Development of Encapsulated UN-UO₂ Accident Tolerant Fuel

Abstract: Accident tolerant fuels (ATFs) are designed to endure a severe accident in the reactor core longer than the standard UO2-Zr alloy systems used in light water reactors (LWRs). Composite fuels such as UN-UO2 are being considered as an ATF concept to address the lower oxidation resistance of the UN fuel from a safety perspective for use in LWRs, whilst improving the in-reactor behaviour of the UO2 fuel. The main objective of this thesis is to fabricate, characterise, and evaluate an innovative ATF concept for LWRs: encapsulated UN spheres as additives for the standard UO2 fuel. Several development steps were applied to understand the influence of the sintering parameters on the UN-UO2 fuel microstructure, evaluate potential coating candidates to encapsulate the UN spheres by different coating methodologies, assess the oxidation resistance of the composites, and estimate the thermal behaviours of uncoated and encapsulated UN-UO2 fuels. All composites were sintered by the spark plasma sintering method and characterised by many complementary microstructural techniques. Molybdenum and tungsten are shown, using a combination of modelling and experiments, to be good material candidates for the protective coating. It is shown that the powder coating methods form a thick, dense, and non-uniform coating layer onto spheres, while the chemical and vapour deposition methods provide thinner and more uniform layers. Finite element modelling indicates that the fuel centreline temperature may be reduced by more than 400 K when 70 wt% of encapsulated spheres are used as compared to the reference UO2. Moreover, the severity of the degradation of the nitride phase is reduced when embedded in a UO2 matrix and may also be reduced even more by the presence of a coating layer. These results contribute to further developments in methodologies for fabricating, characterising, and evaluating accident tolerant fuels within LWRs.