In situ neutron powder diffraction experiments are applied to physical, kinetic, and microstructural characterization of uranium mononitride as a promising light water reactor fuel material. The temperaturevariable coefficient of thermal expansion and isotropic Debye Waller factors are obtained by sequential Rietveld refinement over 499-1873 K. Oxidation of a UN pellet (95.2% density) under flow of 11 mg/min D 2 O is observed to initiate above 623 K and the rate increases by a factor of approximately 10 from 673 to 773 K, with activation energy 50.6 +/- 1.3 kJ/mol; uranium oxide is the only solid corrosion product.Crown Copyright

In-situ neutron diffraction patterns were collected for a sample of as-cast U3Si2 during heating to 1600 degrees C. Anomalous changes were observed above 1000 degrees C, including the formation of a new diffraction peak not belonging to P4/mbm U3Si2, unequal changes in the peak intensities and onset of anisotropic lattice expansion. The large data-set enabled derivation of a function-fitted isotropic thermal expansion coefficient to high precision, in close agreement with previous dilatometry results but reducing linearly with temperature over the studied interval. Anisotropy in the instantaneous lattice thermal expansion corresponded to anomalies reported by White et al. (2015) at a similar temperature.

In this work, a systematic study of the degradation of UN pellets (density range 96%-99.9% and grain size of 6-24 mu m) and UN-10%U3Si2 (wt%) composite in a steam environment is presented. Static steam autoclave tests were performed at 300 degrees C and 9 MPa for period of 0.5-1.5 hours. Microstructural analyses of UN pellets show that, in a high-pressure atmosphere, the fuel collapses principally by intergranular cracking generated by the precipitation of an oxide phase in the grain boundaries. This mechanism leads to a premature mechanical collapse of the fuel pellet, exposing fresh surfaces to steam, and ultimately accelerating the oxidation process. Increasing density (specifically eliminating open porosity) was found to delay the oxidation process, while increasing grain size was found to accelerate the degradation process due to a greater susceptibility to mechanical fracture by way of intergranular oxidation. The performance of the UN-10%U3Si2 composite proved to be better when compared to UN. The U3Si2 phase served to stabilize the UN grain boundary interface and reacted preferentially with the steam, thereby altering the failure mechanism. In this composite material, the cracking was predominantly intra-granular and the exposure of fresh surfaces was limited, resulting in a slower degradation process.

In this study, the oxidation of various accident tolerant fuel candidates produced under different conditions have been evaluated and compared relative to the reference standard–UO2. The candidates considered in this study were UN, U3Si2, U3Si5, and a composite material composed of UN–U3Si2. With the spark plasma sintering (SPS) method, it was possible to fabricate samples of UN with varying porosity, as well as a high-density composite of UN–U3Si2 (10%). Using thermogravimetry in air, the oxidation behaviors of each material and the various microstructures of UN were assessed. These results reveal that it is possible to fabricate UN to very high densities using the SPS method, such that its resistance to oxidation can be improved compared to U3Si5 and UO2, and compete favorably with the principal ATF candidates, U3Si2, which shows a particularly violent reaction under the conditions of this study, and the UN–U3Si2 (10%) composite.

In this study, U3Si2 was synthesized via the use of arc-melting and mixed with UN powders, which together were sintered using the SPS method. The study revealed a number of interesting conclusions regarding the stability of the system - namely the formation of a probable but as yet unidentified ternary phase coupled with the reduction of the stoichiometry in the nitride phase - as well as some insights into the mechanics of the sintering process itself. By milling the silicide powders and reducing its particle size ratio compared to UN, it was possible to form a high density UN-U3Si2 composite, with desirable microstructural characteristics for accident tolerant fuel applications.

In this study, a number of samples of UN sintered by the SPS method have been fabricated, and highly pure samples ranging in density from 68% to 99.8%TD-corresponding to an absolute density of 14.25 g/cm3 out of a theoretical density of 14.28 g/cm3-have been fabricated. By careful adjustment of the sintering parameters of temperature and applied pressure, the production of pellets of specific porosity may now be achieved between these ranges. The pore closure behaviour of the material has also been documented and compared to previous studies of similar materials, which demonstrates that full pore closure using these methods occurs near 97.5% of relative density.