Patent Number: 047028793
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to an improved pressurized water nuclear reactor having an integral means for in-core spraying of a liquid coolant in case of a loss of coolant event, and a passive safety system incorporating an improved reactor vessel. The nuclear power industry has been making efforts to provide for more safe operation of nuclear reactors. While some efforts have taken the form of adding more "active" safety features and then performing probability analyses to show that risks are very small, a portion of the public has remained unconvinced. In such active systems of the prior art, a large number of components such as pumps and fans which may be electrically powered, and optionally diesel powered in the event of electric failure, are provided. Thus, in a pipe break, termed a loss of coolant accident, water is provided to the primary circuit and then the reactor vessel by pumps. Additionally, motor-driven fans as well as pump-operated spray devices, are used to remove containment heat. The reactor decay heat and heat from containment is also transferred into an emergency cooling water system. These safety systems are all required to be redundant in order that failure of a component will not render the safety system ineffective. Thus, the active approach to nuclear plant safety results in plant designs of highly complex and costly design. The need thus exists for a nuclear reactor, of the pressurized water reactor type, which is convincingly safe to one and all without significant increase in costs, which are already high. The most likely approach is thus a system which is of a "passive" nature, i.e. which requires little or no operator action but rather uses gravity or stored energy to perform its functions. It is an object of the present invention to provide a pressurized water nuclear reactor which has a large volume of relatively cold supplementary pressurized water integral within the reactor vessel, which supplementary water is sprayed into the core of the reactor upon occurrence of a loss of coolant accident in the primary coolant circuit, without need for pumps or other active components. It is another object of the present invention to provide a passive safety system incorporating the reactor of the present invention. SUMMARY OF THE INVENTION A pressurized water nuclear reactor having a passive system for in-core spraying of liquid coolant uses an accumulated supply of liquid coolant, a portion of which flashes to form steam upon depressurization of the reactor, to force coolant into the core region of the reactor. The reactor has a substantially cylindrical flow liner that has an open top, a cylindrical wall section, and a bottom wall, with said cylindrical wall section forming therein a lower reactor internals chamber. A cylindrical barrel is disposed in the flow liner spaced from the bottom thereof, to form an annular chamber thereabout and a riser chamber therein, which riser chamber contains the lower reactor internals including the fuel assemblies and control rod assemblies. The flow liner is contained within a pressure vessel that has a removable top, an intermediate cylindrical wall section and a lower wall section, the lower wall section thereof spaced from the flow liner, to form a second annular chamber. The intermediate cylindrical wall section of the pressure vessel has inlet and outlet nozzles that communicate with a cooled coolant return port and a hot coolant discharge port in the flow liner. The hot primary coolant discharge nozzles, and cooled primary inlet nozzles to the annular chamber, are connected to a steam generator, with means provided to circulate the primary coolant therethrough. A supply of supplementary liquid coolant is provided in the second annular chamber. Insulating means are provided to maintain a major portion of the supplementary liquid coolant at a first elevated temperature and a localized minor portion of the supplementary liquid coolant at a second, higher, elevated temperature. Means are provided to effect communication between the second annular chamber and the riser chamber in the cylindrical flow liner and provide for spraying of supplementary liquid coolant into the core in the riser chamber. The communicating means preferably comprise axially aligned openings in the bottom wall of the flow riser and the barrel bottom support plate with tubular elements connecting said openings and elongated tubular elements having spaced apertures therealong disposed within the core assembly. Upon depressurization of the interior of the substantially cylindrical flow liner and concomitantly the second annular chamber, the higher elevated temperature localized portion of the supplementary coolant flashes to a vapor and the increase in volume, so produced, forces the supplementary liquid coolant into the core region in the riser chamber. Means for injecting and removing coolant chemistry control solution, such as a borated water solution, are provided on the pressure resistant vessel, and a pool of liquid is provided about the pressure resistant vessel to further cool the supplementary liquid coolant. The passive safety system incorporates the reactor with means for circulating water solely by natural convection, from the hot leg to the cold leg of the primary coolant system, including a heat exchange means and a means for introducing stored coolant, under superatmospheric pressure, into the substantially cylindrical vessel.