Engineered microstructure composites as means of improving the oxidation resistance of uranium nitride

Abstract: Owing to its high uranium density and good thermophysical properties,uranium nitride (UN) fuel has been considered as a potential Accident TolerantFuel (ATF) candidate for use in Light Water Reactors (LWRs). However,the main disadvantage of UN is its low oxidation resistance in water/steamcontaining atmospheres at the operating temperatures of LWRs.The main objective of this thesis is to investigate a concept of engineeredmicrostructure composites as means of improving the response of UN to watersidecorrosion. The methodology for incorporating the corrosion resistantadditives in the form of metals, nitrides and oxides into the UN matrix hasbeen developed and tested. The additives were proposed to produce coated(no interaction with UN) or doped (incorporation of the additive into theUN bulk) grains, which will be able to shield the UN from the oxidising environmentand slow down the oxygen diffusion through the bulk. The UNcomposite pellets containing the selected additives were sintered using theSpark Plasma Sintering (SPS) technique. The resulting microstructures ofthe composite pellets were well characterised prior to subjecting some of theengineered microstructure representative samples to oxidation testing in airand steam containing environments.The obtained results indicate that the response to air and steam oxidationof the composite samples differs from that of pure UN. Moreover, a delay inthe oxidation onset was observed for the composite samples UN-20CrNpremixand UN-20ZrNpremix in steam and for UN-20CrNpremix pellet in air. Theimproved response to oxidation was accompanied by the formation of theternary oxides, an observation that could be applied to the screening processof the additive candidates for waterproofing of UN.