Patent Number: 053902211
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. Each fuel bundle includes a plurality of upstanding fuel rods 42 surrounded by a channel 44. Spacers 46 surround the fuel rods 42 discretely at several elevations and constitute locations where debris can be trapped, dynamically fretted by the passing coolant, and cause damage to fuel rods 42. Accordingly, and in this disclosure, the filter of this invention is located in any of the illustrated plenums to the rod supporting grid G of the lower tie plate (See FIGS. 2, 3 and 4), or in the fuel support casting C. In the following illustrations, the debris catchers of this invention will be illustrated with location in the lower tie plate flow plenum between the inlet orifice or nozzle N and the rod supporting grid G. Remembering further 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 changes in flow velocity and direction 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. Flow resistance 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 flow resistance should be avoided. Regarding the overall performance of a debris catcher or trap, such structure must be capable of trapping particles small enough to be entrained but large enough to enter through the lower tie plate grid G and in between the fuel rods 42. Such a structure must be structurally sound and especially avoid any failure resulting in loose parts. It is desired that the structure trap and retain debris particles. At the same time, adverse flow conditions into the fuel bundle should not be generated. Finally, the filter should be such that it is not possible under any circumstances for the filter to become clogged and cause appreciable obstruction to the total flow into the fuel bundle B. Accordingly, and in the description of the specific embodiments that follow it will be seen that we utilize a filter structure that does not constitute a continuum of structure across the particular flow plenum being utilized. In each case--assuming that the perforate portions of the filter become complete clogged--it will be seen that unobstructed water coolant flow paths are preserved to the fuel rods. The following designs direct debris particles into screen or mesh paths that intercept only a fraction of the total flow path. This minimizes pressure drop. At the same time, solid portions can be incorporated to the mesh structures to impart required resistance to failure. Referring to FIG. 2, a side elevation section schematic of a lower tie plate assembly T is shown. This lower tie plate includes four walls 52 defining a substantially square volume with tapered substantially conical wall 54 truncated at inlet nozzle N. Nozzle N includes a bail 60 over the nozzle forming the lower most structure of the fuel bundle. In the structure illustrated in FIG. 2, there is included a debris collector ring 70. Ring 70 fastens interiorly of plenum P surrounding nozzle N and projects upwardly into the volume of plenum P. As will be realized hereafter, ring 70 forms between the inside of conical wall 54 and the outside surface of ring 70 a trap for debris. Secondly, located above or preferably within nozzle N is static swirl vane 80. Swirl vane 80 imparts an upwardly spiralling flow to coolant flowing through nozzle N into plenum P. Such spiral flow classifies heavier debris to the outside of ring 70 with the lighter coolant flowing upwardly through rod supporting grid G. Finally, mesh pick off filter 90 including horizontal portion 92 and downward depending ring 94 is placed centrally of the structure. Preferably, the structure is perforate for allowing fluid flow through the mesh pick off structure; it will be understood that portions of this structure can be solid if desired. Operation is easy to understand. Water coolant including debris enters nozzle N and has a swirling motion imparted by a static swirl vane 60. Above the static swirl vane there is an open central flow path. Heavier debris--typically metal particles having 8 to 10 times the density of water--are classified to the exterior of plenum P and trapped--either by ring 70 or overlying mesh pick off filter 90. Debris is retained at these locations. At the same time, the open central flow path is not obstructed by a continuum of filter structure. Obstruction of the filter structure causing impeding of flow to the fuel rods 42 cannot occur. It is anticipated that the length of pitch of static swirl device 80 will be adjusted for optimum performance. Further, dimension of debris collector ring 70 and mesh pick off filter 90 will likewise be adjusted for optimum trapping of debris. It is to be noted that upon cessation of flow, debris trapped at mesh pick off filter 90 will fall. In such a fall, trapping of the debris will occur at ring 70. Thus, and in the case of the illustrated fuel bundle B, with removal of the fuel bundle removal of the debris will occur. Referring to FIG. 3, a structure similar to FIG. 2 is illustrated with the exception of cone deflector 100. This deflector peripherally diverts fluid to the plenum periphery at the cone 100. This cone can be constructed of mesh and/or solid material. Cone 100 extends beyond ring 70 and terminates in depending mesh ring 102. Depending mesh ring 102 is outside of ring 70. Operation is easy to understand. Debris entraining water coolant is deflected at cone 100 with debris being trapped either at debris collector ring 102 or the mesh pick off filter. Debris falling from either location--either during coolant flow or after coolant flow has ceased--will fall into the outside of ring 70 and be trapped by ring 70 within lower tie plate T. Referring to FIG. 4, a structure similar to that illustrated in FIGS. 2 and 3 is illustrated in which upper filter layer 124 and lower filter layer 114 impart the circuitous flow path to the passing fluid. Lower filter structure 114 includes cone filter 110 having a solid central ring 11 to define a central flow path. A peripherally sloping perforate cone 110 truncated at the central flow paths extends to perforate ring 113. Upper filter layer 124 consists of peripheral annular perforate filter section 120 and central inverted conical basket 122. Conical basket 122 includes an inverted perforate cone section 126 and a depending perforate ring section 127. In operation, it will be seen that the disclosed design includes offset over-under placement of layered traps for debris particles. At the same time, open flow passages are preserved so that complete debris or corrosion clogging of the filter cannot occur. The sloped profile of the filter assists migration of the trapped debris to the collecting corners of the upper filter layer 124. Debris falling from these upper layer 124 corners is either trapped by lower filter layer 114 or optional ring 70. In the schematic of the apparatus herein illustrated, a ring, cone, and annulus structure is shown. The reader will understand that a straight structure across plenum P having the overall side elevation of the ring structure illustrated could as well be used.