Patent Number: 059354399
Section: summary

FIELD OF THE INVENTION The present invention relates generally to the field of fluid recirculation systems incorporating suction strainers. More particularly, the present invention relates to internal core tube suction strainers for use with Emergency Core Cooling Systems of nuclear power plants. BACKGROUND OF THE INVENTION A critical function of Emergency Core Cooling Systems (ECCS) and other recirculation systems of nuclear power plants is to move fluids quickly and in large volumes to critical areas of the nuclear power plant in the event of accidents and emergencies. Integral to this critical function is the ability of strainers, filters, screens and other such devices associated with the systems to remove solids from the moving fluids while at the same time maintaining a sufficiently large volume of fluid flow. Suction strainers are used in suppression pools of Boiling Water Reactor (BWR) nuclear power plants to remove solids from the fluid stored in the suppression pools when the fluid is drawn into an Emergency Core Cooling System (ECCS) or other recirculation system. The goal is to have strained fluid substantially free from particulate matter, thereby minimizing pump degradation. In the United States and other countries, there are generally three different types of BWR nuclear power plants. The most common of these is the Mark I, followed by the Mark II and finally the Mark III. Each type of BWR nuclear power plant has a different suppression pool design. Generally speaking, the Mark I has a toroidal-shaped suppression pool, the Mark II has a simple circular tank, and the Mark III can best be described as a moat around the power plant. The differences in suppression pool design, as well as other plant design differences, have made the construction of a universally adaptable suction strainer unfeasible. Moreover, retrofitting upgraded suction strainers in existing BWR nuclear power plants is an extremely difficult task. A universal goal in the nuclear power plant field has been to increase the effective surface area of suction strainers so that the required volumetric flow rate of water can be delivered to the reactor following a loss of coolant accident (LOCA). A LOCA can result when a high pressure pipe ruptures with such great force that large quantities of debris from thermal insulation, coatings, concrete, and other sources can wash into the suppression pool, thereby clogging the suction strainer(s). As a result, the volumetric flow rate of cooling water delivered to the reactor can be drastically reduced which, in turn, can lead to reactor core overheating. The thrust of recent advancements in the suction strainer art has been directed toward designing suction strainers that can adequately filter such debris from the suppression pool fluid without becoming clogged (i.e., without leading to a reduction in ECCS pump volumetric flow rate). Following a LOCA, it is critical that the ECCS pumps can operate undegraded for extended periods of time. To achieve this result, large quantities of fluid, free from solids and other particulate matter, must reach the pumps. Recent advances have yielded suction strainers that can adequately filter debris from the fluid to limit pump degradation, but the goal of increasing the surface area of suction strainers so that greater volumes of water can be delivered to the reactor has been more difficult to achieve in some BWR plants. This is due to the second effect of a LOCA. The second effect of a LOCA in a BWR plant is the generation of post-LOCA hydrodynamic forces. Following a LOCA, high pressure steam is expelled from the reactor through structures known as downcomers which extend into the suppression pool. The resulting hydrodynamic forces created within the suppression pool place extreme loads upon any protruding structure within the pool, including suction strainers. While one function of the suppression pool is to condense this steam and thereby quickly dissipate these high pressures, significant hydrodynamic forces are still applied to the structural features and protrusions within the pool. In general, the greater the length and diameter of the suction strainer, the greater the resulting load on the strainer. For this reason, while it is easy to design a suction strainer having an increased surface area by increasing the overall length and diameter of the suction strainer, it is difficult to support such a strainer and, in many cases, to install such a strainer. Heretofore, various suction strainers have been employed for the general objective of filtering solids from the fluid stored within a suppression pool of a BWR nuclear power plant. One such suction strainer design is the cantilevered suction strainer. Such suction strainers typically extend into the suppression pool, are connected to the ECCS suction pipe at one of its ends, and simply cantilever off that suction pipe end. That is the only means of support. Due to the extreme loads which result from post-LOCA hydrodynamic forces and the limited load carrying capabilities of the ECCS pipe and pipe penetration (that portion of the suppression pool wall adapted to receive the ECCS pipe to place the ECCS pipe in fluid communication with the suppression pool), the overall length and diameter of the cantilevered suction strainer is limited. For a given strainer diameter, if the cantilevered suction strainer is too long, the torque applied to the suction strainer by the post-LOCA hydrodynamic forces can damage the suction pipe to which it is attached and/or the penetration through the suppression pool wall. An advancement in cantilevered suction strainer design is disclosed in U.S. Pat. No. 5,696,801. The suction strainer disclosed in this Application includes a filtering surface defined by a filtering structure that is attached to and built around an internal core tube. Reinforcing structural members extend outward radially from the internal core tube and provide support for the filtering structure. The external filtering structure is formed from a plurality of perforated plate assemblies positioned adjacent one another along the length of the core tube. The plate assemblies extend radially at alternating distances from the internal core tube thereby forming alternating protrusions and troughs. In this way, the surface area of the filtering surface is increased without increasing the overall length of the filtering structure. Openings in the internal core tube allow water from the suppression pool to be drawn through the filtering structure through perforations in the filtering surface. This configuration promotes controlled fluid in-flow along the suction strainer and substantially precludes the establishment of non-uniform localized entrance velocities through the filtering surface. The unique configuration of the external filtering structure enlarges the filtering surface area while minimizing the projected area of the suction strainer. Thus, more water can be drawn through this cantilevered suction strainer without increasing the overall distance this suction strainer extends into the suppression pool. While the overall filtering surface area of cantilevered suction strainers can now be increased, for a given strainer diameter, such suction strainers are still hampered by length limitations. Other advancements in the art have been made by Sulzer Thermtec. Sulzer Thermtec has designed an elongated simple cylindrical strainer that appears to use a rib-type cage to support a perforated plate. The perforated plate performs the straining function while the cage provides structural support for the plate. The strainer extends parallel to and along the wall of the suppression pool and is connected at one end to the suction pipe with a 90.degree. tee. There is no internal core tube. In order to withstand the extreme forces in the pool, the strainer is secured to the suppression pool wall at each of its ribs. Legs extending from each rib are apparently bolted or otherwise attached to the walls of the suppression pool. Again, installation can be time consuming and difficult, particularly if the suppression pool cannot be drained and if welding is required for strainer installation. Also, most BWR plants cannot accommodate a strainer diameter larger than 3 or 4 feet. While the suction strainers described above remove solids from the fluid stored within the suppression pools of BWR nuclear power plants, it appears that neither is capable of handling the LOCA generated debris, being installed within geometrically limited diameters, and being supported adequately. What is needed, therefore, but seemingly unavailable in the art, is a suction system that can (1) handle the postulated debris quantities, (2) be adequately supported and withstand LOCA generated forces, and (3) be installed without modifying the shell in the suppression pools of BWR nuclear power plants. Unlike a BWR nuclear power plant, a Pressure Water Reactor (PWR) nuclear power plant does not utilize a suppression pool. Rather, a PWR nuclear power plant, both light water and heavy water types, has a containment area which remains dry until an accident occurs. In conventional PWR nuclear power plants, an accident results in the containment area being partially flooded with water and the ECCS relying on a sump pump to circulate the water through the reactor. Typically, the water is filtered through a structurally protective trash rack and then through a finer debris screen to separate particulate matter from the water passed through the ECCS. The suction strainer of the type utilized in a BWR nuclear power plant is not typically found connected to a PWR's ECCS suction piping. Typically, the volume and rate of fluid (e.g., water) flow recirculating through the ECCS is dependent upon the size of the sump pit as well as the overall size of the inlet orifice and related trash rack and debris screen. Accordingly, the volumes and rates of fluid flow in a prior art PWR nuclear power plant were limited by the structural limitations of these sump structures and fixtures. What is needed, therefore, is a manner of retrofitting PWR nuclear plants to overcome the surface area limitations of configurations already existing and, thereby, maintain rates of fluid flow through the ECCS that is encumbered by LOCA generated debris. SUMMARY OF THE INVENTION Briefly described, the present invention comprises an improved suction system including, in the genus, a suction strainer and suction pipe assembly mounted to fluid delivery piping of an ECCS of a nuclear power plant or other such fluid delivery system, with the suction strainer being supported between two opposing ends, and in its species a plurality of alternate embodiments of end mounted suction strainers and suction pipe assemblies mounted to BWR suppression pool wall(s) or PWR containment area sumps through various support combinations. The present invention provides an improved suction strainer for use, in its preferred embodiments, in the suppression pools and/or containment areas of nuclear power plants which overcomes the design deficiencies of other suction strainers known in the art. While the discussion of the improved suction strainer and the suction system of this invention focuses heavily on its use and value in connection with BWR nuclear power plants, alternate embodiments of the strainer also have utility when employed with Pressurized Water Reactor (PWR) nuclear power plants of both the light water and heavy water type. Furthermore, it will be understood from these descriptions that the invention will find application in connection with nuclear reactor plants other than BWR and PWR plants, and in connection with other facilities having comparable fluid delivery systems. The suction system of this invention provides an inventive improvement to that suction strainer disclosed in U.S. Pat. No. 5,696,801, which patent is incorporated herein by this reference. The result is a novel method and apparatus for filtering solids and other particulate matter from the fluid (e.g. water) used in the emergency core cooling systems of nuclear power plants and other recirculation systems. The elongated suction strainer of the suction system of the present invention can be used to maintain design volumetric flow capacity through an Emergency Core Cooling System (ECCS) encumbered with LOCA generated debris, and other similarly encumbered recirculation systems. Several ECCS pumps can be connected to the same suction strainer via multiple suction pipes. If one pump fails, the other pumps will continue to draw fluid through a common suction strainer. The present suction system is also designed to be adaptable for use within the suppression pool of any type of existing BWR nuclear power plant, either the Mark I, Mark II or Mark III. Moreover, the suction system of the present invention can easily be adapted for use in other suppression pools for BWR nuclear power plants not yet designed. Alternate embodiments of the suction system of this invention are employable for use with both light water and heavy water pressurized water reactor (PWR) nuclear power plants, typically as part of a larger assembly/system which includes additional piping attached to and projecting from the sunken drain of the PWR. The flexibility of the present system will be further described in greater detail hereafter. These and other advantages which will be discussed more fully below, are attainable due to the novel construction of the suction system of the present invention. The suction strainer of the system is connected to the suction pipe of a recirculation system and removes solids from the fluid from, for example, a suppression pool of a BWR nuclear power plant. The strainer is constructed with an internal hollow core tube and an exterior filtering structure. The internal core tube is formed from a core wall which bounds a hollow core chamber. A plurality of fluid inlets spaced along the core wall place the chamber and suppression pool in fluid communication with each other. The exterior filtering structure is connected to and at least partially bounds the core wall and has a number of very small perforations passing therethrough. The filtering structure is further constructed from a plurality of plate assemblies spaced sequentially along and surrounding the core wall. When ECCS pumps in the recirculation system are activated, fluid from the suppression pool is drawn through the perforations in the exterior filtering structure, then through the fluid inlets in the core wall, and finally, into the core chamber. The strained fluid is then, for example, drawn through the suction pipe to the pump where it is either sprayed onto the reactor core and/or simply recirculated through a closed loop cooling system. The details regarding the structure of the suction strainer as hereinabove described are more fully set forth in U.S. Pat. No. 5,696,801, which has been incorporated herein by reference. This suction strainer structure is applicable to all of the embodiments of the suction system of this invention which will be described in more detail below. The core tube of the present invention has at least two functions. First, it acts as a suction flow control apparatus once fluid from, for example, the suppression pool or containment area has passed through the perforations in the exterior filtering structure. Second, and more pertinent with respect to the novelty of the present invention, the core tube is the primary structural support for the suction strainer of the present invention. Because of the rigidity of the core tube, the suction strainer can be constructed so that when the suction strainer is supported only at its two ends, it spans a length significantly longer than any other suction strainer known in the art. The suction strainer can be a unitary structure or it can be formed from several sections connected end-to-end in series along a common longitudinal axis. Regardless of how the suction strainer is formed, an elongated suction strainer results. When several suction strainer sections are used to create the elongated suction strainer, the adjacent ends of the suction strainer sections can be connected in several ways. In one embodiment of the present invention, each end of the core tube has a truncated core extension depending therefrom. The core tube extensions are, preferably, equipped with a typical pipe flange on the extension end that is remote from the core tube. Each flange is sized and shaped to abut the flange of an adjacent suction strainer core tube extension to facilitate connection of the suction strainer sections. Typically, the core tube extensions protrude away from the strainer plate assemblies a distance sufficient to permit connection of the flanges between the opposed plate assemblies of adjacent suction strainer sections. The flanges can be connected by welding or with any number of suitable devices such as, but not limited to, clamps, brackets, sleeves, bolts, or other fastening mechanisms. The flanges are also sized and shaped to be attached to flanges depending from the end of the suction pipe of the piping system of a BWR nuclear power plant or the ECCS pipe penetration in the suppression pool wall. The flange connections at the suction pipe are made in the same manner as other flange connections between suction strainer sections. When a suction pipe is connected to each end of the elongated suction strainer, the core tube provides support for the entire weight of the elongated suction strainer. When the elongated suction strainer is supported in this manner, the suction pipes and ECCS pipe penetration should be reinforced so that they can withstand the loads which will result from post-LOCA hydrodynamic forces. The flanges attached to the core tube extensions at the ends of the elongated suction strainer can also be secured to a cap that prevents access to the core chamber. These end caps are then fastened to existing structural supports within the suppression pool so that the loads from post-LOCA hydrodynamic forces are transferred from the strainer sections, through the structural supports, and to the suppression pool supports rather than directly to the suction pipes or their penetrations. When the elongated suction strainer is supported at both of its ends in this manner, the suction pipe connections can be made anywhere between the end caps. Generally speaking, the type of BWR/PWR nuclear power plant, the suppression pool (or containment area and sump pit) geometry and the ECCS pipe configuration will dictate how the suction pipe connections are made, the maximum length of the strainer sections used and the maximum length of the resulting elongated suction strainer employed. In certain BWR nuclear power plant suppression pools, the suction pipes are connected to a pipe fitting, such as an elbow or tee, located between an end of the elongated suction strainer and the structural support within the pool. In an alternate embodiment of the present invention the suction strainer sections are formed with flangeless core tube extensions. The suction strainer sections are aligned end-to-end along a common axis so that the core tube extensions of adjacent suction strainer sections are directly in line with and in contact with each other. The core tube extensions are then connected together with welds, brackets, clamps or other fastening devices. The suction pipe connections are then made in the same manner as described above with respect to the first embodiment of the present invention; the one difference being that the core tube extensions at the ends of the elongated suction strainer may not have flanges. Thus, these suction pipe connections will also be made using welds, brackets, clamps or other fastening devices. In another embodiment of the suction system of the present invention, the suction pipe connections to the elongated suction strainer are made at a 90.degree. angle with respect to the longitudinal axis extending through the internal core tube. To make this connection, a weld-o-let T-connection is used. The weld-o-let T-connection preferably has a core tube portion and a suction pipe portion. The core tube portion has a core wall surface that defines a bore therethrough. A plurality of apertures are spaced along a portion of the core wall surface. That portion of the core wall surface having apertures therethrough is bounded by a plurality of partial plate assemblies spaced sequentially along and eccentrically mounted on the core tube portion of the weld-o-let T-connection. The suction pipe portion includes a solid suction wall surface which forms the leg of the weld-o-let T-connection. The suction wall surface defines a channel for connecting the bore to the suction pipe. Typically, the suction pipe portion extends from the non-perforated area of the core wall surface. As a result of this novel weld-o-let T-connection arrangement, the T-connection is a part of the suction system. This novel arrangement allows for an increase in the suction strainer surface area while providing additional suction pipe connections to the elongated suction strainer. Because the weld-o-let T-connections are a part of the suction system, the suction pipe connections can be staggered along the entire length of the elongated suction strainer. The core tube portion of the T-connections provide additional support to the elongated suction strainer, thus the elongated suction strainer can be made even longer. Another advantage of this arrangement is that a number of suction pumps, can be connected to the same suction strainer using separate suction pipe connections. Thus, if one pump falters, the effect on the recirculation system is minimal. While the preceding disclosure focuses primarily on elongated suction strainers constructed from a plurality of suction strainer sections, a single elongated suction strainer, supported at its ends, is a viable alternative due to the structural support provided by the internal core tube. However, forming the elongated suction strainer from a plurality of suction strainer sections provides a number of practical advantages. First, because existing nuclear power plants are to be retrofitted with the suction system of the present invention, it will be more practical to install the system using a number of smaller suction strainer sections. Moreover, because there are different types of ECCS for different BWR/PWR nuclear power plants, using several suction strainer sections will provide more design options. Additionally, it is easier to transport suction strainer sections rather than a single elongated suction strainer. Thus, the costs associated with transportation and installation are reduced. Also, standardized castings can be used to create standard size suction strainer sections. These and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description, read in conjunction with the accompanying drawings.