Disposal MOX—A Potential Immobilization Route for Separated Plutonium

The United Kingdom possesses the world’s largest civil stockpile of separated plutonium (Pu), ≈140T, accumulated from an extensive history of spent fuel reprocessing. Due to associated nuclear proliferation and security risks, it is imperative that this Pu is disposed of in a wasteform that renders it permanently inaccessible, such as by immobilising it within a ceramic wasteform. One candidate wasteform currently under assessment is Disposal MOX (dMOX), a variation of MOX fuel designed for direct Geological Disposal Facility (GDF) disposal instead of irradiation. Key differences between the two include: the incorporation of a neutron poison (Gd) for criticality purposes, an increased density, and potentially a higher Pu loading.<br/><br/>The resistance of dMOX wasteforms to oxidative dissolution under GDF groundwater relevant conditions needs to be understood. Specifically, how the introduction of both Pu and Gd dopants alter the corrosion behaviour of the bulk UO₂matrix needs to be explored. This may indicate ideal dopant concentration levels for dissolution inhibition and could help underpin the deployment of dMOX.<br/><br/>Using Ce as a non-active Pu surrogate, a series of homogenous Gd and Ce doped UO₂ powders have been manufactured from the thermal decomposition of mixed-metal oxalate precursors, containing 1-10mol% of the metal ion dopant. Dopants were successfully homogenously incorporated, with no evidence of phase segregation. These powders were cold pressed, sintered into pellets, and cast in epoxy resin to form electrodes. Electrochemical investigation has indicated that the introduction of the lanthanide dopants inhibits the oxidation of UO₂ to soluble UO₂^2- when in excess of ≈ 7 mol%, evidenced by a reduction in current density when an overpotential of 0.4V is applied (vs a saturated calomel reference electrode). This has been attributed to the clustering of defects at grain boundaries, reducing overall grain boundary conductivity. This observed inhibition in UO₂ corrosion is advantageous for the deployment of dMOX.<br/><br/>Additionally, the effect of new novel sintering routes, namely Spark and Plasma Sintering (SPS) on the corrosion behaviour of UO₂ and the lanthanide doped UO₂ has been explored. Sintering via SPS allows higher final pellet densities to be achieved, at the expense of smaller grains. While a higher density may lead to more dissolution resistance, smaller grains will likely cause an increased rate of corrosion. The overall effect of these competing factors is currently under investigation, with the results to be reported at the conference.