Beyond-Design-Basis Accidents with Vessel Failure

Beyond-Design-Basis Accidents with Vessel Failure... 

In a beyond-design-basis accident, it is assumed that the air-cooled passive decay-heat-cooling system has failed and that significant structural failures (vessel failure, etc.) have occurred. Decay heat continues to heat the reactor core but decreases with time. To avoid the potential for catastrophic accidents (accidents with significant release of radionuclides), the temperature of the fuel must be kept below that of fuel failure by (1) absorption of decay heat in the reactor and silo structure and (2) transfer of decay heat through the silo walls to the environment. For the modular high-temperature gas-cooled reactor (MHTGR), the maximum size of reactor that can withstand this accident without major fuel failure is -600 MW(t). 

Leak of coolant from the reactor mono-block and termination of lead-bismuth coolant circulation through the core in a failure of tightness of the reactor mono-block vessel (a beyond design basis accident) cannot occur due to the presence of the guard vessel outside the reactor mono-block main vessel, with a small free gap between the guard vessel and the main vessel being provided. 

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