UN-UO2 composite fuel is considered an advanced technology fuel (ATF) option to overcome the low oxidation resistance of the UN fuel. However, the interaction between UO2 and UN limits the performance of such composites. A possible way to avoid this interaction is to encapsulate the UN fuel with a material that has a high melting point, high thermal conductivity and reasonably low neutron cross-section. Amongst many candidates, refractory metals can be the first option. In this study, detailed investigations in UN-X-UO2 composite systems (X = V, Nb, Ta, Cr, Mo, W) were performed using SEM/FIB-EDS. The systems were heat-treated at 1773 K and 80 MPa for 10 min in vacuum using the spark plasma sintering method as a pressure-assisted diffusion apparatus. The results suggest that Mo and W are the most promising coating candidates to protect the UN fuel against interactions with UO2. Both metals are inert to N migration and preserve sharp interfaces with the nitride fuel. V, Nb, Ta and Cr strongly interact with UO2 and UN and form their respective nitrides V2N/V8N, Nb2N, and Cr2N. The formation of TaNx was not observed but Ta reacts with UO2 and forms two phases at the UO2-Ta interface (UTa2O7 and Ta2O5), while O from UO2+ x diffuses throughout the Ta foil and oxidise the UN pellet via grain boundary attack. This oxidation mechanism also occurs at the V, Nb and Cr-UN interfaces. Our recent atomic scale modelling of the X-UN interfaces also proposes Mo and W as the optimal candidates. Therefore, these results validate the coating candidates for the UN fuel and may guide further experimental/modelling development in UN-X-UO2 advanced technology fuel.