Patent Number: 050826190
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear reactors and the containment therefor, the invention more particularly being directed to passive heat removal from the containment in the event of one or more types of system malfunctions. Commonly assigned application Ser. No. 07/325,729 filed Mar. 20, 1989 discloses a nuclear reactor system which on occurrence of an accident, e.g., loss of coolant in the reactor vessel, break in main steam pipe etc operates to dissipate initial heat generated incident the occurrence and also to dissipate the decay heat, i.e., the heat produced by fuel rod decay reactions which persist for a period of time of days or even weeks duration. In the prior disclosed system, initial heat can be dissipated by venting steam generated in the pressure vessel to a suppression pool in a closed space wherein the steam condenses and heat is transferred to water in the pool. After the pressure in the reactor vessel reduces to a certain magnitude, water in an elevated pool will flow by force of gravity into the reactor vessel to replace any loss of coolant therein as may have taken place, the elevation height of the pool being such as to provide a flow head sufficient to overcome the reduced pressure value in the reactor vessel. In the prior system, one or more isolation condensers are submerged in a large supply of water disposed elevated with respect to the pressure vessel, the water in turn being open to atmosphere so that any boiling thereof caused by a heat transfer from a heated medium passing through the condenser can be passed to atmosphere. This heat transfer results from venting the pressure vessel to the isolation condenser through an isolation line connecting the pressure vessel with the inlet side of the condenser, the condenser in this case serving to dissipate initial heat. Condensate from the isolation condenser is returned to the pressure vessel by a return line connecting the condenser outlet with the pressure vessel. A depressurization line in the system also can be opened to vent steam to the drywell surrounding the pressure vessel and in time the steam pressure in the drywell will, due to vessel cooling, be higher than that in the vessel. As a result, steam from the drywell will pass through the depressurization line, through the pressure vessel, the isolation line and into the isolation condenser where it will be condensed. In this manner and over a period of time, decay heat is dissipated. The arrangement and operation of the prior disclosed system involves the need during an accident to open valves, either automatically or manually, in one or both of the isolation line and condensate return line, and in a condenser vent line as a condition for cooling to be carried out in the isolation condenser. Since automated controls and/or human intervention are subject to failure, isolation condenser operation is only passive to the extent that these components operate. Also, it is noted that it is only sometime after accident occurrence that heat dissipation from the containment space through the isolation condensers is initiated. OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide improved heat removal capability in a nuclear reactor system for removing heat from the reactor and its containment space when a system malfunction or accident occurs. It is a further object of the invention to provide a fully passive means for effecting heat removal from the containment space so automated and/or human agency involvement which is always susceptible to failure is eliminated. It is a still further object of the invention to provide heat removal means which is disposed in the nuclear reactor containment space and is always in heat exchange communication with that containment space so that heated fluids present in the containment space can always access a cooling means and be cooled before such fluids could effect containment space structure integrity. Briefly stated, the present invention provides a nuclear system of the type which includes a containment wherein a nuclear reactor pressure vessel is located, the system including a suppression pool to which steam can be vented so that it will condense and reduce the pressure in the pressure vessel upon the happening of an accident such as loss of reactor coolant or a steam pipe failure. Steam also can be vented directly to the containment space to further reduce pressure in the reactor. When reactor pressure is lowered to a predetermined pressure value, a gravity supply of water from an elevated pool of water elevated to a height to have a sufficient head to flow against the predetermined pressure in the reactor, will flow into the reactor to submerge the fuel rods therein. One or more isolation condensers are submerged in a large supply of water, this supply being elevated some distance above the pressure vessel. At least one isolation condenser has inlet thereto communicated to an open entry conduit disposed in the containment. Steam in the containment space can enter the isolation condenser to be cooled at any time an accident occurs and steam is vented into the containment. The condensing of steam in the condenser boils water in the large water supply but since this water supply is open to atmosphere, such boil-off is vented to atmosphere without any hazard of environmental contamination. Condensate resulting from the cooling occurring in the isolation condenser is returned to the suppression pool, and non-condensable gasses which were present in the steam are separated from the condensate and vented to the suppression pool. In accordance with these and other objects of the invention, there is provided a nuclear system including a containment having a nuclear reactor therein, the system 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; an open entry conduit within said containment and communicating with said heat exchanger, heated fluid present in the containment flowing into said heat exchanger through said conduit and being cooled in said heat exchanger; and means for returning cooled fluid from said heat exchanger to a collection space in said containment. The use of an open entry conduit to admit heated fluid to the isolation condenser and means to return cooled fluid to a collection space assures that in event of an accident, immediate operating and fully passive means are available to start dissipating heat in the containment space. The collection space conveniently can be the suppression pool and the means for returning cooled fluid to that space can be a non-blockable conduit. A condensate/non-condensable gas collector means can be disposed in the return conduit with non-condensable gasses passing from the collection means to a vent pipe which outlets in the suppression pool. According to a further feature of the invention, there is provided a nuclear system of the type including a containment having a nuclear reactor pressure vessel therein and comprising a plurality of heat exchangers; a pool of water surrounding each of said heat exchangers; means for venting said pool of water to the environment; an open entry conduit within said containment and communicating with at least one of said heat exchangers, heated fluid present in the containment flowing into said one heat exchanger; means for returning cooled fluid from said one heat exchanger to a collection space in the containment; an isolation line connecting said pressure vessel with at least one other of said heat exchangers whereby heated fluid present in the pressure vessel can be conveyed to said other heat exchanger for cooling therein; and an isolation return line connecting said other heat exchanger with said pressure vessel for returning cooled fluid from said other heat exchanger to said pressure vessel. The above, and other objects, features and advantages of the invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals indicate like parts in the description.