Abstract
Uranium nitride (UN)-uranium dioxide (UO2) composite fuels are being considered as an accident tolerant fuel (ATF) option for light water reactors. However, the complexity related to the chemical interactions between UN and UO2 during sintering is still an open problem. Moreover, there is a lack of knowledge regarding the influence of the sintering parameters on the amount and morphology of the α-U2N3 phase formed. In this study, a detailed investigation of the interaction between UN and UO2 is provided and a formation mechanism for the resulting α-U2N3 phase is proposed. Coupled with these analyses, an innovative ATF concept was investigated: UN microspheres and UO2.13 powder were mixed and subsequently sintered by spark plasma sintering. Different temperatures, pressures, times and cooling rates were evaluated. The pellets were characterised by complementary techniques, including XRD, DSC, and SEM-EDS/WDS/EBSD. The UN and UO2 interaction is driven by O diffusion into the UN phase and N diffusion in the opposite direction, forming a long-range solid solution in the UO2 matrix, that can be described as UO2-xNx. The cooling process decreases the N solubility in UO2-xNx, causing then N redistribution and precipitation as α-U2N3 phase along and inside the UO2 grains. This precipitation mechanism occurs at temperatures between 1273 K and 973 K on cooling, following specific crystallographic grain orientation patterns.
Highlights
Uranium dioxide (UO2) and zirconium-based (Zr) cladding are currently used as standard nuclear fuel and cladding within light water reactors (LWRs)
Pure cubic phases were found in both materials, with very low intensity peaks matching the UO2 phase (e.g. 28.2 and 32.7) in uranium nitride (UN) sample, which were from the oxygen impurity
As-fabricated UN microspheres were porous with an average diameter of 847(12) mm, average weight of 2.35 mg, and geometric density of 7.40 g/cm3, which represents about 52% of the theoretical density (TD) [26]
Summary
Uranium dioxide (UO2) and zirconium-based (Zr) cladding are currently used as standard nuclear fuel and cladding within light water reactors (LWRs). Many studies have been carried out aiming to improve the passive safety of the nuclear fuel system by using accident tolerant fuel (ATF) materials These new concepts for fuel must maintain or enhance fuel performance under normal and transient operating conditions, as well as during a potential design basis accident (DBA) and beyond-design basis accident (BDBA) [2]. Due to its high thermal conductivity, the cooler UN pellet is expected to have lower fission gas release during normal operation [4]. These improved properties contribute to lower centreline temperature during operation, providing higher margin for melting. The sol-gel method can be used to fabricate UN microspheres and, the UN pellet via SPS [8]
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