The characteristic confirmation test has been demonstrating by using High Temperature engineering Test Reactor (HTTR). The nuclear heat supply performance test, which is one of the characteristic confirmation test is planned to be carried out after restarting of HTTR. Towards the realization of the industrial utilization of a High-Temperature Gas-cooled Reactor (HTGR) cogeneration system as an extension of a nuclear plant, it is important to ensure the reactor safety in the case that thermal-load of the heat application system is fluctuated or lost. The preliminary analysis for the thermal load fluctuation test, which is one of the nuclear heat supply performance test has been investigated. In the analysis, the reactor outlet temperature can continue to be stable against the reactor inlet temperature changing by the thermal fluctuation. It means that HTGR have the capability of absorbing the thermal fluctuation. This paper focuses on the investigation of the mechanism of absorbing the thermal fluctuation. With the reactor inlet temperature increasing, the graphite moderator reactivity keeps negative though the fuel reactivity becomes active. The large negative graphite moderator reactivity enhances the capability of the absorbing thermal fluctuation. In addition, in the middle of the core, the graphite moderator reactivity insertion trend is inverted. This trend is unique to HTGR because of the large temperature difference between top and bottom of HTGR core.

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