The Very High Temperature Reactor (VHTR) has been selected by the U.S. as the Generation IV technology for the Next Generation Nuclear Plant (NGNP), and both the U.S. and Japan have been developing VHTR concepts based on a prismatic, block-type core design. For these VHTR concepts, the primary coolant (helium) inlet temperature is expected to be in the range 490°C to 590°C and the outlet temperature is expected to be in the range 850°C to 950°C. Passive safety is one of the fundamental requirements for the VHTR, and the VHTR is designed to be passively safe even during Loss of Coolant Accidents (LOCAs) and Loss of Flow Accidents (LOFAs). For the VHTR, these two transient events are referred to as a Low-Pressure Conduction Cooldown (LPCC) and High-Pressure Conduction Cooldown (HPCC), respectively. During both events, the decay heat is conducted through the graphite to the vessel. The heat is transferred from the vessel by thermal radiation and natural convection to a passive Reactor Cavity Cooling System (RCCS). In this paper, we describe parametric studies of LPCC and HPCC events using a 30-degree sector, 3-dimensional ANSYS model of the VHTR, which includes a detailed radiation exchange model between the RPV and RCCS.

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