Passive heat removal from nuclear reactor containment

Abstract: A nuclear systemof the type which includes a contain­ ment (14) wherein a nuclear reactor pressure vessel (12) is located, has a suppression pool (22) to which steam can be vented (20,28) 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 (32) directly to the containment space to further reduce pressure in the reactor. When reactor pressure is lowered to a certain pressure value, a gravity supply of water from an elevated pool of water (36) will have a sufficient head to flow against the pressure in the reactor and into the reactor to submerge the fuel rods in the reactor. One or more isolation condensers (40) are submerged in a large supply of water (50) this supply being elevated some distance above the pressure vessel. At least one isolation con­ denser has inlet thereto communicated to an open entry conduit (42) 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 supply but since the large water supply is open (52) to atmosphere the boil-off is vented. Condensate resulting from cooling occurring in the isolation condenser is returned (46) to the suppression pool and non-condensable gasses present in the steam being condensed are separated (60) from the condensate and vented (62) to the suppression pool.

Description: 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. European Patent Application No. 0 389 231 (Prior art under Article 54(3)EPC) 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 in initiated. 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. According to the invention, there is provided a nuclear system of a type including a containment having a nuclear reactor therein, comprising a heat exchanger disposed in the nuclear reactor containment space; a pool of water surrounding said heat exchanger; means for venting said pool of water to the environment outside said containment; an open entry conduit containing no flow blocking device 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 the said heat exchanger; and conduit means containing no flow blocking device for returning cooled fluid from said heat exchanger to a collection space in said containment connected at one end with the heat exchanger outlet, an opposite conduit end being situated in said collection space. The invention provides 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. In an embodiment of the invention to be described, a nuclear system of the type which includes a containment wherein a nuclear reactor pressure vessel is located, also includes 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 trolation 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. The above, features and advantages of the invention and its described embodiment 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 and in which: Fig. 1 is a simplified schematic depiction of a nuclear reactor system of the construction disclosed in European Application No. 90302950.2. Fig. 2 is a schematic depiction showing a nuclear reactor system having containment space passive heat removal capability as provided by the present invention. In Figure 1 depicting the nuclear reactor system of European Application No. 90302950.2, system 10 includes a pressure vessel 12 disposed in a containment space defined by containment building 14, only a portion of the building being shown but it being understood that same is a reinforced structure having strength to withstand any pressure as may exist therein in consequence of a malfunction or accident. Steam for system service use is delivered through steam line 16 to a point of use, the line 16 being provided with a depressurization valve 32 and a safety relief valve 20. The pressure vessel is situated in the drywell 18 of the containment space and that in turn is surrounded by an annular structure containing a suppression pool 22 of water 24 to which steam in the event of pressure vessel rupture or steam line break at a downstream line location, can be diverted through outlet tube 28 into the suppression pool so the steam will condense and pressure thereby reduced in the pressure vessel. Also high pressure steam entering the containment space from a large steam line break or pressure vessel rupture can enter suppression pool 22 through vent pipes(not shown) communicating between the drywell and horizontal vents leading into the suppression chamber below the top of the water and as described in our European Application No. 90305070.6. The suppression pool is sized to have a substantial air space 26 above water 24 to provide a compressible medium headspace in the pool. A gravity driven cooling system pool 34 including sufficient supply of water 36 to flood the pressure vessel to a depth substantially above the fuel rods in event of vessel rupture is located elevated some distance above the suppression pool. The line connecting pool 34 to the pressure vessel is fitted with a normally closed control valve 38. An equalizing line 54 connects the suppression pool to the pressure vessel, the line having a normally closed stop valve 56 and a check valve 58, the check valve preventing back flow to the suppression pool from the pressure vessel or the gravity pool 34 whenever valve 56 is open. The Figure 1 system also includes an isolation condenser 40' which is submerged in an isolation pool 48 comprised of a large body of water 50, the isolation pool being open to atmosphere by means of vent stack 52. The isolation condenser inlet is connected to the pressure vessel by means of isolation line 42' and intervening isolation valve 44'. An isolation return line 46' connects the condenser outlet with the pressure vessel so tbat steam condensate can be returned from the condenser to the vessel, that line having a valve 47' and a condensation-condensable gas collector chamber 60 therein, the non-condensables being vented from chamber 60 to the suppression pool through vent line 62 which is fitted with a normally closed vent valve 64. On occurrence of a malfunction, either as a loss of coolant in the reactor or a break in thc steam line, valve 20 is opened to direct steam from the reactor to suppression pool 22 thereby to condense the steam and dissipate a significant amount of the initial heat. Diverting the steam in this manner reduces pressure within the reactor and tbat pressure may be further lowered by opening depressurization valve 32 so that tbe bead of pressure in gravity spool 34 is sufficient to overcome the reduced pressure inside the pressure vessel and cooling water will flow from the pool into the reactor to a level above that required to cover the reactor core, the valve 38 having been opened at about the time valve 32 was opened. Dissipation of heat also takes place in the isolation condenser 40'. Normally, isolation valve 44' is open and valves 47' and 64 are closed. When an accident occurs, valves 47' and 64 will be opened so that steam from the pressure vessel will flow into the isolation condenser and be cooled to condensate, heat transfer being to the body of water 50 and any boiling of that body being vented to atmosphere by vent stack 52. Long term or decay heat dissipation will occur in condenser 40'. Condensate from the condenser 42' is returned to the pressure vessel via line 46', and non-condensables to the suppression pool by way of vent line 62. Alter the pressure reduces in the pressure vessel to a level below that present in the containment space and because the depressurization valve is open, steam present in the containment space can enter the pressure vessel through valve 32 and pass on to the condenser 40' for condensing of same. The reactor system can employ plural isolation condensers and most usually will use four such condensers in association with a reactor vessel. The system arrangement of Figure 1 it will be seen establishes heat dissipation communication with the isolation condenser only by way of the reactor vessel including communication of the containment space as such with condenser 40'. Further, it is only after the reactor pressure is reduced below that present in the containment space that this communication takes place. The Fig. 2 system improves the Figure 1 system to provide for immediate availability of isolation condenser cooling of steam present in the containment space upon happening of an accident and by means of a fully passive system arrangement that operates without reliance on any automatically or human controlled device whereby there is assured immediate heat dissipation response in the containment space at the instant an accident occurs. At least one and probably two of the plural isolation condensers included in a Figure 1 system would be constructed in the manner shown in Figure 2 to be described next. Referring to Figure 2, the system 10 embodies elements common to the Figure system, and so the same reference numerals are employed in each Figure but it is to be noted the Figure 2 system does not employ all the valves used in the Figure 1 system. According to the invention, isolation line 42 is not connected to the pressure vessel but rather is an open entry conduit having its open end situated in elevated location within the reactor containment space to maximize convective flow entry of steam to the conduit and on into the isolation condenser 40, the other end of conduit 42 communicating with the condenser inlet. The isolation return line 46 which receives condensate outflow from the isolation condenser is connected at one end to the condenser outlet and at its other end locates in a containment collection space, i.e., in the suppression pool 22. Both isolation line 42 and return conduit 46 are of unblockable character, i.e., neither has a component such as a valve or other device located along its course that could block steam flow into the condenser or condensate flow out therefrom. The other or lower end of conduit 46 is submerged a distance below the normal water level in pool 22. Chamber 60 which is in the conduit line 46 does not contain any part which could block flow into the conduit 46. It simply serves to collect condensate at the condenser outlet end and pass it directly into conduit 46. Chamber 60 also collects non-condensable gasses such as air and they pass into vent pipe 62 and from it into the suppression pool, the lower end of pipe 62 also being submerged below the normal water level in pool 22 but not to the extent to which the lower end of return conduit 42 is. Operation of the suppression pool 22, gravity driven cooling system pool 34, equalizing line 54, valves 20 and 32 and vacuum breaker 30 are the same and for the same purpose as is shown in the Figure 1 system. The principal advantage of providing at least some isolation condensers of the Figure 2 character in a nuclear reactor system is the immediate capability of the system to deal with a large measure of heat invading the reactor containment space, this being possible without dependence on any other system heat dissipating means or devices, the system having instantaneous response to the heat presence because it is fully passive not needing automated or human initiated response. A further embodiment of the invention provides that a nuclear reactor system will employ plural isolation condensers, at least one of these condensers being of the Figure 2 arrangement and at least one other being of the Figure 1 arrangement. Such use of the differently arranged isolation condensers maximizes the flexibility of the system to deal with dissipation of both initial and decay heat loads.

Claims: A nuclear system (10) of a type including a containment (14) having a nuclear reactor (12) therein, comprising: a heat exchanger (40) disposed in the nuclear reactor containment space; a pool (48) of water (50) surrounding said heat exchanger; means (52) for venting said pool of water to the environment outside said containment; an open entry conduit (42) containing no flow blocking device 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 conduit means (46) containing no flow blocking device for returning cooled fluid from said heat exchanger to a collection space (22) in said containment connected at one end with the heat exchanger outlet, an opposite conduit end being situated in said collection space. A nuclear system in accordance with claim 1, further comprising a suppression pool (22) in said containment, said suppression pool including a supply of water (24) therein, the collection space to which cooled fluid is returned comprising said suppression pool. A nuclear system in accordance with claim 2, in which said cooled fluid return conduit opposite end is disposed for outlet therefrom at a location submerged a distance below a normal water level in said suppression pool. A nuclear system in accordance with claim 3, further comprising a condensate/non-condensable gas collector means (60) disposed in said cooled fluid conduit (46), condensate passing from said collector means to said cooled fluid conduit opposite end, the non-condensable gasses passing from said collector means to a vent pipe means (62) connected at one end with said collector means, the pipe means having another end outletting in the suppression pool (22). A nuclear system in accordance with claim 4, in which said pipe means other end outlet is located submerged a distance below a normal water level in said suppression pool. A nuclear system in accordance with claim 5, in which said pipe means other end outlet submergence level below a normal suppression pool water level is less than that of said cooled fluid return conduit opposite end. A nuclear system in accordance with any one of claims 1 to 6, comprising: a plurality of heat exchangers (40,40'); said open entry conduit (42) communicating with at least one of said heat exchangers (40); an isolation line (42') including isolation means (44') connecting said pressure vessel (12) to at least one other of said heat exchangers (40') whereby heated fluid present in the pressure vessel can be conveyed to said other heat exchanger for cooling therein; and an isolation return line (46') including isolation means (47') connecting said other heat exchanger with said pressure vessel for returning cooled fluid from said other heat exchanger to said pressure vessel.

Inventors: DELANY SAWYER CRAIG, DELANY, SAWYER CRAIG
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1990
Main classification: G21C15
IPC codes: G21C9, G21C15