The use of burnable absorbers integrated into TRISO/QUADRISO particles as a reactivity control method in a pebble-bed HTR reactor fuelled with (Th,233U)O2

Abstract

High-temperature gas-cooled reactors (HTGRs) are one of the promising candidates for the next generation of nuclear reactors due to their inherently and passively safe in normal operation and even under accident conditions. There are two HTGR concepts, one based on a pebble-bed core, and the other is a prismatic core. One of the key differences between the pebble-bed core and the prismatic core is the complexity of applying the seed-blanket (SB) concept to the internal production of U-233 from Th-232 in order to extend the cycle length of the reactor. Actually, an enhancement of the operating cycle length in the pebble-bed reactors fuelled with (Th,233U)O2 can be reached by using high U-233 ratios, but these ratios could result in high excess reactivity at the beginning of life (BOL). In order to suppress the excess reactivity phenomenon at the early stage of the fuel cycle, to flatten the reactivity curve as a function of time, a new reactivity control method has been investigated by incorporating burnable absorbers into the fuel particles. This purpose is set up by introducing two varieties of BAs, including Gadolinia and Erbia, either as a homogeneous mixture with the fuel kernel in the TRISO particles or as an additional layer next to the fuel kernel producing the quadruple isotropic (QUADRISO) particles. The present work aims at characterizing the effect of these different loading patterns on some neutronic parameters that define the reactor core under BOL conditions as well as during burnup. In this attempt, the nominal UO2 (17 wt% U-235) fuelled core was used as a reference, while the other models adopt (Th,233U)O2 (17 wt% U-233) fuel with Gadolinia and/or Erbia with different loading patterns. The calculation results show that excess reactivity can be compensated during the early stages of core life and flattened during the cycle by adding Erbia doped with a few ppm of Gadolinia to the TRISO/QUADRISO particles. In these updated configurations, the new core life achieved is approximately 678 Effective Full Power Days (EFPDs), equivalent to 52.5 GWd/t on a single-batch discharge burnup. This means that from the core fuelled with UO2, which has a core life of 537 EFPDs (37 GWd/ton), an average period of 141 EFPDs is extended.