Patent Number: 047175290
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 2, wherein fuel assembly 14, core plate 10, and bottom mounted instrumentation column 42 are configured as previously discussed with respect to FIG. 1, thimble guide 30 of FIG. 1 has been replaced by thimble guide assembly 62 of the present invention. The first or lower element 64 of assembly 62 includes a projecting portion 66 which extends from the upper surface of core plate 10, a right-hand threaded portion 68 whereby element 64 can be screwed into the threaded region 28 of bore 26, and a lower sleeve portion 70 which extends from threaded portion 68 to the flared region 56 of upper element 48. Sleeve portion 70 is provided with an upper annular shoulder 72 which frictionally engages bore 26 and a lower annular shoulder 74 which also frictionally engages bore 26, the region 76 between shoulders 72 and 74 being spaced apart from bore 26. Sleeve portion 70 provides a uniformly dimensioned flow path 78 between upper pipe element 48 and projecting portion 66 despite the different dimensions of bores 54, 58, and 26, as previously discussed. The uniformity of flow path 78 reduces flow-induced vibration and thus wear of thimble 60 (which is not illustrated in FIG. 2). Region 76 is spaced apart from bore 26 in order to keep the insertion force for lower element 64, during installation thereof, at a modest level. With continuing reference to FIG. 2, projecting portion 66 is provided with lower and upper annular recesses 80 and 82, a recessed wrench-engaging region 84 and a left-hand threaded sleeve 86. Thimble channel 87 runs through portion 66 to accommodate thimble 60 (see FIG. 1). Lower spring clip 88 (also see FIG. 3) includes a flange portion 90 which is spot-welded at 92 to core plate 10 and a sleeve portion 94 having an annular protrusion 96 which snaps into recess 80. The second or upper element 98 of assembly 62 has a threaded region 100 which screws onto threaded sleeve 86 of lower element 64. Upper spring clip 102 (also see FIG. 6) is spot-welded at 104 to element 98, and is provided with an annular protrusion 106 which snaps into annular recess 82 of lower element 64. Above recessed wrench-engaging region 108, element 98 is terminated by sloping shoulders 110 and flat annular lip 112. The plane of lip 112 is perpendicular to the axis of the cylindrical thimble channel 113 through assembly 62. The squared-off configuration at the top of element 98, in contrast to the sloping recess at upper lip 35 of thimble guide 30 in FIG. 1, minimizes the turbulence created by fluid discharge. During fabrication of a reactor, an upper element 98 of the desired length is screwed onto a lower element 64, and the elements are tightened together by wrenches inserted into regions 84 and 108. Spring clips 88 and 102 are then forced onto elements 64 and 98 until annular protrusions 96 and 106 snap into place, whereupon welds 104 are applied to form a completed assembly 62. To install the assembly 62 when a reactor is being fabricated, the lower sleeve portion 70 is forced into a bore 26 of the core plate 10 until threaded portion 68 engages threaded portion 28. Assembly 62 is then tightened by a wrench inserted into region 84, and welds 92 are applied. It will be apparent from FIG. 2 that the distance between the upper tip of guide assembly 62 (that is, lip 112) and aperture 22 is considerably less than the distance between the upper tip of thimble guide 30 (that is, lip 35) in FIG. 1 and aperture 22, so that assembly 62 is more effective in shielding the thimble 60 from turbulence. The distance between lip 112 and aperture 22 is preferably less than about an inch (2.54 cm),and may be 0.5 inches (1.27 cm) or, better still, 0.25 inches (0.64 cm). The fact that assembly 62 is fabricated using a lower element 64 and an upper element 98 means that different fuel assembly designs can be accommodated, during construction of a reactor, simply by selecting an upper element 98 of the appropriate length. More importantly, upper elements 98 can subsequently be exchanged in the event that the lengths of legs 18 change as a result of fuel assembly after a reactor has been made. After removal of the fuel assemblies 14 during a refueling operation, remotely controlled tools can be inserted into the reactor vessel to engage regions 108 and unscrew upper elements 98 so that new elements 98 can be installed, again by remote control. The fact that there is a right-hand threaded connection between plate 10 and lower element 64, and a left-hand threaded connection between element 64 and upper element 98, precludes the possibility that both elements might be unscrewed from core plate 10 simultaneously. That is, unscrewing element 98 tends to screw element 64 in more tightly. Spring clips 88 and 102 are present to prevent elements 64 and 98 from becoming unscrewed inadvertently during operation of the reactor due to vibration. The use of spring clips 88 and 102 is preferred but is not essential. Spring clip 102 may have a slot through it to facilitate installation and, similarly, spring clip 88 may have a slot through flange 90 and sleeve portion 94. Other alternatives for spring clip 88 are illustrated in FIGS. 4 and 5. In FIG. 4, lower spring clip 114 has a flange portion 116 and a sleeve portion 118 having a plurality of protrusions 120 rather than the single annular protrusion 96 illustrated in FIG. 3. In FIG. 5, lower spring clip 122 has a flange portion 124 and a sleeve portion 126 having slots 128 which divides it into a plurality of portions. While assembly 62 in FIG. 2 employs recessed wrench-engaging regions 84 and 108, other means may be provided to tighten the upper and lower elements. For example in FIG. 7, thimble guide assembly 128 includes a first or lower element 130 having a protruding wrench-engaging region 132, and an upper element 134 having a protruding wrench-engaging region below planar annular lip 138 and sloping shoulders 140. Wrench-engaging regions 132 and 136, which protrude by different distances, facilitate the use of remote-control machinery if upper element 134 is to be replaced after the reactor has been in use. It will be apparent that other alternatives to the FIG. 2 embodiment, including tool-engaging radial or axial slots in the outer surfaces of the upper and lower elements, or a multisided cavity at the opening of the upper element to accommodate an allen-wrench type tool, could be used instead. It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.