Patent Number: 052232107
Section: summary

FIELD OF THE INVENTION This invention relates to an improvement in a passive cooling system for liquid metal cooled nuclear reactors having a pool of liquid metal coolant with the heat generating fissionable fuel core substantially immersed in the liquid metal pool, such as the type disclosed in U.S. Pat. No. 4,508,677, issued Apr. 2, 1985. BACKGROUND OF THE INVENTION In the operation of liquid sodium or sodium-potassium metal cooled nuclear reactors for power generation, it may be necessary to shut down the fission reaction of the fuel to deal with emergencies or carry out maintenance services. Reactor shut down is attained by inserting neutron absorbing control rods into the core of fissionable fuel to deprive the fuel of the needed fission producing neutrons. However decay of the fuel in the shut down reactor continues to produce heat in significant amounts which must be dissipated from the reactor units. The heat capacity of the liquid metal coolant and adjacent structure aid in dissipating the residual heat. However, the structural materials of the nuclear reactor may not be capable of safely withstanding prolonged high temperatures. For example the concrete of the walls of the typical housing silo may splay and crack when subjected to high temperatures. Accordingly, auxiliary cooling systems are commonly utilized to safely remove heat from the nuclear reactor structure during shut down. Conventional nuclear reactors have utilized a variety of elaborate energy driven cooling systems to dissipate heat from the reactor. In many of the situations warranting a shutdown, the energy supply to the cooling systems make the cooling systems themselves subject to failure. For example, pumps and ventilation systems to cool the core may fail. Furthermore, if operator intervention is necessary, there are foreseeable scenarios in which the operator would be unable to provide the appropriate action. The most reliable and desirable cooling system would be a completely passive system which could continuously remove the residual heat generated after shutdown. Liquid metal cooled reactors such as the modular type disclosed in U.S. Pat. No. 4,508,677, utilizing sodium or sodium-potassium as the coolant provides numerous advantages. Water cooled reactors operate at or near the boiling point of water. Any significant rise in temperature results in the generation of steam and increased pressure. By contrast, sodium or sodium-potassium has an extremely high boiling point, in the range of 1800 degrees Fahrenheit at one atmosphere pressure. The normal operating temperature of the reactor is in the range of about 900 degrees Fahrenheit. Because of the high boiling point of the liquid metal, the pressure problems associated with water cooled reactors and the steam generated thereby are eliminated. The heat capacity of the liquid metal permits the sodium or sodium-potassium to be heated several hundred degrees Fahrenheit without danger of materials failure in the reactor. The reactor vessels for pool-type liquid-metal cooled reactors are essentially open top cylindrical tanks without any perforations to interrupt the integrity of the vessel walls. Sealing of side and bottom walls is essential to prevent the leakage of liquid metal from the primary vessel. The vessel surfaces must also be accessible for the rigorous inspections required by safety considerations. In the typical sodium cooled reactor, two levels of sodium loops are used. Usually, a single primary loop and two or more secondary loops are used. The primary loop contains very radioactive sodium which is heated by the fuel rods. The primary loop passes through heat exchangers to exchange the heat with one of the non-radioactive secondary sodium loops. Upon shutdown of the reactor by fully inserting the control rods, residual heat continues to be produced and dissipated according to the heat capacity of the plant. Assuming that the reactor has been at full power for a long period of time, during the first hour following shutdown, an average of about 2% of full power continues to be generated. The residual heat produced continues to decay with time. This invention comprises an improvement upon the passive cooling system for removing shutdown decay heat from a liquid metal cooled nuclear reactor disclosed and claimed in U.S. Pat. No. 4,678,626, issued Dec. 2, 1985. The disclosed contents of the above noted U.S. Pat. Nos. 4,508,677 and 4,678,626, comprising related background art, are incorporated herein by reference. SUMMARY OF THE INVENTION This invention comprises an improved shut down, passive heat removal system for liquid metal cooled nuclear fission reactors which transfers reactor decay and sensible heat from the fuel core and liquid metal coolant by means of the inherent thermal energy transfer mechanisms of conduction, radiation, convention and natural convection of fluids out to the ambient atmosphere. The improved system of the invention is entirely passive and operates continuously through the inherent phenomenon of natural convection in fluids, conduction, convection, and thermal radiation. The invention particularly includes a primary passive cooling circuit for the flow of cooling air located adjacent to the conventional combination of reactor and containment vessels to transfer thermal energy absorbed from the outer surfaces of the containment vessel to the atmosphere which is combined with a backup secondary passive cooling system for service in the event of significant breach of the reactor and containment vessels. In the event of a reactor shutdown, after the control rods are fully inserted into the fuel core, the heat generated by the fuel rods is transferred through the reactor vessel across an inert gas gap to the surrounding containment vessel primarily by the thermal radiation, with a small fraction of the heat transferred by conduction and convection in the contained inert gas. Surfaces of high thermal emissivity provided on the outside of the reactor vessel and the interior of the containment vessel increase the efficiency of the heat transfer. Heat is then removed from the outside surface of the containment vessel partly by thermal radiation and partly by direct convection to the circulating air in the primary circuit in the passage between the containment vessel and the shield. The energy is then transported to the atmosphere by naturally circulating air. Vessels for modular-type reactors have approximately one third the diameter and are about the same height as conventional nuclear reactor vessels. In modular reactors, the ratio of the surface area to the power generated is approximately three times greater than the surface area to power ratio in a conventional and large reactor. This provides sufficient surface area over which the residual heat may be passively dissipated. The highly emissive exterior surfaces of the containment vessel also enhance the heat transfer. OBJECTS OF THE INVENTION It is a primary object of this invention to provide an improvement in a passive cooling safety system for liquid metal cooled nuclear reactors for the removal of decay and sensible heat under conditions of accidental malfunctions. It is also an object of this invention to provide measures for enhancing the protection afforded by indirect cooling safety means for the passive cooling of liquid metal cooled nuclear reactors comprising a core of fissionable fuel substantially submerged within a pool of liquid metal coolant. It is another object of this invention to provide added protective measures for passive cooling safety systems in liquid metal cooled nuclear reactors comprising an auxiliary backup passive cooling circuit for removing heat upon the occurrence of liquid metal coolant leakage due to a significant break of reactor and containment vessels. It is a further object of this invention to provide means for improving the operating safety of heat removing systems for liquid metal cooled nuclear reactors which are entirely passive and operate by the inherent phenomenon of natural convection in fluids, conduction, convection and thermal radiation. It is a still further object of this invention to provide a backup passive safety system for removing decay and sensible heat produced during shut down or an accidental interruption in a liquid metal cooled nuclear reactor which affords effective protection against the destructive effects of escaping liquid metal coolant and its escape into the atmosphere.