Patent Number: 050009079
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to pressurized water nuclear reactors having a water injection device for delivering a flow of emergency water stored in an external reservoir to the reactor vessel for temporarily compensating the egress of water upon failure of a pipe in the primary cooling circuit. PRIOR ART Reactors provided with such injection devices (FR-A-1 597 057) have been known for long. One of the defects of most prior art devices is that injection of cold emergency water, stored at atmospheric pressure in an external reservoir, causes a thermal shock when it enters the reactor vessel at a high operating temperature. This particularly detrimentally affects the portion of the ring of the vessel situated at the horizontal level of the core for, due to the high level of irradiation received by this portion, its mechanical characteristics may have decreased. It is necessary that the vessel resists the residual internal pressure upon a rupture of the primary circuit and occurrence of emergency injection. In present day reactors, attempts have been made to solve the problem by injecting water at one or more locations of the circuit which are as remote as possible from the vessel wall, so that the cold water mixes with hot water still present in the circuit before it contacts the side wall of the vessel. Another pressurized water nuclear reactor has been proposed in FR-A-2 314 919, which comprises a vessel which is closed by a cover, has cooling water input and output nozzles, and is connected to at least one conduit for the injection of pressurized water from an emergency reservoir; the duct opens into the vessel at the same level as or above the level of all nozzles; the reactor further comprises internals suspended from the vessel, having a core support plate and forming with the side wall of the vessel an annular downward water flow passage from the input or each inlet nozzle to a distribution space situated below the core support plate; a duct extending the conduit downward is located within the dividing structure (baffles and formers) of the internals equipment. In such a reactor, the injection function of the conduit is secondary: its main purpose is to reintroduce water delivered by a water recirculation circuit connected to the outlet nozzle. The duct is used for emergency injection based on the natural idea that, since the duct plays its function only during normal operation of the reactor, it is available for emergency injection should an accident occur. However, since the duct opens above the core support plate, the water reaches the fuel assemblies before it is appreciably heated, and may damage the fuel assemblies and particularly causes fuel rod sheath failure. A nuclear reactor is also known having conduits passing through the vessel and used solely for injecting emergency cooling water, whose output into the vessel has deflectors for directing the jet of incoming water toward the bottom of the vessel (FR-A-2 286 478). This solution does not overcome the thermal shock problem on the side ring of the vessel. On the contrary, it may be enhanced since the deflectors direct the stream of cold water along the vessel, directly toward the most sensitive portion thereof. SUMMARY OF THE INVENTION It is an object of the present invention to provide a nuclear reactor of the above-defined type which simultaneously allows injection of emergency cooling water directly into the vessel, consequently achieving maximum efficiency, and avoids causing a thermal shock on sensitive parts, such as the side ring of the vessel at the horizontal level of the core and the fuel assemblies. To this end, there is provided a reactor in which the duct is isolated thermally from the side wall of the vessel, at least at the same horizontal level as the core, and opens in the distribution space below the core support plate where the temperature of the cold water injected is rapidly increased by mixing with a large amount of water. In a particular embodiment, each duct is fixed rigidly to a casing belonging to the internal equipment and comprises end sealing means for substantially water-tight contact abutment against an outlet of the corresponding conduit; in a modification, each duct is fixed to the side wall of the vessel by means which hold it at a distance therefrom.