2017 | AFC ACCOMPLISHMENTS 68 Development of high density fissile composite fuels: UO2/UBx systems Principal Investigator: Erofili Kardoulaki Collaborators: Andy Nelson, Darrin Byler, Kenneth McClellan - LANL;TiankaiYao, Bowen Gong, Spencer Scott, Jie Lian - RPI Figure 1. SEM backscatter image showing the typical microstructure of a UO2- 30%wt UB2 sample. Developing accident tolerant fuels (ATFs), for use in conventional light water reactors (LWRs) is a major goal of the nuclear energy community. Improved thermal conductivity, oxidation resistance, high fissile density and high melting point are all desirable properties for ATFs. Recent research has aimed to improve the thermal conductivity of ATFs via the addition of a second phase, with better thermal conductivity, in existing UO2 fuels. Difficulties exist during conventional fabrication of some UO2-based ATFs due to phase interactions between the composite constituents at tempera- tures above those of in-service LWRs. Application of field assisted sintering (FAS) techniques to the development of a novel fabrication route for these ATFs is emerging as an important goal. FAS techniques include spark plasma sintering (SPS) and flash sintering (FS), both variations that use electric field and current to sinter powders at reduced times and temperatures. The operating conditions during FAS, therefore, enable a viable and cost- effective fabrication route for ATFs that cannot be fabricated conventionally. Project Description: 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 10 B is a neutron absorber, tailoring the 10 B/11 B enrichment ratio would enable a built-in burnable poison. An impor- tant 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 enrich- ment 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 also, the volume fraction can be tuned to act as an efficient burnable absorber.