JP2006329867A - Fuel assembly for boiling water reactor, group of fuel assembly and reactor core - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain fuel assemblies of uranium-plutonium mixed oxide fuel for boiling water reactors, aimed at high enrichment which fully ensures the margin for maximum burnup of pellet, in using uranium-plutonium mixed oxide fuel rods of single enrichment and also be proper for reactivity control. <P>SOLUTION: In the fuel assemblies for boiling water reactors provided with as a fuel rod group, a plurality of uranium-plutonium mixed oxide fuel rods of fissile plutonium enrichment of one kind and 5 wt% or more, a plurality of uranium oxide fuel rods containing no plutonium and a plurality of uranium oxide fuel rods with burnable poison containing no plutonium, the uranium-plutonium mixed oxide fuel rods are arranged, excluding the outermost preiphery and in the vicinity of water rod positions. The fuel rods with burnable poison are arranged in the region vicinity of water rods. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a boiling water nuclear reactor fuel assembly using a uranium / plutonium mixed oxide, a set of fuel assemblies, and a core.

  A so-called pull thermal project is currently underway to load uranium / plutonium mixed oxide fuel (hereinafter referred to as MOX fuel) mixed with plutonium (hereinafter referred to as Pu) obtained by reprocessing spent nuclear fuel into a light water reactor. . In designing the MOX fuel, a plurality of design concepts can be considered from the viewpoint of the economics of the MOX fuel, Pu consumption, and the like.

  For example, a design concept that emphasizes the reduction in the number of manufactured MOX assemblies and the number of transporters, and a design concept that emphasizes an increase in the amount of core load Pu along with a reduction in the number of MOX fuel rods manufactured are considered effective design concepts. It is done. Here, when a single MOX fuel rod type including a fissile plutonium enrichment (hereinafter referred to as “Puf enrichment”) type is used, there are advantages such as a reduction in fuel rod manufacturing cost (for example, Patent Document 1).

  Therefore, a design with a single enrichment type of MOX fuel rods (single enrichment design) is one of the promising concepts of MOX fuel design. Hereinafter, a single enrichment MOX fuel will be described. In a boiling water reactor, the moderator boiles in the core, and therefore the distribution of the moderator in the assembly is biased, and the power output distribution in the assembly is distorted. In order to alleviate this and flatten the output distribution in the assembly, it is necessary to have a concentration distribution or enrichment distribution in the assembly.

  For a single enriched MOX fuel assembly with the same Puf enrichment of the MOX fuel rods, uranium fuel rods that do not contain Pu and flammable materials that do not contain Pu in order to reduce the power distribution within the assembly It is necessary to arrange fuel rods containing combustible poisons containing poisons in an appropriate manner, and it is necessary to devise the arrangement of MOX fuel rods and uranium fuel rods, and to set the enrichment distribution appropriately for uranium fuel rods. There is a need.

  Therefore, the number of MOX fuel rods in the assembly is generally 1/3 or less of the number of fuel rods in the assembly, that is, the MOX heavy metal ratio in the assembly is 1/3 or less. By the way, in the present light water reactor, it is said that the design similar to the conventional uranium fuel core is possible until the weight ratio of MOX heavy metal in the core is about 1/3.

  From the viewpoint of effective consumption of Pu, it is desirable to increase the weight of MOX heavy metal in the core as much as possible within a range not exceeding 1/3.

  As described above, the MOX heavy metal ratio in the single enriched MOX aggregate is generally 1/3 or less. Therefore, in the core loaded with the single enriched MOX aggregate, the MOX heavy metal ratio in the core is reduced to 1/3. In order to increase as much as possible within a range that does not exceed, the total number of fuel assemblies in the core must be the MOX assemblies.

  In this case, in order to flexibly cope with fluctuations in cycle length, etc., it is highly preferable to prepare a plurality of types of fuels having different numbers of fuel rods containing flammable poisons as a method for facilitating the adjustment of the reactivity. .

  The construction of a core using two types of replacement fuels with different flammable poison addition amounts is called two streams. A set of fuel assemblies designed for these two streams is disclosed in, for example, Japanese Patent No. 2958861. This concept is effective (see, for example, Patent Document 2).

This is because the number of flammable poison-containing fuel rods adjacent to the water rod in the second assembly with a large number of flammable poison-containing fuel rods is larger than that of the first assembly with a small number of flammable poison-containing fuel rods. It is. However, this concept is mainly optimized for fuel assemblies that do not contain MOX fuel rods. In particular, when using single enriched MOX fuel rods, a design that considers the characteristics is desirable. It is.
JP 2003-107183 A Japanese Patent No. 2958861

  On the other hand, in order to increase the extraction energy per MOX fuel assembly and improve the average extraction burn-up, development has been carried out to further increase the Puf enrichment. If the Puf enrichment is increased and the MOX fuel rod enrichment is one kind in the assembly, the output of the MOX fuel rods will increase excessively, and the possibility of adversely affecting the fuel integrity increases. Further, when the Puf enrichment is increased, the neutron spectrum is hardened and the value of the flammable poison is deteriorated. For this reason, it is necessary to appropriately select the arrangement of fuel rods containing flammable poisons.

  As a means for increasing the value of the flammable poison, it is effective to place the fuel rod containing the flammable poison at a position where the spectrum becomes soft, that is, a position adjacent to the water gap (an outermost periphery position of the assembly).

  However, since this method has a concern about adverse effects such as lowering the value of the control rod, priority is given here to a method in which the fuel rod containing the flammable poison is appropriately arranged around the inner periphery of the assembly. In this case, when preparing two types of fuels having different numbers of fuel rods containing combustible poisons, it is necessary to optimize both of them in the same manner.

  Also, for example, in the current uranium fuel, the pellet maximum burnup assumed in the thermal / mechanical design of the fuel rod for stress evaluation and the like is 75000 MWd / t, and the MOX fuel can be designed not to exceed this value. Required.

  This limit value is related to fuel soundness, and unless the verified data is expanded, it is considered difficult to expand at present. When using single enrichment MOX fuel rods, the increase in fuel rod burnup and fuel pellet burnup is a major obstacle to increasing enrichment and has not been fully studied in the past. It is a new issue. In the conventional single enrichment design aiming at an extraction average burnup of 45000 MWd / t, it is possible to satisfy the limit value of pellet burnup of 75000 MWd / t and increase the Puf enrichment to 5.0 wt% or more. It was extremely difficult.

  In the MOX fuel for boiling water reactors aiming at high enrichment, the present invention ensures sufficient margin for the maximum burnup of pellets when using a single enriched MOX fuel rod, and controls reactivity. It is an object of the present invention to obtain a boiling water reactor fuel assembly, a set of fuel assemblies, and a core suitable for the above.

In the fuel assembly for a boiling water reactor according to the first aspect of the present invention, the fuel rod group in which the fuel pellets made of nuclear fuel material are filled in the cladding tube is regularly arranged in a square lattice array, A water rod that is a non-boiling region that occupies an area corresponding to a plurality of fuel rods at a substantially central position of the lattice arrangement,
As the fuel rod group, one or more uranium / plutonium mixed oxide fuel rods having a fissile plutonium enrichment of 5 wt% or more, a plurality of uranium oxide fuel rods not containing plutonium, and plutonium A boiling water reactor fuel assembly comprising a plurality of uranium oxide fuel rods containing flammable poisons,
The uranium / plutonium mixed oxide fuel rods are arranged except for the outermost periphery and excluding the position adjacent to the water rod,
The fuel rod containing the flammable poison is arranged in a region adjacent to the water rod.

The set of boiling water reactor fuel assemblies according to the invention described in claim 2 is the same as that of the boiling water reactor fuel assembly described in claim 1, but the average plutonium dioxide enrichment is mutually A set of fuel assemblies for a boiling water reactor comprising a first and a second fuel assembly constituting a two-stream replacement fuel assembly having the same amount of combustible poisons;
In the first fuel assembly, all of the fuel rods containing flammable poisons are arranged in a region adjacent to the water rod, and the fuel rods containing flammable poisons are not arranged in other regions,
The second fuel assembly has more fuel rods containing the combustible poison than the first fuel assembly, and is disposed in the entire region adjacent to the water rod and the inner peripheral region excluding the outermost periphery. It is characterized by this.

  A boiling water reactor core according to the invention described in claim 3 is loaded with the first fuel assembly and / or the second fuel assembly described in claim 2.

  In the present invention, when a single enriched uranium / plutonium mixed oxide fuel rod having a single fissile plutonium enrichment is used, a margin for the maximum burnup of the fuel rod and fuel pellet is secured. Thus, it is possible to provide a fuel assembly for a boiling water reactor having an increased enrichment level of fissile plutonium. Furthermore, by using a set of two types of fuel assemblies for boiling water reactors with different numbers of fuel rods containing flammable poisons, there is an effect that it is possible to provide a boiling water reactor core in which reactivity control is easy. .

  In the present invention, as a fuel rod group, a plurality of uranium / plutonium mixed oxide fuel rods having one kind of fissile plutonium enrichment and 5 wt% or more, and a plurality of uranium oxide fuel rods not containing plutonium, In the boiling water nuclear reactor fuel assembly comprising a plurality of uranium oxide fuel rods containing flammable poisons that do not contain plutonium, the uranium / plutonium mixed oxide fuel rods, except for the outermost periphery, and Since the fuel rod containing the flammable poison is disposed in a region adjacent to the water rod, the uranium / plutonium mixed oxide fuel rod (hereinafter referred to as MOX fuel rod) is high. Fuel with excellent fuel integrity and economic efficiency when used as a single fissionable plutonium enrichment (hereinafter referred to as Puf enrichment). It can be provided.

  That is, since the MOX fuel rods are arranged except for the outermost periphery and excluding the position adjacent to the water rod, and the fuel rod containing the flammable poison is arranged in the region adjacent to the water rod, Although the local peaking coefficient is large, the value of the local peaking coefficient decreases with the progress of combustion, and becomes smaller than the value of the conventional design example at the end of combustion.

  In the conventional design example, this is because the highly enriched MOX fuel rod is disposed adjacent to the water rod and disposed in a region where the reactivity increases through combustion, whereas in the present invention, the fuel rod containing a combustible poison is used. This is because the reactivity of the fuel rod at the position adjacent to the water rod at the end of combustion is prevented from excessively increasing because the fuel is disposed in the region adjacent to the water rod.

  That is, in the conventional design example, although the arrangement of the fuel rods containing the flammable poison is optimized from the viewpoint of the flammable poison effect, no consideration is given to the tendency of the local peaking coefficient through the combustion. The present invention in which all the fuel rods containing the toxic poisons are arranged at positions adjacent to the water rod is extremely effective in reducing the local peaking coefficient after the middle stage of combustion.

  As a result, in the present invention, it is possible to achieve a further increase in enrichment while providing a sufficient margin for the design value of the maximum pellet burnup. Although the fuel assembly of the present invention has a large local peaking coefficient at the initial stage of combustion, the assembly with a large number of fuel rods has a sufficiently large margin for the limit value of the thermal margin such as the maximum linear power density. Never become.

  In addition, a set of fuel assemblies for boiling water reactors, which is another invention, constitutes a two-stream replacement fuel assembly having an average plutonium dioxide enrichment and different amounts of combustible poisons. The first fuel assembly is disposed in a region where all of the fuel rods containing flammable poisons are adjacent to the water rod, and the second fuel assembly is configured such that the number of fuel rods containing flammable poisons is the first fuel assembly. More than the body, it is arranged in all the areas adjacent to the water rod and the inner peripheral area excluding the outermost periphery.

  Thereby, the adjustment range of the reactivity at the beginning of the cycle can be increased, and the flexibility with respect to fluctuations in the cycle length and the like can be increased. In addition, since the isotope composition of Pu is generally different at the time of design and manufacture, there is a possibility that even if the same Puf enrichment degree, the Puf ratio is different and there is a possibility that a difference in combustion change at an infinite multiplication factor will occur. Flexibility with respect to changes in the Pu isotopic composition is also increased. Furthermore, the local peaking coefficient at the end of combustion can be made smaller than in the conventional design example.

  Furthermore, in the two-stream core loaded with the first fuel assembly and / or the second fuel assembly, a margin for the maximum burnup of the pellet can be increased, and a core with easier reactivity control can be configured. can do.

Comparative Examples 1 and 2
In the cross section of the fuel assembly for a boiling water reactor, the neutron spectrum is soft in the region adjacent to the water gap (the outermost peripheral region of the assembly), and the output tends to be excessive. Further, the corner portion surrounded by the water gap has an excessive output, and in order to avoid this, it is usual to make the corner portion enrichment or Puf enrichment smaller than other regions. If the MOX fuel rods have a single Puf enrichment and the enrichment level is increased, it is difficult to design the MOX fuel rods on the outermost periphery. It is desirable to arrange.

  Further, in the MOX fuel assembly, since the neutron spectrum tends to be hard and the value of the combustible poison tends to be reduced, it is necessary to optimize the arrangement of the fuel rods containing the combustible poison. By placing a fuel rod containing a combustible poison at a position adjacent to the soft water gap in the neutron spectrum, the value of the combustible poison increases, but the deterioration of the control rod value and the sensitivity of the in-core nuclear instrumentation system existing in the water gap. Problems such as deterioration may occur.

  FIG. 8 shows an example of a conventional design that reflects such conditions. A square water rod W is arranged in a region occupying nine fuel rods in the central portion 3 × 3 in a 9 × 9 lattice arrangement. Is. This fuel assembly is a single enriched MOX fuel assembly consisting of type 1 MOX fuel rods, type 2-4 uranium fuel rods and type G1, G2 flammable poisoned fuel rods. The Puf enrichment of Type 1 is as high as 5.5 wt%, and is disposed at a position adjacent to the second layer and the water rod in the outer periphery of the aggregate other than the outermost periphery.

  In addition, the fuel rods containing flammable poisons G1 and G2 are dispersed and arranged at positions excluding the outermost periphery of the assembly, and the number of fuel rods containing flammable poisons is 12 in total including G1 and G2. The base material of Type 1 MOX fuel rod is depleted uranium. Depleted uranium blankets are used at the upper and lower ends of type 2-4 uranium fuel rods and type G1, G2 flammable poison-containing fuel rods.

  The enrichment distribution of the uranium fuel rods uses fuel pellets with A being the highest enrichment, and the enrichments of B, C, and fuel pellets become lower in the following order. The mixing ratio of the combustible poison is α <β <γ, the fuel rod containing the combustible poison does not contain Pu, and the enrichment is the intermediate enrichment B.

  FIG. 9 also shows a conventional design example. Four fuel rods containing combustible poisons are added to the fuel assembly shown in FIG. 8, and the number of fuel rods containing combustible poisons is set to G1 and G2. The number is 16.

  The fuel assemblies in FIGS. 8 and 9 are referred to as Conventional Design Example 1 and Conventional Design Example 2, respectively, and the present invention will be described in detail below with reference to the conventional design examples and the embodiments of the present invention. In both the conventional design example and the example, the Puf ratio is a standard composition of 67 wt%.

Example 1
As a first embodiment of the present invention, FIG. 1 shows a fuel assembly in which a square water rod W is arranged in a region occupying nine fuel rods in a central portion 3 × 3 in a 9 × 9 lattice arrangement. This fuel assembly is a single enriched fuel assembly consisting of type 1 MOX fuel rods, type 2-4 uranium fuel rods and type G1, G2 flammable poison-filled fuel rods. The number of fuel rods, enrichment distribution, and combustible poison mixing ratio are the same as in the conventional design example 1 shown in FIG. 8, but the arrangement of type 1 fuel rods and type G2 fuel rods in the assembly is different.

  That is, in this embodiment, all the type 1 MOX fuel rods are arranged in a region other than the outermost peripheral region and the region adjacent to the water rod, and all the fuel rods containing flammable poisons are arranged in the region adjacent to the water rod. Is.

  FIG. 2 shows the combustion change of the local peaking coefficient in the lower section of the fuel assembly shown in FIGS. In Conventional Design Example 1, the local peaking coefficient is substantially constant throughout combustion, and a high local peaking coefficient value is maintained even at the end of combustion.

  On the other hand, in Example 1 of the present invention, although the local peaking coefficient at the initial stage of combustion is larger than that in the conventional design example 1, the value of the local peaking coefficient decreases with the progress of combustion, and the conventional design is achieved at the end of combustion. Less than the example value. This is because in the conventional design example 1, the highly enriched MOX fuel rod is placed adjacent to the water rod and arranged in a region where the reactivity increases through combustion, whereas in the first design example, a combustible poison is contained. This is because all the fuel rods are arranged at a position adjacent to the water rod so that the reactivity of the fuel rod at the position adjacent to the water rod at the end of combustion is not excessively increased.

  That is, in the conventional design example 1, although the arrangement of the fuel rods containing the combustible poison is optimized from the viewpoint of the combustible poison effect, no consideration is given to the tendency of the local peaking coefficient through the combustion, It can be seen that the design of Example 1 in which all the fuel rods containing flammable poisons are arranged adjacent to the water rod has a great effect on the reduction of the local peaking coefficient after the middle stage of combustion.

  As a result, in the case of the conventional design example 1 in which the local peaking coefficient is large at the end of combustion, the maximum pellet burnup becomes large. For example, when the design value of the maximum pellet burnup is set to 75000 MWd / t, more Puf rich While it is difficult to increase the degree of conversion, in the design of Example 1, it is possible to achieve a further increase in enrichment while providing a sufficient margin for the design value of the maximum pellet burnup. Although the fuel assembly of the present invention has a large local peaking coefficient at the initial stage of combustion, an assembly having a large number of fuel rods such as the 9 × 9 lattice shown in this embodiment has a thermal margin such as the maximum linear power density. The margin for the limit value is sufficiently large and will not be a problem.

Example 2
As a second embodiment of the present invention, FIG. 3 shows a fuel assembly in which a square water rod W is arranged in a region occupying nine fuel rods in a central portion 3 × 3 in a 9 × 9 lattice arrangement. This assembly is obtained by increasing the number of fuel rods containing flammable poisons by four with respect to the fuel assembly of the first embodiment. Type 1 MOX fuel rods, type 2-4 uranium fuel rods, and type G1 , A single enriched fuel assembly consisting of fuel rods containing G2 flammable poisons.

  Further, this fuel assembly has the same number of fuel rods, enrichment distribution, and flammable poison mixing ratio as in the conventional design example 2 shown in FIG. 9, but the type 1 fuel rod and the type G2 fuel. The bar arrangement is different. That is, in this embodiment, all of the type 1 MOX fuel rods are arranged in a region other than the outermost peripheral region and the region adjacent to the water rod, and the fuel rod containing the flammable poison is adjacent to the water rod and the outermost periphery of the assembly. 1 in the region inside one layer.

  FIG. 4 is a diagram showing the combustion change of the local peaking coefficient in the lower section of the fuel assembly shown in FIGS. As shown in FIG. 4, the tendency of the local peaking coefficient is the same as in the comparison between Example 1 and Conventional Design Example 1, but in Example 2, a fuel containing a flammable poison with respect to the assembly of Example 1 When four rods are added, both the infinite multiplication factor in the early stage of combustion and the local peaking coefficient can be suppressed by newly arranging the fuel rod containing a flammable poison at a position where the local peaking coefficient tends to increase.

  FIG. 5 is a diagram showing a combustion change at infinite multiplication factor in the lower cross section of the embodiment shown in FIGS. FIG. 6 is a diagram showing a combustion change at an infinite multiplication factor in the lower section of the conventional design example shown in FIGS. From the comparison between FIG. 5 and FIG. 6, in the case of the two-stream core in which the core is configured with two fuel types having different numbers of fuel rods containing flammable poisons, in the combination of the conventional design examples 1 and 2, the reactivity difference at the initial stage of combustion is about In contrast to the 7% Δk, in the combination of Examples 1 and 2, the reactivity difference at the initial stage of combustion is about 10% Δk.

  Accordingly, when the core is configured by the combination of the first and second embodiments, the adjustment range of the reactivity at the beginning of the cycle is increased by adjusting the number of the fuel assemblies of the first and second embodiments. That is, it means that the flexibility with respect to fluctuations in cycle length and the like is increased. Further, since the isotope composition of Pu is generally different at the time of design and at the time of manufacture, there is a possibility that even if the same Puf enrichment degree, the Puf ratio is different and there is a difference in combustion change at an infinite multiplication factor. The present invention is also very flexible with respect to such changes in Pu isotope composition. Further, the local peaking coefficient at the end of combustion is smaller in Examples 1 and 2 than in Conventional Design Examples 1 and 2.

  As described above, the two-stream core combining the fuel assemblies of Embodiments 1 and 2 can increase the margin for the maximum burnup of the pellets, and constitute a core with easier reactivity control. it can.

  In the equilibrium core loaded with the fuel assembly of the conventional design example, although the maximum linear power density has a sufficient margin with respect to the limit value, the maximum pellet burnup is about 73000 MWd / t, so further enrichment is achieved. In addition to the fact that the degree of increase is not possible, the tolerance to the limit value is small, and it is practically difficult to achieve a Puf enrichment of 5.5 wt%.

  FIG. 7 is a diagram showing the combustion change of the maximum linear power density of the equilibrium core loaded with only the fuel assemblies of Example 1 and Example 2. In the present embodiment, as shown in FIG. 2, the local peaking coefficient at the initial stage of combustion tends to slightly increase as compared with the conventional design example, but the maximum linear power density through the combustion has a sufficient margin for the limit value. Moreover, the maximum burnup of the pellet is less than 69000 MWd / t, and a sufficient margin can be secured for 75000 MWd / t. As a result, the enrichment can be further increased.

It is explanatory drawing of the 9x9 fuel assembly which shows 1st Example of one Example of this invention. FIG. 9 is a diagram showing a combustion change of a local peaking coefficient in a lower section of the fuel assembly shown in FIGS. 1 and 8. It is explanatory drawing of the 9x9 fuel assembly which shows 2nd Example of another Example of this invention. FIG. 10 is a diagram showing a combustion change of a local peaking coefficient in a lower section of the fuel assembly shown in FIGS. 3 and 9. It is a diagram which shows the combustion change of the infinite multiplication factor in the lower cross section of the Example shown in FIG.1 and FIG.3. FIG. 10 is a diagram showing a combustion change at an infinite multiplication factor in a lower section of the conventional design example shown in FIGS. 8 and 9. It is a diagram which shows the combustion change of the maximum linear power density of the equilibrium core which loaded only the fuel assembly of Example 1 and Example 2. FIG. It is explanatory drawing of the 9x9 fuel assembly which shows the conventional design example 1. FIG. It is explanatory drawing of the 9x9 fuel assembly which shows the conventional design example 2. FIG.

Claims (3)

A group of fuel rods filled with fuel pellets made of nuclear fuel material in a cladding tube is regularly arranged in a square lattice array, and is a non-boiling region occupying an area corresponding to a plurality of fuel rods at substantially the center position of the lattice array. With a water rod,
As the fuel rod group, one or more uranium / plutonium mixed oxide fuel rods having a fissile plutonium enrichment of 5 wt% or more, a plurality of uranium oxide fuel rods not containing plutonium, and plutonium A boiling water reactor fuel assembly comprising a plurality of uranium oxide fuel rods containing flammable poisons,
The uranium / plutonium mixed oxide fuel rods are arranged except for the outermost periphery and excluding the position adjacent to the water rod,
A fuel assembly for a boiling water reactor, wherein the fuel rod containing a combustible poison is disposed in a region adjacent to a water rod. Among the fuel assemblies for boiling water reactors according to claim 1, the first and second replacement fuel assemblies constituting the two-stream replacement fuel assemblies having the same average plutonium dioxide enrichment and different amounts of combustible poisons. A set of fuel assemblies for a boiling water reactor comprising a second fuel assembly,
In the first fuel assembly, all of the fuel rods containing flammable poisons are arranged in a region adjacent to the water rod, and the fuel rods containing flammable poisons are not arranged in other regions,
The second fuel assembly has more fuel rods containing the combustible poison than the first fuel assembly, and is disposed in the entire region adjacent to the water rod and the inner peripheral region excluding the outermost periphery. A set of fuel assemblies for boiling water reactors. A boiling water reactor core loaded with the first fuel assembly and / or the second fuel assembly according to claim 2.

Images