The high temperature engineering test reactor (HTTR) in Japan is a 30 MWth helium-cooled and graphite-moderated reactor that was built to demonstrate the technological feasibility and advantages for the next-generation high-temperature gas-cooled reactors. The multi-level heterogeneous configuration of the HTTR core represents challenges to reactor physics calculations. A suitable core model is of vital significance for reliable neutronic analyses of the HTTR. In this study, a detailed HTTR core model is constructed based on the MCNP5 Monte Carlo code and the ENDF/B-VII neutron cross-section library. For code and model validation, several HTTR benchmark calculations were performed first and compared with the experimental results. After that, a variety of fuel design was explored to investigate their effects on the HTTR criticality calculations. The fuel design or parameters under consideration include the moderator-to-fuel ratio in the core, fuel rod dimension, and different fissile materials. The resultant trend of change in system multiplication factor and neutron spectrum was presented and discussed in this paper.

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