Abstract: Heat retarding closure system for partitions having pressure relief openings formed therein especially in nuclear reactor buildings where main coolant nozzles of a reactor pressure vessel penetrate a biological shield, including lightweight construction closure elements having a side facing the reactor and anchors for holding the closure elements, the closure elements being pushable out of the anchors by an overpressure in a given pressure difference direction on the reactor side, and an outer sealing blowout skin, the closure elements being in the form of heat-retarding cassette inserts having a front surface with a peripheral shearing edge formed thereon resting against the blowout skin, and the blowout skin having a given thickness in the given pressure difference direction enabling the cassette insert to shear off the blowout skin and be pushed out of the anchors when a given pressure difference is at least reached.