Understanding Thermal Property Behaviour as a Function of Radiation Damage in UN for Advanced Nuclear Fuel Applications

Uranium nitride (UN) is of interest as an accident tolerant fuel (ATF) and advanced fuel in future higher temperature fission reactors due to its advantageous thermophysical properties. This programme of work looks to examine ion beam irradiation as a surrogate for neutron/fission product damage and perform thin film measurements of the materials thermophysical properties to predict in service behaviour. UN pellets, fabricated at the Nuclear Fuels Centre of Excellence (NFCE) have been irradiated with Ar ions, to replicate fission gas build-up up to 100 displacements per atom (DPA) at 300°C. Post irradiation examine results will be presented, detailing X-ray diffraction (XRD) and Transmission Electron Microscope (TEM) results, detailing the defect types formed as a function of dose. Transient grating spectroscopy (TGS) has been used to probe the first ~µm of ion irradiated material and measure the thermal diffusivity behaviour as a function of dose. This has shown a significant decrease in thermal diffusivity with higher radiation damage doses (>10 DPA) which will be correlated to defect evolution. In-situ annealing within the TGS showed a sharp recovery of thermal diffusivity above 400°C, close to pristine values, which has been correlated with in-situ annealing with TEM to examine defect evolution. The results will be presented within the context of in-pile results and the potential utilisation of the techniques of ion irradiation and TGS to probe the thermal property behaviour in nuclear materials, which could aid the down selection of novel fuel materials prior to in-pile tests and aide technology readiness level development.