An alternative approach for excess reactivity control of TRISO particles

Abstract

This study explores an innovative approach to manage the initial excess reactivity in High-Temperature Gas-Cooled Reactors (HTGRs) by modifying the fuel and non-fuel compositions of TRISO particles. Choosing a single fuel block of the 350 MWth Prismatic HTR as the reference, minor actinides (MAs) in three loading amounts (2 wt.%, 5 wt.%, and 10 wt.%) were introduced into the TRISO fuel kernels across three alternating rings of the assembly. Four more models were developed by replacing the TRISO particles in these rings with QUADRISO, featuring distinct layers of four burnable absorbers (B4C, Er2O3, Eu2O3, and Gd2O3), each having a thickness of 2 µm. These approaches were compared against using conventional B4C poison rods, placed at adjacent and opposite ends of the hexagonal assembly. Depletion analysis over a period of 600 EFPDs was carried out using open-source Monte Carlo code OpenMC. The Eu2O3 layered particles and oppositely placed B4C rods exhibited the most effective reduction in initial reactivity, amounting to 0.28 Δk/k and 0.17 Δk/k respectively. The 10 wt.% MA loading approach proved to be versatile, as it efficiently lowered the initial reactivity by 0.15 Δk/k while also achieving minor actinide transmutation, yielding transmutation rates of 23.1%/yr for 237Np, 51.5%/yr for 241Am, and 30%/yr for 243Am. Fuel cycle parameters were calculated for multi-batch refueling schemes using the linear reactivity model (LRM). While MAs reduced burnup and cycle length, the use of B4C, Er2O3 and Eu2O3 layers improved fuel cycle performance, with B4C demonstrating the best results. Due to its high absorption cross section, Gd2O3 rendered the core sub-critical initially, resulting in negative fuel cycle parameters. The fuel temperature coefficient (FTC), relative pin-power distribution, and energy-dependent neutron flux of the models were also evaluated.