Thermal-fluid characteristics of the transuranics fuel in a deep-burn HTR core

Abstract

The Deep Burn Project is evaluating the feasibility of the DB-HTR (Deep Burn High Temperature Reactor) to achieve a very high utilization of transuranics (TRU) derived from the recycle of LWR spent fuel. This study intends to evaluate the thermal-fluid and safety characteristics of TRU fuel in a DB-HTR core using GAMMA+ code. The key design characteristics of the DB-HTR core are more fuel rings (five fuel-rings), less central reflectors (three rings) and decay power curves due to the TRU fuel compositions that are different from the UO 2 fuel. This study considered three types of TRU kernel compositions such as 100%(PuO 2 + NpO 2 + Am), 99.8%(PuO 1.8, NpO 2) + 0.2%UO 2 + 0.6 mole SiC getter, and 70%(PuO 1.8, NpO 2) + 30%UO 2 + 0.6 mole SiC getter. The first fuel type of TRU kernel produces higher decay power than the UO 2 kernel. For the second and the third fuel types, removing the initial Am isotopes and reducing the volumetric packing fraction of TRISO particles will reduce the decay power. The flow distribution, core temperature and TRISO temperature profiles at the steady state were examined. As a safety performance, this study mainly evaluated the peak fuel temperature during LPCC (low pressure conduction cooling) event with considering the impact of decay power, the annealing effect of the irradiated thermal conductivity of graphite, and the impact of the FB (fuel block) end-flux-peaking. For the 600 MW th DB-HTR core, the peak fuel temperature of 100%(PuO 2 + NpO 2 + Am) TRU was found to be much higher than the transient fuel design limit of 1600 °C due to the lack of heat absorber volume in the central reflector as well as to the increased decay power of the TRU fuel compositions. For a 0.2%UO 2 mixed or a 30%UO 2 mixed TRU, the peak fuel temperature was decreased due to the reduced decay power, however, it was still higher than 1600 °C due to the lack of heat absorber volume in the central reflector.