Patent Number: 056278667
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 1 is a cross-section of a fuel assembly for a boiling water nuclear reactor in accordance with the present invention. The fuel assembly 10 includes a plurality of fuel rods 12, a pair of coolant rods 14 (two coolant rods 14 are illustrated and described in the preferred embodiment, however, a single coolant rod is often used in such fuel assemblies), and a channel 16 surrounding the fuel rods 12 and coolant rods 14. The fuel rods 12 are preferably arranged in a 10.times.10 matrix and are secured against lateral movement in the channel by a plurality of spacers 18. The coolant rods 14 are generally centrally disposed in the fuel rod matrix. Small holes are provided at both the lower and upper ends of the coolant rods 14 allowing water to be driven through the rod, thus introducing moderating material within the fuel rod matrix. One water rod also serves as the spacer-capture rod being mechanically locked to each of the spacers 18, thereby fixing the axial position of each spacer 18. The fuel rod spacers 18 are equipped with InconeI-X springs to maintain rod to rod spacing. The fuel rods 12 and the coolant rods 14 are supported by a lower tie plate 20. An upper tie plate 22 receives the fuel rods 12 and the coolant rods 14 and restricts lateral movement. End plugs of the fuel rods have pins that fit into anchor holes in the tie plates 20, 22. An expansion space located over the top end plug of each fuel rod allows them to expand axially by sliding within the holes in the upper tie plate to accommodate differential axial thermal expansion. In contrast with the conventional structure, none of the fuel rods is threaded into the lower tie plate 20 or the upper tie plate 22. One or both of the coolant rods 14 may be securely threaded into the lower tie plate 20. As discussed above in connection with the prior art, it is not desirable to thread anything into the tie plates 20, 22 as the threads tend to seize over extended submersion times. In contrast with the fuel rods 12, however, the coolant rods 14 need not be removed from the bundle nearly as frequently as the fuel rods 12. Thus, in the present invention, the coolant rods 14 are threaded or otherwise securely attached to one or both of the tie plates 20, 22. A transition member 24 supports the lower tie plate 20 in the channel 16 and serves as a transition to the nose piece 26. The channel 16 is secured to the transition member 24 by any suitable structure. In the illustrated embodiment, a bolt 28 is threaded through the channel and into the transition member 24. Four bolts 28 are preferably threaded one each through each side of the substantially square cross-section of the channel 16. The transition member 24 has corresponding threaded bolt receiving holes 30 in each side of its corresponding square cross-section. The bolts 28 are preferably formed of alloy X-750. Integral with the upper tie plate :22 is a bail handle assembly 32. Referring to FIGS. 1 and 2, the bail handle assembly 32 includes a bail handle 33 and two boss members 34. FIG. 3 is a cross-section through line III--111 in FIG. 2. The boss members 34 include a channel 36 formed therein. A latch pin 38 is movably disposed in the channel 36. A latch pin cap 40 is fixed to an outer end of the latch pin 38 and has a first outermost diameter that is configured to be extendible through a latch pin aperture 42 in the channel 16 and a second innermost diameter that is larger than the latch pin aperture 42 in the channel 16 and that serves as a stop surface for the latch pin 38. An inside surface of the latch pin cap 40 delimits the channel 36 in the boss members 34. A spring 44 is disposed around the latch pin 38 in the channel 36 between the inside surface of the latch pin cap 40 and an end of the channel 36. The spring 44 urges the latch pin 38 to an extended position, engaging the latch pin aperture 42 in the channel 16. In operation, because the transition member 24 is rigidly secured to the channel 16 by means of the bolt 28 and because the latch pin 38 is inserted into the latch pin aperture 42 in the channel 16, when lifting the fuel assembly 10 with the bail handle assembly 32, the channel 16 bears the structural load of the fuel assembly 10. As noted above, the channel 16 has the highest volume to surface area of the components in the fuel assembly 10 and better avoids the effects of corrosion. With continued reference to FIG. 3, a bolt 43 is disposed between the water rods 14 in an aperture 45 in the upper tie plate 22. The bolt 43 extends into a channel 47. A substantially cylindrical member 51 is fixed to the end of the bolt 43 and delimits the channel 47. A spring 53 is disposed surrounding the bolt 43 in the space delimited by the cylindrical member 51 and the top of the channel 47. The spring 53 serves to maintain the upper tie plate 22 spaced from the water rod main spring supports 55 as illustrated in FIG. 3. Moreover, the spring 53 urges the upper tie plate 22 upward such that the latch pins engage an upper end of the latch pin apertures 42. Still further, preload forces of the water rod main springs are diverted from the upper tie plate 22. FIG. 4 is a cross sectional view through line IV--IV in FIG. 1. Through the channel 16 adjacent the transition member 24 are provided a plurality of end gussets or clips 48 that are attached to the channel 16 and that are inserted into corresponding slots 49 in the transition member 24. The clips 48 serve as a backup support for the assembly in the event that the bolts 28 fail. In a preferred embodiment, four end clips 48 are provided, one each attached to each corner of the channel 16. The end clips 48 are preferably welded to the channel. FIG. 5 illustrates a channel guide 50 according to the present invention. The channel guide 50 includes two channel guide arms 52 disposed substantially at 90.degree.. A center plate 54 is disposed substantially between the arms 52. The center plate 54 consists of a gusset having a threaded, aperture 56 therein. The channel guide 50 is configured to surround the channel 16 and is mounted by the aperture 56 in the center plate 54 to a corner post 35 of the upper tie plate 22 with a bolt 66. The corner post 35 is integral with the upper tie plate 22. The bolt 66 securing the channel guide 50 to the corner post 35 of the upper tie plate 22 is illustrated in FIG. 7. Referring to FIG. 6, two spring leaves 58 are disposed in the vicinity of the center plate 54 and are adapted to support the channel guide 50 along the channel 16. At an end of each of the arms 52 is provided a leg 60 configured to extend in the same direction along the channel 16 as the spring leaves 58. An ear 62 protrudes from each leg 60 toward the channel 16. Referring to FIG. 1, the channel also includes ear apertures 64 configured to receive the ears 62. The ear apertures 64 are sized slightly larger than the ears 62 both in the longitudinal direction and the axial direction to enable the ears 62 to be freely inserted into and removed from the ear apertures 64 and to accommodate differential axial thermal expansion. The ears 62 and ear apertures 64 thus provide a redundant support between the upper tie rod 22 and the channel 16 in the event that the spring loaded latch pins 38 fail. If it is desired to remove the upper tie plate from the assembly, the bolt 66 is removed from the corner post 35 of the upper tie plate 22, and the channel guide 50 is removed. The latch pins 38 are then compressed against the force of springs 44, either manually or using a tool, and the tie plate is lifted from the assembly using the bail handle 33. The fuel bundle can then be removed from the channel 16 by attaching a known grapple head to the coolant rod ends, which are specially shaped to facilitate an attachment tool. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.