Patent Application
20090154634
1. Technical Field
The present disclosure relates to a check valve system for a nuclear reactor.
2. Description of Related Art
A conventional valve system for a nuclear reactor may include a vacuum breaker operatively connecting a wetwell and a drywell. The drywell may house a reactor pressure vessel of the nuclear reactor. The wetwell may contain a suppression pool that absorbs excess heat that is released into the drywell. As a result, steam from the drywell may be condensed by the suppression pool in the wetwell. When the pressure of the wetwell becomes greater than the pressure of the drywell, the vacuum breaker opens momentarily to reduce the pressure of the wetwell. However, the vacuum breaker may develop leakage paths over time after repeated opening/closing operations. Consequently, hot steam from the drywell may flow directly into the wetwell through the leakage paths rather than flowing through the intended passive containment cooling system. Accordingly, the leakage paths may result in a relatively high containment pressure following a loss of coolant accident.
The present invention relates to a passive check valve system. In one embodiment, the passive check valve system includes a vacuum breaker operatively connecting a wetwell and a drywell. A discharge pool may be provided in the drywell. A housing is provided in the drywell to enclose the vacuum breaker. The housing may have one or more discharge pipes extending into the discharge pool, and the volume of the discharge pool may be greater than an interior volume of the one or more discharge pipes.
The features and advantages of example embodiments will become more apparent in view of the detailed description in conjunction with the accompanying drawings. The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the intended scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. For purposes of clarity, various dimensions of the drawings may have been exaggerated.
It should be understood that when an element or layer is referred to as being “on”, “connected to”, “coupled to”, or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout the specification. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of example embodiments.
Spatially relative terms, e.g., “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, including those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The drywell 100 may include two volumes: (1) an upper drywell volume surrounding the upper portion of a reactor pressure vessel (not shown) and housing the main steam and feedwater piping (not shown), gravity driven cooling system pools and piping (not shown), passive containment cooling system piping (not shown), isolation condenser system piping (not shown), safety relief valves and piping (not shown), depressurization valves and piping (not shown), drywell coolers and piping (not shown), and other miscellaneous systems; and (2) a lower drywell volume below the reactor pressure vessel support structure (not shown) housing the lower portion of the reactor pressure vessel (not shown), fine motion control rod drives (not shown), other miscellaneous systems and equipment below the reactor pressure vessel (not shown), and vessel bottom drain piping (not shown).
The upper drywell volume may be a cylindrical, reinforced concrete structure with a removable steel head (not shown) and a diaphragm floor 114 constructed of steel girders with concrete fill. The reactor pressure vessel support structure (not shown) may separate the lower drywell volume from the upper drywell volume. There may be an open communication path between the two drywell volumes via upper drywell to lower drywell connecting vents (not shown), which may be built into the reactor pressure vessel support structure (not shown). Penetrations through the liner for the drywell head, equipment hatches, personnel locks, piping, electrical and instrumentation lines may be provided with seals and leak-tight connections.
The drywell 100 is designed to withstand the pressure and temperature transients associated with the rupture of any primary system pipe inside the drywell 100. The drywell 100 is also designed to withstand the negative differential pressures associated with containment depressurization events, e.g., steam in the drywell 100 condensed by the passive containment cooling system (not shown), gravity driven cooling system, fuel and auxiliary pool cooling system (not shown), and cold water cascading from the break following post-loss of coolant accident flooding of the reactor pressure vessel (not shown).
The wetwell 102 may include a gas volume and a suppression pool water volume (not shown). The wetwell 102 may be connected to the drywell 100 by a vent system (not shown) having a plurality of vertical/horizontal vent modules (e.g., 12 modules). Each module may include a vertical flow steel pipe with a plurality (e.g., three) of horizontal vent pipes extending into the suppression pool water (not shown). Each vent module may be built into the vent wall 120, which separates the drywell 100 from the wetwell 102. The wetwell boundary is the annular region between the vent wall 120 and the cylindrical containment wall (not shown) and is bounded above by the drywell diaphragm floor 114. Wetted surfaces of the liner in the wetwell 102 may be stainless steel, while the other surfaces may be carbon steel. The suppression pool water (not shown) may be located inside the wetwell region. The vertical/horizontal vent system (not shown) may connect the drywell 100 to the suppression pool (not shown).
In the event of a pipe break within the drywell 100, the increased pressure inside the drywell 100 may force a mixture of noncondensable gases, steam, and water through either the passive containment cooling system (not shown) or the vertical/horizontal vent pipes (not shown) and into the suppression pool (not shown) where the steam may be rapidly condensed. The noncondensable gases transported with the steam and water may be contained in the free gas space volume of the wetwell 102.
The safety relief valves (not shown) may discharge steam through their discharge piping (equipped with a quencher discharge device) into the suppression pool (not shown). Operation of the safety relief valves may be intermittent, and closure of the valves with subsequent condensation of steam in the valve discharge piping may produce a partial vacuum, thereby drawing suppression pool water into the exhaust pipes (not shown). Vacuum relief valves (not shown) may be provided on the valve discharge piping to limit reflood water levels in the safety relief valve discharge pipes, thus controlling the maximum safety relief valve discharge bubble pressure resulting from a subsequent valve actuation and water clearing transient.
There may be sufficient water volume in the suppression pool (not shown) to provide adequate submergence of the top of the upper row of horizontal vents (not shown) and the passive containment cooling system return vent (not shown). Adequate submergence may be beneficial when the water level in the reactor pressure vessel (not shown) drops to about one meter above the top of the active fuel (not shown) following a loss of coolant accident. In such an event, water from the suppression pool will flow into the reactor pressure vessel (not shown) via the equalization lines between the reactor pressure vessel (not shown) and the suppression pool. The water inventory, including the gravity driven cooling system, may be sufficient to flood the reactor pressure vessel to at least one meter above the top of the active fuel.
The passive containment cooling system (not shown) may remove decay heat from the drywell 100 following a loss of coolant accident. The passive containment cooling system may use a plurality of elevated heat exchangers (e.g., six condensers) located outside the containment in large pools of water at atmospheric pressure to condense steam that has been released to the drywell 100 following a loss of coolant accident. The mixture of steam and noncondensable gases may be channeled to each of the condenser tube-side heat transfer surfaces where the steam condenses, and the condensate may return to one or more gravity driven cooling system pools 118 via gravity. Noncondensable gases may be purged to the suppression pool (not shown) via vent lines (not shown). The passive containment cooling system condensers may be an extension of the containment boundary. The passive containment cooling system condensers may not have isolation valves and may start operating immediately following a loss of coolant accident. These relatively low pressure passive containment cooling system condensers may provide a thermally efficient heat removal mechanism. No forced circulation equipment may be required for operation of the passive containment cooling system. Steam that is produced, due to boil-off in the pools surrounding the passive containment cooling system condensers, may be vented to the atmosphere. The water inventory in the pools may be sufficient to handle at least 72 hours of decay heat removal.
One or more vacuum breakers 104 may be provided between the drywell 100 and wetwell 102. The vacuum breaker 104 may be a process-actuated valve. The purpose of the drywell-to-wetwell vacuum breaker system is to protect the integrity of the diaphragm floor 114 and the vent wall 120 between the drywell 100 and the wetwell 102 as well as the drywell structure and liner. The vacuum breaker 104 may also reduce or prevent back-flooding of the suppression pool water (not shown) into the drywell 100. Redundant vacuum breaker systems (not shown) may be provided to protect against the failure (e.g., failure to open or close when required) of a single vacuum breaker 104.
During the operation of the economic simplified boiling water reactor, a negative pressure differential may develop across the drywell 100 and the wetwell 102 such that the wetwell pressure is greater than the drywell pressure. As a result, the vacuum breaker 104 may open so that gas may flow from the wetwell 102 to the drywell 100 and equalize the drywell and wetwell pressures. Referring to
The spill-over hole 112 in the discharge pool 110 may ensure that the discharge pipe 108 is submerged to a distance of “h” or less. For example, “h” may be about 4-6 inches. The flow area of the discharge pipe 108 may be equal to or greater than the flow path area of the vacuum breaker 104. For example, the flow area of the discharge pipe 108 may be about 1 ft2. The length “H” of the discharge pipe 108 may be equal to or greater than about 2-3 times the submerged length of a passive containment cooling system vent line (not shown). For example, the length “H” of the discharge pipe 108 may be equal to or greater than about 2-3 m. The surface area of the discharge pool 110 may be equal to or greater than about 40-60 times the flow area of the discharge pipe 108. For example, the surface area of the discharge pool 110 may be about 40-60 ft2. Additionally, the volume of water in the discharge pool 110 may be greater than the interior volume of the discharge pipe 108.
As discussed above, the vacuum breaker 104 may open when the wetwell pressure PWW becomes greater than the drywell pressure PDW. When (PWW−PDW)>(0.445 psi+ρgh), where ρ is water density and g is gravitational acceleration, gas from the wetwell 102 may flow through the discharge pipe 108, clearing the column of water in the discharge pipe 108 and discharging into the drywell 100. As mentioned above, because “h” may be about 4-6 inches, the column of water in discharge pipe 108 may be about 4-6 inches.
On the other hand, when the drywell pressure PDW becomes greater than the wetwell pressure PWW in a conventional economic simplified boiling water reactor, gas from the drywell may directly flow into the wetwell if leakage paths have developed in the vacuum breaker. In contrast, with the check valve system according to the present disclosure, when the drywell pressure PDW becomes greater than the wetwell pressure PWW but (PDW−PWW)<ρgH, the higher drywell pressure may push the water from the discharge pool 110 into the discharge pipe 108, resulting in a water column with a static head of ρgx, (where h<x<H). Thus, the water column may counter the higher drywell pressure PDW, thereby preventing gas in the drywell 100 from directly flowing into the wetwell 102, particularly when leakage paths have developed in the vacuum breaker 104. Furthermore, because the length “H” of the discharge pipe 108 may be equal to or greater than 2-3 m, which may be 2-3 times the submerged length of the passive containment cooling system vent line (e.g., about 1 m), the drywell gas may be diverted to flow through the passive containment cooling system (not shown) at higher drywell pressures instead of overcoming the static head in the discharge pipe 108 and flowing through to the vacuum breaker 104.
While example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of example embodiments of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
