Patent Number: 052672913
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a perspective of a nuclear reactor core of a boiling water nuclear reactor is illustrated at four adjacent fuel bundles B1-B4. The four fuel bundles B1-B4 rest on fuel support S at the lower end and extend upwardly to a top guide G. Sealed fuel rods R extend from lower tie plate L above core support S to upper tie plate U. As can be seen lower tie plate L, fuel rods R, and upper tie plate U are all surrounded by channels C. In the case of fuel bundle B3, channel C and rods R are shown broken away without the upper tie plate U. Sufficient breaking away has occurred to expose one spacer S. Flow within the fuel bundles B1-B4 is easy to understand. Water flows into the fuel bundles B1-B4 from below core plate P at support S where the flow is distributed by fuel support S to equal flow among fuel bundles B1-B4. Flow through the lower tie plate of water occurs. The water passes upwardly confined by channels C and finally out of up tie plate U. During this flow the channels C separate flow interior of the fuel bundle from core bypass region R between the fuel bundles. It will be noted that fuel bundles B1-B4 are square in section and held in spaced apart relation with respect to one another. This being the case, it will be understood that the respective fuel bundles define a cruciform interstices of the core bypass region R. In the sketch here, a control blade N is shown partially inserted for control of the reaction. During normal operation, the control blade N is withdrawn, core bypass region R flooded, and reaction continues aided by the moderating effect of water moderator in region R. Flow to core bypass region R is well known and will not be specifically illustrated here. It will suffice to say that certain metering apertures at the bottom of fuel bundles B1-B4 and other reactor leakage paths maintain this region flooded with liquid moderator. Sufficient flow is provided to the region to prevent flashing of the contained water moderator to steam. Referring to FIG. 2, bundle B1 is illustrated in section adjacent spacer S. The fuel bundle comprises a 9 by 9 matrix of fuel rods R with a central water rod W for supplying a high moderator fraction to the central portion of fuel bundle B1. It will be understood that there is a flow region for the generation of steam interior of the fuel bundles while the surrounding region R is flooded with water. Attention can be devoted to the construction of spacer S. The illustrated spacer S is a so-called ferrule type spacer S have a matrix of interconnected ferrules F each surrounding a rod R at the elevation of the spacer. The spacers as a group are surrounded by band 14. FIG. 2 is a cross-section that illustrates the prior art by showing over emphasized the difficulties of the prior art. Specifically, spacer S has moved to the left opening a large gap 18 with respect to one wall of channel C at small gap 20 and closing small gap 20 with respect to the remaining channel wall. As has heretofore been stated, this migration of spacer S as a group causes rods R adjacent channel C to be even more proximate moderator within region R (and become more reactive and generate more heat) while at the same time restricting the flow of moderator within the fuel bundle B1 adjacent the inside wall of channel C at small gap 20. This will cause the critical power of the peripheral fuel rods R at this location to decrease. Once the critical power decreases at one bundle location, the remainder of the fuel bundle B1 is required to be restricted in heat out put so that the critical power limit at a fuel rod R is no where exceeded within a fuel bundle. Referring to FIGS. 3, 4, and 5, modification to band 14 is illustrated to prevent the prior art condition of FIG. 2. Band 14 (of a spacer S--the rest of the spacer being omitted for clarity) is shown with respective near sides 31, 32 and far sides 33, 34. Referring to far sides 33, 34, it will be seen that side 33 is formed with protrusions 45, 46 while side 34 includes formed protrusions 47, 48. Referring to the detail of FIG. 4, it can be seen that these respective protrusions are "bath tub" like indentations impressed to the metal of band B and relatively thick--especially when compared to protrusions 41-44 described hereafter. It is the purpose of the four protrusions 45-48 to occupy a full centering interval between a channel C and a spacer S at band B (See FIG. 6). Referring to near sides 31, 32, it will be seen that side 31 is formed with protrusions 41, 42 while side 32 includes formed protrusions 43, 44. Referring to the detail of FIG. 5, it can be seen that these respective protrusions are again "bath tub" like indentations impressed to the metal of band B and relative thin--especially when compared to protrusions 45-48. It is the purpose of the four protrusions 41-44 to occupy less than a full centering interval between a channel C and a spacer S at band B (See FIG. 6). This interval defined by the protrusions 41-44 with respect to channel C is chosen with two purposes in mind. First, clearance is required to fit the channel C over the remainder of the assembled fuel bundle B during assembly. Accordingly, protrusions 41-44 are less than protrusions 45-48 by a sufficient interval to define this assembly clearance. Second, a minimum interval is defined from the fuel rods R adjacent channel C to define a "worst case" critical power limit. Accordingly, protrusions 41-44 are dimensioned to provide this "worst case" critical power dimension. Finally, it will be seen that near sides 31, 32 include leaf springs 61-64. It will suffice to discuss leaf spring 64 as illustrated in FIG. 5. It will be understood that springs 61-63 are identically constructed. Referring to FIG. 5, leaf spring 64 is attached by some suitable means to band 14 at 65. Leaf spring 64 has an essentially U-shaped configuration imparted by bends at 65, 66, 67 and 68. A large raised central portion 70 resiliently bears against a channel C while the distal end 69 of spring 64 from attached portion 65 fits in sliding relation over the exterior of band 14. Preferably, springs 61-64 are located adjacent the corners of channels C. In this location, they bear on channel C where the channels have maximum strength. Function of springs 61-64 is plain to see in FIG. 6. Simply stated, springs 61-64 bias spacer S at band 14 on sides 31, 32 away from channel C. This causes registration of protrusions 45-48 to channel C with protrusions 41-44 being spaced apart from channel C. Centering of the confined fuel rods R within channel C occurs at the optimum interval established by protrusions 45-48 and the dimension of spacer S with respect to channel C. We prefer the leaf springs 61-64 vertically disposed. Similarly, protrusions formed from "bath tub" like projections in the metal are shown. The reader will understand that protrusions and springs can include a wide variety of equivalents.