Patent Number: 050769995
Section: summary

FIELD OF THE INVENTION This invention relates generally to safety systems for nuclear reactors and more particularly to passive systems for removing decay heat from water-cooled nuclear reactors. BACKGROUND OF THE INVENTION In a water-cooled nuclear reactor that has operated for a significant length of time, radioactive fission products build up in the reactor core. These fission products generate radioactive heat even when the reactor is shut down, with typical decay heat generation being one percent of the reactor full-power heat generation rate. If the reactor core is not cooled after shutdown, either by use of normal shutdown procedures or post-accident shutdown procedures, the reactor core may melt. Depending on details of reactor design and mode of reactor shutdown, decay heat removal may be accomplished by cooling the hot water reactor or allowing the reactor water to boil with steam exiting the reactor and adding makeup water to the reactor. In either case, active cooling systems are used to cool the reactor and prevent a reactor core meltdown. Active systems such as these can fail due to equipment failure or operator error as happened at Three Mile Island. What is needed is a passive decay heat removal system of high reliability that becomes automatically activated upon loss of coolant and which may have its operability verified during normal operations. Various approaches toward devising passive cooling systems for water-cooled reactors are disclosed in the following papers presented at the International Atomic Energy Agency Technical Committee Meeting on Passive Safety Features in Current and Future Water-Cooled Reactors in Moscow on Mar. 21-24, 1989: Application of Passive Systems in WWER-1000 Design Project of Increased Safety: Part I, V. I. Naletov, G. A. Tarakov, E. M. Damrin, and N. B. Trunov. PA1 Application of Passive Systems in WWER-1000 Design Project of Increased Safety: Part II, T. A. Brantova and N. S. Fil. PA1 Analysis of the Possibility to Increase WWER-440 Safety Level on the Base of Passive Systems, B. Dimitrov. The systems disclosed in these papers rely on opening of air doors to obtain cooling or depend upon flooding of the reactor by water located above the reactor containment level. Placement of an open-top box inside the pressure vessel in combination with a closed circuit heat exchanging loop as in the present invention is not disclosed in the prior art known to the applicant. SUMMARY OF THE INVENTION This invention is directed to a passive decay-heat removal system for water-cooled nuclear reactor in which an open-topped insulated box is located inside the reactor vessel and positioned below the water level that exists during normal operations of the reactor. A heat exchanger which evaporates fluid within a closed loop is disposed within the box, forming one end of a closed heat-transfer loop communicating with a condenser located outside of the pressure vessel and above the reactor water level. Heat removal by the heat-transfer loop during normal operations is limited owing to the immersion of the evaporator heat exchanger under water and to cooling of the water in the box by natural circulation heat transfer and thermal stratification in the box, which separates cold water therein from an upper layer of hot water, isolating coils of the evaporator from hot reactor water. Upon dropping of the level of the reactor coolant water, as might incur in an emergency situation, water drains from the box, exposing the heat exchanger therein to high temperature steam. The vapor produced in the evaporator then circulates through the loop to the condenser where it is condensed to liquid on the outside of the containment vessel, releasing heat to the open environment (atmosphere, lake, or soil) in the process. The resulting condensed liquid then flows by gravity back to the evaporator. This provides for continuous removal of decay heat by a system with no moving parts that might malfunction. Owing to the minimized flow, which occurs during normal reactor operation, loss of heat and the resulting loss of reactor efficiency is held to a very low level. It is, therefore, an object of this invention to provide a passive system for removal of decay heat from a water-cooled nuclear reactor. Another object is to provide such a system that becomes operable upon loss of coolant water without operator invention. Yet another object is to provide a passive heat removal system that has no moving parts. Still another object is to provide a system in which removal of heat from the reactor during normal operation is minimized. Other objects and advantages of the invention will be apparent from the following detailed description and the appended claims.