On excess reactivity control of the advanced high temperature reactor (AHTR)

Abstract

The Advanced High Temperature Reactor (AHTR) is a large Fluoride salt cooled High temperature Reactor (FHR) with a thermal power of 3400 MWt, utilizing hexagonal fuel elements with TRISO fuel spheres embedded in graphite fuel plates. While its cycle length is relatively short (less than one year), its specific power is high, resulting in high cycle burnup and consequently high initial excess reactivity that needs to be controlled. This is achieved by using both Eu2O3 burnable poison spheres and molybdenum-hafnium-carbon control rods. This work details studies developing and implementing these two reactivity control features over representative AHTR fuel cycles. Various Eu2O3 densities are considered to evaluate both the achieved reactivity control and the residual reactivity penalty of burnable poison at end of cycle. Control rod studies are performed to find integral and partial insertion reactivity worth for control rod banks at various radial locations. Axial power impacts are also evaluated. Focus then shifts toward control rod movement strategies. A manually generated insertion scheme leading to a critical configuration is first investigated to serve as a reference point for automated schemes to follow. Next, automated insertion and withdrawal strategies are evaluated based on their radial power peaking performance. Control rod insertions into the core locations with the highest local power are shown to be more stable than control rod withdrawals from core locations with the lowest local power. Finally, a methodology is introduced for finding a critical control rod insertion configuration. The methodology is intended to be used over a depletion sequence where the positions of the control rods are automatically adjusted to maintain criticality over cycle. In summary, this work presents a possible implementation of AHTR reactivity control and introduces automated control rod movement strategies for use in multi-physics depletion analyses.