Patent Number: 050826190
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 depicts the nuclear reactor system of pending application Ser. No. 07/325,729, that system 10 including 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 commonly-owned application Ser. No. 07/350,189 filed May 11, 1989, now U.S. Pat. No. 4,950,448 and entitled PASSIVE HEAT REMOVAL FROM CONTAINMENT. 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 FIG. 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 that steam condensate can be returned from the condenser to the vessel, that line having a valve 47 and a condensate/non-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 the 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 that pressure may be further lowered by opening depressurization valve 32 so that the head of pressure in gravity pool 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 40 is returned to the pressure vessel via line 46, and non-condensables to the suppression pool by way of vent line 62. After 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 FIG. 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 present invention improves the FIG. 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. The invention contemplates that at least one and probably two of the plural isolation condensers included in a FIG. 1 system would be constructed in the manner shown in FIG. 2 to be described next. Referring to FIG. 2, the system 10 embodies elements common to the FIG. 1 system, and so the same reference numerals are employed in each Figure but it is to be noted the FIG. 2 system does not employ all the valves used in the FIG. 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 FIG. 1 system. The principal advantage of providing at least some isolation condensers of the FIG. 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 FIG. 2 arrangement and at least one other being of the FIG. 1 arrangement. Thus isolation condenser 40 will be employed for direct drywell passive cooling upon happening of a loss-of-coolant accident, whereas, isolation condenser 40-1 can be used for cooling of steam from the pressure vessel 12 flowing thereto via isolation line 42 thereby facilitating depressurization of the vessel. 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. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.