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Conference Article: Distribution of the decay heat in various module HTRs and influence on peak fuel temperatures

Abstract

A unique feature of modular high-temperature reactors (MODUL-HTRs) is their benign response to a Loss-Of-Coolant Accident (LOCA). The reactor inherently becomes subcritical. The decay power partly heats up the reactor and partly is removed to the environment via thermal conduction and radiation, while avoiding overheating of the fuel. Production, storage, and removal of the decay heat is studied for different MODUL-HTR concepts having annular-core designs and thermal-powers of 350 MW_t. Based on use of Low-Enriched-Uranium/Thorium (LEU/Th) fuel cycles in Prismatic-Fueled Reactors (PFRs), and LEU fuel cycles in Pebble-Bed-Reactors (PBRs), the following has been determined: (1) Comparison of a PFR and a PBR having essentially the same design shows higher decay heat production in the PFR due to a higher fission-product inventory and to the decay of ²³³Pa bred from ²³²Th. Comparison also shows lower heat-transport rates from the pebble-bed core during a LOCA due to the lower thermal conductivity of the core. (2) Changing the PBR design to utilize carbon bricks and an additional coolant gap in the outer regions of the reactor adds significant barriers to the transport of decay heat to the Reactor Cavity Cooling System (RCCS) which is external to the reactor vessel. (3) In HTRs for Gas-Turbine (GT) applications, the operating temperature of the reactor is higher than in Steam turbine Cycle (SC) HTRs. Consequently, under LOCAs the relative heat transport to the RCCS is higher in GT-HTRs. As a result, during a LOCA the amount of heat energy stored in the GT-HTR cores was only about 50% of the amount stored in SC-HTR cores.

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Reference:

Specialists meeting on decay heat removal and heat transfer under normal and accident conditions in gas cooled reactors. Juelich (Germany). 6-8 Jul 1992

International Atomic Energy Agency, Vienna (Austria)

IAEA-TECDOC--757, pp:53-62