Equilibrium Modeling of a MOX-Iron Corrosion System

Abstract

Anoxic iron corrosion is expected to play a key role in limiting radionuclide release from spent nuclear fuel following a canister breach, making it important to under stand and model the interaction between nuclear fuel and water with increased iron levels. This thesis develops an equilibrium model of the nuclear fuel-water system as a step towards a more predictive model of MOX fuel corrosion under repository con ditions. The experimental system is defined by two main processes: the corrosion of iron and the addition of the MOX pellet containing UO2, PuO2 and trace amounts of AmO2. The developed model, based on this system, focuses on iron corrosion, controlling uranium solubility and potential precipitation of secondary phases. To decide the optimal iron corrosion configuration, equilibration with an unconstrained or a constrained amount of available metallic iron was studied. The MOX pellet was modeled as a solid solution. Different phases containing uranium and ameri cium were equilibrated with the solution to bring their dissolved molalities down to physically reasonable levels; uraninite and Am(OH)3(am) were found to work best. Precipitation of secondary phases was tested by placing them in equilibrium with the solution, first one by one, and then in pairs of two. The simulation showed that the constrained iron model matched the experiment more closely than the uncon strained model, based on pH, pE, and the molalities of Fe(II) and Fe(III). Addition of the MOX pellet further increased the reducing conditions of the system. As the secondary phases were tested, all were thermodynamically possible, but no clear cor relations were found to decide whether they were present or not. The lack of kinetic parameters limited the model, as it was developed as an equilibrium model and thus shows the system’s conditions at a specific point in time rather than its continuous development. The results of the simulation showed reasonable agreement with the experimental system it was based on, though the model’s consistently more reducing conditions suggest that further development would be needed for closer replication or the experimental results.