Patent Number: 054886341
Section: summary

TECHNICAL FIELD The present invention relates to a lower tie plate grid for a nuclear reactor fuel bundle and particularly relates to a unitary one-piece lower tie plate grid forming part of a lower tie plate assembly, the grid having a lower portion with a plurality of small openings for separating debris from the flow of water coolant through the tie plate, and an upper portion which, in conjunction with the lower portion, support the fuel bundle. The grid is constructed to afford a minimum pressure loss for the water coolant flow through the tie plate grid into the region downstream of the tie plate assembly. BACKGROUND Boiling water nuclear reactors have been in operation for many years. Commencing with their initial construction and throughout their service lives, these reactors may accumulate debris in their closed circulation moderator systems. This debris can become an operating hazard if the debris is allowed to enter into the fuel bundle core region containing the heat generating fuel rods. In order to understand this problem, a summary of reactor construction as it relates to the accumulation of debris in the core needs first to be given. Thereafter, fuel bundle construction will be set forth. Emphasis will be given to the need to preserve substantially unchanged the regions of pressure drop within the fuel bundles. Thereafter, the effects caused by debris entering into the fuel rod region of the fuel bundles will be summarized. In boiling water nuclear reactor construction, the reactor is provided with a large, central core. Liquid water coolant/moderator flow enters the core from the bottom and exits the core as a water steam mixture from the top. The core includes many side-by-side fuel bundles, each containing a plurality of fuel rods. Water is introduced into each fuel bundle through a fuel bundle support casting from a high pressure plenum situated below the core. Water passes in a distributed flow through the individual fuel bundles and about the fuel rods, is heated to generate steam, and exits the upper portion of the core as a two-phase water steam mixture from which the steam is extracted for the generation of energy. The core support castings and fuel bundles are a source of pressure loss in the circulation of water through the core. By properly controlling such pressure losses substantially even distribution of flow across the individual fuel bundles of the reactor core is achieved. When it is remembered that there are as many as 750 individual fuel bundles in a reactor core, it can be appreciated that assurance of the uniformity of flow distribution is important. To interfere with the pressure drop within the fuel bundles could affect the overall distribution of coolant/moderator within the fuel bundles of the reactor core. The fuel bundles for a boiling water nuclear reactor include a fuel rod supporting lower tie plate assembly. Typically, this is a one-piece cast structure including an upper grid, a lower inlet nozzle and a structure providing a transition region from the inlet to the grid. The inlet nozzle provides for coolant entry to an enlarged flow volume within the flow transition region of the lower tie plate assembly. At the upper end of the flow volume, there is located a tie plate grid defining with the nozzle a flow volume. The tie plate grid has two purposes. First, it provides the mechanical support connection for the weight of the individual fuel rods to be transmitted through the entire lower tie plate assembly to the fuel support casting. Secondly, the tie plate grid provides a path for liquid water moderator to flow into the fuel bundle for passage between the side-by-side supported fuel rods. Above the lower tie plate grid, each fuel bundle includes a matrix of upstanding fuel rods--sealed tubes each containing fissionable material which when undergoing nuclear reaction transfers energy to the flowing water to produce the power generating steam. The matrix of upstanding fuel rods includes at its upper end an upper tie plate assembly. This upper tie plate assembly holds at least some of the fuel rods in vertical side-by-side alignment. Some of the fuel rods are attached to both the upper and lower tie plate assemblies. Usually, water rods are also included between the upper and lower tie plate assemblies for improvement of the water moderator to fuel ratio, particularly in the upper region of the fuel bundle. Fuel bundles also include a number of fuel rod spacers at varying elevations along the length of the fuel bundle. These spacers are required because the fuel rods are long (about 160 inches) and slender (about 0.4 to 0.5 inches in diameter), and would come into abrading contact under the dynamics of fluid flow and nuclear power generation within the fuel bundles. The spacers provide appropriate lateral restraints for each fuel rod at their respective elevations and thus prevent abrading contact between the fuel rods and maintain the fuel rods at uniform spacing relative to one another along the length of the fuel bundle for optimum performance. It will be appreciated that these spacers are sites where debris can be trapped and damage the fuel rods. Each fuel bundle is surrounded by a channel. This channel causes water flowing between the upper and lower tie plate assemblies to be restricted to only one bundle in an isolated flow path between the tie plate assemblies. The channel also serves to separate the steam generating flow path through the fuel bundles from the surrounding core bypass region, this region being utilized for the penetration of the control rods. The water in the bypass region also provides neutron moderation. In the operation of a boiling water nuclear reactor, maintenance of the originally designed flow distribution is very important. Specifically, from the lower (high pressure) plenum inlet to the core to the outlet from the core of the steam and water mixture through the upper tie plate assemblies of the fuel bundles, about 20 pounds per square inch (psi) of the pressure drop is encountered at typical flow operating conditions. About 7 to 8 psi of this pressure drop occurs through the fuel support casting. This pressure drop is mainly to assure the uniform distribution of coolant/moderator flow through the many fuel bundles making up the core of the reactor and is related to the prevention of operating instabilities within the reactor at certain power rates. At the lower tie plate assembly of each fuel bundle, from the inlet nozzle into the flow volume and through the tie plate grid, about 1 to 11/2 psi pressure drop occurs which contributes to establishing flow distribution between the individual fuel rods of each fuel bundle. Finally, through the fuel bundle itself--from the exit of the lower tie plate assembly to the exit at the upper tie plate assembly--about 11 psi of pressure drop usually occurs. When new fuel bundles are introduced into a reactor core, these pressure drops must be preserved. Otherwise, the coolant/moderator flow distribution could be compromised. With respect to the tie plate grid of the lower tie plate assembly, a matrix of cylindrical bosses and webs generally form the grid. The bosses are sized to receive the fuel rod end plugs. The spacing and thickness of the bosses and webs are primary factors in controlling pressure drop resulting from water flow through the grid. In early grid constructions, since the fuel rods had greater cross-sectional diameters, the bosses were large. In more recent grid constructions, since the fuel rods have smaller cross-sectional diameters, the bosses are smaller. Also, in early constructions, fewer fuel rods formed a fuel bundle than in recent constructions. Even with all of these changes in grid and bundle construction, however, it is necessary to avoid significantly altering pressure drop. For example, a core may be composed of older (8.times.8) bundles and newer (11.times.11) bundles, and the pressure drop through each bundle preferably is uniform. One challenge with new fuel bundle constructions, and particularly, lower tie plate grid constructions, is to accommodate more fuel rods and perform debris catching functions yet maintain a pressure drop equivalent to the pressure drop resulting from older bundle constructions. Typically, debris within boiling water nuclear reactors can include extraneous materials left over from reactor construction, debris liberated from corrosion during the reactor lifetime, and during the numerous outages and repairs, further debris accumulates. Because nuclear reactors constitute closed circulation systems, it will be appreciated that debris will essentially accumulate with increasing age and use of the reactor. A particularly vexing and usual place for the accumulation of debris is in the fuel bundles between the fuel rods, particularly in the vicinity of the fuel rod spacers. It will be recalled that each fuel rod is surrounded by a spacer at the particular elevation of the spacer. Debris particles tend to lodge between the spacer structure and the fuel rods and often dynamically vibrate with the coolant/moderator flow in abrading contact to the sealed cladding of the fuel rods. SUMMARY OF THE INVENTION The present invention provides a lower tie plate assembly including a debris catcher forming part of a grid. The grid has lower and upper portions, the lower portion serving to catch debris above a predetermined size, while simultaneously providing minimal pressure drop of water coolant through the grid. The grid also supports the fuel rods in a manner enabling a smooth, substantially uniform expansion of the flow into the fuel bundle. To accomplish the latter, a plurality of laterally spaced, generally cylindrical bosses, defining through openings, extend between upper and lower surfaces of the lower tie plate grid and receive lower ends of the fuel rods. Webs also extend between those surfaces and interconnect the bosses. The bosses and webs include respective portions thereof which extend upwardly from the lower portion of the grid and lie coextensively with the upper portion of the grid to define in the upper grid portion a plurality of flow spaces. The bosses are arranged on vertical centerlines arranged at the corners of square matrices, with the webs extending linearly between the bosses along the sides of the square matrices. Convex portions of the cylindrical bosses extend between the right angularly related webs of each matrix. Thus, the webs and the convex portions of the bosses of the upper portion of the lower tie plate grid define the flow spaces. This lower grid portion has a plurality of openings extending therethrough and which open into the flow spaces. These lower grid portion openings separate debris above a certain size from the water flowing through those openings into the flow spaces between the boss and web portions in the upper grid portion. In a preferred embodiment of the present invention, a plurality of openings extend through the lower grid portion and open into each of the flow spaces. In order to minimize the pressure loss and maximize the debris catching function, the openings are specifically oriented, shaped and dimensioned. For example, given the shape of certain of the flow spaces defined by the web and boss portions, a first array of generally square openings with linear sides and radiussed corners are located in the lower grid portion such that vertical centerlines through these square openings intersect a diagonal of a square matrix and which diagonal passes through the vertical centerlines of the cylindrical bosses. A second array of openings having a plurality of sides in excess of four sides, preferably five generally linear sides, with adjacent sides having a radius therebetween, is oriented such that each opening of the second array has a side oriented generally parallel to a web. Preferably, each opening in the lower grid portion has an optimal minimum area, e.g., a throat area, serving to catch debris larger than the minimum area and prevent it from passing through the grid. The openings in the lower portion transition from the minimum or throat areas into the flow spaces and preferably have walls laterally divergent in a direction toward the upper surface of the lower tie plate grid such that each opening defines a venturi in a direction toward the associated flow space. Consequently, the flow pattern for each opening obtains a substantially uniform velocity over its cross-section and which flow pattern flares as it transitions from the lower portion and enters the associated flow space in the upper grid portion to reduce the pressure loss and enable the flow to expand smoothly into the flow spaces. To further facilitate the debris catching function with minimum pressure drop, the lower grid portion has a thickness in the direction of flow less than about 25% of the overall thickness of the tie plate. Preferably, the ratio of the overall thickness of the tie plate grid to the thickness of the lower grid portion is within a range of 5-7 to 1. Additionally, the thickness of the lower grid portion is preferably less than about two times the shortest hole size dimension. In a preferred embodiment according to the present invention, there is provided in a nuclear fuel assembly, a unitary one-piece lower tie plate grid comprising a lower grid portion and an upper grid portion, means for supporting fuel rods above the lower tie plate grid including the upper and lower grid portions, the supporting means comprising a plurality of laterally spaced bosses having portions extending upwardly from the lower grid portion, with the bosses being sized for receiving lower ends of the fuel rods. The supporting means further include web portions extending upwardly from the lower grid portion and interconnecting the boss portions to define with the boss portions a plurality of flow spaces in the upper grid portion extending from the lower grid portion and opening through an upper surface of the lower tie plate grid, the lower grid portion of the lower tie plate grid including a plurality of openings extending therethrough and opening into the flow spaces for separating debris from a coolant flowing through the lower grid portion into the flow spaces between the boss and the web portions. In a further preferred embodiment according to the present invention, there is provided in a nuclear fuel assembly, a fuel rod support structure, comprising a lower tie plate assembly including an inlet nozzle, a unitary one-piece lower tie plate grid and a transition structure defining a flow volume for receiving coolant entering the nozzle and flowing coolant to the lower tie plate grid, the unitary one-piece lower tie plate grid having a lower grid portion and an upper grid portion. The lower tie plate grid comprises a plurality of laterally spaced bosses having portions extending upwardly from the lower grid portion, the bosses being sized for receiving lower ends of the fuel rods, the lower tie plate grid further including web portions extending upwardly from the lower grid portion and interconnecting the boss portions to define with the boss portions a plurality of flow spaces in the upper grid portion extending from the lower grid portion and opening through an upper surface of the lower tie plate grid, the lower grid portion of the lower tie plate grid including a plurality of openings extending therethrough and opening into the flow spaces for receiving the coolant from the flow volume and flowing the coolant through the lower grid portion into the flow spaces between the boss and the web portions. In a further preferred embodiment according to the present invention, there is provided a nuclear fuel bundle and support therefor comprising upper and lower tie plate assemblies, a nuclear fuel bundle between the upper and lower tie plate assemblies and including a plurality of fuel rods, the lower tie plate assembly including means for supporting the nuclear fuel bundles, the lower tie plate assembly further including a lower tie plate grid having a lower grid portion and an upper grid portion. The lower tie plate grid comprises a plurality of laterally spaced bosses having portions extending upwardly from the lower grid portion, the bosses being sized for receiving lower ends of the fuel rods, the supporting means further including web portions extending upwardly from the lower grid portion and interconnecting the boss portions to define with the boss portions a plurality of flow spaces in the upper grid portion extending from the lower grid portion and opening through an upper surface of the lower tie plate grid. The lower grid portion of the lower tie plate grid includes a plurality of openings extending therethrough and opening into the flow spaces for flowing a coolant flowing through the lower grid portion into the flow spaces between the boss and the web portions. Accordingly, it is a primary object of the present invention to provide a novel and improved unitary one-piece lower tie plate grid for supporting a nuclear fuel bundle and having a lower portion for limiting the passage of debris in the moderator flow through the tie plate grid with minimal pressure drop.