The Reactor Cavity Cooling System (RCCS) of a High-Temperature Gas-Cooled Reactor (HTGR) may be required to operate in a “passive” mode when heat is removed from the reactor cavity by letting RCCS water inventory to boil off to atmosphere. Overheating of the reactor cavity concrete walls may lead to a failure of the reactor vessel support structures and its shift off the design position. Dislocation of the massive reactor vessel may cause multiple ruptures of pipes, connected to both the upper and lower vessel heads. Such breaks of the reactor pressure boundary will enable air ingress into the core, fuel oxidation and overheating, and possible release of fission products into the environment. The computer code TINTE [3] was used to simulate a “Depressurized Loss Of Flow Circulation (DLOFC) Without Reactor Scram” accident and determine from its results the magnitude, axial distribution, and time dependence of the heat flux on the RCCS cooling panels. The computer code RELAP [4] was used to model the operation in “passive” mode of one RCCS train, consisting of a tank and four standpipes. This paper describes the application of both RELAP and TINTE for the simulation of RCCS operation in passive mode. The main conclusion from this analysis is that the proposed way of using both codes is suitable to perform scoping studies and design evaluation of RCCS of a pebble-bed HTGR.

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