The High Temperature engineering Test Reactor (HTTR) in Japan is a 30 MWth helium-cooled graphite-moderated reactor. Similar to most light water reactors (LWRs), the HTTR was designed to burn low-enriched UO2 fuel. Due to its superior safety features and ultra-high burnup tolerance of the coated fuel particles, it is deemed interesting to explore the possibility and performance of burning plutonium and minor actinides (PuMA) in HTTR. A detailed HTTR core model was established using the MCNP5/X Monte Carlo code with ENDF/B-VII cross-section library. The model has also routed for validation by performing benchmark calculations and comparing with reference results. This work presents a preliminary study on neutronic effects of replacing uranium in the design fuel with typical plutonium reprocessed from LWR spent fuels. The results revealed that the core characteristics such as neutron spectrum, spatial flux/power distribution, multiplication factor versus fuel-to-moderator atomic ratio, and safety related reactivity temperature coefficients are strongly dependent on the fuel content to various extents. In particular, PuMA tends to harden the neutron spectrum and cause the subsequent effects accordingly.

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