Patent Number: 052788830
Section: description

DETAILED DESCRIPTION Referring to FIG. 1, a boiling water reactor fuel assembly is generally shown at 10 having elongated fuel rods 12 containing nuclear fuel pellets. The fuel rods are supported between a lower tie plate 14 and upper tie plate 16. Fuel rods 12 pass through low pressure drop spacer grids 18, only two of which are shown in this fragmentary view. Assuming present reactor designs, up to nine low pressure drop spacer grids could be used. Low pressure drop spacer grids 18 provide intermediate support of rods 12 over the length of fuel assembly 10 and maintain them in a spaced relationship while restraining them from lateral vibration. A central rectangular channel 44 is at the center of the array of fuel rods 12. Outer rectangular channel 11 is shown around the fuel rods 12 and spacers 18. FIG. 2 is a perspective fragmentary view looking from the side and down upon assembly 10 in FIG. 1 and shows one of the low pressure drop spacer grids 18 with the outer channel 11 partially removed. Assembly 10 houses a 9.times.9 fuel array although most of the fuel rods 12 are not shown in FIG. 2 for clarity of illustration. Although reference is made in the specification to a 9.times.9 fuel rod array, such an array has been selected for illustrative purposes only. The present invention can be used with other arrays including, but not limited, to 8.times.8, 10.times.10, and 11.times.11. Referring to FIG. 2, low pressure drop spacer grid 18 includes a perimeter strip 20 which is a band forming four walls 20a, 20b, 20c, and 20d. Each of walls 20a, 20b, 20c, 20d have a leading or upstream edge 21b and a trailing or downstream edge 21a. The direction of coolant flow through the assembly is shown by the arrow marked F in FIG. 2. Grid members 22 and 24 are arranged at right angles to one another and divide the region within perimeter strip into nine separate subregions. Grid members 22 and 24 are secured to perimeter strip 20. Grid members 22, 24 have a leading or upstream edge (not shown) as well as a trailing or downstream edge 22a, 24a, respectively. Both the upstream and downstream edges of the walls 20a, 20b, 20c, 20d as well as grid members 22 and 24 can be convexly contoured to decrease the resistance to coolant flow. Eight of the subregions house fuel rods 12 and the central region houses central water channel 44 which is formed by channel walls 46. Most of the fuel rods are not shown in FIG. 2 for clarity. A square central water channel 44 is shown in FIG. 2 although other shapes are known and can be used. Central channel walls 46 extend the length of fuel assembly 10 allowing water to flow through central water channel 44 from the bottom to the top of fuel assembly 10. Perimeter strip walls 20a, 20b, 20c, and 20d, and grid members 22 and grid members 24 make up a basic egg-crate structure shown in FIG. 4. Zircaloy, an alloy of zirconium, is a preferred material for the egg-crate structure because of its low neutron absorption characteristics. Two of the most commonly known forms of zircaloy are Zircaloy 2 and Zircaloy 4 which are described in ASTM standard B350-91 (1991); compositions R60802 and R60804, respectively and is hereby incorporated by reference. Each low pressure drop spacer 18 includes four spring forks, two of which are lower spring forks 36, and two are the upper spring forks 34. Each spring fork is formed of six pairs of spring strips of three different lengths 38a, 38b, 38c attached to an end support 39 and is shown in FIG. 3. Upper spring forks 34 extend into the egg-crate structure of the spacer through apertures 40 positioned in the upper portion of perimeter strip walls 20a, 20c and grid members 24. Shown in FIG. 2 is a portion of the upper spring fork 34 with spring strip pairs 38a which extend through apertures 40 in side wall 20a. Spring strip pairs 38a further extend through apertures 40 in grid member 24 and into the adjacent subregion. The length of spring strips 38a are such that they extend into but not completely through the adjacent subregion. Spring strips 38b of upper spring fork 34 extend through apertures 40 in side wall 20a and into the middle subregion. Although not shown in FIG. 2 for clarity, spring strips 38c of upper fork 34 similarly extend through apertures 40 in side wall 20a, into and through the corner subregion formed in part by walls 20a and 20b, and extend further through apertures 40 in grid member 24 into the next adjacent subregion. A second upper spring fork 34 (not shown in FIG. 2 for clarity) similarly extends into the egg-crate structure through apertures 40 in perimeter strip wall 20c. Both upper spring forks 34 are shown in FIG. 4A. Pairs of spring strips 38a, 38b and 38c extend into the subregions adjacent to sidewall 20c. As is further shown in FIG. 4A, two upper spring forks 34 are positioned in the fuel assembly in the same plane in which their spring strip pairs extend. In addition to upper spring forks 34, lower spring forks 36 are provided which are positioned below the upper spring forks. Pairs of spring strips 38a, 38b and 38c of lower spring fork 36 extend through apertures 42 which are positioned in the lower or upstream portion of perimeter wall 20b and guide members 22 (FIGS. 2 and 4B). Apertures 40 in sidewalls 20a, 20c and grid members 24 are positioned at a higher axial location than apertures 42 in sidewalls 20b, 20d and grid members 22. Although not shown in FIG. 2 for clarity, a second lower spring fork 36 extends into the egg-crate structure through apertures 42 in sidewall 20d and through apertures 42 in guide members 22 (FIG. 4B). Each spring strip pair of both of the upper spring forks 34 and both of the lower spring forks 36 extend into the fuel assembly. The two lower spring forks 36 are positioned in a plane which is parallel to the plane occupied the upper spring forks. As is shown in FIGS. 2, 4A and 4B, the spring strips of the lower spring forks extend in a direction orthogonal to the direction of the spring strips of the upper spring fork. The intersection and superposition of spring forks 34, 36, grid members 22, 24, and perimeter strip 20 form fuel rod passageways 55 (FIG. 4C) through which fuel rods 12 extend. Seventy two passageways 55 are defined for the 9.times.9 fuel assembly shown in FIG. 2. Although a fuel rod occupies each passageway 55 in a fully loaded fuel assembly, one fuel rod from each of the eight subregions has been removed from the view shown in FIG. 4C for purposes of illustration. Each spring strip 38 of each of spring forks 34, 36 has spring members 41 which act against fuel rods 12. FIG. 3 is a top plan view of the spring fork and shows the position of spring members 41 in each spring strip. At least one spring member 41 from each of upper spring fork 34 and lower spring fork 36 provide reaction loads in at least two different directions perpendicular to each other and restrain fuel rod 12 within passageway 55 against lateral vibration (FIG. 4C). Each fuel rod 12 is restrained in passageway 55 in four directions solely by spring members 41, or by spring members 41 in conjunction with dimples 43a and 43b which are formed in perimeter strips 20 and grid members 22, 24 (FIGS. 2 and 4C). Dimples 43b are positioned towards the upstream or trailing edges of perimeter strip 20 and grid members 22, 24, whereas dimples 43a are positioned towards the downstream or leading edge of perimeter strip 20 and grid members 22, 24. A dimple pair is formed by a dimple 43a and a dimple 43b. Each dimple pair is positioned at the same radial location in the fuel assembly. Dimples 43a and 43b are preferably of the "flow-through" type, i.e., open at their tops and bottoms to reduce the pressure drop. Referring to FIG. 4C, each fuel rod 12 within the assembly is restrained within a passageway 55 by the combined action of either: (a) two spring members 41 and two dimple pairs; (b) three spring members and one dimple pair; or (c) four spring members. Although the present invention is shown and described for purposes of illustration with a square 9.times.9 fuel assembly, it is apparent to those skilled in the art that a fuel configuration other than a square or rectangular array can be selected and that the number of spring members and dimples which restrain each fuel rod within passageway 55 will vary. FIGS. 5A-5D show one pair of spring strips 38a of the spring fork shown in FIG. 3. Spring strip pairs 38b and 38c have been omitted from FIGS. 5A-5D for clarity but have the same structure as spring strip pairs 38a except that they are shorter and have fewer spring members 41. Referring to FIG. 5A, spring strips 38a have spring members 41 which are formed of convolutions 58 which alternate in opposite directions. Spring members 41 and convolusions 58 extend vertically to form a three sided groove or channel. Once loaded with fuel rods, convolusions 58 exert a retaining force on the fuel rods. Pairs of spring strips 38a are joined by spot welds 60 between alternating abutting matching convolutions 58 with the opposing convolutions forming unobstructed flow spaces 62 as shown in FIG. 5A. Unobstructed flow spaces 62 formed by convolutions 58 are hexagonal in shape and allow coolant to flow unobstructed through low pressure drop spacer 18. The matching convolutions 58 which are not spot welded are positioned to form gaps 64 at trailing edge of spring strips 38a due to the shape of and resilience of spring strips 38a. The leading or upstream edge 68 of each spring strip 38a except for the leading edge of spring members 41 form abutting edges 70 (FIG. 5D). In a preferred embodiment, abutting edges of leading or bottom edge 68 are welded together. The preferred material for the spring fork is Inconel, a nickel alloy, which has high strength and exhibits less spring relaxation during irradiation. The strip material is preferably relatively thin, 8 to 12 mils in thickness, to minimize the resistance to coolant flow and to reduce the mass and volume of neutron absorbing material. In the unloaded condition (without fuel rods inserted), convolutions 58 are not in line along a row, but rather displaced and tilted by the integral spring force provided by spring strips 38a. When a fuel rod 12 is positioned in a passageway 55, convolusions 58 exert a retaining force on fuel rods 12 and gap 64 is closed as shown in FIG. 5B. The leading edge 68 and trailing edge 69 of each spring strip 38 can be convexly contoured to minimize the resistance to coolant flow and reduce the pressure drop. Abutting edges 70 can be convexly contoured to further reduce the pressure drop. In order to reduce the volume of neutron absorbing material, windows 72 may be formed in spring strips 38a. Each pair of spring strips is attached to an end support 39 by welds 73. End support 39 extends vertically, and in a preferred embodiment, is contoured to provide a seal surface 82. The height of end support 39 extends beyond that of spring strip pairs 38a and provides spring resilience to keep seal surface 82 seated against the inner wall of outer channel 11 to reduce, if not eliminate, bypass flow. For each low pressure drop spacer 18, upper spring forks 34 and lower spring forks 36 are positioned at slightly different elevations relative to each other. Bypass low which would ordinarily pass between perimeter walls 20a, 20b, 20c, 20d of spacer 18 and the inner wall of outer channel 11 is reduced. However, it may be desirable to permit bypass flow, particularly in the regions of the four corner fuel rods. The corner fuel rods of a fuel assembly are ordinarily undercooled because the coolant flow rate through the corner regions of the fuel assembly is usually low. In order to compensate for such low flow rate, the enrichment of the corner fuel rods is ordinarily reduced to prevent them from overheating. If bypass flow is selectively provided to the corner fuel rods, then the enrichment of such rods need not be reduced as much which results in a net increase in the linear power rate for such rods. By utilizing a spring fork with a seal surface which does not extend the full width of the fuel assembly, selectively limited bypass flow to the corner fuel rods and optimum flow in the corner subregions of the assembly can be obtained. Such optimum flow maximizes power production from the corner fuel rods as well as the other fuel rods in the assembly. Spring forks 34, 36 are preferably made of a material much stronger than perimeter walls 20a, b, c, d and grid members 22 and 24 which form the egg-crate structure. Because limited relative motion is possible between the spring forks 34, 36 and the egg-crate structure, the possibility of damaging low pressure drop spacer 18 from the installation or removal of channel 11 is reduced. The possibility of damaging the spacer from the installation or removal of channel 11 is further reduced by making the spring forks of material(s) stronger and less embrittled by irradiation as well as more resilient than the material of the egg-crate structure. The possibility of damaging the spacer due to the installation or removal of the outer channel is reduced even further because relative motion between the seal and the outer channel as well as between the spring forks and fuel rods is permitted. As stated above, one way in which improved uranium utilization as well as increased moderation in the upper region of the reactor core can be achieved is to decrease the diameter(s) of the fuel rods in the upper half of the reactor core. If one or more fuel rods fail during reactor operation, it is often desirable to replace such failed rods to avoid the costly premature discharge of the entire fuel assembly from the reactor. After the discharge from the reactor, repair of a fuel assembly involves the removal of the fuel assembly upper tie plate, upward withdrawal of the failed fuel rod(s), replacement of the failed rods, and replacement of the tie plate. A fuel assembly which utilizes a spacer grid design which accommodates fuel rods having reduced diameters at their upper end cannot be repaired using conventional methods to replace the failed fuel rod because the larger diameter lower portion of the fuel rod cannot be removed through the upper spacer grids. Repair methods of fuel assemblies utilizing fuel rods with such variable diameters involve upending the fuel assembly, and removing the lower instead of the upper tie plate. The upending of the fuel assembly is difficult and expensive, and can require nuclear fuel assembly redesign and subsequent operational risks. Spring forks 34 and 36 are neither welded nor bolted to other elements of the low pressure drop spacer and may be easily removed. Both single and/or multidiameter fuel rods may be removed from the top of the fuel assembly due to the resilience of spring forks 34, 36 without having to first remove the spring forks. If as a result of reactor operations, one or more fuel rods undergo physical changes or deformations which prohibit removal from the fuel assembly, the present invention makes conventional repair methods possible for a fuel assembly with or without reduced fuel rod diameters. According to the present invention, and referring to FIG. 2, the spring forks 34, 36 which restrain the failed fuel rods 12 are removed from the egg-crate structure. The removal of the spring forks to such failed fuel rods enables the upward withdrawal of failed fuel rods without damage to the fuel assembly or the spacer grid. After fuel rod replacement, spring forks 34, 36 are reinserted into the egg-crate structure. Low pressure drop spacer grid 18 is secured to the central water channel by any conventional method. In a preferred embodiment, retainer strips 86 are secured to central water channel walls 46 by spot welds 88 at locations just below and above the desired axial position of spacer grid 18. Four retainer strips 86 are located just above spacer grid 18 (two are shown in FIG. 2) and have flow tabs 90 extending into the coolant stream which serve the additional function of directing liquid condensing on central water channel walls 46 towards fuel rods 12 where it can collect as a water film on the rods and thereby improve local heat transfer. Retainer strips are similarly positioned and secured just below the axial position of spacer grid 18. As stated above, the present invention has several advantages. The low pressure drop spacer enables the use of larger fuel rod diameters improving the fuel cycle cost. Increased moderation of the fuel assembly is achieved by decreasing the diameters of the fuel rods in the upper portion of the core which improves the fuel cycle cost by approximately $20/kg equivalent fuel weight. Replacement of conventional fuel rods or fuel rods in assemblies incorporating reduced diameter fuel rods in the upper region of the fuel assembly is achieved without damage to the spacer or the other fuel rods. The power capability of the fuel assembly is increased by improving the transfer of condensing liquid water from the surrounding outer channel and central water channel to the fuel rod surfaces. Because of the reduced pressure drop at least one extra spacer can be installed within a fuel assembly which decreases the span between spacer grids in the upper region of the core, and as a result, improves even further the heat transfer to the coolant. While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing form the true spirit and scope of the present invention.