Patent Number: 048271390
Section: description

DETAILED DESCRIPTION In FIG. 1 there is shown, for purposes of illustration and to facilitate an understanding of the ensuing discussion, a typical spent nuclear fuel shipping cask 11, mounted in its shipping cradle 12. The outer shell of cask 11 may be of steel or other suitable strong material. Within cask 11 is contained a spent nuclear fuel basket 13, consisting of a number of elongated tubes or cells 14 into which the nuclear fuel rod assemblies (not shown) are inserted for transport. Any empty spaces between the basket 13 and the shell of cask 11 may be filled with filler blocks 16 of suitable material to hold the basket 13 in place within the cask 11. The cask 11 is sealed with end plates 17 and 18 for holding the basket assembly in place longitudinally. The cask itself may include an outer jacket 19, having a plurality of channels 21, 21 containing water, for example, for containing neutrons emitted from the fuel cells and also for cooling. In FIG. 2 there is shown, in perspective, a tube member 22 for use in the basket of the present invention. For illustrative purposes, a portion of a fuel rod assembly 23 is shown, consisting of a square base 24 and fuel rods 26, 26 mounted thereon. Tube 22 is preferably made by extrusion of aluminum or a boron-aluminum alloy to form a seamless tube forming a hollow, square holding cell of dimensions such that the fuel rod assembly 23 is virtually slip-fitted therein so that it is held snugly within tube 22. Instead of extrusion, the tube 22 may be formed by swaging, or by welding along one edge thereof. This last expedient is the least desirable, but, as will be apparent hereinafter, in the overall construction of the basket the single, strong weld will not be overly deleterious to the function and stress resistance of the basket. It is also possible to make the tube 22 of stainless steel or other non-corrosive material, although aluminum is preferred for a variety of reasons, among which is its better heat conductivity, and the fact that, when alloyed with boron, it functions as a neutron poisoning material, thereby restricting large amounts of neutron interaction between the fuel cells. FIG. 3 depicts a fuel basket and cask assembly 31 embodying the principles of the present invention. The cask 32 of assembly 31 comprises three spaced concentric rings or shells 33, 3, and 36 of, for example, steel. The space 37 between rings 33 and 34 may be filled, for example, with water, which functions to suppress neutron radiation, and, to some extent also functions as a coolant. Alternatively, the space 37 may be filled with a hydrogen containing material such as, for example, Bisco.RTM. NS4FR. Also, the hydrogen containing material may have ducts or passages (not shown) containing water. If water alone is used in space 37, the spacing between rings 33 and 34 may be maintained by suitable webs (not shown) of sufficient number to maintain a high degree of structural strength. The space 38 between rings 34 and 36 is preferably filled with lead, which blocks gamma radiation from the fuel rods. Inside of ring 36 is the basket assembly embodying the principles of the present invention. The basket 39 comprise a plurality of fuel containing tubes 22 arranged in the stack pattern shown to maximize the number of tubes within the inner ring or shell 36. Each tube is totally independent of every other tube, there being no physical connection between any of the tubes, although the individual tubes are adapted to maintain adjacent tubes in position along at least one axis in the pattern shown. Between adjacent tubes in the assembly are inserted spacer slabs 41, 41, of a neutron poising material such as an alloy of boron and aluminum. One such material is known as Boral.RTM.. The slabe 41, may be attached to an adjacent tube, for ease of assembly, but in no case is a slab 41 connected to two adjacent tubes. The entire assembly of tubes 22 with spacers 41 is maintained firmly in position relative to each other and the inner shell 36 of the cask by filler blocks 42, 43, and 44, which are preferably extruded from a neutron poisoning material such as an alloy of aluminum and boron, e.g., Boral.RTM., and inserted into the spacer formed by the pattern of the assembly and the circular surrounding wall 35. Because various tolerances are involved in the tube dimensions and the wall or ring 36, some slight machining of the filler blocks 42, 43, 44 may be necessary to achieve a substantially slip fit in the assembly. Blocks 42, 43, and 44 function not only as neutron poisoning members, but also as heat conductors. When the filler blocks have been inserted into the assembly, the basket 39, although made up of a number of totally independent components, is, to all intents and purposes, a rigid structure, and under normal transporting and handling conditions, is as rigid as various prior art structures. Under other than normal conditions, i.e., where various stresses are introduced, the structure of the present invention, where substantially all of the elements are independent of each other, is better able to withstand these stresses. Referring now to FIG. 3 and Tables I and II, the affect of various stresses on a typical prior art basket and the basket of the present invention is shown, based upon a stress analysis in which three different impact conditions and one condition of differential thermal expansion were considered. The analysis showed that for a side drop of the cask (as opposed to an end drop, where stresses are quite small), stress was at a maximum for a 45.degree. orientation of the basket to the point of impact, and the maximum at the point B, shown in FIG. 3, for both the prior art basket and the basket of the present invention. However, as shown in Table II, the relative stress at point B was approximately 12% less for the basket of the present invention. In the same manner, for a zero degree orientation, maximum stress occurred at point A as seen in FIG. 3, but the relative stress was approximately 9% less in the basket of the present invention. For a 90.degree. orientation, relative stresses were the same, although for the prior art basket, maximum stress occurred at point C, whereas it occurred at point D for the basket of the invention. This last indicates the efficacy of the theory underlying the structure of the present invention. Although the point of impact was at 90.degree. as shown in FIG. 3, for the prior art basket, the stress travelled through the structures, increasing to a maximum at point C, because being a solid, connected structure, the stress was transmitted through a number of tubes to the point of maximum stress, thus representing potential, and most probably actual damage to all of the intervening tubes. On the other hand, for the basket of the invention, the single tube at point D absorbed most of the impact because is was, or is, independent of the other tubes which are themselves independent, so that stresses are not readily transmitted. Because the tubes are independent of each other, under conditions of extreme stress there is presented at their boundaries, i.e., walls, a high impedance to the transmission of the stress to adjacent tubes. As can be seen in Table II, the behavior under conditions of differential thermal expansion are even more pronounced. Assuming a maximum differential thermal expansion stress at point E, the stress for a solid or welded basket structure is approximately three times as great as for the structure of the invention. This can be at least partially explained by the fact that in a solid or welded structure all of the tubes are interdependent, actually presenting a unitary structure to the stress, while in applicants' basket, all tubes are independent of each other and stress on one does not imply stress on all. Earlier it was mentioned that the tube 22 could be fabricated by welding along one edge. Welding is not desirable, since welds are susceptible to stresses and tend to crack and break under heavy stress. However, because all of the tubes 22 are independent of each other, the damage caused by a breaking weld is limited to the tube on which the weld is located and the remaining tubes are substantially unaffected. FIG. 4 depicts a tube 22 to which the aluminum-boron spacer slabs 41, 41 are affixed as by welding, brazing or other suitable means, to only two sides. It can be seen that in a pattern of tubes such as is shown in FIG. 3, this is all that is necessary to insure that there will be a spacer between any pair of adjacent tubes. In this configuration, the spacer becomes a part of the tube, which still remains independent of all other tubes and spacers. The arrangement of FIG. 4 facilitates assembly of the basket in the desired pattern, eliminating the difficulty of inserting the spacer slabs into place. FIG. 5 depicts a modified version of the tubes 22, also intended to facilitate assembly and insure proper location of the spacers 41. Each of the tubes 22 is provided with two pairs of locating tabs 51, 51, and 52, 52, extending the length of the tube. The tabs are formed during the extrusion formation of the tube 22. The depth of the tab is substantially the same as the thickness of the spacer 41, for example, 0.170 inches. The tabs, as can be seen from FIG. 5, in conjunction with the untabbed wall of the adjacent tube, form a pocket for insertion of the spacer, which is substantially a slip fit therein. As can be seen in FIG. 5, only two sides of each tube 22 need to be tabbed, thereby insuring that there will be a spacer between each pair of adjacent tubes. It is readily apparent from the foregoing that the invention comprises a new spent fuel basket assembly that is less susceptible to damage or failure arising from the application of dynamic stresses to the cask or basket. While the foregoing illustrative embodiments of the invention represent preferred forms thereof, various modifications and changes may occur to persons skilled in the art without departure from the spirit and scope of the invention.