Accelerated Dissolution of Doped UO₂-Based Model Systems as Analogues for Modern Spent Nuclear Fuel Under Repository Conditions

Decades of research have demonstrated that spent nuclear fuel is a good waste form supporting its direct disposal in a deep geological repository. Due to the use of doped nuclear fuels (doped with Gd, Al, Cr, etc.) in recent years, a verification of the favourable behaviour of these modern fuels was needed. Here we have studied tailor-made UO₂ model materials that can provide additional insights into the influence of doping on the dissolution behaviour of spent nuclear fuels. Thus, within this work single-effect dissolution studies were carried out using systematically produced and carefully characterised UO₂-based model materials, to provide additional insights into the dissolution behaviour of spent modern LWR-fuels under the post-closure conditions expected in a DGR.<br/>The first step in this study was the development of an optimised synthesis route for the production of UO₂ reference materials, Cr-doped UO₂as well as Nd- and Gd-doped UO₂ based materials. A special focus was set on a high density of the sintered pellets, control of grain growth and homogeneous distribution of the dopants in the UO₂ matrix.<br/>Here, we present recent results on the dissolution behaviour of these tailor made UO₂ model materials in accelerated static batch experiments using H₂O₂ as simulant for radiolytic oxidants, present in long-term disposal scenarios for SNF in failed container conditions due to the alpha-irradiation of water. In these dissolution experiments, pure UO₂ reference pellets exhibiting different densities and grain sizes, as well as Cr-, Nd-, and Gd-doped UO₂pellets with varying doping levels, produced using different doping methods and having different grain sizes, were used. In addition, industrially produced Cr- and Cr/Nd-doped UO₂ pellets were used. The dissolution experiments were performed under strictly controlled conditions with respect to exclusion of oxygen, temperature control, and exclusion of light.<br/>This bottom-up approach was followed to understand how the addition of the different dopants into the fuel matrix affects SNF dissolution behaviour. The results of the dissolution experiments indicate that the addition of Cr and the consequential modification of the fuel matrix does not lead to a significant change of the dissolution behaviour of these model materials compared to pure UO₂ reference materials (i.e., dissolution rates agree within an order of magnitude). This is in strong contrast to the dissolution rates of Ln³⁺-doped pellets, which are significantly lower than those of pure and Cr-doped pellets.