In this study, the multiplication factor and neutron spectrum behaviors were investigated against the moderator-to-fuel ratio, the fuel loading height, and the detector location in high-temperature gas-cooled reactor (HTR)-10. The MCNP5 computer code (version 1.51) was employed to perform all the simulation computations. The results revealed that the multiplication factor varies significantly depending on the moderator-to-fuel ratio and the fuel loading height due to the competition among the neutron moderation and absorption abilities of the moderator as well as the neutron production ability of the fuel. Due to its inherent stability, HTR-10 is deliberately designed such that the multiplication factor decreases and the neutron spectrum softens as the moderator-to-fuel ratio increases. The average neutron energy level in the HTR-10 fuel balls is approximately 240 keV and ranges from smallest to largest at the middle, bottom, and top of the reactor core, respectively.

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