Patent Number: 051065711
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear reactors and, more specifically, to techniques for cooling nuclear reactors, and their containments, in the event of one or more types of malfunctions. During normal operation of a nuclear reactor, the nuclear fuel in a reactor vessel remains covered with water to generate steam. The nuclear fuel consists of fuel rods which develop substantial internal heat. After shutdown of the reactor, decay reactions continue to generate heat in the fuel rods for a lengthy period. It is important to environmental safety to ensure that radioactive materials are not released during normal or abnormal operation. Such radioactive materials include, for example, steam generated in the reactor vessel or water that is condensed from such steam. Abnormal operations to be discussed in the following disclosure include a loss-of-coolant accident which may occur due to a break in a component or piping such as, for example, a steam pipe within the reactor building. The three requirements in such a situation are 1) to replace water in the reactor pressure vessel to cover the fuel rods, 2) to dissipate the heat existing immediately following the break, and 3) to remove the decay heat over an extended period (days or weeks) following the break, such that structural integrity of the containment vessel is maintained. In the prior art, the movement of cooling water to satisfy the three foregoing requirements is provided as a result of forced circulation by high-pressure water pumps driven by electricity or other external power source. In the event of failure of the normal electrical grid supplying electric power to the plant, diesel generators are provided to take over the task of supplying power for driving the pumps. It is a fact, however, that there is a small but finite probability, that diesel generators can fail to function at a critical time, or that human errors can incapacitate systems. Such failure following a serious loss of coolant accident such as, for example, a break in a steam pipe, can be considered a worst-case scenario. An electric generator driven by steam from a nuclear reactor can experience a sudden loss of load for a number of causes. In such a situation, a concomitant reduction in demand for steam occurs at a rate that exceeds the ability of a reactor control system, and the normal power-driven cooling system, to accommodate. In the past, an isolation condenser disposed in a pool of water receives and condenses excess steam until reactor control reduces the production of heat to a value within the capability of the cooling system, or until the generator load is resumed. The pool of water is open to the atmosphere, but the steam and condensate remain in the heat exchanger, isolated from the atmosphere. The prior art appears innocent of any teaching of the use of an isolation condenser, or similar heat exchanger, as part of a passive system for dissipating heat following a reactor operating transient or an accident. OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide a nuclear reactor system which overcomes the drawbacks of the prior art. It is a further object of the invention to provide a safe nuclear reactor system that removes decay heat passively. It is a still further object of the invention to provide a nuclear reactor having a passive cooling system capable of removing initial heat from a reactor vessel and further capable of removing decay heat from the reactor over an extended period following an accident and transferring such heat across the containment boundary for release to the environment. It is a further object of the invention to provide a nuclear reactor system having a gravity driven cooling system pool disposed at a location providing a gravity head sufficient to feed water to a reactor vessel in a quantity sufficient to cover and cool the reactor core after a loss-of-coolant accident. It is a still further object of the invention to provide a nuclear reactor system wherein a suppression pool in a containment vessel absorbs an initial high heat load as steam is released into a containment vessel. Water from a gravity driven cooling system flows by gravity to cover and cool the reactor core. An isolation condenser in a cooling pool of water that is open to the atmosphere continues to condense steam and return cooled condensate to the reactor vessel for as long as is necessary to dissipate decay heat from the reactor core. The water in the cooling pool is permitted to boil off to the atmosphere. An initial supply of water in the cooling pool is great enough to ensure that ample time is available for its replenishment. In one embodiment of the invention, the cooling pool is capable of dissipating reactor decay heat for at least 72 hours without replenishment. Briefly stated, the present invention provides a gravity driven cooling system pool disposed at an elevated location with respect to the locations of nuclear fuel rods in a pressure vessel. In the event of a loss of coolant in the pressure vessel, steam pressure is initially reduced by venting the steam into a closed suppression pool containing a quantity of water under a large air space. The suppression pool condenses sufficient steam to lower the steam pressure in the pressure vessel so that water can flow by gravity from a gravity driven cooling system pool to flood the fuel rods in the pressure vessel. An isolation condenser is submerged in a large supply of water elevated with respect to the pressure vessel. Steam is admitted to the isolation condenser, where it is cooled by boiling the water surrounding it. The resulting steam is vented to the atmosphere. A depressurization valve vents steam from the pressure vessel into the drywell to aid pressure reduction, and thus to aid the gravity flow of coolant. Later, as the pressure in the pressure vessel is reduced by condensation of steam in the isolation condenser, steam in the drywell is permitted to flow from the drywell, and into the depressurization valve and thence to the isolation condenser. It is a further object of the invention to provide a passive heat removal system based on natural convection and evaporation heat transfer that is capable of removing heat directly from a reactor and indirectly from the containment and discharge the heat across the containment boundary in either isolation transients or loss-of-coolant events. A single system removes heat for plant transients and for transients, instead of the design using two different systems. It is a still further object of the invention to provide a nuclear reactor system wherein a suppression pool absorbs initial heat energy released from a nuclear reactor into its containment. During accidents, a passive system, driven by gravity, provides liquid inventory flow into the reactor to keep it covered and cooled. Depressurization valves are opened following an accident to permit passive heat removal through the isolation condensers by taking suction from both the nuclear reactor and drywell for decay heat removal. According to an embodiment of the invention, there is provided a nuclear system of a type including a containment having a nuclear reactor therein, comprising: a heat exchanger, a pool of water surrounding the heat exchanger, means for venting the pool of water to the environment outside the containment, means for admitting a heated fluid from within the containment to the heat exchanger, whereby the heated fluid is cooled, and means for returning cooled fluid from the heat exchanger to the containment. According to a feature of the invention, there is provided a nuclear system of a type including a containment having a nuclear reactor therein, comprising: a pool of coolant, the pool of coolant being disposed at an elevation sufficient to permit a flow of the coolant into the nuclear reactor against a predetermined pressure within the nuclear reactor, and means for reducing a pressure in the nuclear reactor to a value less than the predetermined pressure in the event of a nuclear accident. The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.