Optimizing Micro-PeLUIt reactor with UO2-ThO2 fuel mixtures and improved graphite moderation

Abstract

Micro-PeLUIt is a High-Temperature Reactor (HTR) design similar to China’s HTR-10 reactor, developed by Indonesia to meet commercial and industrial power demands with flexible operational power ranging from 10 MWt to 40 MWt. The latest Micro-PeLUIt pebble fuel design is proposed to feature lower ²³⁵U enrichment levels and higher heavy metal (HM) content per pebble compared to the standard HTR-10 fuel. These conditions may pose challenges regarding the criticality lifetime of the fuel, particularly due to reduced moderation effects. Another issue with this design is the potential increase in plutonium production, which raises concerns about fuel waste management and nuclear proliferation. This study proposes the use of ²³²Th as a UO₂-ThO₂ fuel mixture to reduce neutron absorption by ²³⁸U, thereby limiting plutonium production. Two UO₂-ThO₂ mixing methods are evaluated: mixing in a single TRISO kernel as compound (CM) and mixing in different TRISO kernels (TM). The optimal UO₂-ThO₂ design is also evaluated with adjusted ²³⁵U enrichment specifications for Micro-PeLUIt, as well as graphite density adjustments in the pebble matrix to enhance neutron moderation. The results show six optimal UO₂-ThO₂ fuel variations capable of reducing plutonium production by 10%–25% and decreasing total fissile material per pebble by 9%–26%. The use of UO₂-ThO₂ mixed fuel with graphite density adjustment in the pebble matrix can result in performance similar to normal fuel in the HTR-10 reactor. Increased graphite density enhances neutron moderation within the pebble, effectively maintaining the criticality of the system without significantly increasing the HM loading per pebble. Furthermore, this design provides better criticality potential compared to UO₂ fuel, making it more efficient in operation.