Patent Number: 
Section: description

Preferred embodiments of the present invention will be described with reference to the accompanying drawings. First, a state where only reload fuels are placed in the reactor core of a nuclear reactor will be explained. FIG. 1 is a quarter sectional view of a fuel loading pattern for an equilibrium cycle formed by a method representing a first embodiment according to the present invention. Referring to FIG. 1, an equilibrium core comprises 872 reload fuel assemblies including 196 fuel assemblies for the first cycle (hereinafter referred to as xe2x80x9cfirst cycle fuelsxe2x80x9d), 196 fuel assemblies for the second cycle (hereinafter referred to as xe2x80x9csecond cycle fuelsxe2x80x9d), 196 fuel assemblies for the third cycle (hereinafter referred to as xe2x80x9cthird cycle fuelsxe2x80x9d), 196 fuel assemblies for the fourth cycle (hereinafter referred to as xe2x80x9cfourth cycle fuelsxe2x80x9dn), and 88 fuel assemblies for the fifth cycle (hereinafter referred to a xe2x80x9cfifth cycle fuelsxe2x80x9d). The mean enrichment factor of the reload fuel assemblies is about 3.8 wt %. The reactor core is provided with twenty-four first control cells each consisting of four little fissioned fuel assemblies having a relatively large infinite multiplication factor, and thirteen second control cells 20 each consisting of four considerably fissioned fuel assemblies having a relatively small infinite multiplication factor. The first control cell has first cycle fuels 1, second cycle fuels 2 and third cycle fuels 3. The second control cell 20 has fourth cycle fuels 4 and fifth cycle fuels 5. The first control cells are divided into two groups differing from each other in the operation of the control rods; twelve first control cells 11 of a group A (hereinafter referred to a xe2x80x9cfirst control cells (A)xe2x80x9d) and twelve first control cells 12 of a group B (hereinafter referred to as xe2x80x9cfirst control cells (B)xe2x80x9d). The first cycle fuels 1, the second cycle fuels 2 and the third cycle fuels 3 are disposed in regions other than an outer peripheral region of the reactor core and regions for the second control cells 20. The fifth cycle fuels 5 are disposed in the outer peripheral region and in the regions for the second control cells 20. The fourth cycle fuels 4 are disposed in the outer peripheral region of the reactor core, the regions for the second control cells 20, regions providing relatively high output and outer regions of the reactor core. After an operation cycle has been completed, the first cycle fuels 1 are moved to positions from which the second cycle fuels 2 have been removed, the second cycle fuels 2 are moved to positions from which the third cycle fuels have been removed, the third cycle fuels 3 are moved to positions from which the fourth cycle fuels 4 have been removed and new reload fuels are disposed at positions from which the first cycle fuels 1 have been removed. After the completion of the operation cycle, the 116 considerably fissioned fourth cycle fuels 4 among the fourth cycle fuels 4 and all the fifth cycle fuels 5 are removed from the reactor core, and the remaining 80 fourth cycle fuels 4 are moved to positions from which the fifth cycle fuels 5 have been removed. An operation is continued for most of the period of an operation cycle with cruciform control rods, not shown, inserted in the first control cells (A) 11 and (B) 12 and without inserting any control rods in the second control cells 20; that is, the control rods are inserted in only the first control cells (A) 11 and (B) 12 for most of the period of the operation cycle and the nuclear reactor is operated. A control rod pattern, i.e., a pattern indicating the control rods inserted in the control cells, in the first embodiment will be described with reference to FIGS. 3(a), 3(b) and 3(c) are quarter sectional views of control rod patterns in which, which control rods are arranged by the method in the first embodiment. In FIGS. 3(a), 3(b) and 3(c), the control rods are inserted in the shaded control cells. In an initial stage of the cycle, in which burn-up is in the range of 0 to 2.2 GWd/t, the nuclear reactor is operated with the control rods inserted in the thirteen second control cells 20 as shown in FIG. 3(a) In most of the remaining period of the cycle, in which burn-up is in the range of 3.3 to 9.4 GWd/t, the nuclear reactor is operated with the control rods inserted in only the twenty four first control cells (A) 11 and (B) 12 as shown in FIG. 3(b). In the last stage of operation of the nuclear reactor, in which burn-up is 10.4 GWd/t, all the control rods are extracted from the reactor core as shown in FIG. 3(c). In most of the period of the cycle, the nuclear reactor is operated with the control rods inserted alternately in the first control cells (A) 11 and the first control cells (B) 12. First, the control rods are inserted in only the first control cells (A) 11 for a period in which the nuclear reactor operates at a burn-up of several gigawatts day per ton, and then the control rods are extracted from all the first control cells (A) 11 and control rods are inserted in only the first control cells (B) 12 for a period in which the nuclear reactor operates at several gigawatts day per ton. Thereafter, this operating mode is repeated. Generally, asymmetric burn-up occurs in the fuels of the fuel assemblies when the control rod is inserted in the control cell. Therefore,when the control rod is extracted, then output increases locally in the fuel rods adjacent to the control rod, causing control blade historical effect which adversely affects the soundness of the fuel rods. This embodiment limits burn-up in a period in which the control rods are inserted continuously in the first control cells (A) 11 and the first control cells (B) 12 to several gigawatts day per ton to reduce the control blade historical effect which occurs when the control rods are extracted. A method in a comparative example for verifying the effect of the first embodiment operates the nuclear reactor through a cycle with the control rods inserted in only the second control cells 20 each consisting of the four fuel assemblies having relatively small infinite multiplication factors. An equilibrium core for carrying out the comparative example comprises 872 reload fuel assemblies including 200 first cycle fuels 1, 200 second cycle fuels 2, 200 third cycle fuels 3, 200 fourth cycle fuels 4, and 72 fifth cycle fuels 5. The mean enrichment factor of the reload fuels, the numbers and the arrangement of the first control cells (A) 11 and (B) 12, and the number and the arrangement of the second control cells 20 are the same as those in the reactor core for carrying out the first embodiment. In this reactor core, the first cycle fuels 1, the second cycle fuels 2 and the third cycle fuels 3 are disposed in regions other than an outer peripheral region of the reactor core and regions for the second control cells 20. The fifth cycle fuels 5 are disposed in the outer peripheral region and in the regions for the second control cells 20. The fourth cycle fuels 4 are disposed in the outer peripheral region of the reactor core, the regions for the second control cells 20, regions providing relatively high output and outer regions of the reactor core. After an operation cycle has been completed, the first cycle fuels 1 are moved to positions from which the second cycle fuels 2 have been removed, the second cycle fuels 2 are moved to positions from which the third cycle fuels have been removed, and the third cycle fuels are moved to positions from which the fourth cycle fuels 4 have been removed. New reload fuels are disposed at positions from which the first cycle fuels have been removed. After the operation cycle has been completed, 128 considerably fissioned fourth cycle fuels 4 having a relatively small infinite multiplication factor among the fourth cycle fuels 4 are removed from the reactor core, and the remaining 72 fourth cycle fuels 4 are moved to positions from which the fifth cycle fuels 5 have been removed. All the fifth cycle fuels 5 are removed from the reactor core. In this comparative example, the control rods are inserted in the second control cells 20 throughout the cycle of operation of the nuclear reactor (FIGS. 3(a) and 3(b)), and all the control rods are extracted from the reactor core in the last stage of the cycle (FIG. 3(c)). When the nuclear reactor is operated by the method in the first embodiment, the number of new reload fuels to be loaded into the reactor core after the completion of the cycle is smaller by four than that of the new reload fuels to be loaded into the reactor core after the completion of the cycle when the nuclear reactor is operated by the method in the comparative example. Such an effect can be provided by operating the nuclear reactor with the control rods inserted in the first control cells for a period (FIG. 3(b)) longer than half of the period of the cycle (FIGS. 3(a) to 3(c)); that is, the effect can be provided even if the control rods are inserted in the first control cells throughout the cycle (FIGS. 3(a) and 3(b)). A fuel arrangement including both the initial loading fuels and the reload fuels for the third cycle will be explained by way of example. FIG. 4 is a quarter sectional view of a fuel loading pattern for the third cycle formed by a method representing a second embodiment according to the present invention. The reactor core of a nuclear reactor is provided with 208 low enrichment fuel assemblies (hereinafter referred to as xe2x80x9clow-enrichment fuelsxe2x80x9d) having a mean enrichment factor of about 1.5 wt % at initial loading and 664 high-enrichment fuel assemblies (hereinafter referrer to as xe2x80x9chigh enrichment fuelsxe2x80x9d) having a mean enrichment factor of about 4.1 wt % at initial loading, and no reload fuels are loaded into the reactor core in the second cycle. The reactor core in the third cycle as shown in FIG. 4 comprises 872 fuel assemblies including 132 first cycle fuels 1, 76 low-enrichment fuels la (initial loading fuels) and 664 high-enrichment fuels 1b (initial loading fuels). The first cycle fuels 1, i.e., reload fuels, are fuel assemblies having a mean enrichment factor of about 3.8 wt %. Forty first control cells each comprising the first cycle fuels 1 and the fissioned high-enrichment fuels having relatively large infinite multiplication factors, and thirty-seven second control cells 40 each comprising considerably fissioned high-enrichment fuels having relatively small infinite multiplication factors are formed in the reactor core. The first control cells are divided into two groups differing from each other in the operation of the control rods; sixteen first control cells (A) 31 and twenty-four first control cells (B) 32. The first cycle fuels 1 are disposed in a central region of the reactor core, the low-enrichment fuels 1a are arranged in an outer peripheral region of the reactor core, and the high-enrichment fuels 1b are disposed in the rest remaining regions in the reactor core. After the completion of this operation cycle, i.e., the third cycle, the 216 fuels including all the 76 low enrichment fuels 1a, and the 140 considerably fissioned high-enrichment fuels 1b having small infinite multiplication factors among the high-enrichment fuels 1b are removed from the reactor core and 216 new reload fuels are loaded into the reactor core. In most of the period of the third cycle, the nuclear reactor is operated with cruciform control rods, not shown, inserted in the first control cells (A) 31 and (B) 32, and without inserting any control rods in the second control cells 40. That is, the nuclear reactor is operated for the most part of the period of the third cycle with the control rods inserted in only the first control cells (A) 31 and (B) 32. A control rod pattern in the second embodiment will be described with reference to FIGS. 5(a), 5(b) and 5(c), which are quarter sectional views of control rod patterns in which control rods are arranged by the method in the second embodiment. In FIGS. 5(a). 5(b) and 5(c), the control rods are inserted in the shaded control cells. In an initial stage of the cycle, in which burn-up is in the range of 0 to 2.2 GWd/t, the nuclear reactor is operated with the control rods inserted in the twenty second control cells 40 as shown in FIG. 5(a). In most of the remaining period of the cycle, in which burn-up is in the range of 3.3 to 9.4 GWd/t, the nuclear reactor is operated with the control rods inserted in only the forty first control cells (A) 31 and (B) 32 as shown in FIG. 5(b). In the last stage of the cycle, in which burn-up is 10.4 GWd/t, all the control rods are extracted from the reactor core as shown in FIG. 5(c). In most of the period of the cycle, the method in the second embodiment, similarly to the method in the first embodiment, operates the nuclear reactor by inserting the control rods alternately in the first control cells (A) 31 and the first control cells (B) 32 for a period in which burn-up is several gigawatts day per ton. Thus, the second embodiment, similarly to the first embodiment, reduces control blade historical effect that occurs when the control rods are extracted. A method in a comparative example for verifying the effect of the second embodiment operates the nuclear reactor through a cycle with the control rods inserted in only the second control cells 40 each consisting of the four fuel assemblies having the same loading patterns and relatively small infinite multiplication factors. The mode of operation in the first and the second cycle and the movement of the fuel assemblies after the completion of the second cycle by the method in the comparative example is the same as those by the method in the second embodiment. The method in the comparative example operates the nuclear reactor by inserting the control rods only in the second control cells 40 throughout the third cycle (FIGS. 5(a) and 5(b)), and extracts all the control rods from the reactor core in the last stage of the cycle (FIG. 5(c)). The method in the comparative example removes 220 fuels including all the low-enrichment fuels 1a and 144 considerably fissioned high-enrichment fuels 1b having small infinite multiplication factors among the high enrichment fuels 1b after the completion of the third cycle and loads 220 new reload fuels into the reactor core. Accordingly, when the nuclear reactor is operated by the method in the second embodiment, the number of the new reload fuels to be loaded into the reactor core after the completion of the cycle is smaller by four than that of the new reload fuels to be loaded into the reactor core after the completion of the cycle when the nuclear reactor is operated by the method in the comparative example. Such an effect can be provided by operating the nuclear reactor with the control rods inserted in the first control cells for a period (FIG. 5(b)) longer than half of the period of the cycle (FIGS. 5(a) to 5(c)); that is, the effect can be provided even if the control rods are inserted in the first control cells throughout the cycle (FIGS. 5(a) and 5(b)). The difference in the number of the reload fuels between the first embodiment and the second embodiment, and the comparative examples, and the criticality of the nuclear reactor will be explained below. As mentioned above, the burn-up of the fuel assemblies forming the control cells into which the control rods are inserted is suppressed by the control action of the control rods. The method in the foregoing embodiment operates the nuclear reactor with the control rods inserted in the control cells each comprising the large-infinite-multiplication-factor fuels for most of the period of one operation cycle. Accordingly, the fuel assemblies of the first control cells still have enough infinite multiplication factors after the completion of the operation cycle. Since the fuel assemblies having large infinite multiplication factors can be used in the next cycle, the nuclear reactor can be kept in a critical state in the next cycle even if the number of the new reload fuels is smaller than that of the new reload fuels needed by the method in the comparative example. In the foregoing embodiment, most or all of the fuel assemblies taken out of the reactor core after the completion of the cycle are those disposed in regions other than those in which the control cells are disposed. Accordingly, the burn-up of the fuel assemblies to be taken out of the nuclear reactor is promoted and the burn-up can be enhanced.