Patent Number: 055984509
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION This invention relates to an improved fuel bundle assembly, one of which is shown in FIG. 1 at 10. It will be understood that the fuel bundle assembly is not shown in its true length, and instead is broken away so as to illustrate the bottom and top portions of the bundle only. The assembly includes an upper handle 12 and a lower nose piece 14. A channel 16 extends upwardly from the nose piece end substantially the full length of the fuel bundle assembly 10. Individual fuel rods 20 are disposed in a matrix interior of the fuel assembly, i.e., surrounded by the channel 16. The full length fuel rods (FLR's) 20 extend between a lower tie plate 22 and an upper tie plate 24 in a well known manner, whereas partial length fuel rods (PLR's) 20' extend upwardly from the lower tie plate 22 but terminate short of the upper tie plate 24 as explained in greater detail below. The rods 20 (including PLR's 20') are normally arrayed in rows and columns. Further, and because of the length of the fuel assemblies (on the order of 160 inches), spacers, e.g., S.sub.1, S.sub.2 -S.sub.N are placed along the length of the fuel assembly to retain the rods in the desired array and to minimize or eliminate lateral vibration of the fuel rods. Typically, seven such spacers, roughly evenly spaced at 20 inch intervals, extend from the top to the bottom of the fuel assembly. As has been mentioned above, this invention relates to a new fuel bundle design for placement interior of the channels 16. Referring particularly to FIG. 2, fuel rods 20 are arranged in a 9.times.9 matrix or array. Were it not for the presence of the central water rod 26, eighty-one (81) individual fuel rods would extend the length of the matrix of fuel rods shown in FIG. 2. The invention constitutes modifying preferably the upper two thirds of the fuel assembly. Specifically, the invention here relates to the modification of the PLR's 20' which extend at least one-half of the length of the fuel assembly. As already noted, the PLR's 20' extend from the bottom tie plate 22 and extend upwardly toward the upper tie plate 24. The PLR's terminate short of the upper tie plate 24, however, preferably adjacent a spacer, e.g., S.sub.2 as shown in FIGS. 1-3; and S.sub.3 as shown in FIG. 4. For purposes of this invention, it is not important where the PLR's terminate, although PLR's will typically extend into the upper two phase region of the bundle. It is preferred that the PLR's 20' be located at least within the second row of the array removed from the channel 16. In order to illustrate the location of these PLR's in the perspective of FIG. 2, the first row of rods has been omitted from that portion of the perspective that is towards the viewer. What the viewer sees, then, is the second row of rods. The invention here, however, is not limited to any particular location for the PLR's within the bundle. With reference now to FIG. 4, a removable extension rod, or unfueled follower 32, is secured to the upper end of a PLR 20', extending upwardly from spacer S.sub.3 through spacers S.sub.1 and S.sub.2 to the upper tie plate 24 where they are secured by any suitable means. A similar follower or extension would be installed for each PLR 20' in the bundle. The followers or extension rods 32 may be secured to the uppermost ends of the PLR's 20' by any suitable fastening means such as a bayonet joint, screw threads or the like. The followers or extension rods 32 may be secured at their upper ends directly to the upper tie plate 24, or they may terminate or a few inches above the uppermost spacer S.sub.1, depending on the design of the upper tie plate. In other words, if the upper tie plate 24 is not designed to accommodate such followers, the latter can simply terminate short of the tie plate, adjacent the uppermost spacer S.sub.1. The followers or extension rods 32 may be tubular or solid in form, depending on desired reactivity characteristics as discussed below. It also may be desirable to have a number of such unfueled followers or rod extensions 32 attached to each other to facilitate easy removal from the bundle during a reactor outage. From a geometric standpoint, it will be appreciated that PLR's 20' with extensions 32 mimic FLR's 20 in terms of coolant pressure drop particularly in the two phase region of the channel, by increasing flow resistance and decreasing flow. In further accordance with the invention, power of reload bundles incorporating the extensions 32 can be adjusted to better match power requirements of the existing fuel bundles in the core. For example, the extensions 32 may be provided as hollow Zircaloy tubes and filled almost entirely with a single phase water in order to increase reactivity, i.e., increase power. On the other hand, the extensions 32 (solid or hollow) may be devoid of any hydrogen bearing material to thereby decrease reactivity and decrease power. Reactivity may be further reduced via addition of gadolinium (via natural uranium, for example) to a hollow Zircaloy follower. By thus being able to adjust both bundle flow and reactivity characteristics, control rod movement can be minimized and operating margins improved. 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.