Patent Number: 046541918
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a pressure release arrangement for a container, especially for the safety containment of a pressurized water nuclear reactor in connection with which it is utilized to provide for safe pressure release before the occurrence of a containment explosion. Pressure reduction in containers may be achieved by operational measures which, during failures, always impart certain insecurities. It is generally safer to install in the safety containment a suitable pressure relief valve which is independent of operational actions. It is the object of the present invention to provide a release valve which remains securely closed during normal operating pressures in the safety containment and which opens only just before rupture of the containment would otherwise occur since otherwise, that is, if the valve could open earlier, containment integrity during the normal, more likely accidents for which the containment was designed might be questionable. It is also to be taken into consideration that testing of the valve for proper operation is not possible when such a valve is installed since the valve opening pressure is above the operating pressure for which the containment is designed. Such a valve therefore needs to be composed of simple structural elements which can be exactly calculated by well established stress analysis techniques. SUMMARY OF THE INVENTION In a pressure release arrangement for a pressure container such as the pressure containment of a nuclear reactor, a pressure release pipe extends through the containment walls and is closed at its end within the containment by a closure plate. A drawbar is with one end operatively connected to the closure plate and with its other end to a distant point of the containment walls. Within the containment, the release pipe has a design rupture wall area adapted to be ruptured by the drawbar when the vessel walls are stretched beyond a predetermined degree so as to release the pressure to prevent explosion of the containment but to release the pressure only when absolutely necessary as evidenced by stretching of the containment walls. With this arrangement, pressure and, if contained in the containment, radioactive material are released to the environment safely and in a simple manner only when necessary to avoid a more serious accident as it would surely happen by explosion of the containment. The advantages of a relief valve controlled by stress-induced deformation of materials are as follows: During the plastic deformation of the safety containment, a substantial amount of mechanical energy is consumed. A portion of the energy can be utilized by means of the simple elements for the opening of the pressure relief valve. Auxiliary operating means are not necessary. As a result the pressure relief valve is a simple structure which is fully passively operative and insures high reliability without requiring servicing. PA1 Growing plastic deformation of the safety containment is a reliable indication that a critical state of complete rupture of the containment is approaching. PA1 In contrast, the containment pressure is a less reliable indicator since deformation occurs as a result of overpressurization and the interrelation between pressure and deformation is of complex nature with inherent inaccuracies so that the pressure at which a relief valve must be opened cannot exactly be determined. If the pressure would be used as a basis for the control of the pressure relief valves, it would be easily possible, for example, that the pressure relief valve would be permitted to open unnecessarily upon occurrence of momentary pressure peaks which, because of the mass inertia of the containment, would not generate critical deformation of the containment, resulting possibly in unnecessary release of radioactive material. PA1 Plastic deformation passes a plurality of stages at a certain pressure before the rupture of a container occurs. Selection of the particular stage at which the valve is open is therefore not critical. On the other hand, the respective pressure has only a relatively small delay period before failure of the containment would occur because of the non-linearity of the stress-strain curve. Inaccuracies of the values controlling a pressure dependent relief valve would therefore be of great influence. As a result, pressure control of the relief valve would not be advantageous. PA1 The relief valve according to the invention has no seals or gaskets which may cause leakage long before opening of the valve. There are essentially no movable components. Only the drawbar moves slightly in an opening in the operating beam until a given free motion length is accommodated. Furthermore, the valve opens only far enough to prevent a further rise in pressure in the safety containment. Undesired opening of the valve, if possible, is possible only by strong mechanical impacts. However, they can be avoided by suitable selection of the valve location or by providing protective panels.