Patent Number: 052590089
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, there are two sources of coolant to make up for losses of coolant in the nuclear reactor 22. A high pressure make up tank 32 is coupled by valves 34 between the coolant inlet or cold leg 38 and a reactor vessel injection inlet 42. However, the volume of the high pressure make up tank 32 is limited. A much larger quantity of coolant water is available from the in-containment refueling water storage tank (IRWST) 50, at atmospheric pressure due to vent 52, which opens from tank 50 into the interior of the containment shell 55. A valve 56 and a series of check valves 58 are provided for draining water from the refueling water storage tank 50 to the coolant circuit 62. Additional check valves and/or motor operated valves 64 are provided for recovering water from a sump 68 located inside the containment. However, these valves 58, 64 require that the reactor be fully depressurized in order to allow injection of coolant. According to the embodiment of the invention shown in FIGS. 1 and 2, a nuclear reactor 22 is depressurized by venting the coolant circuit into the containment shell 55 in a number of stages of decreasing pressure. For example, three initial stages are achieved by opening valves 82 coupled via spargers 84 between the coolant circuit 62 and the interior of the containment shell 55, the respective valves 82 in each leg being opened at successively lower levels of the high pressure makeup tank 122 and preferably being coupled in parallel legs along conduits 86, which are progressively larger for the successive stages. A final stage of depressurization is achieved by opening a valve means 92 which couples the coolant circuit 62 directly into the containment shell 55. The goal is to reduce the pressure to atmospheric pressure as quickly as practicable, whereupon coolant can be added to the coolant circuit by gravity feed at ambient pressure in the containment 55, while preventing severe thermal and hydraulic loads, undue loss of coolant, and other dangerous effects. Preferably, in each case, two serial valves are provided for each valve means along a respective conduit leg, as redundant shutoff means for safety purposes. Such valves are normally intended to be operated simultaneously, and accordingly such valve pairs or sets are described herein simply as valves. Staged depressurization helps to reduce thermal and hydraulic loading. It also makes inadvertent use of the depressurization system 96 less severe. A relatively small depressurization flowpath is opened initially, and the flowpath is enlarged in stages. This is accomplished by opening successively larger flowpaths upon reaching decreased pressure setpoints, and also can be accomplished by timed opening of the valves 82, 92 for individual flowpaths. FIGS. 1 and 2 are schematic representations of the depressurization system 96 and the reactor 22, and FIGS. 3 and 4 are physical representations. The same reference numerals have been used in all the figures to identify corresponding elements. The depressurization system 96 reduces the pressure in a nuclear reactor 22 having a reactor vessel 46 disposed in a containment shell 55 and inlet piping 102 and outlet piping 104 coupled to the reactor vessel 46. At least one steam generator 110 is coupled between the outlet 104 and the inlet 102 for extracting useful power, typically driving an electrical generator. The reactor vessel 46, inlet/outlet conduits 102, 104 and steam generator connections together define a recirculating coolant path or circuit in which water heated by the nuclear fuel is circulated under pressure. A plurality of depressurizer valves 82, 92 are coupled in fluid communication with the coolant circuit 62 and at least one sparger 84 in fluid communication with an inside of the containment shell 55. The valves 82, 92 are provided with control means and/or operators for successively opening the depressurizer valves 82, 92 to effect depressurization. As additional valves or groups of valves open in stages, the coupling between the coolant circuit 62 and the inside of the containment shell 55 is increased. At the same time, the pressure in the coolant circuit 62 decreases. The depressurizer valves 82, 92 can be successively opened on actuation signals from pressure responsive valve controls or, preferably, via level responsive controls including sensors 122 associated with the high pressure makeup tank and arranged such that each successive one or each successive group of depressurizer valves 82, 92 opens at a progressively lower level setpoint. Preferably, a pressurizer tank 130 is disposed in the containment shell 55, the pressurizer tank 130 having a bottom head 132 coupled by a conduit 134 to a coolant outlet 140 of the reactor, also referred to herein as the hot leg of the coolant circuit 62. A top head 142 of the pressurizer tank 130 is coupled to at least one of the depressurizer valves 82. The depressurization valves 82 for the higher level opening stages, as shown in FIG. 1, are coupled to the inside of the containment shell 55 through a sparger 84, i.e., a fluid outlet opening at a submerged point in a water tank. The sparger has a series of holes 114 submerged in a tank of water 50, namely the in-containment refueling water storage tank. Tank 50 is vented to the inside of the containment shell 55, i.e., the tank 50 can be at atmospheric pressure. Preferably, the tank 50 is arranged by suitable valving 152 to empty via gravity into one or more of the reactor vessel 46, the coolant circuit 62 and/or the sump 68 in the bottom of the containment shell 55, thereby to cool the core in the event of an accident such as a loss of coolant accident. Inasmuch as the depressurization valves 82 for the higher level stages vent into the refueling water storage tank 50 and the tank 50 vents into the containment shell 55, the pressure of the reactor coolant circuit 62 is vented through to the containment shell 55 in this arrangement. The depressurization valves 82, 92 are coupled to the containment shell 55 via conduits 162, 164 which are progressively larger for depressurization valves 82, 92 opening at progressively lower pressures. Therefore, not only is the coupling between the coolant circuit 62 and the containment shell 55 increased due to additional conduits 162, 164 opening as pressure in the coolant circuit 62 decreases. Furthermore, the size of the flowpath for a given opening stage is larger than the flow path opening of the previously opening stage. The result is a gradual but expeditious decrease in coolant pressure. To render the change more gradual, the depressurization valves 82, 92 can be motor operated or otherwise arranged to open from fully closed to fully open over a period of time, whereby peak flows through the depressurization valves 82, 92 are limited. At least one of the depressurization valves 92 or group of valves 92 defines the last stage of depressurization, i.e., this valve or group is openable at a lowest level of the high pressure makeup tank. Valve(s) 92 couple directly between the coolant circuit 62 and the containment shell 55 by conduits 164. Preferably, this last stage opens a flowpath between a coolant outlet 140 of the reactor (the hot leg of the coolant circuit) and the containment shell 55. The depressurization valves 82, 92 preferably include a plurality of parallel valve legs 162 coupled between the coolant circuit 62 and spargers 84, submerged in the tank 50, or the containment 55 through conduit 164, coupling progressively larger ones of conduits 162, 164 to vent the pressure, and openable at progressively lower pressures as indicated by the level in the high pressure makeup tank 32. Spargers for boiling water reactors, intended to reduce pressure in the coolant circuit but not to bring the pressure to the atmospheric pressure in the containment, are typically located about 17 feet underwater, which creates a large back pressure. Additionally, flow resistance causes a further back pressure. The spargers 84 according to the invention are also submerged. However, in addition to venting into the containment 55 through spargers 84, the depressurization system 96 of the invention also vents directly to the containment 55 in the last stage of depressurization. The first stage, which vents into a sparger 84 from a pressurizer tank 130 coupled to the coolant outlet 140 of the reactor 22, has a relatively small conduit size, and thus reduces initial shock to the coolant circuit 62 when depressurization commences. The subsequent stages use larger conduit sizes. Additionally, it is possible to use a relatively slow opening form of valve 82, 92 to soften the impact of depressurization. Preferably, a first stage valve 82 opens a 4 inch (10 cm) internal diameter conduit 162 and takes approximately 20 seconds to open fully: and the second and third stage valves 82 open 8 inch (20 cm) conduits and take about 90 seconds to open fully. A slow opening form of valve reduces the peak flow rate upon opening of the valve, and thus conserves coolant. With the opening of the valves, and the successive opening of further stages having progressively larger conduit size, there is a slow and gradual increase in coupling between the coolant system 62 being depressurized and the inside of the containment shell 55. The foregoing conduit sizes are exemplary, preferred for a reactor having a capacity of about 600 MWe. The sizes can be scaled up or down to accommodate other reactor capacities and the like. Similarly, the staging can have a different number of stages than the number of stages shown in connection with the presently preferred embodiment. The final stage, which in the illustrated embodiment is the fourth stage, vents the coolant circuit 62 directly into the containment 55. Preferably, the fourth stage valve 92 opens a 12 inch (30 cm) conduit 164. This last stage valve or group of valves 92, and the conduit arrangement 164 therefor, are different in several respects from the first three stages. The fourth stage is coupled from the hot leg 140 of the coolant circuit 62 (i.e., the reactor output) directly into the containment shell 55, instead of through the pressurizer tank 130 and/or through underwater spargers 84. Although all the stages are coupled in fluid communication between the containment 55 and the hot leg 140 of the reactor, the stages with smaller conduits and/or which couple through the pressurizer 130 and through the spargers 84 have a characteristic back pressure due to the flow restrictions inherent therein. The fourth or final stage effectively brings the coolant circuit 62 down substantially to the atmospheric pressure existing in the containment shell 55. The hot leg 140 of the coolant circuit 62 (i.e., the reactor outlet) is the point where the coolant water is hottest in the circuit. Under operational conditions, the water in the hot leg is about 600.degree. F. (330.degree. C.). The water returning from the steam generator 110 along the cold leg 38 is at approximately 550.degree. F. (290.degree. C). Whereas the water is taken off by the depressurization system at its highest temperature, the reactor coolant in more effectively utilized in providing cooling. Both water and steam may be vented. As soon as the first stage opens, the system starts to draw water out of the hot leg 140, into the pressurizer 130, then out of the system and into the containment 55. Although the pressurizer 130 is a rather large tank, even with venting through the spargers 84 the flow through the pressurizer 130 is not sufficient to carry all the contents of the pressurizer 130 through to the refueling water supply tank 50. The water in the pressurizer 130 thus produces a back pressure which limits flow during the first three stages of depressurization. The first three stages thus have back pressure characteristics which the fourth stage does not have. These include the fluid pressure head of the water in the refueling water tank 50 above the spargers 84, the water elevation in the pressurizer 130, and the line flow resistance caused by the relatively smaller size of conduit 162 as compared to the final stage conduit 164. The depressurization valves for effecting respective stages of depressurization, and/or the operators which are arranged to open the valves, can be chosen with respect to the differential pressure at which the valves are expected to open during depressurization. In particular, the valve and/or operator for the final stage of depressurization is preferably openable only below a predetermined differential pressure, thereby minimizing the possibility that the last stage will be opened prematurely. Steam and water in the hot leg 140 of the reactor 22, as vented directly in the fourth stage, is carried through a sufficiently large opening defining the hot leg 140 that very little flow resistance occurs between the reactor vessel 46 and the point of venting through the fourth stage conduit 164 and valve 92. Thus the direct connection of the hot leg 140 with the containment 55 in the fourth stage, and the large diameter of the hot leg 140 and fourth stage conduit 164, are such that the pressure in the reactor vessel 46 and the coolant circuit 62 comes substantially fully down to the ambient atmospheric pressure in the containment 55. It is of course the pressure in the reactor vessel 46 which is most important, because the intent of depressurization is to enable water to be drained into the reactor by low pressure means, especially by gravity from the in-containment refueling water storage tank 50. Preferably, the stages each have a plurality of valves 82, 92 coupled in a series, for example as shown by valves 168. Both valves 168 in the series connection are normally closed. Having two valves in series minimizes leakage and makes it unlikely that any of the stages will be opened or left open inadvertently. Both valves 168 in each stage can be opened via actuation signals from a controller 172. The first three stages can have motor controlled valves 82, powered or geared to achieve the timed opening discussed above. Preferably these valves 82 are powered from batteries. The fourth stage can use a different type of valve to prevent common mode failure and thereby increasing reliability. For example, the fourth valve 92 can be operated pneumatically from a dedicated air cylinder. Alternatively, the fourth stage valve 92 can be explosively operated. For further redundancy, in a reactor having more than one steam generator circuit, a valve and different type of operator can be associated with each of a plurality of hot legs 140 coming out of the reactor vessel 46, again avoiding the possibility of common mode failure. Where there are two loops in the steam generator portion of the reactor design, the pressurizer 130 and its initial stage valves 82 can be coupled to one hot leg, and the final (e.g., fourth) stage valve 92 can be coupled to the other hot leg. By coupling the initial stages through the pressurizer and the final stage directly, flow is limited during the initial stages, and the pressurizer surge line can be smaller. Referring to FIGS. 3 and 4, each of the coolant loops is partly enclosed in a coolant loop compartment 175 having concrete walls 177. The fourth stage can open into a respective coolant loop compartment 175. The concrete walls 177 provide shielding for plant personnel, and it is unlikely that plant personnel will be in the area of the coolant loop compartment 175 during plant operations, because radiation levels there are high. One disadvantage, however, is due to the fact that there is some equipment housed in the cooling loop compartment, such as instrumentation coupled to the loops to monitor temperature and flow, and electrical connections for the reactor coolant pumps. Whereas the discharge from the depressurization valve 92 may wet such equipment, some cleanup would be needed before restarting the plant following depressurization. As an alternative, the fourth stage can be coupled to an outlet at the refueling cavity 181 (see FIGS. 3 and 4). The refueling cavity 181 is a stainless steel lined pit designed to be flooded during refueling operations, and is an advantageous place to direct the output of the fourth stage. The reduction of pressure through the first three stages can reduce the pressure in the coolant circuit 62 to about 50 psi prior to opening of the fourth stage. The opening of a relatively large conduit 164 to the containment in the fourth stage brings the coolant circuit 62 down to a low pressure, without producing large discharge forces and flow rates. Depending on the extent of boiling still occurring in the reactor vessel 46, some minimal pressure may remain over atmospheric pressure; however, the staged depressurization efficiently reduces the pressure to a point where it is possible to add coolant from a refueling tank at atmospheric pressure. Sufficient water is contained in the refueling water storage tank that its gravity head alone can overcome this remaining pressure. Another feature of the embodiment shown in the drawings is that the respective valves 82, 92 in each stage can be opened individually and then closed during normal plant operating conditions to verify that the valves 82, 92 are operating properly. As shown in FIG. 2, several small solenoid valves 192 can be provided in test legs 194 leading to the sparger 84, operable to isolate each of the serially coupled valves in the three stages for testing at reduced pressure conditions. Pressure conditions are reduced because the test conduits 196 comprise small (e.g., 0.75 inch or 2.0 cm) internal diameter conduits. Testing at low differential pressure reduces the possibility of causing valve leakage as a result of a test. For the first three stages, two test valves 192 can isolate and test any of the six valves in the first three stages. A similar arrangement for testing the fourth stage is shown in FIG. 3. In connection with testing of the fourth stage, a test valve arrangement of this type is required for testing, because the fourth stage valves are designed for operation at lower pressures than those typical of plant operation. The test valves can be used during a cool-down leading to a refueling outage, for conducting a more rigorous test of the automatic depressurization system and its valves. The test can be conducted, for example, at intermediate pressures in the range of 400 to 600 psig. For such a test, the test valves 192 can be arranged such that each tested valve 82 or 92 along the depressurization paths opens under a large differential pressure, by operating selected ones of the test valves 192. For example, the upstream depressurization valve in each pair can be tested by opening the test valve downstream thereof, leading to the spargers 84. The downstream depressurization valve in the pair can by tested by opening the upstream test valve to obtain elevated pressure leading into the depressurization valve. In each case, the small conduit size of the test system limits the flow occurring during the test, and reduces the impact on the downstream refueling water storage tank as well as the atmosphere in the containment, in particular keeping radiation to acceptable levels. Routing the flow to an appropriate drain means apart from the refueling water storage tank 50 is another possibility. The invention having been disclosed, a number of alternatives will now become apparent to those skilled in the art. The foregoing embodiments are illustrative, and are not intended to limit the particulars of the invention in which exclusive rights are claimed. Reference should be made to the appended claims rather than the discussion of preferred embodiments, in order to assess the scope of the invention in which exclusive rights are claimed.