Patent Number: 046438718
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to an emergency standby cooling device for the core of a pressurized water reactor. The known cooling devices of this types used in reactors generally comprise accumulators filled with pressurized water, which are connected with the primary cooling circuit of the reactor via isolating valves, which are sensitive to a given lower value of the pressure in the primary circuit. The water filling the accumulators is pressurized with nitrogen. These known devices have a number of disadvantages. Thus, the injection of water from the standby cooling accumulators of the core does not necessarily take place at the most favourable moment. Thus, when the pressure drop in the primary circuit is due to a relatively small fracture of the piping there may be a draining of the primary circuit without an adequate reduction in the primary pressure to bring about the draining of the accumulators. The primary circuit can then be filled by the "high pressure" pumps, however, the latter constitute active systems which may be erroneously manipulated by the operators. In addition, the draining of the accumulators is linked with a value of the primary circuit pressure and not a vessel level, which is the most significant physical magnitude of the need for filling the primary circuit with water. Moreover, the injection of water into the primary circuit is generally terminated by an injection of nitrogen serving to pressurize the water in the accumulators. This gas can then be trapped either in the inverted U-tubes of the steam generators, or in the upper parts of the hot branches of the primary circuit when the steam generators are of the "single pass" type, in which the primary fluid enters the generator at its upper end and leaves it at its lower end. Thus, in both cases, there is a loss of efficiency of the cold source constituted by the steam generators. BRIEF SUMMARY OF THE INVENTION The object of the invention is to provide an emergency cooling device which does not have the disadvantages of the prior art devices and which more particularly makes it possible to inject cold water into the core at the most opportune moment. Therefore, the present invention relates to a device for the emergency or standby cooling of the core of a pressurized water reactor in which the reactor core is located within a tightly sealed vessel connected by the hot and cold branches of a primary cooling circuit with at least one steam generator, wherein the device comprises at least one water-filled, high pressure reservoir and whose bottom is positioned above the tubes by which the hot and cold branches of the primary circuit enter the vessel, said reservoir being permanently connected to the reactor vessel by a first pipe issuing into the vessel between the said tubes and the high level of the core and by a second pipe issuing into the vessel below the first pipe. As a result of the device according to the invention, the injection of the emergency water is no longer controlled by isolating valves sensitive to a given lower pressure within the primary circuit and is instead controlled by a reduction of the primary fluid level in the reactor vessel. The starting level is determined by the first pipe connecting the reservoir to the reactor vessel, the steam formed in the case of a reduction in the level entering the said pipe to start the draining of the reservoir. According to a first variant of the invention, the two pipes enter the vessel at substantially the same level. Preferably, the two pipes then have horizontal portions which are either not or only slightly thermally insulated of an adequate length to prevent any initiation of the flow by natural convection between the vessel and the reservoir under normal operating conditions. None of the pipes connecting the vessel and the reservoir is equipped with a valve. According to a second variant of the invention, when it is desired to create a flow by natural convection between the vessel and the reservoir, the two pipes enter the vessel at different levels. According to another feature of the invention, the second pipe may have within the vessel a downwardly directed bend or elbow and entering a funnel extended by a tube making it possible for the injection water to on the one hand partly mix with the hot water of the vessel and on the other to arrive directly at the bottom of the vessel. According to another feature of the invention, the device also comprises a cooling circuit having a heat exchanger located within the reservoir. This cooling circuit can be the low pressure emergency injection circuit which is used for ensuring the pressure drop of the primary circuit in the case when a crack or opening occurs thereon. As soon as the primary pressure has dropped sufficiently, this cooling circuit ensures the filling of the primary circuit via a three-way valve. The device according to the invention can also have two pipes for cooling the reactor on shutdown respectively connecting the upper part of the reservoir with at least one hot branch via at least one electrovalve and the lower part of the reservoir with at least one cold branch via an electrovalve and a circulating pump. As a result of this characteristic, the device according to the invention can also be used as a high pressure device for cooling the reactor on shutdown. For the latter function, the water reservoir of the cooling circuit must itself be cooled. The device according to the invention may also comprise a heating circuit for the water contained in the high pressure reservoir, said circuit having a coil located in the reservoir and whose ends are respectively connected to at least one hot branch and at least one cold branch by means of at least one electrovalve. According to yet another feature of the invention, the water contained in the reservoirs can be a boric acid solution, whose boric acid concentration is controlled and adjusted by means of a connecting circuit by which the reservoirs are connected to an external chemical and volumetric control circuit.