Understanding the Moderator Temperature Coefficient of Reactivity of Prismatic Graphite-moderated Gas-cooled Reactors with Burnable Absorbers

This study comprehensively analyses the Moderator Temperature Coefficient (MTC) of a conventional graphite-moderated High-Temperature Gas-cooled Reactor (HTGR) loaded with a B4C burnable absorber. Emphasizing significant neutron spectrum differences, the investigation progresses from spectrum changes to their impact on reaction rates, particularly at End of Cycle (EOC), considering the depleted fuel composition. The study concludes with an isotopic reaction rate analysis, identifying primary contributors to a positive MTC at EOC for HTGRs. Assumptions are validated through direct manual computation of the MTC. The single fuel assembly level study highlights the role of spectrum evolution during depletion, revealing a shift from a strongly negative MTC at Beginning of Cycle (BOC) to slightly positive values at EOC. Graphite capture emerges as a major MTC contributor at both BOC and EOC, with beneficial and then adverse effects. At EOC, along with graphite capture as the primary factor, other prevalent contributions from 239Pu and 241Pu isotopes. The findings offer valuable insights for higher power density HTGRs. All calculations were performed utilizing the Continuous Monte Carlo Code Serpent 2 with the primary library ENDF/B VII.1.