Uranium Silicide/Uranium Diboride Composite Fuel Development

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Light Water Reactors (LWRs) comprise around 90% of the world's current nuclear generating capacity. The use of Accident Tolerant Fuels (ATFs) could offer significantly improved high temperature capability and operational safety. One way this can be achieved is by reducing the potential for reactivity with high temperature steam by replacing zirconium cladding with a ceramic composite material. This necessitates the replacement of existing UO2 fuel with a silcide; U3Si2, which is the focus of a concerted international research effort and aggressive commercial deployment plan \(see for example "Fuel of the Future", Nuclear Engineering International 2018). This research effort includes a national research programme within the United States, including large efforts at Idaho National Laboratory, Oak Ridge National Laboratory, The University of South Carolina, Westinghouse and General Atomics (amongst others). It includes a range of ATF concepts, some very close to current fuel designs (UO2, engineered to have a larger grain structure and coated claddings) as well as more research intensive efforts to deploy a true step-change (U3Si2 fuel pellets with composite silicon carbide cladding). International fuel vendors (Westinghouse, GE, Areva) are particuarly interested in devloping ATFs, as they may also provide the key to reducing nuclear capital cost by removing the need for currently expensive and multiply-redundant safety systems. The aim of this research is to establish if the introduction of a boride phase to U3Si2 could improve the materials high temperature water performance.This is a key challenge currently facing the ATF community Recent work performed by Sooby-Wood et al at Oak Ridge National Laboratory has demonstrated that U3Si2 rapidly and energetically pulverises when exposed to high temperature steam (E. Sooby Wood et al (2018). Journal of Nuclear Materials, 501, 404-412). Atomic scale modelling at the Universities of Bangor suggests that this is the result of a hydrogen reaction within the material, which leads to a rapid volume change and increases the surface area available for oxidation.Similar atomic modelling of the UB2 system performed at the University of Bangor (in support of the present work and currently being prepared for joint publication) suggests it will not be susceptible to this damage mechanism, and so could provide protection to U3Si2 while maintaining the benefits of an ATF fuel material (high thermal conductivity, high uranium density etc). Preliminary work has produced several test pellets by coarse mixing U3Si2 and UB2 powders in various weight percentages. On investigation, the boride and silicide phases do not have a detectable interaction layer around them, and the UB2 phase appears to have been successfully sintered within the U3Si2 phase. Preliminary steam tests on these pellets shows the steam reaction with composite pellets onsets at significantly higher temperatures than pure U3Si2 and appears to react much more slowly. This suggests that the presence of UB2 within U3Si2 may do more than simply limit the physical pathways by which U3Si2 is attacked by steam, as it appears to change the reaction mechanism in some way as noted by the lack of any reaction below 500C. The proposed work will build upon these preliminary studies and assess the feasibility of utilising UB2 within U3Si2 fuel material to improves its steam-corrosion resistance to acceptable levels. This will include pellet manufacture and characterisation, steam testing, proton irradiation tests, autoclave studies and characterisation of the effects of temperature cycles.