Patent Number: 051768771
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is made first to FIGS. 11 to 13 which illustrate the effects which may be achieved by application of the invention. FIG. 11 shows the change in the control rod worth at cold when the ratio of the relative amount of U-235 contained in the region b to that in region c is increased. FIG. 11 is for the case where the relative amount of U-235 contained in the region a is the same as in the representative case and is the minimum of all the regions. Here, the relative amount of U-235 contained in the region b is increased by making the enrichment of the fuel rods, which are not adjacent to the larger water rod 3 positioned at the center of the fuel assembly 1, higher than that of the representative case. The relative amount of U-235 contained in a region is assumed to be expressed by: [Amount of U-235 Contained in Region]/[Amount of U-235 of Fuel Assembly]. The control rod worth at the cold is increased to an extent of 0.7% .DELTA.k/k by increasing the relative amount of U-235 contained in the region b by about 30% relative to the representative case to a level higher than that of U-235 contained in the region c. According to the design standards of the boiling water reactor at present, the shut-down margin is 1% .DELTA.k or more. This illustrates that the present invention can provide a large improvement. On the other hand, FIG. 12 plots the change in the reactivity difference between the run state and the cold state when the ratio of the relative amount of U-235 contained in the region b to that of region c is increased. If the relative amount of U-235 contained in the region b is increased to exceed that of U-235 contained in the region c, the reactivity difference between the run and the cold is decreased. FIG. 13 plots the change in the local power peaking factor. The increase in the amount of U-235 in the region b does not exert a large influence upon the increase in the local power peaking factor. Even if the relative amount of U-235 contained in the region b is increased by about 30% relative to the representative case, the increase in the local power peaking factor is about 3% at most. This is because the relative amount of U-235 is minimized in the region adjacent to the wider gap water region, as has been described above. Since the relative amount of U-235 contained in the region b is increased, it is possible to improve the control rod worth at the cold and the reactivity difference between the run and the cold and accordingly the cold shutdown margin while suppressing the increase in the local power peaking factor. A specific method for increasing the relative amount of U-235 contained in the region b can be realized by: (1) Establishing an enrichment distribution; and PA1 (2) Changing the fuel inventory among the regions. This second method can be realized by moving a large water rod, which is inserted to prevent the distribution of the thermal neutron flux in the fuel assembly from becoming heterogeneous at the central portion of the fuel assembly, to the region c to increase the number of the fuel rods contained in the region b, in addition to the following methods: (a) increasing the fuel pellet diameters; PA2 (b) increasing the number of fuel rods contained in the regions; and PA2 (c) increasing the density of fuel pellets. If the large water rod is moved towards the narrower water gap region providing less moderation, the flatness of the thermal neutron flux distribution in the fuel assembly can be improved, and the reactivity is improved. If the ratio of the thermal neutron flux in the narrower water gap region to that in the wider water gap region are compared, it is at 0.626 in the standard case whereas it can be at 0.738 in the present invention so that the flatness of the thermal neutron flux distribution is improved. As a result, the reactivity is improved by about 0.2% .DELTA.k. If, on the other hand, the relative amount of U-235 contained in the region b is greatly increased by the aforementioned method (1), the local power peaking factor in the region b may rise. According to the method of increasing the relative amount of U-235 contained in the region b by moving the aforementioned large water rod, however, the rise in the local power peaking factor can be suppressed more than by the aforementioned method (1). Specific embodiments of the invention will now be described with reference to FIGS. 1-6. In the various figures of the drawings, the same reference numerals are used for corresponding parts, and repeated description of these parts is avoided. The figures showing cross-sectional views of fuel assemblies are in a conventional form in order to show the arrangement of the different fuel rods in the array. Such fuel assemblies are generally well known to those skilled in the art, and their details need not be described. For a general view, see for example FIG. 3 of EP-A-284 016. FIG. 1 of the present drawings is a horizontal section showing the fuel assembly forming a first embodiment of the present invention. The fuel assembly 1 is constructed by arranging fuel rods 2 side-by-side in a square lattice or array (8.times.8) and by arranging a large water rod 3 at the center of the fuel assembly 1. The fuel rods 2 are of several types, indicated by A,B,C --. In the present embodiment, the enrichment of the fuel rods 2 with U-235 decreases in the order of A,B,C--, as enumerated in Table 1: TABLE 1 ______________________________________ Rod type A B C D E F G ______________________________________ Enrichment 4.40 4.10 3.50 3.00 2.80 2.30 1.90 (Wt. %) ______________________________________ In the present embodiment, the amounts of U-235 contained in individual notional regions 4 (region a), 5 (region b), 6 (region c) and 7 (region d) already defined above are adjusted in accordance with the desired enrichment distribution. As explained above, region 4 (region a) is adjacent the wide water gap (control rod region) and the region 7 (region d) is adjacent the narrow water gap (non-control rod region). By raising the enrichment of the fuel rods adjacent to the large water rod 3, according to the present embodiment, the relative amount of U-235 contained in the region b is maximized. In the present embodiment, this relative amount is such that the region b contains 27.0% of the total enrichment in the fuel assembly so that the amount of U-235 in region b is about 20% more than that of the representative case shown in FIG. 10. On the other hand, the relative amount of U-235 in the region a is about 16% equal to that of the representative case of FIG. 10. Like the fuel enrichment distribution which is adopted in the fuel assembly loaded in the D-lattice core being run at present, the average fuel enrichment in the region d facing the narrow water gap region is made higher than that of the region a facing the wide water gap so that the local power peaking factor can be suppressed. In this fuel assembly in which the relative amount of U-235 contained in the region b is increased, as has been described above, the control rod worth at cold, i.e. in the cold condition of the core, is higher by about 0.6% .DELTA.k/k than in the representative case of FIG. 10. Moreover, the reactivity difference between the run state and the cold state is reduced by about 0.04% .DELTA.k/k. On the other hand, the increase in the local power peaking factor is suppressed to about 3%. According to the present invention, more specifically, the relative amount of U-235 at the control rod side can be increased, as compared with that of the representative case of FIG. 10, thereby augmenting the control rod worth at cold while suppressing the increase in the local power peaking factor. The water rod 3 is central, i.e. is symmetrically located with respect to both diagonal lines of the array. FIG. 2 is a horizontal section showing a fuel assembly of a second embodiment of the present invention. The embodiment of FIG. 2 is modified from that of FIG. 1 by moving the larger water rod 3, which is positioned in the center of the fuel assembly 1 in FIG. 1, to the region c at the narrower water gap side to increase the number of fuel rods 2 in the region b thereby to increase the relative amount of U-235 contained in the region b. Thus in FIG. 2, the water rod 3 lies wholly within the region c, and is located symmetrically on the upper left to lower right diagonal line joining the corner at the middle of region a to the corner at the middle of region d. Like the structure of FIG. 1, the reference letters A, B, C--, and so on designate the enrichments of the fuel rods 2. In order to increase the relative amount of U-235 contained in the region b, this region b is arranged with a higher ratio of about 0.9 of the fuel rods having a maximum fuel enrichment to all the fuel rods in the region b. As a result, the relative amount of U-235 contained in the region b is 29.1% of the total amount in the fuel assembly. The than in the representative case of FIG. 10. On the other hand, the amount of U-235 in the region a is equal to that in the representative case of FIG. 10. Because of this arrangement in which the large water rod 3 is moved towards the narrower gap water side to increase the amount of U-235 in the region b more than in the embodiment of FIG. 1, the control rod worth at cold is augmented. The control rod worth at the cold of the present embodiment is more by about 0.7% .DELTA. k/k than in the representative case of FIG. 10 and by about 0.1% .DELTA.k/k than in the embodiment of FIG. 1. Since, moreover, the large water rod 3 is moved from the center of the fuel assembly 1 to the region c so as to increase the relative amount of U-235 contained in the region b, the reactivity difference between the run state and the cold state can be reduced in the present embodiment by about 0.2 % .DELTA.k/k compared with the representative case of FIG. 10. On the other hand, the increase in the local power peaking factor can be suppressed to about 3% at most, as in the embodiment of FIG. 1, by reason of the enrichment distribution. As has been described above in general discussion, moreover, the reactivity can be improved by about 0.3% .DELTA.k. FIG. 3 is a horizontal section showing a third embodiment of the present invention. In this case, the fuel rods 2 are arrayed in the form of a square 9.times.9 lattice, and a plurality of large water rods 3, in this case two, are used. The fuel rods 3 are both wholly in the region c and, taken together, are symmetrical about the upper left to lower right diagonal line. The enrichments of the fuel rods 2 of the present embodiment decrease in the order of rod types A', B', C', --and so on, and the symbol G' indicates the gadolinium-containing fuel rods 101. Gd is a burnable poison. The Gd.sub.2 O.sub.3 content of rods G' is 4.3 wt. %. The fuel enrichments of the fuel rods are enumerated in Table 2. TABLE 2 ______________________________________ Rod type A' B' C' D' E' F' G' ______________________________________ Enrichment 4.90 4.50 3.80 3.50 3.10 2.60 3.95 (Wt. %) ______________________________________ The amount of U-235 contained in the region b is about 31% relative to the whole fuel assembly. This value is higher by about 15% than that in the representative case of a 9.times.9 lattice of FIG. 14. The amount of U-235 contained in the region a is 17% and equal to that of the representative case of FIG. 14. In this embodiment having the 9.times.9 fuel rod lattice and the plurality of large water rods, the control rod worth at cold can be made higher by 0.4% .DELTA.k/k than in the representative case of FIG. 14, with a local power peaking factor the same as that of the representative case of FIG. 14. Moreover, the reactivity difference between the run state and the cold state can be reduced by about 0.1 % .DELTA.k/k. The large water rods are utilized to increase the relative amount of U-235 in the region b in this embodiment, but the average fuel density in the rods of region b can be higher than that of any other region, or the average outer diameter of the fuel pellets in the rods of region b may be larger than that of any other region. In the foregoing embodiment, the control rod worth at cold is increased by about 0.9% .DELTA.k/k by increasing the fuel density of the portion of the fuel belonging to the region b by about 8%. FIGS. 4(a) and 4(b) are a horizontal section showing the fuel assembly of a fourth embodiment of the present invention and a distribution diagram of its fuel enrichment (in weight %), respectively. Some of the fuel rods have an uneven axial distribution of the fuel enrichment. FIG. 4(b) also shows the proportions (in 24ths) of the fuel rod height in which the fuel is distributed. In FIG. 4(a) and also in FIG. 5(a) below, the regions a,b,c,d defined above are not indicated, but the orientation of the diagram is the same as that of FIGS. 1 to 3. The enrichments (wt. %) of the individual fuel rods types in the present embodiment are designated by A.sub.1, B.sub.1, C.sub.1 --G.sub.1 and G.sub.2 FIG. 4(b) shows that there is uneven axial distribution of the fuel enrichment (wt. %) of the fuel rod symbols B.sub.1 and C.sub.1. The fuel enrichments of the individual fuel rods are shown in FIG. 4(b). For the upper and lower portions of the fuel assembly, the relative amount of U-235 contained in the region b is about 31% compared with the total amount. The relative amount of U-235 contained in the region a (i.e. 18% in the upper portion of the fuel assembly, and 17% in the lower portion) is substantially equal to that of the representative case of a 9.times.9 lattice of FIG. 14. In the present embodiment, too, the control rod worth at cold and the reactivity difference between the run state and the cold state can be improved as in the embodiment of FIG. 3. Moreover, the axial power distribution can be flattened, and the reactivity difference between the run state and the cold state can be improved, permitting reduction of the amount of gadolinium added and improving the neutron economy. FIGS. 5(a) and 5(b) are a horizontal section showing the fuel assembly of a fifth embodiment of the present invention and a distribution diagram of its fuel enrichment (in weight %), respectively. FIG. 5(a) shows a fuel assembly in which the present invention is applied to an upper portion of the reactor core, when the fuel assembly is loaded. In the present embodiment, the portion from 11/24 to 22/24 of the effective fuel length from the lower end of the fuel rods has the structure of the present invention. As shown, the fuel assembly of the present invention is obtained by changing the fuel enrichments (wt. %) of the fuel rods B.sub.2 and C.sub.2 in the upper portion of the fuel assembly. In the present embodiment, the relative amount of U-235 contained in the region b at the axial upper portion is about 31% compared with the total amount of that portion. In this portion of the fuel assembly, which corresponds to the upper portion of the core, the reactivity difference between the run state and the cold state can be improved, as has been described. The cold shut-down margin is one of the design restricting conditions of the core. The cold shut-down margin at the end of the cycle is seriously influenced by the reactivity difference between the run state and the cold state of the fuel at the upper core portion. The cold shut-down margin becomes more significant for a larger reactivity difference between the run state and the cold state of the fuel assembly. By applying the present invention to the upper portion of the core, according to the present embodiment, the reactivity difference between the run state and the cold state can be reduced without deteriorating another core running characteristic such as the maximum linear heat generation ratio, thus improving the cold shut-down margin. In the embodiments of the invention described above, the ratios of the average concentration of fissile material per fuel rod in region b to that in region c in each case are as follows: ______________________________________ FIG. l 1.10 FIG. 2 1.13 FIG. 3 1.08 FIG. 4 (upper portion) 1.08 FIG. 5 (upper portion) 1.08 ______________________________________ FIG. 6(a) is a diagram showing a part of a D-lattice core, in which a fuel assembly of the present invention e.g. as shown in FIG. 3 and three fuel assemblies 121 of the prior art are arranged around a cross-shaped control rod 122. FIG. 6(b) shows one embodiment of a boiling water reactor core 131 when the fuel assembly (of FIG. 6a) of the present invention is loaded in the core. FIG. 6(a) shows that the region a, i.e., the outer fuel rod group, in which the relative amount of the fissile material relative to that of all the regions is minimized, faces the cross control rod 122. The present embodiment is directed to a transfer core in which the three fuel assemblies 121 of the prior art and one fuel assembly of the present invention are loaded around the cross control rod 122. Although the transfer core is presented as the embodiment, the core may be constructed exclusively of the fuel assemblies of the present invention. Since the fuel assembly of the present invention can improve the control rod worth at cold and the reactivity difference between the run state and the cold state, it is possible to improve the cold shut-down margin of the core which is loaded with the fuel assembly of the present invention. As has been described in the general description, moreover, the fuel assembly of the present invention can flatten the neutron flux distribution in the assembly. As a result, it is possible to reduce the degree of radiation damage of the channel box enclosing the fuel assembly. If, moreover, the supply of plutonium grows sufficient in the future to allow the use of MOX fuels, (mixed oxide fuels) the effects of the present invention can be further enhanced by using Pu-239 at least partly in place of the amounts of U-235 of the region b in the embodiment of FIG. 1, for example.