Abstract: An illustrative embodiment of the invention provides pressure relief valve means for the core support cylinder of a nuclear reactor vessel during a failure or accident of the nuclear reactor system. The valve means is responsive to differential pressure across the valve which in one direction sealably seats the valve plate against the valve body, and which, in the other direction opens the valve for pressure relief of the cylinder. Moreover, the valve means is provided with energy absorbing means which limit the impact load of an "explosively" opening valve on the reactor vessel wall.