JP2011128103A - Core of boiling water reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a core that enables 14-month or longer safe operation and where uranium/MOX fuel assemblies with the pattern of control rods simplified are intermingled. <P>SOLUTION: In the core of a boiling water reactor which is loaded with uranium fuel assemblies containing only uranium as a fuel material and MOX fuel assemblies containing uranium-plutonium mixed oxide as a nuclear fuel material at the same time and which is designed so that the average discharged burnup of the MOX fuel assemblies can be lower than that of the uranium fuel assemblies, the MOX fuel assemblies are located in 80% or more of the outermost region and three or more are uranium fuel assemblies in a CC (control cell) comprising four or more fuel assemblies. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a fuel assembly containing only uranium as a nuclear fuel material (hereinafter referred to as “uranium fuel assembly”), and a fuel assembly containing uranium-plutonium mixed oxide (hereinafter referred to as “MOX fuel assembly”). In a long-term cycle operation of a boiling water nuclear reactor (hereinafter also referred to as “BWR”) core loaded with a BWR core that can be safely operated.

  The core of a light water reactor is configured by arranging a large number of fuel assemblies in a square lattice pattern. Generally, enriched uranium enriched with natural uranium is used for the fuel assemblies. After this fuel assembly has passed a predetermined operation period (usually about 13 months), about 1/4 to 1/5 of the fuel loaded in the core is replaced with a new fuel assembly. That is, a plurality of fuel assemblies having different residence times in the reactor are mixed in the core.

  There are many arrangement methods (loading patterns) of the fuel assemblies having different residence times in the furnace in the furnace, and a fuel loading pattern corresponding to the purpose is selected. For example, there are a loading pattern that minimizes the labor of fuel replacement, a loading pattern that suppresses the maximum linear power density of the fuel rod, a loading pattern that simplifies the control rod pattern, and the like.

  By the way, in BWR, an attempt is made to increase the average enrichment of uranium 235, increase the average take-off burnup, and reduce the fuel cycle cost with respect to the uranium fuel assembly. A fuel that achieves about 45 GWd / t as a burn-up burnup has been put into practical use, and there is a plan to increase it to 50 GWd / t in the future.

In addition, BWR takes out plutonium obtained by reprocessing spent fuel, loads MOX fuel assemblies in which plutonium dioxide (PuO ₂ ) and uranium dioxide (UO ₂ ) are mixed into the furnace to save resources. In addition, a so-called “pull thermal plan” for plutonium consumption is underway. When the MOX fuel assembly is loaded into the furnace, there are a method in which the fuel in the furnace is entirely MOX fuel assembly, and a method in which only a part of the fuel in the furnace is MOX fuel assembly.

  For this MOX fuel assembly, in order to efficiently use plutonium, studies have been made to increase the plutonium enrichment per fuel assembly, which is usually the average burnup of the MOX fuel assembly. Leads to an increase.

  However, the MOX fuel assembly is used less than the uranium fuel assembly, and the current situation is that the high burnup is delayed. When only a part of the furnace is used as the MOX fuel assembly, Is likely to use a MOX fuel assembly designed to have a lower average extraction burn-up than a uranium fuel assembly.

  Furthermore, in the BWR, a long-term cycle operation plan is progressing in which the operation period is extended and the efficiency of the reactor is improved by improving the utilization factor of the reactor. In this case, although the average fuel burn-out decreases, the improvement in equipment utilization rate can be expected to improve the economic efficiency by making up for it. In general, in order to perform long-term cycle operation, it is necessary to increase the number of fuel exchangers, and the degree of freedom of the loading pattern in the furnace is significantly reduced.

  On the other hand, as for the fuel loading pattern, a loading pattern that simplifies the control rod pattern (for example, see Patent Document 1), a loading pattern that minimizes the effort of fuel replacement (for example, see Patent Document 2), and the like have already been proposed. ing.

  Here, the technical idea disclosed in Patent Document 1 is often used in the current nuclear reactor core. In this technical concept, a low-reactivity fuel assembly called a “control cell (CC)” is arranged in a control rod cell at a predetermined position, and the control rod is inserted exclusively into the CC during operation. This is a technical idea for performing operation with a control rod pattern simplified.

  The core constituting the CC is called a “control cell core” (CCC), and is still one of the mainstream core configurations at present. In the case of CCC, only a relatively small number of control rods can be deeply inserted for operation.

  For example, by inserting this deep insertion control rod, the power distribution of the core can reach the lower peak, and plutonium accumulation can be expected as the void rate rises during operation. Since the accumulated plutonium can be burned by reducing the void ratio of the core by increasing it or the like, it is possible to configure a core that is excellent in economic efficiency. Such an operation method is called “spectral shift operation”.

  As in this “spectrum shift operation”, the use of CCC is extremely useful for operation for improving economic efficiency. On the other hand, in the case of a CCC core, since the number of control rods is limited, it is preferable that the control rods inserted into the CC have as much control rod value as possible. Further, in the case of CCC, there is a tendency to insert the deep control rod for a long period of time, and the fuel in the CC is burnt in one side (a state in which the combustion of the control rod side half, particularly the corner fuel rod does not proceed).

  When these control rods are pulled out to compensate for the reactivity at the end of the cycle, the corner fuel rods have an excessive output, which may impair fuel integrity. In order to avoid this, especially in the case of a long-term operation cycle, such as changing the control rod used in the middle, such as not inserting the control rod for a long term, etc. are devised. For this reason, the longer the operation cycle, the greater the number of CCs prepared in advance in the core design.

  In addition, as a loading pattern of the core in which the uranium fuel assembly and the MOX fuel assembly are loaded at the same time, for example, a loading that efficiently minimizes the fuel exchange by loading the MOX fuel assembly in the outer peripheral region. Pattern (Patent Document 3): Arrangement of each fuel assembly for obtaining a highly safe BWR core by ensuring a thermal operation margin by flattening the power distribution in the core without impairing the economy A loading pattern as defined (Patent Document 4), a loading pattern in which the loading rate per core unit volume of the MOX fuel assembly is larger in the core peripheral region than in the core central region (Patent Document 5), A loading pattern (Patent Document 6) that determines the arrangement of each fuel assembly to obtain a BWR core that can obtain a reactivity gain by utilizing the difference, MOX without sacrificing safety Charge loading pattern to define a layout of a nuclear fuel assembly for obtaining a BWR core with Pu enrichment can be raised of the assembly (Patent Document 7) or the like has been proposed.

Japanese Patent Application Laid-Open No. 55-070792 Japanese Patent Laid-Open No. 60-100086 JP 60-262090 A JP 2002-372594 A JP-A-63-16292 JP 2003-185776 A JP 2006-284278 A

  In the replacement core of the BWR, a method of loading a fuel assembly having low reactivity with advanced combustion is used on the outermost peripheral portion of the core. This technical idea is also referred to as a low-leakage core, and can suppress the leakage of neutrons outside the reactor, increase the core reactivity, and thus increase the fuel life and improve the economic efficiency. On the other hand, as the output of the fuel assembly loaded on the outermost periphery decreases, the output sharing in the core increases, so the thermal operating margin acts in a severe direction. In order to secure an appropriate thermal operation margin, measures such as the above-described CCC technology and advanced fuel design are used in combination.

  As the need for long-term operation cycles increases, it is possible to obtain safe and good core characteristics without degrading economy in achieving long-term operation cycles in a core where uranium fuel assemblies and MOX fuel assemblies are mixed. Providing a reactor core that can be used will greatly contribute to the future utilization of nuclear reactors. As described above, the degree of burnup of the MOX fuel assembly tends to be designed to be lower than that of the uranium fuel assembly. For this reason, in the core in which MOX is mixed, the core is composed of only the uranium fuel assembly. As a result, the number of new fuels (number of replacement bodies) necessary to achieve the same cycle length is inevitably increased.

  Originally, assuming a long-term operation cycle that requires a larger number of replacement bodies, the number of replacement bodies in the core in which MOX fuel assemblies and uranium fuel assemblies are mixed reaches a considerable number. Increasing the number of new fuels significantly reduces the degree of freedom in core design. Also, when performing core design on the premise of CCC, the long-term cycle requires a large number of CCs for the reasons described above, and the degree of freedom in core design is further restricted, and there is a thermal operation margin and a reactor shutdown margin. It is extremely difficult to ensure the said safety restrictions.

  In other words, the conventional technology shown above does not give consideration to the long-term cycle operation in which the degree of freedom of the furnace interior load pattern is significantly reduced. Therefore, the uranium fuel assembly and the MOX fuel assembly are mixed only with the conventional technology. A long-term operation cycle cannot be realized for the resulting core.

  It is an object of the present invention to obtain a reactor core that can be operated safely for 14 months or more and has a mixed uranium / MOX fuel assembly with a simplified control rod pattern.

The boiling water reactor core according to the first aspect of the present invention includes a uranium fuel assembly containing only uranium as a nuclear fuel material and a MOX fuel assembly containing a uranium-plutonium mixed oxide as a nuclear fuel material. In the boiling water reactor core in which the average extraction burnup of the MOX fuel assembly is designed to be smaller than that of the uranium fuel assembly,
Place MOX fuel assemblies in more than 80% of the outermost peripheral area,
Of the CCs composed of four fuel assemblies, three or more of them are uranium fuel assemblies.

In the boiling water reactor core according to the invention described in claim 2, the uranium fuel assembly according to claim 1 is a fuel rod square lattice array of 9 rows and 9 columns of fuel rods,
The MOX fuel assembly has a 9-by-9 fuel rod square lattice arrangement.

  The present invention has an effect that a uranium / MOX mixed core having a simplified control rod pattern that can be operated safely for 14 months or more can be obtained.

19 months in which a uranium fuel assembly designed to have a removal burnup of 45 GWd / t during 13-month operation and a MOX fuel assembly designed to have a take-off burnup of 35 GWd / t during 13-month operation It is explanatory drawing which showed the fuel loading pattern in the nuclear reactor core (4th quadrant) supposing operation. It is explanatory drawing which showed the control rod plan. It is the diagram which showed the thermal suitable limit value (maximum linear power density) of the core. It is the diagram which showed the thermal suitable limit value (minimum limit power ratio) of a core. It is the diagram which showed the excess reactivity of the core. It is the diagram which showed the effective multiplication factor of the core when one control rod with the maximum control rod value is pulled out. The fuel loading pattern when the operation period is 19 months in a reactor loaded with a uranium fuel assembly designed for 45 GWd / t and a MOX fuel assembly designed for 40 GWd / t is shown. FIG. It is the diagram which showed the combustion transition of the infinite multiplication factor of the uranium fuel assembly designed to 45 GWd / t and the MOX fuel assembly designed to 35 GWd / t.

  The present invention is arranged at the outermost periphery in a CCC in which a uranium fuel assembly and a MOX fuel assembly designed to have a lower average take-off burn rate than the uranium fuel assembly are loaded at the same time and the operation period is 14 months or more. This is a loading pattern in which 80% or more of the fuels are MOX fuel assemblies and three or more of the CCs composed of four fuel assemblies are uranium fuel assemblies. As a result, it is possible to obtain a reactor core that can be safely operated for 14 months or more and includes a uranium / MOX fuel assembly in which the control rod pattern is simplified.

  First, the characteristics of the long-term cycle operation core will be described. In the long-term cycle operation, the number of fuel replacement bodies increases, and the types of fuel having different stay periods existing in the furnace decrease, so the degree of freedom of the loading pattern in the furnace is significantly reduced. When operating a nuclear reactor, it is necessary to avoid an excessive increase in fuel output, but when there are a large number of replacement bodies and CCs are prepared, the degree of freedom in the core design will be reduced in the first place. Therefore, it is difficult to suppress output peaking by devising a local loading pattern.

Even in such a case, it is necessary to consider the following four points to consider.
1) To contribute to the reduction of peaking over the long term during the operation period,
2) Do not cause a decrease in the economics of the core,
3) It should not deteriorate other core characteristics such as stop margin,
Furthermore, since the number of fuel replacement bodies increases with the extension of the operation period, the surplus reactivity in the middle of the cycle increases, so it is necessary to sufficiently control the reactivity.
4) A reactor core that does not excessively increase excess reactivity and has excellent controllability.

  Compared with uranium fuel, MOX fuel contains plutonium that absorbs thermal neutrons that promotes fission efficiently, so there is a shortage of thermal neutrons compared to uranium fuel and the neutron spectrum tends to harden. Further, due to the hardening of the neutron spectrum, the decrease in reactivity due to the combustion of the MOX fuel becomes more gradual than that of the uranium fuel.

  By loading the MOX fuel on the outermost periphery of the reactor core, the hardening of the neutron spectrum can be suppressed by a large amount of moderator present around the core, so the reactivity of the MOX fuel increases. Furthermore, by disposing MOX fuel, which has a moderate decrease in reactivity due to combustion, on the outermost periphery of the core, it is possible to flatten the core power distribution throughout the operation period.

  As a result of the hardening of the neutron spectrum, the MOX fuel also has a feature that the value of the control rod that controls the reactivity mainly by absorbing thermal neutrons is lower than that of the uranium fuel. For this reason, it is possible to increase the value of the control rod by keeping the MOX fuel not adjacent to the control rod to be used as much as possible (that is, adjoining the uranium fuel).

  In MOX fuel, the control rod value is lower than that of uranium fuel, so if more than half of one CC is a MOX fuel assembly, the control rod value of the CC will be reduced and the reaction of the core by CC The amount of decrease is not sufficient. In that case, it becomes impossible to operate only with the control rod in the CC, and it is necessary to use a control rod other than the CC. If it becomes so, it will cause the fall of economical efficiency that the effect of the spectrum shift driving | operation by CC cannot fully be performed.

  In addition, by making most of the CC (three or more) uranium fuel assemblies, surplus reactivity is likely to be high, even in long-term cycle operation cores in which MOX fuel assemblies are mixed. It is possible to control the reactivity.

  With the above configuration, the above noted points 1) to 4) can be solved, and a MOX mixed core suitable for a long-term operation cycle can be configured.

  FIGS. 1 to 6 show that the operation period is 19 months in a nuclear reactor loaded with a uranium fuel assembly designed for 45 GWd / t extraction burnup and a MOX fuel assembly designed for 35 GWd / t. This shows the fuel loading pattern, control rod pattern, and core characteristics.

  FIG. 1 shows a mixture of a uranium fuel assembly designed to have a removal burnup of 45 GWd / t at 13 months of operation and a MOX fuel assembly designed to have a takeoff burnup of 35 GWd / t at 13 months of operation. It is explanatory drawing which shows the fuel arrangement | positioning of the reactor core supposing the 19-month driving | running | working. In FIG. 1, since the core has symmetry, only the fourth quadrant is shown. The other quadrants are rotationally symmetric with respect to the fourth quadrant. The second cycle MOX fuel assemblies are all arranged on the outermost periphery, and only uranium fuel assemblies are used for CC.

  Only the uranium fuel assembly is disposed in the control cell (CC) indicated by a thick frame in the figure, and only the MOX fuel assembly in the second cycle is disposed on the outermost periphery of the core. In this embodiment, 444 of the 764 bodies are uranium fuel assemblies, and 320 are MOX fuel assemblies. This is a so-called 1/3 MOX core in which the weight of the mixed oxide occupies about one third of the weight of the heavy metal. In the existing boiling water reactor, the weight of the mixed oxide fuel is up to one third of the weight of the heavy metal. This is an example with the highest loading ratio of MOX fuel assemblies.

  The present invention is a case where the removal burn-up of the MOX fuel assembly is lower than the removal burn-up of the uranium fuel assembly, and reducing the loading ratio of the MOX fuel assembly from this embodiment is highly reactive. As the loading ratio of uranium fuel assemblies increases, the number of new fuel bodies decreases, and the degree of freedom of fuel placement in the reactor increases, so that when the loading ratio of MOX is lower than in this embodiment, the core can be more easily Can be configured.

  FIG. 2 shows a control rod plan for an operating nuclear reactor. In the figure, a cell indicated by “−” indicates that the control rod is in a fully extracted state, and the numeral indicates the amount of control rod extraction when the fully extracted state is 48. That is, it is planned to gradually change the CC to be used together with combustion from a large number of CCs prepared, so that control rods are not inserted into the CC over an excessively long period. The insertion period of the control rod is about 10 GWd / t or less at maximum. This corresponds to a case where the operation period is 13 months.

  3 and 4 show the combustion transition of the optimum thermal limit values (maximum linear power density, minimum critical power ratio) of the above embodiment. Due to the effect of the present invention, a sufficient margin is secured for the operation limit value throughout the operation period. FIG. 5 shows the combustion transition of the excess reactivity of the core. It can be seen that the excess reactivity becomes maximum in the middle of the cycle, but the excess reactivity does not increase excessively.

  FIG. 6 shows the effective multiplication factor of the core when one control rod having the maximum control rod value is pulled out. Due to the effect of the present invention, a sufficient margin for the limit value is secured. In the case of the present embodiment, the average take-off burnups of the uranium fuel assembly and the MOX fuel assembly are 41 GWd / t and 28 GWd / t, respectively.

  FIG. 7 shows the fuel loading when the operation period is 19 months in a nuclear reactor loaded with a uranium fuel assembly designed for 45 GWd / t extraction burnup and a MOX fuel assembly designed for 40 GWd / t. It shows a pattern. In this case as well, a sufficient driving margin can be ensured as in the embodiment shown in FIGS.

  FIG. 8 is a graph showing the transition of combustion at an infinite multiplication factor between a uranium fuel assembly designed to have an extraction burn-up of 45 GWd / t and a MOX fuel assembly designed to 35 GWd / t. U1, M1, etc. in the figure indicate the burnup reached when the uranium fuel assembly burns for one cycle, and the burnup reached when the MOX fuel assembly burns for one cycle.

  From the figure, as described above, the infinite multiplication factor of the MOX fuel assembly is smaller than that of the uranium fuel assembly, and the amount of decrease in reactivity due to combustion is small. The body is gentler. It can also be seen that the MOX fuel assembly having a low extraction burn-up has a lower reactivity.

  Further, all the fuels arranged on the outermost periphery in FIG. 7 are the MOX fuel assemblies in the second cycle, and this corresponds to the combustion period from M1 to M2 in FIG. If all the outermost fuel is the uranium fuel assembly in the third cycle, it corresponds to U2 to U3 in FIG. 2, but the output is lower than M1 to M2 of the MOX fuel assembly in the second cycle, and further combustion Accordingly, since the output sharply decreases, it is clear that the power distribution of the core does not become flat compared with the case where the outermost fuel is the MOX fuel assembly in the second cycle.

Claims (2)

A uranium fuel assembly containing only uranium as a nuclear fuel material and a MOX fuel assembly containing uranium-plutonium mixed oxide as a nuclear fuel material are loaded at the same time, and the average removal burnup of the MOX fuel assembly is the uranium fuel assembly. In a boiling water reactor core designed to be smaller than
Place MOX fuel assemblies in more than 80% of the outermost peripheral area,
A boiling water reactor core characterized in that three or more of the control cells composed of four fuel assemblies are uranium fuel assemblies. The uranium fuel assembly is a fuel rod square lattice array of 9 rows and 9 columns of fuel rods,
2. The boiling water reactor core according to claim 1, wherein the MOX fuel assembly is a 9 × 9 fuel rod square lattice array. 3.

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