In a high temperature gas-cooled reactor (HTGR), one can postulate simultaneous ruptures
of the pressure boundary at points near the top and bottom of the reactor vessel. In this
extremely low probability event, depressurization and heat-up of the core can lead to ingress
of air into the vessel driven by the buoyancy of the hot helium relative to the colder, cavity air
('chimney effect'). Although this is considered a 'beyond design basis accidents'(BDBA), it
receives high attention because it may lead to graphite oxidation of reflectors and fuel …