1. Technical Field of the Invention
The present invention relates generally to strainer devices and, more particularly, to a strainer system for use in a nuclear sump. The strainer system of the present invention includes at least one primary strainer module which defines a primary strainer/filter surface. The primary strainer surface of the primary strainer module includes a debris interceptor which is cooperatively engaged thereto, and comprises a screen or mesh layer which may be outfitted with one or more pressure released or activated membranes.
2. Description of the Related Art
A nuclear power plant typically includes an emergency core cooling system that circulates large quantities of cooling water to critical reactor areas in the event of accidents. A boiling water reactor or BWR commonly draws water from one or more reservoirs, known as suppression pools, in the event of a loss of coolant accident. More particularly, water is pumped from the suppression pool to the reactor core and then circulated back to the suppression pool in a closed loop. A loss of coolant accident can involve the failure of reactor components that introduce large quantities of solid matter into the cooling water, which entrains the solids and carries them back to the suppression pool. For example, if a loss of coolant accident results from the rupture of a high pressure pipe, quantities of thermal insulation, concrete, paint chips and other debris can be entrained in the cooling water.
In contrast to a BWR, a pressurized water reactor or PWR, after a loss of coolant accident, typically draws cooling water from a reactor water storage tank and, after a signal, shuts off the flow from the storage tank and re-circulates this water through the reactor. In this regard, the pressurized water reactor has a containment area that is dry until it is flooded by the occurrence of an accident, with the emergency core cooling system using a pump connected to a sump in the containment area to circulate the water through the reactor. Nevertheless, the water that is pumped in the event of an accident will also usually contain entrained solids that typically include insulation, paint chips, and particulates. Thus, in both types of reactors (i.e., boiling water reactors and pressurized water reactors), cooling water is drawn from a reservoir and pumped to the reactor core, with entrained solids or debris potentially impairing cooling and damaging the emergency core cooling system pumps if permitted to circulate with the water.
In recognition of the potential problems which can occur as a result of the presence of entrained solids or debris in the coolant water of the emergency core cooling system, it is known in the prior art to place strainers in the coolant flow path upstream of the pumps, usually by immersing them in the cooling water reservoir. It is critical that these strainers be able to remove unacceptably large solids without unduly retarding the flow of coolant. In this regard, the pressure (head) loss across the strainer must be kept below the minimum required net positive suction head levels necessary to safely operate the recirculation pumps and maintain adequate cooling water flow to the reactor core. Strainers are commonly mounted to pipes that are part of the emergency core cooling system and that extend into the suppression pool or sump, with the emergency core cooling system pumps drawing water through the strainers and introducing the water to the reactor core. There has been considerable effort expended in the prior art in relation to the design of strainers to minimize both head loss across the strainer and debris which can bypass the strainer, for the desired coolant flow. Existing strainers often include a series of stacked perforated hollow discs or flat perforated plates and a central plenum through which water is drawn by the emergency core cooling system pump. The perforated discs or plates prevent debris larger than a given size from passing the strainer perforations and reaching the pumps.
As is apparent from the foregoing, large amounts of fibrous material can enter the circulating coolant water in the event of a reactor accident. This fibrous material, which often originates with reactor pipe or component insulation damaged during the design basis event enters the emergency core cooling system coolant stream in the event of a loss of coolant accidents indicated above, typically accumulates on the strainer surfaces and captures other particulate, paint chips, and other matter entrained in the flow. The resulting fibrous debris bed on the strainer surfaces can quickly block the flow through the strainer, even though the trapped particulates may be small enough to pass through the strainer perforations. More particularly, the debris accumulates in a fluffy density in and on the strainer until the strainer becomes completely covered with a fiber and particulate debris bed. Depending on the break location and its surrounding insulation configuration, the strainer may become so fully filled and coated with the fibrous insulation debris that it loses its complex geometric surface advantages, thus becoming a simple strainer. Throughout the event, some debris and components in the containment typically dissolve into solution and interact with chemical buffers present in the containment. At the same time, containment temperatures are trending down. This phenomenon causes certain chemical precipitates to form which eventually make their way to the strainer. Once they reach the strainer surface, these chemical precipitates effectively fill the interstitial spaces in the fiber debris bed and dramatically increase the debris bed density. This increase in density significantly increases the effective pressure drop experienced across the strainer. Additionally, there are numerous remaining unknown phenomenon, the effect of radionuclide's on the debris bed, micro-organism blooms in the warm, moist environment of the containment, etc. which must also be accounted for in a strainer's ability to meet its required performance criteria.
The prior art has attempted to address the above-described flow blockage effect by making the strainer larger, the goal being to distribute the trapped debris over more area, reducing the velocity through the debris bed, and further reducing the head loss across the strainer as a whole. This solution, however, is often undesirable since the available space in a reactor for a suction strainer is usually limited, and further because larger strainers are typically more costly. As a result, the situation sometimes arises wherein the expected debris load after a loss of coolant accident can dictate a need for strainers that are too large for the available envelope for them in the containment area. Moreover, large strainers are often more difficult work with and thus more costly to install. In addition, prior art emergency core cooling system strainers have been constructed in ways that make them somewhat expensive to fabricate.
In an attempt to address the aforementioned deficiencies associated with existing nuclear power plant strainer designs, Applicant developed the improved strainer systems described in its co-pending U.S. patent application Ser. No. 12/578,413 entitled INCREASED EFFICIENCY STRAINER SYSTEM filed Oct. 13, 2009, and Ser. No. 12/835,509 entitled INCREASED EFFICIENCY STRAINER SYSTEM filed Jul. 13, 2010, the disclosures of which are incorporated herein by reference. The present invention represents an improvement to these systems by providing a strainer system design which is specifically suited to manage the differential pressure experienced across the strainer system in nuclear power plants with various and unique debris loads, available space, flow rates, minimum water levels, etc. through the use of a uniquely configured debris interceptor in conjunction with a primary strainer module. Such differential pressure management ensures that the recirculation pumps continue to function in an uninhibited manner without the plant needing to perform large insulation modifications, with the design of the strainer system also being intended to reduce the strainer surface area required to meet differential pressure and debris bypass strainer performance criteria. Various features and advantages of the present invention will be described in more detail below.