Boiling water nuclear reactor fuel assembly divided into four sub-assemblies and comprising a channel

Abstract: A fuel bundle assembly (16) for a boding water nuclear reactor includes an bundlended tubular channel subdivided into four quadrants by at least two interior partitions (15,20), each quadrant having a sub-fuel bundle assembly (26) having a plurality of fuel rods (46) extending between upper and lower tie plates (48). An inter-bundle support plate (40) receives a lower end of the channel (16) and has four flow openings (54) at an upper end thereof, such that the lower tie plate (40) of each sub-fuel bundle (26) supported in a respective one of the openings in the support plate (40). In one embodiment, the sub-fuel bundles within a channel are separated by a cruciform shaped coolant passage (86). In a second embodiment, the cruciform coolant passage is omitted to thus provide a homogeneous sub-fuel bundle configuration.

Description: This invention relates to the fuel bundles for boiling water nuclear reactors, and more particularly to an improved and enlarged fuel bundle channel design. BACKGROUND A Boiling Water Reactor (BWR) generates steam in its core. This core is composed of an array of side-by-side, vertically upstanding square sectioned fuel bundles. These fuel bundles divide the core region of the reactor into the so-called core bypass region exterior of the fuel bundles, and the core region interior of the fuel bundles. The flow region interior of the fuel bundles is at a higher pressure than the bypass region. Typically, water is forced to circulate through the fuel bundles by pumping. The flow region exterior of the fuel bundles contains non-boiling water and is used to provide increased presence of water for the moderation of high speed neutrons to low speed neutrons so that the chain reaction in the boiling water reactor can continue. Fuel bundle construction can be summarized in a simplified format sufficient for the understanding of this invention. A fuel bundle consists of a group of fuel rods between an upper tie- plate and lower tie-plate. The upper tie-plate and the lower tie-plate and the fuel rods extending therebetween are provided with a polygon section, which section is preferably square. This section is surrounded by a water impervious channel which forms a water tight boundary from the lower tie-plate to the upper tie-plate. In a BWR, the fuel channels perform three distinct and separate functions. First, the channels form individual coolant cells in which the fuel rods or fuel assemblies are located, thus separating boiling coolant from moderator coolant within the core region. Second, adjacent channel sides form the control rod blade guiding annulus. Third, the channels position and laterally support the fuel assemblies. BWR fuel assembly size has remained basically the same for approximately three decades. A typical size is 140.16 by 140.16 mm square (5.518 by 5.518 inches square) by 4.239 m (166.9 inches) long. Initially, bundle size was determined by the capability of the reactivity control system to keep the core in a state of cold shut-down with sufficient reactivity margin, considering one control rod projecting out of the core. This size was appropriate considering the known D-lattice, C-lattice and N-lattice designs in which one cruciform control blade is inserted between every four bundles in the core. More recent advances in the use of burnable poisons and axial enrichment variation have made it possible to increase the fuel assembly size beyond the typical BWR size. Furthermore, by designing the core such that two control blades are adjacent to two opposite comers of the fuel assembly during shut-down, the bundle size can be increased even more from the viewpoint of reactivity control. This arrangement is called the K-lattice core. The bundle width for this application is approximately 161.9 mm (6.375 inches) or slightly bigger than the bundle width for other lattices as noted above. A BWR having a first assembly generally in accordance with the preamble of claim 1 hereof is described in DE-A-3427372 (equivalent to US-A-4,632,804). Fuel assemblies with increased numbers of fuel rods are described in JP-A-04296693. Recently, there has been interest in further increasing the size of the BWR fuel bundle for future BWR plant designs. The motivation for increasing the size is to reduce the total number of control rod drives (CRD's) required for reactivity control, and to reduce the amount of fuel handling and shuffling during the refueling outage. Bundle widths as much as two times the typical existing BWR bundle pitch are under consideration. The invention here explains how such an enlarged bundle can be constructed so that fuel design issues including channel bulge and availability of two phase flow thermalhydraulic test data are addressed. DISCLOSURE OF THE INVENTION The present invention provides a boiling water nuclear reactor which includes a plurality of fuel bundle assemblies as claimed in claim 1 hereof. In accordance with this invention, a large fuel channel design is provided which consists of a large channel with inner channels or cross ties which divide the large channel into four quadrants. Each quadrant serves as a cell for a sub-fuel bundle. Each sub-fuel bundle is similar in size to a traditional BWR fuel bundle. By arranging four sub-fuel bundles within a single channel, and by arranging four of these enlarged channels about each CRD, it will be appreciated that the total number of required CRD's can be reduced significantly. The lower section of the large fuel channel has an attached inter-bundle fuel support to vertically hold the sub-fuel bundles. Thus, each channel and inter-bundle fuel support serves as a "basket" for the four sub-fuel bundles. The embodiment of the invention described herein is referred to as the "water cross type bundle approach". The water cross type bundle approach includes a large channel divided by inner channel parts or cross ties which divide the channel into four discrete quadrants. These inner channel parts or cross ties provide or define a cruciform passage centrally located vis-a-vis the channel (isolated from the sub-fuel bundles), and through which coolant is adapted to flow. One feature of this water cross type bundle is that water gaps that are maintained on all four sides of each sub-bundle similar to existing BWR lattice configurations in the core. Specifically, the water cross separates the sub-fuel bundles from each other, axially along the entire length of the channel. At the same time, the spacing between adjacent large bundles within the core is the same as the spacing between the sub-bundles of each large bundle, as defined by the water cross. It is this arrangement that provides substantially equal cooling flow along all four sides of each sub-fuel bundle. In addition, the arrangement of CRD's between the various large bundles is such that opposite corners of each large bundle are bracketed by a pair of CRD blades or wings. Four inlet nozzles are provided (one for each sub-bundle) in the inter-bundle support plate, simplified with coolant via a common inlet opening. This embodiment has the ability to orifice each sub-bundle separately in order to assure good thermal-hydraulic-nuclear stability. Thus, there is provided a fuel bundle assembly for a boiling water nuclear reactor comprising an open ended tubular channel subdivided into four quadrants by four interior partitions, each quadrant having a sub-fuel bundle assembly comprising a plurality of fuel rods extending between upper and lower tie plates; an inter-bundle support plate receiving a lower end of the channel and having four flow openings at an upper end thereof, the lower tie plate of the sub-fuel bundle supported in a respective on of the openings. The channel design described herein reduces the time required for fuel handling by allowing roughly four times more fuel to be moved per lift compared to current BWR fuel channel designs. In addition to improving fuel handling, the large channel design has an increased load carrying capability under seismic events, and is structurally more resistant to channel bow and bulge deformation. It will also be appreciated that the new large channel design in accordance with this invention supports the three distinct and separate functions of conventional channels, discussed hereinabove. Advantages of the invention in addition to those described above, will become apparent from the detailed description which follows. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a partial perspective of a boiling water reactor core illustrating a large channel for a water cross type bundle in accordance with a first embodiment of the invention, shown in associated with a control rod drive; FIGURE 2 is a partial plan of four large channels and associated sub-fuel bundles of the type shown in Figure 1, along with adjacent control rod drives; FIGURE 3 is a partial perspective of a large fuel bundle channel with an inter-bundle fuel support at its lower end in accordance with the first embodiment of the invention; FIGURE 4 is a partial side section illustrating inter-bundle fuel support for the large fuel bundle in accordance with the first embodiment of the invention; FIGURE 5 is a partial side section similar to Figure 4 but illustrating the manner in which the large fuel bundle can be removed from the inter-bundle fuel support; and FIGURE 6 is a plan view of the inter-bundle fuel support illustrated in Figure 4. BEST MODE FOR CARRYING OUT THE INVENTION With reference to Figure 1, part of a Boiling Water Reactor (BWR) core 10 is shown, including a cruciform Control Rod Drive (CRD) 12 projecting upwardly through a core plate P. The CRD 12 is centrally located relative to four fuel rod bundle assemblies, only one of which is shown at 14 in Figure 1, but see also Figure 2. The assembly 14 includes a large, open-ended tubular channel 16 provided with inner channels or cross ties 18, 20, 22 and 24 which divide the channel 16 into four corresponding quadrants A, B, C and D. The large channel 16 is substantially square in section, but with rounded corners. Each quadrant serves as a cell for a sub-fuel bundle 26 of otherwise conventional BWR bundle construction, except as noted below. In the embodiment shown, each sub-fuel bundle comprises an 8 X 8 array of fuel rods. Thus, unlike the conventional arrangement where each bundle 26 would have its own fuel channel, the present invention utilizes a single large channel 16 to embrace as many as four sub-fuel bundles 26. Thus, it will be appreciated that the substantially square channel 16 may have side dimensions about twice that of the conventional channel. In this embodiment, referred to as a water cross type bundle, the inner cross ties 18, 20, 22 and 24 are'arranged to provide a cruciform moderator passage (or water cross) 28 between the sub-fuel bundles 26, and specifically between the cross ties 18, 20, 22 and 24, with moderator introduced into the passage 28 via ports 30 located in the inter-bundle fuel support described below. Each cruciform passage 28 is provided with a plurality of reinforcements 29 in each section of the passage, and arranged at vertically spaced intervals along the length of the channel. The reinforcements 29 and cross ties 18, 20, 22 and 24 serve to reinforce the channel 16 and thus resist undesirable channel bowing or bulging. It should be noted that in the arrangement shown in Figure 2, four large channels 16 and associated group of four sub-fuel bundle assemblies 26 are shown in place about the CRD 12 which includes four wings or blades in mutually perpendicular pairs. Additional CRD's are shown at 32, 34, 36 and 38 to illustrate the manner in which two control rod blades are located adjacent two opposite comers of each large channel 16. At the same time, it may be seen that each sub-fuel bundle is cooled by in-channel flow on two sides and out-of-channel flow on the remaining two sides. By maintaining spacing between adjacent channels 16 substantially equal to the spacing between the inner cross ties 18, 20, 22 and 24, the flow gap or space on all four sides of each sub-fuel bundle is essentially the same, similar to existing BWR lattice configurations. Turning now to Figure 3, it will be seen that the large channel 16 is supported on an inter-bundle fuel support plate 40 which, in turn, is supported on a cup 42 (Fig. 4). The latter is fixed to the core plate P. With regard to Figure 4, it is initially apparent that each sub-fuel bundle 26 comprises a plurality of fuel rods 46 supported at their lower ends by a fuel rod supporting grid 48 of a lower tie plate assembly 50. The fuel rods 46 are secured at their upper ends by an upper tie plate 51 (see Figures 1 and 2). The lower inlet nozzle portion 52 of each lower tie plate 50 is circular in shape, and is seated within a corresponding opening 54 in the inter-bundle support plate 40. With reference also to Figure 6, it may be seen that each plate 40 is provided with four such openings 54 to accommodate the lower tie plates of the four sub-fuel bundles supported on each plate 40. Plate 40 is otherwise of generally square shape and of a cross sectional size similar to the large channel 16. The four openings 54 in plate 40 each taper downwardly and inwardly along surfaces 56 such that the vertical centerlines of the openings 54 are offset relative to the vertical centerline of a common inlet opening 62, such that coolant flow is caused to change direction as it flows upwardly through the plate 40 into the individual sub-fuel bundles 26. Upstream of the openings 54, a common inlet opening 62 as defined by the mounting sleeve portion 65 of the plate 40 supplies liquid coolant to the four openings 54. This sleeve portion 64 is formed exteriorly with a tapered annular seating surface at 66 and a horizontally oriented seating flange 68 which engages complementary surfaces 70, 72, respectively, on the support cup 42, fixed to the core plate P and seated within the plate P in a similar manner. The openings 54 of the support plate 40 are surrounded by tapered seating surfaces 74 and a horizontal top surface 76 upon which mating surfaces 78, 80, respectively, of the lower tie plates 50 are seated. The large channel 16 is received over the upper edge 82 of peripheral side wall 84 of the support plate 40, as best seen in Figure 4. With reference also to Figures 4-6, the top of the support plate is also formed with a cruciform groove or slot 86 which receives the inner channels or cross ties 18, 20, 22 and 24 and through which coolant is supplied via ports 30 (Figure 2). Utilizing handles 90 secured to the upper end of the large channel 16 as shown in Figure 1, the entire sub-fuel bundle assembly 14 including channel 16 and four sub-fuel bundle assemblies 26 can be lifted from the support cup 44 (as shown in Figure 5) during refuel or repair procedures.

Claims: A boiling water nuclear reactor (10) including a plurality of fuel bundle assemblies (14) each comprising an open ended tubular channel (16) subdivided into four-quadrants (A,B,C,D) by four substantially solid interior partitions (18, 20, 22, 24) arranged to provide a cruciform passage (28) extending in mutually perpendicular directions and axially along the length of the channel; each quadrant having a sub-fuel bundle assembly (26) enclosed on two adjacent sides by said open ended tubular channel and on two remaining adjacent sides by said interior partitions; each sub-fuel bundle assembly (26) comprising an array of fuel rods (46) extending between upper and lower (50) tie plates; and an inter sub-fuel bundle support plate (40) receiving a lower end of said channel and having a flow opening (54) at an upper end thereof for each sub-fuel bundle assembly, the lower tie plate (50) of each such sub-fuel bundle assembly being supported in a respective one of said openings, CHARACTERIZED BY : a group of four fuel bundle assemblies (14) being arranged in a square array so as to define a control rod space separating said group of fuel bundle assemblies (14) and a cruciform control rod (12) located within said control rod space such that two control blades are adjacent to two adjacent sides of a fuel bundle assembly so that four fuel bundle assemblies are arranged about each cruciform control rod; and, that the spacing between fuel bundle assembly channels is the same as the spacing between said sub-fuel bundle assembly interior partitions. The reactor of claim 1 wherein said inter sub-fuel bundle support plate (40) has a cruciform groove (86) aligned with said sub-fuel bundle assembly cruciform passage (28), said groove supplied by coolant from at least one coolant supply port (30). The reactor of claim 1 wherein said inter sub-fuel bundle support plate (40) is formed with an inlet opening (62) upstream of and common to said four flow openings (54). The reactor of claim 1 wherein said inter sub-fuel bundle support plate is formed with a horizontal seating surface (72) adapted to engage a support cup (42) fixed to a reactor core plate (P). The reactor of claim 1 wherein each sub-fuel bundle assembly contains an 8 x 8 array of fuel rods.

Inventors: DALKE CHARLES ARTHUR, DILLMANN CHARLES WARNER, RELF CHARLES WILLIAM, DALKE, CHARLES ARTHUR, DILLMANN, CHARLES WARNER, RELF, CHARLES WILLIAM
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1995
Main classification: G21C3
IPC codes: G21C3, G21C5