2, Rensselaer Polytechnic Institute, Troy, New York, United States
Improved thermal conductivity, oxidation resistance, high fissile density and high melting point are all very desirable properties for accident tolerant fuels (ATFs). Recent research has aimed to improve the thermal conductivity of ATFs via the addition of a second phase, with better thermal conductivity, in the existing uranium dioxide fuels. Fabrication of accident tolerant UO2 based composite fuels is of interest since the primary nuclear fuel fabrication infrastructure is developed around UO2. Examples of such UO2 based composites include the addition of nitride, silicide and boride phases. In particular, composites of UO2 with phases of uranium borides, namely UB2 and UB4, can be proven to increase thermal conductivity thus providing better safety margins in an accident scenario. Since 10B is a neutron absorber, the boron contents of the proposed ATF composites would have to be enriched to 11B which has a lower neutron cross section. An important parameter when assessing the feasibility of UO2-UBx composites is the required phase fraction of UBx to provide a significant increase in thermal conductivity, compared to UO2. If large phase fractions are required, then the necessary enrichment process may make this fuel too costly to implement. Nonetheless, UBx can still be included as a third phase in a different ATF composite system where the required volume fraction would be small enough that no enrichment would be necessary. In this case, the volume fraction can be tuned to act as an efficient burnable absorber. In this work, UB2 and UB4 fuel pellets have been sintered to high densities (>90% TD) via spark plasma sintering (SPS). A range of SPS conditions was tested to identify which resulted in the highest density pellets and the initial microstructural characterization along with measured thermal conductivity data are presented here.