Patent Number: 044735286
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention Nuclear power plants, because of the potential accidental release of radioactive materials, are required by practice to be designed in such a manner that the health and safety of the public is assured even in the event of the most adverse accident that can be postulated. In nuclear power plants utilizing light water as a coolant, the most adverse accident possible is considered to be a double-ended break of the largest pipe in the reactor coolant system and such an accident is commonly termed the Loss Of Coolant Accident, hereinafter sometimes referred to as LOCA. For accident protection, plants utilizing light water as the coolant employ containment systems designed to physically contain water, steam and any entrained fission products that may escape from the reactor coolant system. The containment system is normally considered to encompass all structures, systems and devices that provide ultimate reliability and complete protection for any accident that may occur. Engineering safety systems are specifically designed to mitigate the consequences of an accident, and the design goal of a containment system is that no radioactive material will escape from the nuclear power plant in the event of an accident. The passive containment system disclosed herein provides this desired level of protection for a loss of coolant accident and for other types of accident that are considered as a basis of design, and the concepts of the invention are considered to be effective for nuclear power plants employing either pressurized water reactors or boiling water reactors. 2. The Prior Art In order to provide containment for light water cooled nuclear power plants prior art techniques have basically utilized either full-pressure "dry-type" containment or pressure suppression containment. In a full-pressure containment the reactor building, completely enclosing the reactor coolant system, is capable of withstanding the pressure and temperature rise expected to occur in the event of a LOCA. The builidng is usually constructed either of steel or steel-lined reinforced concrete or prestressed concrete. Full-pressure containment systems may include double leakage control barriers and subatmospheric pressure operation. For the double leakage-control barrier any leakage into the control annulus is either pumped back into the primary containment, or the leakage is treated before being exhausted to the outside atmosphere. For subatmospheric operation the containment is normally maintained at partial vacuum, and following the LOCA, the pressure is reduced back to less than the outside atmosphere utilizing active engineered safety systems to terminate any potential release of radioactivity to the environment. The pressure-suppression containment consists of a drywell that houses the reactor coolant system, a pressure-suppression chamber containing a pool of water, and a vent system connecting the drywell to the pool of water. This containment structure is constructed of steel enclosed by reinforced concrete, or is steel-lined with reinforced concrete. The pressure-suppression containment is housed within a reactor building. In the event of a LOCA, the reactor coolant partially flashes to steam within the drywell, and the air, steam and liquid coolant flow through the connecting vents into the pool of water in the suppression chamber. The steam is condensed by the water and decreases the potential pressure rise in the containment. The air rises into the free space above the pool of water in the suppression chamber. Refinements in pressure-suppression containment utilizing water includes the inerting of the containment atmosphere. Inerting is aimed at preventing the burning of hydrogen produced from metal-water reaction of overheated nuclear fuel. A different type of pressure-suppression containment utilizes an ice-condenser. The ice is maintained in a refrigerated compartment surrounding the reactor coolant system. The ice-condenser containment is divided into an upper chamber and a lower chamber with the reactor coolant system in the latter. In the event of a LOCA a pressure rise of the lower chamber causes access panels located at the bottom of the ice-storage compartment to open. This provides a flow path for air and steam through the ice bed. The steam is condensed by the ice and decreases the potential pressure rise in the containment. The air passes into the upper chamber through top access panels forced open by the flow of air. Full-pressure containment and pressure-suppression containment are passive structures that require support systems for containment of the accident. Active systems such as residual heat removal systems and containment spray systems are used to dissipate heat to the environs. This prevents the containment design pressure and temperature from being exceeded and in the process, the containment pressure is reduced to limit the leakage of fission products. Active filtration systems are required in conjunction with the spray systems to reduce fission product concentration in the containment atmosphere. This also limits the amount of fission products that can leak out of the containment to the environs. Hydrogen recombiners are also being utilized to protect the containment from developing explosive concentrations of hydrogen. To be effective, both the full-pressure containment and the pressure-suppression require additional engineered safety systems that provide emergency cooling of the nuclear fuel. Pressurized water reactors require passive accummulator systems in addition to active high and low pressure injection systems to maintain an adequate amount of liquid coolant at the nuclear fuel. The residual heat removal systems used for containment pressure reduction also reject decay heat to the environs. Pressure suppression with gravity flooding has also been proposed as an engineering safety system for a LOCA. Active engineered safety systems are inherently required to function effectively in order to maintain the integrity of the containment system in the LOCA. Active systems require high integrity instrumentation and control equipment, rotating machinery, electric power sources and power distribution equipment. These systems need to function properly as part of a larger system under adverse containment environment conditions of high-pressure, high-temperature, high-humidity, high-radioactivity, and eroded thermal insulation. Malfunctioning of any active engineered safety system imposes even more adverse conditions on the operable system. For instance, an inadequate sourch of electric power may result in the malfunctioning of the emergency core cooling system for the nuclear fuel. Overheating of the fuel can result in melting of the fuel cladding with metal-water reactions occuring. The fuel core may slump and portions could collapse and overheat the bottom of the reactor vessel. Hydrogen is released from metal-water reactions and is subject to burning. The added energy from the metal-water reactions and from the burning of hydrogen imposes even more severe requirements on containment structure. Overheating of the fuel and melting of the cladding results in a gross release of fission products that are available for leakage from the containment system. This example points to the critical nature of active engineering safety systems that are an essential part of the containment system of the prior art. The prior art has proposed a variety of solutions to the containment of a nuclear power plant in the event of a LOCA, and in my U.S. Pat. Nos. 3,984,282 and 4,050,983, I have proposed passive containment systems for confinement of the coolant in the event of a LOCA, and for cooling the reactor assembly in the event of such an accident. Further, in my U.S. Pat. No. 3,865,688 I have disclosed a passive confinement system utilizing many of the concepts herein set forth, and this invention constitutes an improvement over that specifically set forth in U.S. Pat. No. 3,865,688. SUMMARY OF THE INVENTION The invention relates to a nuclear reactor containment arrangement, and more particularly, to an entirely passive containment system which encloses a reactor system using a high-pressure, high-temperature coolant and/or moderator such as light or heavy water. In this invention, the passive containment system is used to safely contain even the most adverse reactor accident wherein a sudden rupture of the reactor piping occurs resulting in the loss of coolant. The passive containment system herein provides equal protection for nuclear reactor system of the pressurized water or boiling water types. The containment system of the invention as used for a pressurized water reactor consists of interconnected cells; each cell housing a major component of the nuclear reactor system; i.e., reactor vessel, steam generators, pumps, pressurizer, regenerative heat exchanger, and piping. Cells are also provided for the engineered safety system components. Water-filled deluge tanks, quench tanks and reactor vessel refill tanks are located entirely within containment cells at an elevation above the reactor coolant system piping. Within the containment cells a primary container formed from interconnected steel shells encloses the entire reactor coolant system. The primary container is encased by reinforced or prestressed concrete. The water used within the reactor vessel refill tanks, within the deluge tanks, and within the quench tanks, is specially treated for accident containment purposes. The water is degassed and contains chemicals in solution that serve as a poision to neutrons, inhibitors of corrosion, oxygen "getters", and radio-nuclide getters. The water within the tanks is retained in a chilled condition by suitable refrigeration means such as a steam-jet refrigeration system or other refrigeration system. The passive containment system is housed within a reactor building. The arrangement of the cell structures permits the relocation of spent fuel storage pools and a refueling cavity and other equipment enclosures within the reactor building. In a typical response of the passive containment system hereof to a LOCA, decompression of the reactor coolant through the pipe break produces steam within the primary container which is normally maintained at a high vacuum. The steam pressurizes the containment and the steam overpressure is vented into the deluge and quench tanks. During reactor coolant blowdown, the hydrostatic pressure within the reactor vessel refill tanks causes check valves in the high-pressure injection pipe to lift, and treated water is injected into the reactor coolant system. The decompression of the refill tanks causes check valves in the steamlines between the steam generator secondaries and the refill tanks to lift. This initiates steam flow from the steam generators through jet injectors and steam flow through the injectors entrains treated water from the refill tanks. The steam and water are intimately mixed on passage through the diffuser sections of the injectors to provide a homogeneous solution of treated water that quenches the fuel elements, refills the reactor vessel and overflows through the pipe break into the containment. The elevated deluge and quench tanks include steam vent conduits communicating with the cooling liquid therein and with the containment. Thus, upon the containment being pressurized with steam due to the LOCA the steam within the containment will enter the deluge and quench tanks through their vent conduits and the chilled water in these tanks absorbs the heat energy within the steam. When coolant blowdown is arrested a gravity flow of the borated water from the deluge tanks continues emergency core cooling with flow through the pipe break that resulted in the loss of coolant. All stored energy within the reactor system is absorbed by the refill and deluge water flow, and sufficient heat capacity is provided in the chilled, stored water within the refill, deluge and quench tanks to reduce temperatures to low levels. The containment atmosphere is restored to the normal high-vacuum condition by the vapor carryover. The heat-sink capacity of the water in the quench tanks provides a vented containment for the term of the accident, and the borated water in the deluge tanks will provide four hours of passive decay heat removal. In the disclosed embodiment a four loop system is disclosed in conjunction with a single reactor vessel. Accordingly, four steam generators, four refill tanks, four deluge tanks, and four quench tanks are used with the preferred embodiment. A single pressurizer is employed to maintain the pressure within the reactor coolant system. Each steam generator includes a primary system receiving heat from the reactor coolant system and the steam generators transfer this heat to their secondary system which produces steam for utilization purposes, such as powering a turbine. In addition to utilizing the deluge tanks and quench tanks for steam venting and absorption purposes with respect to steam within the containment, these tanks also include steam absorbing means connected to the associated steam generator secondary system through electrically operated valves. Thus, thermal energy can be selectively absorbed within the deluge and quench tanks from the steam generator secondary system by operation of selective valves, and with certain types of malfunctions or leakage, this type of reactor cooldown is utilized. In such instance the transfer of heat from the generator secondary systems likewise cools the reactor coolant through the primary system and the heat absorption capacity of the deluge and quench tanks is sufficient to adequately cool the system for control purposes. In a major LOCA it is possible to use the thermal energy within one steam generator secondary system for the introduction of coolant into the reactor coolant system from refill tank injectors, while the energy within the generator secondary systems of other generators is being dissipated through direct injection of secondary steam into the associated deluge and quench tanks, thereby providing a simultaneous replenishing of reactor coolant and dissipation of the energy within the power plant. The quench tanks, in addition to absorbing vented steam, and steam injected therein from a secondary system, also include a steam-powered injector supplied with steam from the associated generator secondary system having a discharge communicating through a check valve with the associated generator secondary feedwater system. Thus, operation of the quench tank injector introduces auxiliary feedwater into the associated generator secondary system, and this operation is employed in the event of feedwater malfunctioning assuring a supply of feedwater in the event the accident restricts or eliminates the normal feedwater source. OBJECTS OF THE INVENTION It is a general object of the invention to provide a new and improved containment method and apparatus for any energy, toxic or radioactive materials released from a process system accommodated therein. It is a more particular object of the invention to provide a passive containment system process and apparatus for a nuclear reactor power plant system. Another object of the invention is to provide functional improvements in the complete containment of a nuclear reactor system through passive means actuated, controlled, powered and maintained by the forces of nature that are designed to be intrinsic to the containment system. A further object of the invention is to provide a reactor containment system which is less expensive to construct than similar prior systems in that the primary containment free volume is effectively reduced and less expensive materials are required. Another object of the invention is to provide a passive containment system that utilizes the forces of physics to provide the ultimate level of reliability in the containment of nuclear power plants. An additional object of the invention is to provide passive emergency core cooling utilizing passive reactor vessel refill decay heat transfer utilizing the energy within the reactor power plant system. Another object of the invention is to provide a nuclear power plant containment system which permits plant recovery from all design basis accident including the loss of coolant accident. An additional object of the invention is to provide a nuclear power plant heat removal system utilizing a plurality of coolant reservoirs wherein the coolant within the reservoirs may be selectively used for heat absorption by the venting of steam therein, and selected reservoirs permit coolant to be supplied directly to a reactor coolant system, in all events, the cooling capacity within the reservoirs being sufficient to achieve reactor cooldown. Yet another object of the invention is to provide a a nuclear reactor power plant system employing a plurality of steam generators and coolant reservoirs wherein energy within the steam generators may be selectively dissipated within the reservoirs, and energy within the generators may also be employed to introduce coolant directly into the reactor coolant system, and feedwater in the secondary system.