Coupled Experimental-Modelling Development of High Performance Composite UN-UO2 Fuels

We here present the results of a coupled experimental and modelling campaign focused on developing high-performance composite UN-UO2 fuels. Composite UN-UO2 has many interesting advantages in terms of enhanced neutronics, good oxidation resistance and improved thermal conductivity with respect to canonical UO2. However, it has been shown that a pure composite will lead to significant deleterious interaction between oxide and nitride. In order to avoid this interaction we have developed a ternary composite where the UN is protected by a thin layer of a refractory metal. We have studied V, Nb, Ta, Cr, Mo and W as candidate metals. Through electronic structure calculations coupled with atomic transport theory, we predicted that Mo and W would be the best refractory metal candidates given that they do not interact and efficiently hinder interdiffusion between oxide and nitride, while V, Nb, Ta and Cr will interact and form different kinds of intermediate phases which limit their use as a protective interdiffusion block. We verified these predictions experimentally using a novel pressure assisted diffusion experiment and analysed in depth the experimental interactions between the different metals with the nitride and oxide. The agreement between modelling and experiment is striking, down to predicting what type of phases will form and in what way the interactions will take place. Furthermore, we have used finite element analysis and Monte Carlo neutronics to model the thermal conductivity, neutronics and in-pile performance of such a composite fuel, while varying geometrical parameters, in order to optimize fabrication parameters and operation conditions. We will show how such a multi-disciplinary research program can efficiently streamline development of advanced high-performance fuel technology.