Adjustment of long-cycle fast spectrum SMR performance by using gadolinia burnable poison in a moderated outer core driver fuel zone

Abstract

This study investigates the use of ZrH1.6 as the radial reflector material and in a heterogeneous fuel pin design with internal moderating layer for a lead-bismuth cooled, fast-spectrum, long-cycle, small modular reactor (SMR). In the local regions of the outer core driver fuel zones, the presence of the moderating material in sufficient quantities and geometric arrangements shifts the flux spectrum in the thermal and epithermal regions to support effective depletion of gadolinia burnable poison integrated in the heterogeneous fuel pin. The breed-andburn performance of the long-cycle SMR for 30 effective full-power years without refueling or fuel shuffling is assessed through pin-wise depletion analysis of an R-theta model using a continuous-energy neutron transport Monte Carlo code. A parametric study of moderating reflector, moderator content in heterogeneous fuel pin, burnable poison weight percentage, and fuel enrichment and enrichment zoning shows fast reactor performance metrics of reactivity swing, fuel burnup, global power distribution, and power peaking in the core center at end-of-cycle are highly sensitive to the simultaneous variation of the design parameters. A severe local power peaking issue (several hundreds of percent) in some of the high-enriched outer core fuel pins that directly see a thermalized neutron current from the reflectors can be partially mitigated by the use of burnable poison.