The Advanced High-Temperature Reactor (AHTR) is a new nuclear power reactor concept being investigated in some countries including the United States. The coolant is a liquid salt with a melting point of 460°C and a boiling point of 1430°C. The AHTR uses Silo Cooling System (SCS) as the decay heat removal system in a Beyond-Design-Basis Accident (BDBA). SCS has two accident mitigations. The first component is low-cost, and thick steel rings which conduct heating up the silo wall for BDBA. The second component is an annular ring of an inexpensive, solidified BDBA salt, which is heated from the bottom and melts when the temperature of the salt increases above the melting point, then flows into the silo, and floods the whole silo to its top level. SCS could make AHTR free from catastrophic accidents, where core melting or vessel failure never takes place since the BDBA salt near the top of silo passively absorbs decay heat. On the other hand, AHTR decreases its heat removal ability to avoid freezing of the salt and blocking the flow of the liquid when the temperatures are low. We performed the numerical calculation of AHTR heat removal system and evaluated whether it has the ability to remove decay heat with the robustness for a long-time cooling operation after BDBA. Furthermore, we need to build up and optimize the operation plan of SCS in AHTR, taking its thermal characteristics of this system into account. It is essential to avoid severe accidents which we can suppose as the possible catastrophic scenario.

In this paper, we calculated temperature distributions using the thermal-hydraulics code developed for AHTR, and assessed the performance in a long term cooling period under BDBA conditions. Finally, we investigated the temperature distributions of the whole plant, predicting the accident scenario without air-cooled passive decay heat-removal system. We obtained important conclusion about SCS of the AHTR that its heat removal ability was enough to avoid catastrophic accidents under Loss of Heat Sink (LOHS) conditions.

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