Patent Number: 047042478
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a typical fuel rod assembly 10 includes individual fuel rods 11 (64 rods are shown in FIG. 1), guide rods 13 and a handle member 14. The individual fuel rods (also called fuel pins) 11 are about 0.4-0.6 inch in diameter and about eight feet long in one type of nuclear reactor installation, about 15 feet long in another type of nuclear reactor installation. The fuel rod assembly 10 is withdrawn from a nuclear reactor after the nuclear fuel within the fuel rods 11 has been spent. Thereafter, the fuel rod assembly 10 is stored in appropriate storage racks under water in storage pools until its activity is dissipated. The purpose of the present invention is to compact the fuel rods 11 after their activity has dissipated and to store the fuel rods in a new and different container wherein their spacing is altered. As shown in FIG. 2, a fuel rod assembly 10 initially has its upper end removed so that the top ends 15 of the individual fuel rods 11 are exposed. The top end is removed by cutting or otherwise. One way of removing the top end is to cut the top elements with an air-powered underwater band saw. In some fuel rod assemblies, the top end may be dismantled by removing the bolts or other devices which connect it to the main frame. After the top end of the assembly is removed, the top ends 15 of the individual fuel rods 11 are exposed as shown in FIG. 2. FIG. 3 is a cross-section view taken along the line 3-3 of FIG. 2 showing the spacing pattern (array) of the fuel rods 11 within the fuel rod assembly 10. Placed in tandem with fuel rod assembly 10 is a transition funnel 20 which has a lower end 21 and an upper end 22. The lower end 21 is shown in a plan view in FIG. 4 as a generally square grid corresponding to the cross-section of the fuel rods 11, as shown in FIG. 3. The lower end 21 is a grid 23 having openings for individual tubes 24 corresponding in number and array with the top ends 15 of the fuel rods. The transition funnel tapers from its lower end 21 toward its upper end 22. At the upper end 22, the transition funnel 20 has a plan view as shown in FIG. 5 having a grid 25 with openings for receiving the top ends of the tubes 24 in a desired array. It will be observed that the array of the tube openings 24 in the grid 25 is equilateral triangular--a preferred array. Above the transition funnel 20 is a container 30 having outer dimensions corresponding to the outer dimensions of the fuel rod assembly 10. The container 30 preferably is a metal rectangular box having a length slightly greater than the length of the fuel rods 11 and having sufficient cross-sectional area to receive the compacted fuel rods from a fuel rod assembly 10 in approximately half of its cross-sectional area. In one embodiment, a vertical baffle 31 may be provided to divide the container 30 into parallel chambers 32, 34. All of the fuel rods 11 from a fuel rod assembly 10 can be confined in the chamber 32 as shown in FIG. 2. All of the fuel rods from another fuel rod assembly can be confined in the chamber 34. Extending downwardly through the container 30 are a number of individual wires 40 corresponding to the number of fuel rods 11 in the fuel rod assembly 10. The individual wires 40 are connected at their upper end to a tensioning device 41 such as a tensioning reel or individual tensioning reels for each wire 40 or for groups of wires 40. The wires 40 extend through the chamber 32 and enter, one each, into one of the tubes 24 within the transition funnel 20. Each of the wires 40 extends through the grid 23 at the bottom of the transition funnel 20 and terminates in a fuel rod gripping device 42. The fuel rod gripping devices preferably are helical sleeve tension grippers which are secured at their upper ends to the wires 40 and which at their lower ends depend as a sleeve which can be engaged with the top end 15 of an individual fuel rod 11. The operator, employing remote control devices, connects each of the fuel rod gripping devices 42 to a corresponding fuel rod upper end 15. After all of the devices 42 have been connected, the tensioning device 41 is activated and the wires 40 are drawn upwardly through the transition funnel 20 and the chamber 32. Each of the fuel rods 11 is withdrawn from the fuel rod assembly 10 upwardly through an individual tube 24 such that portions of the fuel rods reside in both the fuel rod assembly 10 and the fuel rod directing chamber 12, and, later, in both the fuel rod directing chamber 12 and the container 30 and into an altered array, preferably an equilateral triangular array as shown in FIG. 5. The fuel rods 11 preferably are drawn at a rate such that their upper ends 15 enter into the chamber 32 concurrently and whereby the compacted nesting of the fuel rods 11 is readily achieved within the chamber 32. The tension on each fuel rod required for withdrawal is from about 20 to 200 pounds. After the wires 40 have been withdrawn to the top 33 of the container 30, the individual gripping elements 42 are separated from the fuel rods 11. The chamber 32 is thereafter filled with fuel rods in a compact array. The fuel rod assembly 10 no longer contains fuel rods 11 and can be withdrawn from the water pool for storage and ultimate disposal in an appropriate fashion. The container 30 is subsequently advanced to another fuel rod assembly along with the transition funnel 20. The wires 40 are introduced through the alternate chamber 34 and the transition funnel 20. The process is repeated and the alternate chamber 34 is filled with fuel rods. The container 30, holding fuel rods in a compacted array, can be stored under water in the water storage pool in the same type storage rack which formerly housed the fuel rod assembly 10. The storage capacity of a water storage pool can be nearly doubled by practicing this method. The precise construction of the transition funnel 20 is such that the tubes 24 merge from the lower end 21 to the upper end 22. As the fuel rods are drawn upwardly through the tubes 24, the fuel rods cannot increase their rod-to-rod spacing but, instead, are merged into an ever-increasing density whereby the reactivity of the array is continuously reduced. Thus the possibility of developing a critical spacing of the fuel rods is precluded throughout the controlled densifying operation.