The potential of thorium fertile fuel has been evaluated for a graphite-moderated MHR (Modular Helium Reactor) from the perspective of a self-sustainable U233-Th fuel cycle in the MHR. The 3-D core analyses have been performed with the thermal-hydraulic-coupled computer code systems (HELIOS-MASTER). The feasibility of a self-sustainable U233-Th fuel cycle in MHR was evaluated for a simplified equilibrium fuel cycle. A mixed oxide fuel (ThO2-UO2) was used. Whole-core analysis was performed with the MASTER code for various core configurations. In the core analysis, a 3-batch radial fuel shuffling scheme was adopted to find an equilibrium fuel cycle. Three types of fuel blocks were considered: a homogeneous fuel arrangement and two seed-blanket arrangements. It was found that a near self-sustainable U233-Th fuel cycle (conversion ratio = 0.95∼0.97) is feasible for the MHRs with the appropriate U-233 and Th-232 loadings. To achieve a high conversion ratio while maintaining a long cycle length, it is essential to maximize the thorium loading (∼30 tons) and at the same time soften the neutron spectrum to achieve sufficient reactivity. In order to achieve conversion ratios over 0.95 and an 18-month cycle length, the moderator volume needs to be increased with respect to the regular MHR design. Also, removing the inner graphite reflectors increases noticeably the core performance in terms of the conversion ratio and cycle length. A special seed-blanket block configuration (ISB, with seed and blanket fuels in the inner and outer regions of a block, respectively) provides a superior conversion ratio with respect to a homogeneously fueled block, whereas reversing the placement of seed and blanket in the ISB block configuration (with seed fuels in the outer zone) results in a worse performance. In the case of the U233-Th fuel cycle, the fissile (U233+U235) fraction in the discharged fuel is almost 90%. Denaturing of the uranium vector in the self-sustainable U233-Th fuel was investigated by adding 10% LEU in the fuel, to make the initial fissile fraction ∼20.5%. Neutronic analysis of the operation with denatured fuel reveals that the conversion ratio is substantially reduced and the available cycle length is much shorter.

This content is only available via PDF.
You do not currently have access to this content.