Patent Number: 048308153
Section: description

DETAILED DESCRIPTION OF THE DRAWINGS The invention will now be explained by reference to the prior art cooling systems and then by reference to various embodiments and advantages of the invention. Referring to FIG. 1, FIG. 1 shows a nuclear power generating system 2 having a nuclear boiling water reactor 10 according to the prior art. The nuclear boiling water reactor 10 includes a reactor core 20 which heats water to generate a two-phase steam/water mixture. A core shroud head 22 overlies the reactor core 20 and receives the two-phase steam/water mixture. A plurality of standpipes 18 convey the steam/water mixture to a steam separator assembly 16. The steam separator assembly 16 classifies the steam from water. The separated steam is piped from an outlet 26 over pipeline 5 through a valve 6 to a turbine generator 7. The spent steam from turbine generator 7 is provided to a condenser 8. Condensate from condenser 8 is returned to the reactor 10 at an inlet 15 by a feedwater injection system that includes a pump 9. The water separated by separator assembly 16 flows back into the reactor pressure vessel and forms a part of the reactor coolant inventory 24. FIG. 1 also shows an isolation cooling system and a shutdown cooling system that include elements that are common to the prior art as shown in this FIG. 1 and to one embodiment of the invention as shown in FIG. 2. These common elements include a piping network for conveying pressurized steam from the reactor during isolation cooling; a condenser for receiving the heated steam during isolation cooling; a piping network for conveying heated reactor cooling water from the reactor during shutdown cooling; a secondary heat exchanger for removing heat from the reactor cooling water during shutdown cooling; a cooling pond for providing an additional means for removing heat from the reactor cooling water during shutdown cooling; and a recirculation system for conveying cooled/condensed steam and cooled reactor coolant back to the nuclear reactor. The piping network for conveying heated steam from the reactor includes a pipeline 30 which conducts steam from nuclear reactor steam outlet 12 to an isolation valve 31 during an isolation cooling mode. Isolation valve 31 is opened and closed by a motor 32 which is responsive to a remote motor control unit 33. The nuclear reactor vessel 10, the pipeline 30 and the valve, motors and remote control units are typically located inside the reactor containment 4. Pipeline 30 continues from valve 31 to another isolation valve 34 which is located outside the reactor containment 4. Isolation valve 34 is controlled by a motor 35 which is responsive to a remote motor control unit 36. An output of valve 34 is coupled to a pipeline 50 which conveys steam to a supply side of isolation condenser 100. Isolation condenser 100 includes a housing or shell 104, a quantity of isolation condenser coolant 102, and a heat exchange surface 54. Isolation condenser 100 is also provided with a vent or duct system 99 which conveys steam to the atmosphere. Heat exchange surface 54 includes an input side 53 and a discharge side 55. The discharge side 55 is coupled to a return pipeline 68 which conveys cooled, condensed steam to the reactor pressure vessel. Pipeline 68 is connected to an input of an isolation valve 61. Isolation valve 61 is controlled by a motor 62 which is responsive to a remote motor control unit 69. The output of isolation valve 61 is connected to a recirculation pipeline 81 inside the reactor containment 4. Recirculation pipeline 81 is connected from an outlet 80 of the reactor pressure vessel to an input side of a recirculation pump 82. Recirculation pump 82 continuously circulates coolant inside the reactor when the reactor is in a cooling mode. An output of recirculation pump 82 is connected by a recirculation line 83 to an inlet 84 to the reactor vessel. Referring still to FIG. 1 there is shown a separate shutdown cooling system according to the prior art. As shown in FIG. 1, a coolant outlet 14 for the reactor 10 conveys coolant from the reactor 10 to a reactor coolant isolation valve 41. Isolation valve 41 has an output coupled to reactor coolant circulation pump 42 by a pipeline 40. Pump 42 circulates reactor coolant from the reactor 10 to a shutdown cooling system when the reactor is in a shutdown cooling mode. Coolant pump 42 is connected by pipeline 43 to inlet 210 of shutdown cooling exchanger 200. The reactor coolant is circulated through the tube side of a heat exchange surface 204. Heat exchange surface 204 is immersed in an intermediate coolant 202. The reactor coolant is removed from exchanger 200 at outlet 212. The cooled reactor coolant is circulated from outlet 212 by pipeline 213 to return pipeline 68. The returned cooled reactor coolant is thereafter circulated back into the reactor in the same manner as the cooled condensed steam. Referring still to FIG. 1. a secondary heat exchanger 120 is used in certain nuclear power station applications as a buffer between the shutdown cooling heat exchanger 200 and a cooling pond 130. For these configurations, the intermediate coolant 202 in the shutdown cooling heat exchanger 200 is conveyed from a discharge outlet 206 by pipeline 218 to a valve 220. Valve 220 is controlled by motor 222 which is responsive to remote motor control unit 224. The output of valve 220 is connected by pipes to secondary heat exchanger 120. Secondary heat exchanger 120 includes a heat exchange surface 122. The intermediate coolant 202 from shutdown cooling heat exchanger 200 is circulated to the shell side of heat exchange surface 122 and is removed from secondary heat exchanger 120 at an outlet 125. Baffles 124 mix coolant 202. Outlet 125 is connected to a circulation pump 140. The output of circulation pump 140 is coupled to an isolation valve 141. Isolation valve 141 is controlled by a motor 142 which is responsive to a remote motor control unit 143. The cooled intermediate coolant is conveyed from the output side of valve 141 to inlet 208 of shutdown cooling heat exchanger 200 where it is used again to cool the reactor coolant injected at inlet 210. Coolant 202 is commonly maintained at an operating pressure higher than the pressure of the reactor coolant in reactor 10 during the operating mode. The coolant 202 is maintained at this higher pressure in order to prevent any outward transport of radioactivity that might otherwise occur if tube leaks in heat transfer surface 204 were to develop. Since intermediate coolant 202 is commonly operated at such a higher pressure, all equipment associated with intermediate coolant 202 must be designed to withstand such higher pressure conditions. Secondary heat exchanger 120 includes a quantity of raw water coolant 132 from cooling pond 130. Coolant 132 flows through the tube side of heat exchange surface 122. The raw water coolant 132 in cooling pond 130 is circulated by a circulation pump 134 through pipeline 136 to an inlet 126 to heat exchange surface 122. An outlet 127 provides return, heated raw water coolant to a pipeline 131 which conveys the heated raw water coolant back to cooling pond 130. Refer now to FIG. 3. FIG. 3 is a partial illustration of a nuclear power generating system having a boiling water reactor 10 and a gravity-driven cooling system 150. Such a system is representative of advanced nuclear boiling water reactor designs having the isolation cooling system integrated into the containment pressure suppression pool 156. As shown in FIG. 3, the gravity-driven cooling system 150 includes a suppression pool 156 which contains a quantity of emergency water coolant 158. This isolation cooling system design includes a steam supply line 153, heat exchange surfaces 155 and condensate discharge line 157. Steam supply line 153 is connected to the tube side of a plurality of heat exchange surfaces 155. Steam supply line 153 conveys steam from reactor pressure vessel 10 to the inlet to heat exchange surfaces 155. Heat exchange surfaces 155 have a discharge side connected to pipeline 157. Pipeline 157 is connected back to reactor 10 to provide cooled condensed steam for replenishing reactor coolant and thus for cooling reactor core 20. As previously explained, the prior art systems as depicted in FIGS. 1 and 3 suffer from several drawbacks. The system of FIG. 1 requires a number of shells for the various shutdown heat exchangers. These shells must meet costly high pressure design requirements. The heat exchange surfaces operate with a low heat transfer temperature differential. Thus, the shells and surfaces are large and expensive. The shells are generally included within the reactor building and therefore occupy a large amount of space. Even more space must be provided to permit maintenance of the heat exchange surfaces included in the heat exchangers and the isolation condenser. The gravity-driven cooling system design requires an increase in the size of the reactor containment which adds significantly to the overall nuclear power generating system costs. Refer now to FIG. 2 which is a detailed diagram of a nuclear power generating system 2 having improved isolation cooling and shutdown cooling systems according to one embodiment of the invention. This system includes the isolation condenser 100, the secondary heat exchanger 120, a cooling pond 130, and circulation pipe, pumps and valves in common with the prior art system of FIG. 1. However, the system according to the invention dispenses with the separate shutdown cooling heat exchanger 200, permitting significant cost, maintenance and space savings. The arrangement of the improved cooling system will now be explained first by reference to FIG. 2 and then by reference to FIG. 4. It should be understood, however, that other arrangements or embodiments are possible without departing from the scope or spirit of the invention. As shown in FIG. 2, the isolation condenser 100 now includes a plurality of heat exchange surfaces 54, 64 and 74. These heat exchange surfaces are immersed in isolation condenser water coolant 102 and are sized for shutdown cooling duty. As used herein, "isolation condenser water coolant" means the initial water inventory in isolation condenser 100 as well as makeup water added to offset boiling and venting. Steam supply line 50 is coupled to an input 53 of heat exchange surface 54. Cooled condensed reactor steam is coupled from an outlet 55 of heat exchange surface 54 over pipeline 56 to return line 68. The reactor coolant pipeline 43 from the nuclear reactor 10 now branches into three reactor coolant input pipelines 44, 45, and 46 which are in turn coupled to inputs to the heat exchange surfaces 54, 64, and 74 included in isolation condenser 100. Pipelines 44, 45 to 46 each carry about 1/3 the capacity of line 43 and thus may be of smaller size than line 43. Pipeline 46 branches from pipeline 43 to the input side of an isolation valve 47. The output of isolation valve 47 is coupled to pipeline 50 which, as previously mentioned, couples heated steam from the nuclear reactor 10. Thus, heated reactor coolant conveyed over pipeline 46 is coupled to input 53 of heat exchange surface 54. Pipeline 45 is connected to input 63 of heat exchange surface 64. The output 65 of heat exchange surface 64 is coupled by pipeline 66 to return pipeline 68. Likewise, pipeline 44 is connected to input 73 of heat exchange surface 74. The output 75 of heat exchange surface 74 is connected by pipeline 76 to return line 68. Isolation condenser 100 (i.e., the cooling apparatus) also has a shell or housing 104 which contains isolation condenser coolant 102. Housing 104 may be a conventional "shell and tube, multiple water box" heat exchanger. Heat exchange surfaces 54, 64 and 74 are immersed in isolation condenser coolant 102. Cooling apparatus 100 also has a vent or duct system 99 which conveys isolation coolant 102 that has been heated into steam to the atmosphere. Isolation condenser coolant 102 has a free surface 103 to enhance steam boiloff when isolation condenser 102 is rapidly heated during the isolation cooling mode and during the early phases of the shutdown cooling mode. Cooling apparatus 100 may also include a draining system 90. Drain system 90 may be a conventional drain system and include a pipeline and associated drain valves. Condensed steam and reactor coolant are connected by pipeline 68 back to the recirculation line 81 according to the prior art. The isolation condenser 102 in apparatus 100 is used to remove heat from the hot steam and reactor coolant. The isolation condenser coolant 102 and cooling apparatus 100 may be demineralized water, for example, reactor cooling water or water from the station makeup water system that has been purified. The isolation condenser coolant 102 in cooling apparatus 100 is itself cooled by circulation through a secondary heat exchanger 120. The isolation condenser 102 is conveyed from an outlet 112 on housing 104 through a pipeline 121 to an input 123 of secondary heat exchanger 120. Since coolant 102 is clean, it may be provided on the shell side of heat exchange surface 122 in secondary heat exchanger 120. Heat exchange surface 122 is thus disposed in isolation condenser coolant 102. The warm isolation condenser coolant is cooled as it flows in intimate contact with heat exchange surface 122 as directed by internal baffle plates 124. The cooled isolation condenser coolant 102 is returned to an inlet 110 of cooling apparatus 104 using pumps 140 and associated pipelines and valves as previously discussed for the prior art. Secondary heat exchanger 120 uses a quantity of raw water coolant 132 obtained from cooling pond 130 for cooling the isolation condenser coolant 102. This raw water coolant is circulated as previously discussed for the prior art. As shown in FIG. 2, the nuclear reactor 10, the recirculation loop 140, and the gravity driven cooling system 150 and associated pipes and isolation valves are generally contained within the reactor containment 4. The remainder of the cooling system is contained within the reactor building 3 with the exception of the cooling pond and associated pipes and pumps. Although the embodiment shown in FIG. 2 includes only one cooling apparatus, in normal practice a plurality of such cooling apparatus may be used within the scope or spirit of the invention. In like fashion, a greater plurality of heat exchange surfaces may also be used inside the cooling apparatus 100 within the scope and spirit of the invention. Refer now to FIG. 4 which shows a mechanical diagram of a cooling apparatus according to one embodiment of the invention. As shown in FIG. 4, cooling apparatus 100 includes a shell 104 having ends 105 and 107, and including isolation condenser coolant 102 up to a free water level 103. A plurality of vents 99, 99A and 99B permit steam which is generated from boiling isolation condenser coolant 102 to be conveyed to the atmosphere. A steam dryer assembly 98 conveys moisture classified by the dryer from the vapor/moisture mixture rising from free surface 103 back into housing 104. The cooling apparatus 100 in FIG. 4 also includes a plurality of heat exchange surface assemblies 54, 64, 74 and 54A, 64A, and 74A. Each heat exchange surface assembly includes an integral tube sheet-and-water box member together with a tube bundle comprised of a plurality of U tubes. The tube sheets, water boxes. and U tubes may be fabricated and configured as for conventional isolation condensers and will include additional parts not shown according to the technology, design and manufacturing practices of conventional equipment suppliers. It should be understood that the embodiment in FIG. 4 employs six total tube sheet assemblies. Assemblies 54, 64 (not shown) and 74 are disposed at end 107 of cooling apparatus 100 in an inverted triangular pitch configuration with assembly 54 being the lowermost positioned assembly at end 107. Assemblies 54A, 64A and 74A (not shown) are disposed at opposing end 105 of housing 104 in an opposing triangular configuration to heat exchange surface assemblies 54, 64 and 74 respectively. Isolation condenser coolant 102 is provided to shell 104 by an input pipe 110. Input pipe 110 connects to end 107 of apparatus 100, somewhat above the horizontal center line of shell 104. Input line 110 conveys coolant from the secondary heat exchange system (not shown) to the cooling apparatus 100. A return pipeline 112 is coupled to the bottom 108 of apparatus 100. Return line 112 returns heated isolation condenser coolant 102 to a secondary heat exchange system (not shown) for heat removal. For a conventional reactor, the heat exchange surface area required for shutdown cooling is about three times the heat exchange surface area required for isolation condensing cooling. Thus, for the embodiment shown in FIG. 4, heat exchange surfaces 54 and 54A may be used for isolation cooling whereas all six transfer surfaces may be used for shutdown cooling duty. The shell of a conventional isolation condenser defines a volume large enough to house the shutdown cooling heat exchange surfaces. The positioning of the isolation condenser cooling heat exchange assemblies toward the bottom of the isolation condenser shell provides the longest operation period for isolation cooling heat exchange. Following a reactor isolation condition, isolation coolant inventory draw-down is caused by steam generation and venting of isolation coolant to atmosphere. This isolation coolant draw-down could gradually uncover upper tubes in the U-tube assembly and require replenishment of isolation coolant. Thus, the isolation condenser heat exchange assemblies may be positioned toward the bottom of the isolation condenser shell to provide the longest operation period possible before it is necessary to replenish isolation condenser coolant. Valves may also be included in the system to permit the steam to be provided exclusively to the lowermost heat exchange surfaces during isolation cooling. In this connection, it should be observed that isolation cooling occurs at very high pressures and temperatures, potentially up to reactor design pressures of 1250 psig and over 500.degree. F. As a result, the heat exchange assemblies for isolation cooling, i.e., tube assemblies, must be of a material and wall thickness capable of withstanding these operating conditions. On the other hand, the shutdown cooling mode occurs under moderately high pressures and temperatures, for example, 175 psig and below 212.degree. F. Thus, the heat exchange assemblies that are used exclusively for shutdown cooling may be fabricated from materials having lower pressure ratings and temperatures than those used for isolation cooling. Valves could also be used, (i.e.. pressure relief valves) to prevent exposure of the lower rated shutdown cooling heat surfaces to the isolation cooling operating conditions. It is to be appreciated that the invention capitalizes on the fact that the reactor does not require simultaneous shutdown cooling and isolation cooling. It is possible, however, for the shutdown cooling mode to follow an isolation cooling mode if the plant were shutdown because of the load rejection event. The conventional reactor does not normally initiate shutdown cooling until the reactor has been significantly depressurized and cooled down by the isolation condenser (or by some other pressure unloading system). This depressurization and cool down normally requires up to two or three hours. Thus, there is ample time for smoothly switching the reactor decay removal function from the isolation cooling mode to the shutdown cooling mode. Although the invention has been explained with reference to the foregoing embodiments, it should be understood that other changes and modifications may be made to the foregoing embodiments without departing from the scope or spirit of the invention. Alternative configurations may be used, such as a greater or smaller number of heat exchange surface assemblies, without departing from the scope of the invention. A plurality of cooling apparatus could be used depending on design requirements and overall cost considerations. The isolation condenser coolant and the cooling apparatus could be raw water coolant and could be piped to the shell side of the heat exchange surfaces under emergency conditions to give greater long term capacity for decay heat removal when auxiliary power supplies are limited. Secondary coolant could be piped to the shell side of the cooling apparatus or the secondary heat exchanger rather than to the tube side as previously discussed. The placement of isolation condenser coolant inlets, discharges, steam vent lines, drain lines, and so forth could also be optimized to meet specific applications. The isolation condenser coolant may also be circulated through the secondary heat exchange coolant during the isolation cooling mode to provide increased cooling of the hot reactor steam before the condensate is returned to the reactor as makeup coolant. This approach would augment the effectiveness of other emergency core cooling systems employed during shutdown. It should thus be understood that the invention is limited only by the appended claims.