JP2000180574A - Fuel assembly for boiling-water reactor and reactor core using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a fuel assembly and a reactor core with improved operation property by arranging a uranium fuel rod containing a flammable toxic substance at the outermost periphery part of the assembly and arranging the assembly at linearly symmetrical positions for each diagonal line at four regions that are divided by two diagonal lines and at 1/4 rotary-asymmetrical positions for the intersection of diagonal lines. SOLUTION: An MOX fuel rod 1 with the maximum degree of plutonium enrichment, an MOX fuel rod 2 with a high degree of enrichment, an MOX fuel rod 3 with the medium degree of enrichment, an MOX fuel rod 4 with a low degree of enrichment, an uranium fuel rod G1 with high-concentration gadolinia, and an uranium fuel rod G2 with low-concentration gadolinia are arranged. The uranium fuel rods G1 and G2 with gadolinia are arranged at the adjacent positions of two outermost corner positions excluding the outermost corner position of the assembly and that at the side of a control rod and at the side of an anti-control rod, thus enabling them to be linearly symmetrical for each diagonal line at four regions being divided by two diagonal lines and 1/4 rotary-asymmetrical for both the diagonal lines.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a uranium-plutonium mixed oxide (hereinafter referred to as "MOX") fuel assembly and a boiling water reactor used in a boiling water reactor (hereinafter referred to as "BWR"). It relates to the core of a nuclear reactor.

[0002]

2. Description of the Related Art In a BWR using a uranium fuel assembly,
In addition to controlling the control rods and the core flow rate, burnable poisons such as gadolinia (Gd ₂ O ₃ ) are put into the fuel rods in the fuel assembly to control the reactivity of the core. The reactivity control by gadolinia is characterized by the number of gadolinia-containing fuel rods (Gd fuel rods) and their concentration.

That is, increasing the number of Gd fuel rods in the assembly has the effect of reducing the infinite multiplication factor at the beginning of combustion, and the higher the gadolinia concentration of the Gd fuel rod, the lower the peak value of the infinite multiplication factor during the middle period of combustion. This has the effect of increasing the burn-up at which the peak appears. Using the characteristics of the reactivity control by gadolinia, a core having an appropriate and flat change in the excess reactivity during the operation cycle is designed.

[0004] The reactor is operated while adjusting the insertion amount of the control rod so as to compensate for the combustion change of the excess reactivity.
The greater the change in surplus reactivity, the greater the number of control rod operations. When the control rod operation is performed, the output is reduced. Therefore, if the control rod operation is performed frequently, the operating rate of the reactor decreases. Therefore, from the viewpoint of safety and economy of reactor operation,
The core is preferably such that the excess reactivity during the operation cycle is flat and has an appropriate value.

At present, in the BWR, a high gadolinia (high Gd) fuel assembly having a large number of Gd fuel rods per assembly and a G
A fuel rod loaded with a low gadolinia (low Gd) fuel assembly having a small number of d fuel rods is used. Such a core is called a two-stream core. As a result, the above-mentioned object can be achieved, and particularly at the beginning of the operation cycle (B
The excess reactivity of OC) can be set to an appropriate value. Furthermore, a high Gd fuel assembly and a low Gd fuel assembly as disclosed in Japanese Patent Publication No. Hei 7-92513 have been designed, and the concept is as follows.

[0006] At present, as one of measures to improve the economic efficiency of nuclear power generation, it is considered to lengthen the operation cycle of a nuclear reactor. FIG. 5 is a diagram showing a change in excess reactivity with respect to the burnup during a normal operation cycle. FIG.
FIG. 4 is a diagram showing a change in surplus reactivity when a core assembly having a normal operation cycle is used for a core having a long operation cycle and a change in optimal surplus reactivity in a core having a long operation cycle. . In each figure, the vertical axis is the excess reactivity,
The horizontal axis shows the cycle burnup. In FIG. 6, a solid line indicates a change in surplus reactivity when a core assembly having a normal operation cycle is used for a core having a long operation cycle, and a dashed line indicates an optimum surplus reactivity of a core having a long operation cycle. Indicates a change.

As shown in FIGS. 5 and 6, when a fuel assembly designed so that the excess reactivity becomes flat with respect to a normal operation cycle is used in a long operation cycle, the number of fuel replacement bodies is increased. As a result, the ratio of the number of loaded bodies of the new fuel assembly necessarily increases. Therefore, since the excess reactivity in the BOC decreases and the excess reactivity in the middle stage of the operation cycle (MOC) increases, the change in the excess reactivity from the BOC to the MOC becomes large.

[0008] In some cases, it may not be possible to obtain a sufficient reactivity for performing the rated output operation in the BOC. Japanese Patent Publication No. 7-92513 proposes to use a low Gd fuel assembly design in which the concentration of gadolinia is increased as the second fuel assembly in order to solve the above-mentioned problem caused by a prolonged operation cycle.

The use of a low Gd fuel assembly having a small number of Gd fuel rods increases the infinite multiplication factor at the beginning of combustion. Moreover, by increasing the concentration of gadolinia, the peak of the infinite multiplication factor in the middle stage of combustion can be reduced. Therefore, when this second fuel assembly is used, the excess reactivity in BOC is increased, and the excess reaction in MOC is increased. This is because the reactivity can be reduced, and as a result, the excess reactivity can be flattened.

[0010]

SUMMARY OF THE INVENTION The present invention aims to realize such a reactivity control method using gadolinia in a BWR core using a MOX fuel assembly.

That is, since the neutron spectrum in the MOX fuel assembly is harder than that in the uranium fuel assembly (the ratio of the thermal neutron flux is small), the effect of suppressing the reactivity per Gd fuel rod at the initial stage of combustion is reduced. , And the burning speed of gadolinia is reduced. Further, the MOX fuel assembly has a feature that the reactivity is low in the initial stage of combustion due to a hard neutron spectrum, but the reactivity is hardly reduced due to the combustion.

Therefore, when the same number of Gd fuel rods having the same gadolinia concentration are used, the infinite multiplication factor in the initial stage of combustion is larger in the MOX fuel assembly than in the uranium fuel assembly.
In addition, the peak of the infinite multiplication factor in the middle stage of combustion is lower in the MOX fuel assembly.

On the other hand, in the MOX fuel assembly, the infinite multiplication factor accompanying the combustion decreases more slowly than in the uranium fuel assembly, and the infinite multiplication factor in the early stage of combustion is not so large. Similarly, when the Gd fuel rods are arranged at the center of the MOX fuel assembly, the number of Gd fuel rods per assembly must be increased in order to suppress an appropriate excess reactivity in the BOC.

By the way, since it is assumed that a uranium fuel rod is used as the Gd fuel rod of the MOX fuel assembly,
Increasing Gd fuel rods means lower plutonium loading per assembly.

[0015] From the viewpoint of plutonium utilization and economy, the larger the plutonium filling amount per aggregate, the better.
An increase in Gd fuel rods is not desirable. Further, disposing the Gd fuel rod at the center of the assembly distorts the power distribution, and flattens the power distribution by increasing the types of enrichment of the MOX fuel rods. Increasing the number of types impairs economic efficiency.

In order to solve such a problem, the present invention provides a MOX fuel assembly having excellent operability in a boiling water reactor (BWR) using a uranium-plutonium mixed oxide (MOX) fuel assembly. In addition, the MOX fuel assembly used in the BWR is intended to obtain a MOX fuel assembly having various reactivity characteristics without losing the plutonium filling amount per assembly. And a core of a boiling water reactor having excellent temperature characteristics.

[0017]

According to the first aspect of the present invention, there is provided a BWR fuel assembly comprising a plurality of plutonium-free uranium fuel rods and a plurality of MOX fuel rods. In the loaded BWR fuel assembly,
The uranium fuel rods are disposed at the outermost peripheral portion of the aggregate including at least the outermost peripheral corner position of the aggregate including at least part of the flammable poison, and two diagonal lines connecting the corner positions of the aggregate. The four divided regions are arranged so as to be line-symmetric with respect to each diagonal line but not quarter-rotationally symmetric with respect to the intersection of both diagonal lines.

The BWR according to the second aspect of the present invention.
In the fuel assembly for use, uranium used in the uranium fuel rod according to claim 1 is natural uranium (NU) or depleted uranium (DU).

The BWR according to the third aspect of the present invention.
2. A BWR core loaded with the fuel assembly for BWR according to claim 1, wherein the fuel assembly has a predetermined number of uranium fuel rods containing burnable poisons. And a second fuel assembly having a smaller number of burnable poison-bearing uranium fuel rods than the above number.
It has a stream configuration.

The BWR according to the invention described in claim 4
In the core, the uranium fuel rods at the outermost peripheral corner position of the second fuel assembly according to the third aspect do not contain any burnable poison.

The BWR according to the fifth aspect of the present invention.
In the core, the uranium fuel rods at the outermost peripheral corner position of the second fuel assembly according to claim 3 all contain burnable poisons.

The BWR according to the invention described in claim 6
In the core, uranium used in the uranium fuel rod according to any one of claims 3 to 5 is natural uranium (NU) or depleted uranium (DU).

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION 1) Regarding the outermost periphery of a MOX fuel assembly, since the outermost periphery of the assembly is close to gap water filled with non-boiling water, the thermal neutron flux tends to increase,
The neutron spectrum tends to be soft. Therefore, the outermost peripheral portion of the aggregate is a portion where the output is most likely to be highest and the gadolinia reactivity suppression effect is the highest.

The part where the output tends to be high is as follows.
When arranging MOX fuel rods, those having relatively low plutonium enrichment will be arranged. That is, when the Gd fuel rods are arranged, the outermost peripheral portion is a portion where the decrease in the plutonium filling amount can be suppressed relatively low. Since the outer periphery of this assembly is highly effective in suppressing the reactivity of gadolinia,
By disposing a small number of Gd fuel rods, the infinite multiplication factor at the beginning of combustion can be sufficiently reduced.

As a result, it is not necessary to dispose the Gd fuel rod at the center of the assembly, so that the power distribution is not distorted by the Gd fuel rod at the center. Therefore, it is possible to increase the types of MOX fuel enrichment,
Since there is no need to change the fuel design at the center of the d fuel assembly, the design of the high Gd fuel assembly and the design of the low Gd fuel assembly can be shared. Therefore, the cost of designing and manufacturing the assembly can be reduced.

Next, the effect of suppressing reactivity by gadolinia on the outermost periphery will be described. As an example, when comparing the outermost corner position of the assembly with its adjacent position, the outermost corner position is broader in the region surrounded by the non-boiling water region, so the neutron spectrum is softer than its adjacent position. , The thermal neutron flux increases. As a result, the gadolinia reactivity suppression effect is smaller at the adjacent position than at the outermost corner position, and the gadolinia combustion speed is lower.

That is, when the Gd fuel rod is disposed at the outermost corner, the infinite multiplication factor at the beginning of combustion can be reduced. In addition, Gd is placed at a position adjacent to the outermost corner position.
By arranging the fuel rods, the burnup at which the peak of the infinite multiplication factor appears in the middle stage of combustion can be further increased. Therefore, by changing the position of the Gd fuel rod at the outermost periphery,
The reactivity control effect as when the number of d fuel rods or gadolinia concentration is changed can be obtained. From this, it is possible to design a fuel assembly having more various reactivity characteristics.

2) The effects of the 1/4 asymmetric arrangement of uranium fuel rods for 1/4 asymmetric and 2 streams will be described. In a conventional design for an assembly in a core with an equal gap water width outside the channel box, when the assembly is divided into four by two diagonals connecting the corner positions of the assembly, all four parts are A fuel rod arrangement that is rotationally symmetric is employed. This is called 1/4 symmetry. On the other hand, although it is axisymmetric with respect to each of the two diagonal lines connecting the corner positions of the aggregate, one point with respect to the intersection of the two diagonal lines
A material that is not 回 転 rotationally symmetric is referred to as 非 対 称 asymmetric. FIG. 7 is an explanatory view showing a 1/4 symmetric fuel rod arrangement, and FIG.
FIG. 4 is an explanatory view showing a 1/4 asymmetric fuel rod arrangement.

In each figure, 1, 2, 3, 4, G1, and G2 indicate different fuel rods. For example, 1 indicates MOX fuel rod (plutonium enrichment: highest), and 2 indicates MOX fuel rod (plutonium rich). 3) MOX fuel rod (plutonium enrichment: medium), 4: MOX fuel rod (plutonium enrichment: low), G1: uranium (NU) fuel rod with high concentration gadolinia, G2: low 2 shows a uranium (NU) fuel rod with a concentration gadolinia.

In FIG. 7, A, B,
The quarter regions of the C and D fuel assemblies are quarter rotationally symmetric. On the other hand, as shown in FIG. 8, the quarter region of the fuel assembly of E, F, G, and H surrounded by the broken line is not a quarter rotational symmetry, but is a line with respect to the broken lines I and J. Become symmetric and E
And G are rotationally symmetric, and F and H are rotationally symmetric.

As with the uranium core, the MOX core may be a two-stream core using a high Gd fuel assembly and a low Gd fuel assembly. It is assumed that a uranium fuel rod is used as the Gd fuel rod of the MOX fuel assembly. The two-stream core consists of a high Gd fuel assembly and a low Gd
By changing the ratio of the number of loaded fuel assemblies, the excess reactivity can be easily adjusted even in a normal operation cycle or a prolonged operation cycle, so that the optimal excess reactivity characteristics can be obtained. The flexibility of design and core operation will be higher, and ultimately the economy will be better.

When the Gd fuel rods are arranged on the outer peripheral portion of the MOX assembly, the infinite multiplication factor in the initial stage of combustion can be set to an appropriate value with a small number of rods. Can not ensure sufficient reactivity.
In addition, from the viewpoint of appropriate setting of reactor shutdown margin, excess reactivity, and economic efficiency, the maximum number of Gd fuel rods in the high Gd fuel assembly cannot be so large, and the high Gd fuel assembly and the low Gd fuel assembly cannot be used. The difference in the number of Gd fuel rods is small, usually within four. Further, since uranium fuel rods are assumed as Gd fuel rods, it is not preferable to include too many Gd fuel rods in the MOX fuel assembly from the viewpoint of utilization of plutonium and economy.

Therefore, two designs of a high Gd fuel assembly and a low Gd fuel assembly having sufficient performance using a small number of Gd fuel rods while maintaining 1/4 symmetry require that the fuel arrangement be high. It is difficult because it is limited. A 1/4 asymmetry increases the degree of freedom for fuel rod arrangement, so that even a small number of uranium fuel rods on the outermost periphery of the assembly have the same enrichment for the high Gd fuel assembly and the low Gd fuel assembly having sufficient performance. The design can be made using the arrangement and with a sufficiently large plutonium loading.

3) Regarding the present invention, that is, in the present invention,
A part or all of the uranium fuel rods are arranged at the outermost periphery of the assembly, and at least the fuel rods at the outermost corners of the assembly are uranium fuel rods, and a part or all of these uranium fuel rods are provided with burnable poisons. The four regions divided by two diagonals connecting the corner positions of the assembly are line-symmetric with respect to each diagonal but are not quarter-rotationally symmetric with respect to the intersection of both diagonals. That is, the uranium fuel rods are arranged asymmetrically with respect to 1/4. As a result, it is possible to design a high Gd fuel assembly and a low Gd fuel assembly with sufficient performance using a small number of uranium fuel rods without impairing the plutonium filling amount per assembly. By using the assembly and the low Gd fuel assembly, it is possible to design a two-stream core having appropriate surplus reactivity characteristics according to the operation cycle.

When natural uranium (NU) or depleted uranium (DU) is used instead of enriched uranium for the uranium fuel rods, the reactivity of the entire assembly is reduced due to a decrease in the output of the uranium fuel rods. The decrease in the aggregate reactivity will be compensated by increasing the plutonium enrichment of the MOX fuel rod,
The plutonium loading per assembly will be increased, thereby improving the economics of the MOX fuel assembly.

[0036]

Embodiments of the present invention will be described below. Embodiment 1 FIG. High GdMOX Fuel Assembly FIG. 1 is an explanatory view showing the fuel rod arrangement of the MOX fuel assembly according to the first embodiment of the present invention. In the figure, 1 is M
OX fuel rod (plutonium enrichment: highest), 2 is MOX
Fuel rod (plutonium enrichment: high), 3 is MOX fuel rod (plutonium enrichment: medium), 4 is MOX fuel rod (plutonium enrichment: low), G1 is uranium (NU) fuel with high concentration gadolinia The rod, G2, represents a uranium (NU) fuel rod with low concentration gadolinia.

As shown in the figure, in the first embodiment, in the assembly in which the MOX fuel rods are arranged, the outermost corner positions of the assembly and the outermost corner positions of the control rod side and the non-control rod side are excluded. The fuel assembly is a quarter-asymmetric fuel assembly in which gadolinia-containing uranium fuel rods are arranged adjacent to two outermost corner positions.

That is, the number of gadolinia-containing uranium fuel rods in the corner region is one, three, one, and three when counted clockwise from the control rod side corner region. Therefore, the total number of gadolinia-containing uranium fuel rods per assembly is 8
It is a book. In the present embodiment, the gadolinium concentration of the Gd fuel rod at the outermost corner position is higher than that of the Gd fuel rods other than the outermost corner position. The uranium in the gadolinia-containing uranium fuel rod used in this example is NU, but may be depleted uranium (DU).

FIG. 2 is a graph showing the infinite multiplication factor characteristics of the MOX fuel assembly of this embodiment. As shown in FIG.
Since the peak of the infinite multiplication factor appears in the middle stage of combustion as in the case of the ４ arrangement, the surplus reactivity characteristics of the core also show the same characteristics as the conventional one, and the assembly of the first embodiment is the same as the conventional one. It can be used similarly. Further, in this embodiment, eight gadolinia-containing uranium fuel rods are arranged in the outermost corner region, minimizing the decrease in the plutonium filling amount per aggregate, and using NU for uranium to make plutonium per aggregate. The filling amount has been increased.

Embodiment 2 FIG. Low GdMOX fuel assemblies 1 and 2 stream core The second embodiment uses two types of assemblies having different numbers of gadolinia-containing uranium fuel rods, that is, a high Gd fuel assembly and a low Gd fuel assembly. It is the core of BWR. still,
The high-Gd fuel assembly according to the second embodiment is the assembly described in the first embodiment.

FIG. 3 shows a low G according to the second embodiment of the present invention.
It is explanatory drawing which shows the fuel rod arrangement | positioning of a dMOX fuel assembly.
In the figure, 1 is a MOX fuel rod (plutonium enrichment:
2) MOX fuel rod (plutonium enrichment:
High), 3 is MOX fuel rod (plutonium enrichment: medium),
4 is a MOX fuel rod (plutonium enrichment: low), 5 is a uranium (NU) fuel rod, and G is a uranium (NU) fuel rod containing high-concentration gadolinia. G is the same as G1 in FIG.

As shown in the figure, the low-Gd fuel assembly according to the second embodiment has uranium fuel rods containing no gadolinia at the outermost corner of the assembly where the MOX fuel rods are arranged, and This is an assembly in which gadolinia-containing uranium fuel rods are arranged at positions adjacent to two outermost corner positions excluding the outermost corner positions on the control rod side and the non-control rod side. That is, the number of gadolinia-containing uranium fuel rods in the corner region and the number of uranium fuel rods not containing gadolinia are counted clockwise from the control rod side corner region,
One, three, one, and three gadolinia-containing uranium fuel rods are zero, two, zero, and two. Therefore, the total number of gadolinia-containing uranium fuel rods in the low Gd fuel assembly of this embodiment is four.

The position adjacent to the outermost corner of the low Gd fuel assembly in the second embodiment and the outermost periphery of the high Gd fuel assembly (the assembly described in the first embodiment) in the second embodiment. The gadolinia-containing uranium fuel rod at the corner position has the same gadolinia concentration. The uranium of the gadolinia-containing uranium fuel rod and the gadolinia-free uranium fuel rod used in this embodiment is natural uranium (NU), but may be depleted uranium (DU).

In this embodiment, the low Gd fuel assembly and the high Gd fuel assembly are identical in design except for the arrangement of gadolinia-containing uranium fuel rods and gadolinia-free uranium fuel rods. Costs have been reduced.

The infinite multiplication factor of the low Gd fuel assembly according to the second embodiment is also shown in FIG. 1/4 of conventional
Since the peak of the infinite multiplication factor appears in the middle stage of combustion as in the case of the symmetrical arrangement, the surplus reactivity characteristics of the core show the same characteristics as the conventional one, and the low-Gd fuel assembly of the second embodiment Can be used in the same manner. In addition, the infinite multiplication factor of the low Gd fuel assembly of the second embodiment at the initial stage of combustion is high G
Since it is larger than the d fuel assembly, the excess reactivity in BOC can be adjusted to an appropriate value by using a two-stream core.

Further, in this embodiment, a total of eight gadolinia-containing uranium fuel rods and gadolinia-free uranium fuel rods are arranged in the outermost corner area, and the reduction of the plutonium filling amount per assembly is minimized. And NU for uranium
To increase the plutonium loading per aggregate. The gadolinia-containing uranium fuel rod of the low-Gd fuel assembly of the second embodiment is the same as the high-concentration gadolinia-containing uranium fuel rod of the high-Gd fuel assembly of the second embodiment. When the high Gd fuel assembly is compared with the high Gd fuel assembly, the peak of the infinite multiplication factor of the low Gd fuel assembly is low (FIG. 2), and the same effect as when the gadolinia concentration is increased is obtained.

That is, in this embodiment, a low-Gd fuel assembly containing a high-concentration gadolinia as disclosed in Japanese Patent Publication No. 7-92513 is realized without changing the concentration of gadolinia.
Therefore, the core of the second embodiment is described in JP-B-7-92513.
It is possible to cope with a prolonged operation cycle similarly to the publication. In the second embodiment, the high-concentration gadolinia-containing uranium fuel rod used in the high-Gd fuel assembly is used in the low-Gd fuel assembly from the viewpoint of economy. However, in order to obtain the optimum surplus reactivity characteristics, the low-Gd fuel It goes without saying that the concentration of gadolinia used in the aggregate can be adjusted.

Embodiment 3 FIG. Low GdMOX Fuel Assembly 2 and Two-Stream Core The third embodiment has a different number of gadolinia-containing uranium fuel rods, ie, a BWR using two types of assemblies, a high Gd fuel assembly and a low Gd fuel assembly. Of the core. The high-Gd fuel assembly according to the third embodiment is the assembly described in the first embodiment.

FIG. 4 shows a low G according to the third embodiment of the present invention.
It is explanatory drawing which shows the fuel rod arrangement | positioning of a dMOX fuel assembly.
In the figure, 1 is a MOX fuel rod (plutonium enrichment:
2) MOX fuel rod (plutonium enrichment:
High), 3 is MOX fuel rod (plutonium enrichment: medium),
4 is a MOX fuel rod (plutonium enrichment: low), 5 is a uranium (NU) fuel rod, and G is a uranium (NU) fuel rod containing high-concentration gadolinia. G is the same as G1 in FIG.

As shown in the figure, the low-Gd fuel assembly according to the third embodiment has a gadolinia-containing uranium fuel rod arranged at the outermost corner of the assembly in which the MOX fuel rods are arranged, and the control rod side. And a uranium fuel rod that does not include gadolinia is disposed adjacent to the two outermost corner positions except for the outermost corner position on the side opposite to the control rod.

That is, the numbers of gadolinia-containing uranium fuel rods and gadolinia-free uranium fuel rods in the corner region are counted clockwise from the control rod side corner region.
One, three, one, three, and among them, the number of gadolinia-containing uranium fuel rods is one, one, one, one. Therefore, the total number of gadolinia-containing uranium fuel rods in this embodiment is four. Low Gd fuel assembly outermost corner position in the third embodiment and high Gd in the third embodiment
The gadolinia-containing uranium fuel rods at the outermost corner of the fuel assembly have the same gadolinia concentration.

The uranium in the gadolinia-containing uranium fuel rod and the uranium fuel rod not containing gadolinia used in this embodiment is NU, but may be depleted uranium (DU).
In this embodiment, the low Gd fuel assembly and the high Gd fuel assembly design are the same except for the arrangement of gadolinia-containing uranium fuel rods and uranium fuel rods that do not include gadolinia, thereby reducing the cost of assembly design and manufacturing. Has been reduced.

The infinite multiplication factor characteristic of the low Gd fuel assembly in the third embodiment is also shown in FIG. Since the peak of the infinite multiplication factor appears in the middle stage of combustion as in the case of the conventional arrangement of 1/4 symmetry, the excess reactivity characteristic of the core also shows the same characteristic as the conventional one, and the low Gd of the third embodiment. The fuel assembly can be used as in the prior art. Also, the low G of the third embodiment
Although the infinite multiplication factor at the initial stage of combustion of the d fuel assembly is not as large as that of the second embodiment, it is larger than that of the high Gd fuel assembly. can do.

Further, in the present embodiment, a total of eight gadolinia-containing uranium fuel rods and gadolinia-free uranium fuel rods are arranged in the outermost corner area, and the reduction of the plutonium filling amount per assembly is minimized. And NU for uranium
To increase the plutonium loading per aggregate. In the third embodiment, since the gadolinia-containing uranium fuel rod of the low Gd fuel assembly is arranged at the outermost corner, the fuel rods adjacent to this part, particularly the corners on the control rod side and the counter-control rod side, are arranged. The output of the adjacent fuel rods is reduced, which has the effect of flattening the output distribution. Needless to say, the second
In order to obtain the optimal surplus reactivity characteristics as in the example of
The concentration of gadolinia used in the low Gd fuel assembly can be adjusted.

As described above, in the present invention, a small number of gadolinia-containing uranium fuel rods are arranged at the outermost periphery of the assembly, and the difference in gadolinia neutron characteristics at the outermost periphery is utilized. High Gd fuel assemblies with low reactivity and high reactivity with uranium fuel rods
d By providing a fuel assembly and providing a two-stream core using those assemblies, the reactor design can be maintained without changing the assembly design not only during normal operating cycles but also when operating cycles are lengthened. It is possible to design a reactor core having optimal surplus reactivity characteristics during operation. This reduces assembly design and manufacturing costs, enhances operability during reactor operation, and improves reactor safety and economy by increasing reactor availability.

According to the present invention, the number of MOX fuel rods is increased by arranging uranium fuel rods asymmetrically by 1/4 and arranging gadolinia-containing uranium fuel rods at the outermost periphery of the assembly. The use of NU or DU for uranium increases the plutonium loading per aggregate. Increasing the plutonium loading per assembly increases plutonium utilization and reduces MOX fuel assembly manufacturing and maintenance costs.
The fuel assembly is more economical.

[0057]

As described above, the present invention relates to a boiling water reactor (BWR) using a uranium-plutonium mixed oxide (MOX) fuel assembly, which has excellent operability.
The purpose of the present invention is to obtain an X fuel assembly and a BWR core, and furthermore, in a MOX fuel assembly used in a BWR, a MOX fuel assembly having various reactivity characteristics without impairing the plutonium filling amount per assembly. There is an effect that it is possible to provide a body of a boiling water reactor with excellent surplus reactivity characteristics.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a fuel rod arrangement of a MOX fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an infinite multiplication factor characteristic of the MOX fuel assembly of the present embodiment.

FIG. 3 is an explanatory diagram showing a fuel rod arrangement of a low GdMOX fuel assembly according to a second embodiment of the present invention.

FIG. 4 is an explanatory view showing a fuel rod arrangement of a low GdMOX fuel assembly according to a third embodiment of the present invention.

FIG. 5 is a graph showing a change in excess reactivity with respect to burnup during a normal operation cycle period.

FIG. 6 is a line showing a change in surplus reactivity when a core assembly having a normal operation cycle is used for a core having a long operation cycle and a change in optimum surplus reactivity of a core having a long operation cycle. FIG.

FIG. 7 is an explanatory view showing a 1/4 symmetric fuel rod arrangement.

FIG. 8 is an explanatory view showing a 1/4 asymmetric fuel rod arrangement.

Claims (6)

[Claims] 1. A fuel assembly for a boiling water reactor comprising a plurality of plutonium-free uranium fuel rods and a plurality of plutonium / uranium mixed oxide fuel rods loaded in a bundle, wherein the uranium fuel rods Is disposed at the outermost periphery of the aggregate including at least the outermost corner position of the aggregate and at least partially including the burnable poison, and is divided by two diagonal lines connecting the corner positions of the aggregate. A fuel assembly for a boiling water reactor, characterized in that two regions are symmetrical with respect to each diagonal line but are not symmetrical with respect to the intersection of the two diagonal lines. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein the uranium used in the uranium fuel rod is natural uranium (NU) or depleted uranium (DU). 3. A boiling water reactor core loaded with the fuel assembly for a boiling water reactor according to claim 1, wherein the fuel assembly contains a predetermined number of burnable poisons. A boiling water comprising a first fuel assembly having uranium fuel rods and a second fuel assembly having a smaller number of burnable poison-containing uranium fuel rods than the above number. Type reactor core. 4. The boiling water reactor core according to claim 3, wherein all of the uranium fuel rods at the outermost peripheral corner positions of the second fuel assembly do not contain burnable poisons. 5. The boiling water reactor core according to claim 3, wherein all of the uranium fuel rods at the outermost corners of the second fuel assembly contain burnable poisons. 6. The boiling water reactor according to claim 3, wherein the uranium used in the uranium fuel rod is natural uranium (NU) or depleted uranium (DU). Core.