the unirradiated MOX fuel. A strong inhibiting effect on fuel dissolution was noted for chukanovite, while magnetite could not counteract the radiolytic dissolution of the unirradiated MOX pellet.