Patent Number: 048184756
Section: description

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified block diagram for a simplified boiling water reactor 2 of prior art configuration. Reactor 2 includes a reactor pressure vessel 4 which has disposed therein a reactor core 6. The reactor core is covered by cooling water 8 which is supplied and circulated during normal operation. Normal operation can be simply summarized. As shown in FIG. 1, steam from the reactor vessel 4 is input to turbine 24. Turbine 24 is coupled to generat or 30 through the main rotating shaft 32 of turbine 24. The power output of generator 30 is coupled to the main station power supply 50. Exhaust of turbine 24, in the form of wet steam, is fed to condenser 44. Condensate from condenser 44 flows to the suction of condensate pump 18. Condensate discharged from condensate pump 18 is fed to the suction of feed pump 16. Feed pump 16 elevates the head of the feedwater to exceed reactor vessel pressure and supplies feedwater through feedwater line 38 back to the reactor vessel 4, thereby completing the steam cycle. When a loss-of-coolant inventory accident occurs, the reactor vessel is depressurized through depressurization valve 90 and vent line 92 to suppression pool 10. When depressurization has progressed to an appropriate degree, reactor 2 becomes cooled by the gravity injection of suppression pool coolant through check valve 94. Backup cooling is conventionally provided using power from a main power supply 50 to power feedwater (cooling) system 200. The emergency power may be provided from either the main coupled generator 30, the grid, or from diesel generators (not shown). FIG. 2 is an illustration of an improved emergency core cooling system according to one embodiment of the invention. FIG. 2 shows the conventional prior art boiling water reactor 2 having the emergency core cooling system according to one embodiment of the invention. FIG. 2 shows the conventional prior art boiling water reactor 2 having the emergency core cooling system featuring a low pressure coolant injection capability. A steam output from the turbine-generator 24 inputs to condenser 44. A condensate storage tank 41 supplements the inventory of condensate within the condenser 44 to replenish water inventory within condenser 44 whenever reactor steam supply becomes isolated. Output of condenser 44 is coupled to condensate pump 18. The output of condensate pump 18 has two separate destinations. The first conventional destination is to the suction of feed pump 16. The second destination is to the upstream side of a check valve 120 on a bypass line 22. The output of check valve 120 is coupled to the interior of reactor vessel 4. The bypass line 22 and check valve 120 may be configured to tie into feedwater line 38 or into a dedicated injection inlet to vessel 4. During normal operation, pressure in the reactor vessel 4 exceeds pressure at the output of condensate pump 18. Check valve 120 in line 22 prevents reverse flow from reactor vessel 4 toward condensate pump 18. This condensate pump 18 and feed pump 16 function normally in series to provide conventional feedwater flow. As further shown in FIG. 2, an auxiliary generator 34 is coupled to a main shaft of the coupled main generator 30 and turbine 24. The output of auxiliary generator 34 is coupled to an input to power supply 36. Power supply 36 is dedicated to driving pump motor 28. This condensate pump 18 has a dedicated power supply from generator 34. Pump motor 28 drives condensate pump 18 using power generated by auxiliary generator 34. Power supply 36 is normally directly connected to motor 28 without any intervening switching or bus transfer required. Auxiliary generator 34 provides normal short-term-response power for motor 28 when condensate pump 18 is used during normal initial core cooling. Auxiliary generator 34 converts the rotational energy of main turbine and main coupled generator into electric power, including converting the spindown momentum during loss-of-coolant inventory accidents. Operation of the condensate pump 18, bypass line 22 and check valve 120 during a loss-of-coolant inventory accident can be understood. Specifically, and even though main generator 30 is inoperative and completely decoupled from the main station power supply 50, auxiliary generator 34 will continue to generate power from the available and coupled spindown momentum. This being the case, condensate pump 18 will continue to operate. Discharge of the condensate pump 18 will temporarily be interrupted. Such interruption will occur because main feed pump 16 will likely be offline because of the power failure. Thus condensate pump will output its discharge head to check valve 120. Because of the loss-of-coolant accident pressure in the reactor will fall. When pressure in the reactor reaches a pressure below the shutoff head of the condensate pump, the flow of coolant into the reactor will resume. Such flow will be from the discharge of the condensate pump 18, through line 22 and check valve 120, and directly into the reactor vessel. As will be hereafter emphasized, this flow of coolant to the reactor replacing lost coolant will occur much earlier than has heretofore been possible; it will occur from the moment when reactor pressure falls below the shutoff head of condensate pump 18. I have preferably used the resident condensate pump 18 to supply coolant to the reactor vessel 4. The reader will realize that in some nuclear reactor designs it may be desirable to have a separate dedicated low pressure injection pump 48 and driving motor 58 to accomplish this function. Such an embodiment is illustrated in FIG. 3. FIG. 3 is a coolant flow of an alternative embodiment of the invention. Low pressure injection pump 48 intakes coolant derived from condenser 44 and discharges the coolant through injection line 23 and injection check valve 21 into reactor vessel 4. It is required that low pressure injection motor 58 and pump 48 be signalled and brought on line responsive to conventional prior art reactor water level indicators. Auxiliary generator 34 provides power to motor 58 driving low pressure injection pump 48. While this alternative embodiment represents potential cost increases resulting from the addition of a new pump/motor unit and its connecting piping, there are potential major net cost reductions to the resultant overall system depending on the sizing of pump/motor unit 48/58. Operation of the embodiment of FIG. 3 is easily understood. A bypass low pressure coolant injection (LPCI) line 23 is provided. Line 23 incorporates a normally-closed LPCI flow injection valve 21 located upstream of an LPCI injection nozzle. LPCI injection nozzle is positioned on the reactor vessel 4 and communicated to the discharge side of LPCI pump 48. Under certain conditions, a loss-of-coolant accident, loss-of-station power, loss-of-coolant inventory accident, or another such emergency core cooling event could (in worst-case scenario) cause loss of the normal feedwater supply to the reactor. Reactor 4 through conventional prior art sensors senses a loss-of-coolant inventory condition and begins depressurization through sequentially-opened depressurization valves 90 once the water level inside the reactor 4 reaches the Level-1 level. When reactor 4 has depressurized to approximately the pump shutoff head developed by pump 48, the bypass line injection valve 21 opens to admit pumped condensate to the reactor. As the reactor depressurizes further, the LPCI flow tends to increase--this effect being caused by the characteristic of centrifugal pumps to provide increased volume throughout as pump back-pressure decreases--but may (depending on LPCI motor controls) be partially offset by the reduction in rotational speed (referred to as coastdown or spindown) of the main turbine-generator as a consequence both of turbine-generator bearings and windage losses as well as energy removed for pumping. For the most-challenging design accident scenarios, the depressurization from reactor normal conditions (1020 psig) down to the pressure at which LPCI flow injection can begin (600 psig or lower, depending on design optimization for LPCI pump 48) takes approximately one minute. Thereafter, LPCI flow into the reactor vessel begins. For the same challenging accident scenarios, the reactor depressurizes over the next four minutes down to a pressure at which water from suppression pool 10 begins flowing into the reactor. Referring back to FIG. 1 for conventional simplified boiling water reactors, the main turbine-generator would supply power to the main station (site) power supply. The site power supply 50 would supply power to feed pump 16 and to a condensate pump 18 during normal operations. For backup coolant inventory replenishment, grid power sources and/or non-safety-grade diesel generators, as available, are coupled via bus transfer to the feed pump and to the condensate pump to provide alternate power for the requisite pumping during a loss-of-coolant inventory accident. It will therefore be realized the power supply for backup emergency cooling according to the disclosed invention is inherently more reliable over the duration of power supply need, because of the avoidance of requiring start-up of diesel generators and/or because of the avoidance of bus transfers from electrical buses that are subject to externally-caused power interruptions. As shown in FIG. 2, the emergency coolant injection power supply for the low pressure coolant injection capability is furnished by dedicated, unswitched normal and emergency power from auxiliary generator 34 to the condensate pump 18. The auxiliary generator 34 also can supply normal and emergency power to selected other emergency core cooling system loads 60. During normal operation, the feed pumps, which draw substantial power (on the order of several megawatts) are fed from the site power supply 50 over normal lines to a power input to drive motor 26 of feed pump 16. According to the invention, it is possible to couple dedicated normal and short term emergency power from the auxiliary generator 34 over a power supply line to an input to a plurality of individual motors and their associated coolant injection pumps. For example, where feedwater is used to power the recirculation flow in a boiling water reactor--such as in the case of a feedwater-driven jet pump recirculation system BWR--the feature of having short-term continued feedwater injection capability is highly desirable. The invention brings about the capability of being able to maintain coolant forced circulation in such reactors over the short term of depressurization experienced in a loss of coolant accident. FIG. 4 is a graph that depicts the reactor depressurization curve for a conventional simplified boiling water reactor that uses a venting system together with a gravity-driven cooling system. This same graph also depicts improved system in accordance with the invention. This improved system uses a venting system, a gravity-driven system, and short term low pressure coolant injection capability to inject condensate into the reactor vessel during the depressurization phase, at the early part of a loss-of-coolant inventory accident. As shown in FIG. 4, time t.sub.0 represents the time at which an event requiring emergency core cooling occurs. Before and until time t.sub.0 the pressure in the reactor vessel will be approximately 1000 psig at the point in time when venting is initiated. According to the prior art boiling water reactor gravity-driven cooling system, the reactor vessel would be depressurized down to about 30 psig over approximately a 10-12 minute interval using the venting system. The improved performance of this invention is illustrated in FIG. 4 in broken lines. Using the protocol of either FIG. 2 or FIG. 3, condensate is pumped back into the reactor vessel using the emergency power supply system and condensate (low pressure) pumps. Such introduction of condensate occurs when the reactor/injection vessel pressure reaches the shutoff head for the condensate (low pressure) pump which is around 600 psig. With this earlier induced coolant flow, it is important to realize the depressurization curve for the reactor vessel can be accelerated. Specifically, the reactor vessel can be depressurized down to 30 psig at some t.sub.2 which is several minutes earlier than for the conventional system. The injection of coolant and the more rapid resulting depressurization facilitates a reduction in the volume of coolant required in the TAF to Level-1 zone and in the suppression pool. This reduced depressurization also permits a reduction in the number of valves and the venting capacity required for the depressurization system. The water provided by the LPCI pump (per FIG. 3 embodiment) or by the condensate pump (per FIG. 2 embodiment) using the spindown energy of the turbine generator during the four-minute period provides reliable, low cost, short term emergency coolant. This coolant undergoes injection with considerable margin relative to the volumetric inventory between TAF and Level-1. The reader will understand further that the total volume of water in the reactor is subjeot to reduction. This reduction occurs for at least the following three reasons. First each LPCI pump (condensate pump) is producing nominally 50% rated feedwater flow. Second, the depressurization period necessary to bring the vessel pressure down to 30 psig, over which reactor inventory depletion occurs by venting coolant through the depressurization valves, is limited to four minutes as compared to 10 to 12 minutes with no injection. Finally, multiple LPCI and/or condensate pumps are available to recharge the reactor with coolant. As earlier discussed, only one minute of rated feedwater flow is necessary to supply the requisite water volume. The volume represented in the conventional SBWR reactor between TAF-and-Level-1 can be correspondingly reduced. Thus, the resulting flow rate (50%) times the duration (4 minutes) provides a minimum of 2 minutes of rated-power-flow even assuming that only one LPCI pump (or condensate pump) is available. It is noted that during the initial moments of an event requiring emergency core cooling, prior to start of LPCI injection, the recirculation line customarily provided on such pumps recycles a small portion of discharge flow back around to pump suction. (This type of piping configuration line is a conventional engineering practice which prevents unwelcomed overheating of deadheaded pumped fluid). According to the invention, the amount of required injection flow is thus seen to be small relative to the BWR/3 through the BWR/6 model BWR designs that require coolant flow injections uninterrupted for indefinitely long time periods. Since the integrated pumping energy demand over the period of interest--said to be no longer than five minutes even under worst-case event scenarios--is demonstrably small, the invention is able to use the spindown energy of the main turbine-generator as an assured, virtually cost free source of emergency power. The turbine-generator of the typical BWR power station, separated from its load, typically requires no less than 40 minutes to spindown to speeds at which the turbine-generator turning gear cuts-in to maintain slow revolutions on the turbine-generator shaft. This coastdown is produced by the combination of frictional drag from bearings, plus windage losses by the turbine-generator blades spinning in the low pressure (typically 2-3 ins.Hg) maintained by the main condenser. For example, for a 600 MWe turbine-generator, approximately 1.5 MWe-equivalent drag is produced at the 1500 rpm (50 cycle)/1800 rpm (60 cycle) initial free-rotation speed. (Actually, when the turbine-generator is separated from its load, residual steam in the turbine-generator casing momentarily causes the turbine-generator to go into an overspeed mode, so that coastdown actually begins from a still higher rpm.) The energy extracted by the shaft-coupled auxiliary generator(s) and consumed by the electrically-coupled LPCI and/or condensate pumps amounts to the same order-of-magnitude rate as for the turbine-generator bearing and windage losses. Thus, the 4 or 5 minutes integrated energy drawn by the LPCI and/or condensate pumps can be seen to be modest relative to the integrated energy available from the turbine-generator coastdown. In the event certain specific applications of the invention were to find an insufficiency of rotational energy in the turbine-generator system to accomplish the full desired short-duration coolant injection pumping burden, a properly sized flywheel can be added to the turbine-generator system to provide the additional rotational energy required. It is within the scope of the invention to provide emergency power from the auxiliary shaft-coupled generator to supply short-term power to the adjustable speed drives of the feedwater pumps which are located downstream from the condensate pumps in the feedwater train. As a result, feedwater flow can be continued into the reactor under loss-of-offsite-power events in which the reactor feedwater injection lines do not become shut closed. Under these conditions, the continued supply of feedwater could be critical to avoiding violation of certain safety-limit margin conditions. As one example, if the core recirculation flow for the SBWR reactor were of the forced-circulat ion type based on feedwater-driven jet pumps, then supplying drive power to the feedwater pumps in the manner described by this invention would produce the cited advantage. It is also possible to use the auxiliary generators to power other existing, or new, emergency core cooling loads. These loads can include opening or closing certain motor-operated valves, or providing power for forced injection of coolant from the elevated suppression pool into the reactor vessel to accelerate depressurization of the reactor vessel. It is also possible to conserve the useful energy of the short term power supply, for example, by not switching the LPCI pump on (in the FIG. 3 embodiment) until the reactor vessel pressure has fallen below the shutoff head for the LPCI pump. Further, it is also possible to use feedwater pumps and condensate pumps either in combination or in a staggered timing relationship depending on design constraints. It is also possible within the scope of the invention to use separate dedicated LPCI pumps and injection lines that tie into the normal condensate line. It is also within the scope of the invention to couple power from the auxiliary generator to a main station transfer bus. It is possible to supply during normal operation the condensate pump with power derived directly from the main coupled generator feeding this main transfer bus. After occurrence of a loss-of-coolant inventory accident, the main bus could switch so that power would be provided from the auxiliary generators. This approach obviously lacks the higher reliability feature of those embodiments not requiring any switching. Other changes and modifications to the disclosed embodiments other than the foregoing may be made as will be readily apparent to those skilled in the art within the scope and spirit of the invention. Accordingly, it is applicants' intention that the invention be therefore limited only by the appended claims.