Patent Number: 053902203
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a pertinent detail of a portion of a reactor core is shown. Control rod drive housing H has fuel support casting C supported thereon. Fuel support casting C includes arm 16 which orients casting C with respect to pin 14 in core plate P. Core plate P divides high pressure lower plenum L from core R, preserving the necessary pressure differential barrier to cause the controlled circulation within the many individual fuel bundles of the reactor. Fuel support casting C includes four apertures 20 onto which four fuel bundles B at their respective lower tie plate assemblies T are placed. Each lower tie plate assembly T is disposed to cause its inlet nozzle N to communicate to one of the apertures 20 of the fuel support casting. Fuel support casting C also includes apertures through which control rods 22 penetrate to the interstices of the four fuel bundles sitting on top of the fuel support casting C. Since the action of the control rods is not important with respect to this invention, further discussion of this aspect of the reactor will not be included. Remembering that only four out of a possible 750 fuel bundles are illustrated, it will be understood that the pressure drop across core plate P is important. Accordingly, a review of the pressure drop within a boiling water nuclear reactor can be instructive. First, and through an orifice (not shown) in the fuel support casting C, an approximate 7 to 8 psi pressure drop occurs at typical 100% power/100% flow operating conditions. This pressure drop is utilized to ensure uniform distribution of bundle coolant flow through the many (up to 750) fuel bundles within a boiling water nuclear reactor. Secondly, at in the lower tie plate of the fuel bundles on each fuel support casting C, approximately 11/2 psi of pressure drop occurs. This pressure drop is a result primarily of the change in flow velocity occurring through this complex geometry structure. Finally, and as the coolant rises and generates steam within the fuel bundle, approximately 10 to 12 psi of pressure drop is incurred. This pressure drop is distributed throughout the length of the fuel bundle - and is important to the operating stability of both the individual fuel bundles and the collective fuel bundles constituting the core of the nuclear reactor. The reader should understand that the summary of pressure drop given above is an over simplification. This is a very complex part of the design and operation of a nuclear reactor. Having said this much, one point must be stressed. Pressure drop within the individual fuel bundles of a boiling water must remain substantially unchanged. Accordingly, if apparatus for preventing debris entrainment into the fuel bundles is going to be utilized, appreciable change in overall fuel bundle pressure drop should be avoided. Having carefully reviewed the requirements for the avoidance of increased pressure drop in debris restricting devices, several further comments can be made. First, any debris catching arrangement should be sufficiently rigid so that the excluding apparatus does not under any circumstance break apart, fail to stop debris, and become the source of further debris itself. For this reason, wire screens are not used. Instead, perforated metal is in all cases utilized in the examples that follow. Second, we have found that it is desirable to restrict pressure drop to a minimum. This can be done by making the velocity of flow through the apertures themselves as low as feasible. A second reason for this limitation is the entrainment of the debris in the flow. Assuming entrainment of debris in the flow, if any possible angle of attack can be realized that will enable debris to pass through an aperture, given sufficient time, passage through the aperture will eventually occur. By maintaining slow velocity at the respective apertures, entrainment of debris is less likely to occur. Further, it has been found that a reorientation of the flow at a rejecting hole to an angle where debris passage is less likely can be achieved. Consequently, flow velocity at restricting apertures is restricted to the minimum possible value. Third, we find that modification of the rod supporting grid--a technique utilized in the prior art--is not satisfactory. Specifically, we prefer to use straining apertures that are as small as possible--down to a dimension of 0.050 of an inch diameter. Unfortunately, the rod supporting grid is a member that must have the required static and dynamic properties to support the fuel rods under all conceivable conditions. Utilizing a matrix of such holes in the rod supporting grid at the pitches required for low pressure drop in the lower tie plate is not practicable. First, since the small apertures would be confined to the plane of the rod supporting grid, a total reduction of flow area will be present that would lead both to unacceptable pressure drop as well as high flow velocities through the individual holes in rod supporting grid. Further, such a matrix of small apertures in the rod supporting grid would reduce the strength of the grid to a level below that required for support of the fuel rods. We have identified the so-called flow volume of the lower tie plate assembly as a primary candidate for the location of debris rejection apparatus--preferably the perforated metal utilized with this construction. In boiling water nuclear reactor fuel bundles at the lower tie plate assembly, there is defined between the nozzle at the lower end and the fuel rod supporting grid at the upper end, a relatively large open flow volume. This flow volume is sufficiently large to accommodate a three dimensional structure--with one side of the three dimension structure communicated to the nozzle inlet and the other side of the three dimensional structure communicated to the rod supporting grid. At the same time, periphery of the three dimensional supporting structure can be attached to the sides of the lower tie plate assembly--so that all fluids passing through the flow volume of the lower tie plate simply must pass through the restricting apertures of the perforated plate. Only small modification to the lower tie plate assembly is required. The flow volume in the lower tie plate assembly has an additional advantage. Specifically, and if the flow restricting grid has to be confined to a plane extending across the lower tie plate flow volume, the apertures in the plate would define a total flow area less than the plane in which the perforated plate was disposed. Where a perforated plate is utilized to manufacture a three dimensional structure, the area of the available apertures can increase beyond that total area possible when the perforated plate is confined to a flat plane. In fact, where sufficient structure is utilized, the total flow area available in the aggregated holes of the three dimensional structure can approach and even exceed the total cross sectional area across the flow volume of the lower tie plate assembly before the insertion of the debris restricting assembly. In addition a properly designed debris catcher assembly could improve the flow distribution at the inlet to the fuel bundle. Having set forth these considerations, attention can be directed to the embodiments of the invention. Referring to FIG. 6, a typical filter insert I for placement of the filter apparatus of this invention to a lower tie plate T within the plenum P is illustrated. FIGS. 2A and 2B contain the respective plan and side elevation views of the disclosed helical spring matrix strainer. Referring to FIG. 2A, a plurality of side-by-side springs 100 are shown fastened between opposite walls 114, 116. Looking at FIG. 3, it can be seen that an additional second layer 102 of side-by-side helical springs has been added. These side-by-side helical springs extend between walls 115, 117 at an approximate 90.degree. angle with respect to springs 100. Referring to FIG. 4, an expanded view of springs 100, 102 is illustrated. It can be seen that these respective spring intersect one another at intersections 101. It is preferred that the springs be attached as by welding at these junctures. This attachment forms a unitary mass from the spring matrix to prevent the individual springs from becoming debris themselves should failure of the individual spring matrix occur. Referring to FIG. 5, it is preferred that four layers of springs 100, 102, 104, and 106 be utilized. As illustrated, layers 100, 104 are at 90.degree. angles with respect to layers 102, 106 with the layers alternating in their angularity. Again--and where the layers cross one another --fastening of the springs one to another occurs at points 101, 103, and 105 so that the springs as finally bound together form a unitary mass. The preferred method of such fastening is presently by welding--although other forms of fastening may be obviously used. Referring further to FIG. 5, just as the springs form a matrix, the filter formed has a series of endlessly interconnected passages. This being the case, local fill with debris will not appreciably interrupt intended flow as alternate channels permitting such flow communicate across the entire filter.