Patent Number: 042696615
Section: description

Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIGS. 1 and 2 the upper portion of a fuel assembly 10 including multiple fuel rods 12 and control guide tubes 14 held in spaced relationship with each other by multiple grids 16 disposed along the fuel assembly length. The fuel assembly is supported on a bottom nozzle and lower core plate, not shown, in accordance with conventional practices. As shown in FIG. 3, the top nozzle includes a number of guide tube extensions 18, one for each control rod guide tube, each of which is threaded at its bottom end into orifice plate 20 and secured firmly in place by welds 22. In order to firmly anchor the control rod guide tubes 14 to the top nozzle, FIG. 3, the guide tube extension is provided with a pair of spaced grooves 24 and the control rod guide tube 14 inserted thereinto is squeezed into the openings or grooves 24 which deforms the metal and thus provides an inseparable fit between the parts. The upper end of the guide tube extensions 18 extend through corresponding openings formed in a hold-down plate 26, FIGS. 1 and 2, which also include flow channels 42 through which coolant flows after cooling the fuel assembly. The ends of the guide tube extensions also project upwardly into upper core plate 28. Helical springs 30 are concentrically disposed on the guide tube extensions and bear at their lower ends against the orifice plate 20 and at their upper ends against the hold-down plate 26, thus urging the hold-down plate into contact with the lower surface of upper core plate 28. It will be noted that the upper core plate 28 includes a substantially large opening 32 into which the guide tube extensions extend. It further will be understood that control rods 33 are adapted for vertical movement in the control rod guide tubes 14 during the course of controlling reactor operation. In order to limit the upward movement of the fuel assembly 10, each of the guide tube extensions is equipped with an axially extending slot 34, FIG. 4, in which a pin 36 is adapted to ride when the fuel assembly is moved upwardly. The plan view of the assembly shown in FIG. 2 more clearly shows the design of the hold-down plate 26 and how the pins 36 are adapted to ride in the slots 34 located in each of the four corner guide tube extensions 18. As more clearly shown in FIGS. 2 and 5, the central control rod guide tube extension 18 located in the center of the four corner guide tubes carries a pair of oppositely disposed slots 38 and corresponding pins 40 in the hold-down plate 26. The large number of openings 42 formed by ligaments 44 are utilized to facilitate the flow of coolant through the fuel assembly. In practice, the axis of fuel rods will lie immediately below the ligaments in order to eliminate the possibility of a fuel rod being ejected from the fuel assembly in the event it encounters unusually heavy coolant flows or other forces which may move a fuel rod upwardly relative to the fuel assembly components. In operation, after all the fuel assemblies have been set in position in the reactor, the upper core plate 28 is lowered into position such that its lower surface 46 engages the upper surface of hold-down plate 26. In so doing, the springs are compressed to a slight degree thereby applying a downward force on the orifice plate 20 and control rod guide tubes 14. The guide tubes 14 transmit this load through the lower nozzle to the base of the reactor. The design of the springs and the distance between the orifice plate and hold-down plates are chosen such that the springs will never be fully compressed during reactor operation. In the event a heavy hydraulic lifting force is applied to the fuel assembly, the complete fuel assembly will move upwardly thus compressing springs 30 and causing the pins 36 and 40 respectively to ride in their slots 34 and 38 formed on the guide tube extensions. The springs therefore will absorb the lifting forces and transmit the same through the hold-down plate 26 and into the upper core plate 28 for further distribution through the internal structure of the reactor. As the assembly moves upwardly and thus compresses the springs, the guide tube extension slots will ride on the pins until, for each outer guide tube extension, the bottom of the slot is contacted by the pin, and for the center guide tube extension the bottom surface of the hold-down plate is contacted by the upper surface of the center guide tube extension shoulder 39. Since this will uniformly occur in all of the guide tube extensions, uniform loading of the guide tubes will occur but only after the springs have been compressed to the desired degree. As indicated previously, the springs have a constant such that they never are compressed to the point where all coils in each spring contact one another since this then would involve a solid structure which is intended to be avoided. The modification of FIG. 6 illustrates an arrangement where pads 50 are used at each of the four corners of the assembly, and at the center, where each control rod guide tube extension 18 projects upwardly through the hold-down plate 26. Each of these pads on the orifice plate 20 project upwardly a slight distance while the bottom surface of the hold-down plate likewise has four downwardly projecting pads 52 arranged to contact the lower pads when the fuel assembly is lifted. These nozzle pads limit the amount of fuel assembly lift in the event of an accident, such as a blow down and the four corner pads provide lead in chamfers 54 for fuel handling. The nozzle pads are sized such that they contact the hold-down plate prior to the springs being compressed solid and thereby present the nozzle guide tube extensions from topping out in the blind holes in the upper core plate. This arrangement helps assure uniform loading of the guide tubes during any accident which causes fuel assembly lift. It will be apparent that many modifications and variations are possible in light of the above teachings. It therefore is to be understood that within the scope of the appended claims the invention may be practiced other than as specifically described.