JP2000338281A - Reactor core of nuclear reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve fuel economy by increasing the ratio where the total of a fuel assembly with the longest residence period in a reactor and that with the second longest residence period occupies in the outer periphery and the second layer-region as compared with that for occupying in an internal region. SOLUTION: In the reactor core of a boiling-water nuclear reactor or the like, the outermost periphery region is arranged while fuel at the fifth cycle occupies all, and the second-layer region is arranged while fuel at the fourth cycle occupies all, thus reducing the leakage of neutrons from the reactor core. The fuel at the fourth cycle is also loaded into a control cell for composing a control region. The second-layer region is separated from the control region by a third-layer region to prevent the decrease in output due to the insertion of a control rod during operation from affecting the second-layer region. The total of the fuel at the first cycle and that at the second cycle is for example approximately 80% at the third-layer region and is larger than the total of the fuel at the first cycle and that at the second cycle excluding the third layer out of the inner region, thus flattening an output distribution in a diameter direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reactor core of a boiling water reactor, and more particularly to a reactor core configured to improve thermal economy while securing a thermal margin.

[0002]

2. Description of the Related Art The core of a conventional boiling water reactor will be described with reference to FIG. FIG. 7 is a 1/4 transverse sectional view of the core. FIG.
As shown in (1), several hundreds of fuel assemblies are regularly arranged and loaded in a square lattice, and are generally formed in a substantially cylindrical shape.

At the time of refueling, about one-fourth of all fuel assemblies in the core are replaced with new fuel assemblies. In a certain operation cycle, fuel assemblies having different loading times are loaded in the core. I have. In FIG. 7, the number assigned to each fuel assembly indicates the number of cycles in the furnace.

As an example of a fuel assembly, see, for example,
FIG. 8 shows a 9-row / 9-column high burnup fuel assembly disclosed in Japanese Patent No. 234293. 8A is a longitudinal sectional view of the fuel assembly 1, FIG. 8B is a sectional view taken along the line BB in FIG. 8A,
(C) is a sectional view taken along the line CC in (a).

[0005] In FIG. 8, a fuel assembly 1 indicated by reference numeral 1 has a long fuel rod 2, a short fuel rod 3, and a water rod 6 connected to a fuel rod 7.
It is fixed to the upper tie plate 4 and the lower tie plate 5 by being bundled with the spacers 8 arranged at equal intervals, and is surrounded by the channel box 7. The spacers 8 are arranged at appropriate intervals to suppress the vibration of the fuel rod due to the flow of the coolant. The short fuel rods 3 reduce the pressure loss by enlarging the coolant flow path above the fuel assembly where the proportion of voids in the coolant is large.

In the core, a cruciform control rod is removably arranged in a cruciform gap located at the center of four fuel assemblies arranged in two rows and two columns. Control rods inserted during operation of a nuclear reactor are often control rods arranged at specific positions. In FIG. 7, the control rods described above are used between the four fuel assemblies indicated by thick frames.

When these control rods are inserted and removed during operation,
A local power fluctuation is given to the surrounding fuel assembly. Therefore, a fuel assembly having a long reactor stay period and a low reactivity is arranged to reduce the influence of power fluctuation. The four low-reactivity fuel assemblies and one control rod surrounded by the fuel assemblies are collectively called a control cell.

In the core of the nuclear reactor shown in FIG. 7, a fuel assembly having the longest residence time in the reactor is arranged, that is, loaded in the outermost peripheral region. The reactivity of these fuel assemblies is low because the burnup is advanced, and the neutron leakage from the core is reduced to increase the effective multiplication factor. Such a fuel arrangement is called a low leakage core.

[0009]

The low-reactivity fuel assembly is considered to be arranged not only at the outermost periphery but also at the innermost part of the core, thereby further reducing neutron leakage from the core and improving economic efficiency. Can be However, in such an ultra-low leakage core, the thermal margin is reduced due to an increase in power peaking, and the soundness of the fuel is reduced.

Here, as the thermal margin, the maximum linear output density and the minimum limit output ratio are important. The linear power density is a calorific value per unit length of the fuel rod, and needs to be sufficiently low to reduce the interaction (PCI) between the fuel pellets constituting the fuel rod and the cladding tube. The critical power ratio is the ratio between the critical power that causes a boiling transition and the fuel assembly power, and needs to be sufficiently large.

[0011] In the ultra-low-leakage core, the output of the fuel assembly loaded inside the core increases, and the burnup increases accordingly. As a result, at the end of the operation cycle, the reactivity of the fuel assemblies loaded inside the core is rapidly reduced, and the effective multiplication factor does not increase as expected.
In other words, the economic efficiency cannot be sufficiently improved simply by using the ultra-low leakage core.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has a sufficient thermal margin, reduces neutron leakage from the reactor core, suppresses fuel combustion inside the reactor core, and provides an economical solution. An object of the present invention is to provide a nuclear reactor core capable of improving the performance.

[0013]

In order to solve the above-mentioned problems, a first aspect of the present invention comprises a plurality of fuel assemblies bundled with a plurality of spacers and surrounded by a channel box and surrounded by a channel box. In the core of the nuclear reactor, an outermost peripheral region composed of a plurality of fuel assemblies arranged at the outermost periphery of the core, and a second layer region composed of the fuel assemblies arranged at a second layer from the outermost peripheral region. A control region including a plurality of fuel assemblies provided toward the axis from the second layer region and facing a control rod inserted during the operation of the reactor; and a plurality of control regions extending from the control region toward the axis. Divided into internal areas,
The ratio of the sum of the fuel assembly having the longest residence time in the furnace and the fuel assembly having the second longest period in the outermost region and the second layer region is larger than the ratio in the inner region. .

The reactivity of the fuel assembly decreases as combustion proceeds. Therefore, by arranging a large number of fuel assemblies having a long reactivity in the reactor and having a low reactivity in the second layer region in addition to the outermost peripheral region, the leakage of neutrons in the core diameter direction is reduced and the effective multiplication factor is increased. Can be enhanced.

FIG. 9 shows a case where the fuel of the fifth cycle in the outermost peripheral area is fixed and the fuel of the fourth cycle is arranged in the second layer area in the core of FIG. In the case where fuel of the fourth cycle is all
It shows the relationship between the deterioration of the thermal margin and the increase of the effective multiplication factor.

FIG. 9 shows that even if the fuel in the fourth cycle is arranged in the third layer area, the effective multiplication factor does not improve despite the decrease in the thermal margin. Therefore, in order to improve the economy without excessively reducing the thermal margin, it is effective to arrange the low-reactivity fuel assemblies in the outermost peripheral region and the second-layer region.

The invention according to claim 2 is characterized in that the second layer area and the control area are separated from each other by an internal area different from the control area and the internal area installed in the axial direction. I do.

It is important to increase the effective multiplication factor at the end of the operation cycle in order to improve the economy. When the fuel assemblies with reduced reactivity are arranged in the outermost peripheral region or the second layer region, the output of the fuel assemblies loaded in the internal region increases, and the burnup increases accordingly. As a result, at the end of the operation cycle, the reactivity of the fuel assembly in the inner region does not increase as expected, and the increase in the effective multiplication factor is not sufficient.

Therefore, by separating the control region from the second layer region, the output reduction due to the insertion of the control rod is suppressed by reaching only the inner region, while the output is suppressed in the second layer region. Increase the output by not allowing it.

As a result, at the end of the operation cycle, the reactivity of the fuel assembly is sufficiently reduced due to the progress of the combustion in the outermost peripheral region and the second layer region, while the fuel assembly is not promoted in the internal region, and thus the fuel assembly is not advanced. Since the reactivity does not decrease, the effective multiplication factor can be sufficiently increased.

According to a third aspect of the present invention, in the fuel cell system according to the first aspect of the invention, the inner region includes a fuel assembly disposed at a third layer from the outermost peripheral region.
The ratio of the sum of the fuel assembly that is divided into a layer region and a central region other than the third layer region and that has the shortest residence time in the core and the second shortest fuel assembly in the third layer region is It is characterized in that it is larger than the ratio occupying the central region.

When the fuel assemblies having reduced reactivity are arranged up to the second layer region, the output of the third layer region is also suppressed. Therefore, a fuel assembly with a short reactor stay period and high reactivity is
By arranging more in the layer region, the output of the third layer region can be increased, and the radial power distribution of the core can be flattened. At the same time, the output of the fuel assemblies in the outermost peripheral region and the second layer region can also be increased, so that the burnup at the end of the operation cycle can be increased to reduce the reactivity, thereby increasing the effective multiplication factor.

According to a fourth aspect of the present invention, the first group of fuel assemblies and the second group of fuel assemblies having a smaller number of burnable poison-containing fuel rods than the first group of fuel assemblies are arranged. The ratio of the second group of fuel assemblies and the first group of fuel assemblies having the shortest residence time in the core in the core of the reactor is greater in the third layer region than in the central region. It is characterized by.

The action of the burnable poison disappears at the end of the first cycle, but during the first cycle, the fuel assemblies of the second group having a smaller number of fuel rods containing the burnable poison are in the first group. Higher reactivity than fuel assemblies. Therefore, by arranging a large number of the second group of fuel assemblies in the third layer region,
The power in the layer region can be increased, and the radial power distribution of the core can be flattened. At the same time, the output of the fuel assemblies in the outermost peripheral region and the second layer region can also be increased, so that the burnup at the end of the operation cycle can be increased to reduce the reactivity, thereby increasing the effective multiplication factor.

According to a fifth aspect of the present invention, in the fuel assembly loaded in the internal region, a plurality of fuel rods are bundled in a square grid of 10 rows and 10 columns and are surrounded by the channel box. I do.

The linear power density is reduced in inverse proportion to the number of fuel rods. In addition, the limit power increases in proportion to the fuel rod surface area. Therefore, by loading the fuel assemblies of 10 rows and 10 columns at least in the internal region where the output increases, a sufficient thermal margin can be secured.

According to a sixth aspect of the present invention, in the fuel assembly loaded in the internal region, a plurality of fuel rods are bundled in a square grid of 9 rows and 9 columns or 10 rows and 10 columns by eight or more spacers. It is surrounded by a channel box, and the spacer interval is smaller at the upper part than at the lower part.

The spacer has the function of stirring the flow of the coolant and increasing the thickness of the liquid film on the fuel rod surface downstream of the coolant. Therefore, the limit output is improved by increasing the number of spacers and reducing the spacer interval. Since the boiling transition is more likely to occur in the upper portion of the fuel assembly having a high void fraction than in the lower portion, it is effective to make the spacer interval particularly close in the upper portion of the fuel assembly.

According to a seventh aspect of the present invention, in the fuel assembly disposed in the internal region, a long fuel rod and a short fuel rod having an effective length shorter than the long fuel rod are bundled and surrounded by the channel box. It is characterized by having been done.

In the fuel assembly according to the fifth or sixth aspect, the number of fuel rods or the number of spacers is increased, and any of these may cause an increase in pressure loss. By using short fuel rods as a part of the fuel rods constituting the fuel assembly, an increase in pressure loss can be suppressed.

According to an eighth aspect of the present invention, in the fuel assembly disposed in the internal region, the long fuel rods are disposed at four corners of a second layer from the outermost periphery in a cross section, and two layers from the outermost periphery. The long fuel rods arranged at the four corners of the eye contain a burnable poison at least in most parts except upper and lower ends.

In the 9-row / 9-column fuel assembly according to the sixth aspect, the limit output is improved by increasing the number of spacers, but the linear power density is not improved, so that the output peaking in the cross section of the fuel assembly is reduced. It may appear on the fuel rods at the four outermost corners. Therefore, according to the invention of claim 8, the four corners of the second layer from the outermost periphery are made to contain burnable poison as long fuel rods, thereby reducing the output of the fuel rods at the outermost four corners adjacent to these. can do. As a result, the maximum linear output density can be reduced.

According to a ninth aspect of the present invention, in the fuel assembly disposed in the internal region, some of the fuel rods contain burnable poisons only in the lower portion, or the lower cross-sectional average enrichment in the upper portion is higher. It is characterized by being smaller than the average cross-sectional enrichment.

When the output of the fuel assembly in the inner region increases, the void fraction increases in the fuel assembly, and the axial power distribution has a lower peak. Therefore, by suppressing the output at the lower part of the fuel assembly, the axial output distribution can be flattened, and the maximum linear output density can be reduced.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) (Corresponding to claims 1, 2, 3, 5, 7) FIGS. 1 and 2
According to (a) and (b), the first core of the reactor according to the present invention is used.
An embodiment will be described. FIG. 1 is a 1/4 cross-sectional view showing the core of a nuclear reactor according to the present embodiment, and FIGS. 2A and 2B are cross-sectional views of a fuel assembly disposed in the core. The core of the nuclear reactor according to the present embodiment is a core used for a boiling water reactor having the same size as that of the conventional example in FIG. 7, and a fuel assembly disposed in this core is a 10 × 10 fuel assembly. The overall structure of this fuel assembly is substantially the same as that shown in FIG. 8A, except for the cross-sectional views taken along the lines BB and CC in FIG. 8A. As shown in b), the shape of the water rod 6 is changed from a cylindrical shape to a square shape of a square.

As shown in FIG. 1, the core of the nuclear reactor according to the present embodiment is arranged such that the fuel in the fifth cycle occupies the entire outermost region and the fuel in the fourth cycle occupies the whole in the second layer region. The occupation of the reactor core reduces neutron leakage from the reactor core. The fourth cycle fuel is also loaded in the control cells indicated by the thick frames constituting the control regions of the seventh and eighth layers, the eleventh and twelfth layers, and the innermost region. 2
The layer area and the control area are separated by a third layer area, so that a decrease in output due to insertion of a control rod during operation does not reach the second layer area.

The total of the first cycle fuel and the second cycle fuel occupies about 80% in the third layer area, and is larger than about 59% in the central area of the internal area excluding the third layer area. Since the fuel assemblies having a short residence time in the furnace have a high output, the radial power distribution is flattened.

As shown in FIGS. 2A and 2B, 10 rows 10
The row fuel assembly has two square water rods 6 and twelve short fuel rods 3. This fuel assembly is shown in FIG.
The number of fuel rods is larger and the surface area of the fuel rods is larger than that of the conventional 9-row / 9-column fuel assembly shown in FIG. The linear power density has a margin of 20% or more in inverse proportion to the number of fuel rods.

Further, the heat transfer area is increased by about 10% due to the increase in the surface area of the fuel rod, and the limit power is improved by about 5%.
In this fuel assembly, the pressure loss increases as the number of fuel rods increases.
The ratio of short fuel rods has been increased from eight to twelve compared to the row type fuel assembly.

(Second Embodiment) (corresponding to claims 1, 2, 3, 6, 7, 8, 9) FIG.
(C) and FIGS. 4 (a) and 4 (b), a second embodiment of the reactor core according to the present invention will be described. The configuration of the reactor core of the reactor according to the second embodiment is the same as that of the first embodiment, but the configuration of the fuel assembly disposed in the reactor core is different.

That is, as shown in FIGS. 3A to 3C, the fuel assemblies arranged in the second embodiment have nine rows and nine rows.
The difference between the row-type fuel assemblies is that the number of spacers 8 is increased from the conventional seven to eight, and one is increased, and the gap L2 between the upper spacers 8 is reduced. In addition, FIG.
In (c), the same parts as those in FIGS. 8A to 8C are denoted by the same reference numerals, and the description of the overlapping parts will be omitted.

That is, in the present embodiment, as shown in FIG. 3A, the gap L1 between the lower spacers 8 is the same as the conventional example shown in FIG. 8, and the gap L2 between the upper spacers is smaller than L1. Boiling transitions are more likely to occur at the top of the fuel assembly, so the fuel assembly has about a 5% increase in critical power.

Since the pressure loss increases with the increase in the number of the spacers 8, the length of the short fuel rods 3 is shortened in the present embodiment as compared with the conventional example shown in FIG. That is, the upper end of the short fuel rod 3 is located at the fifth spacer position from the bottom in the conventional example of FIG. 7, but is located at the fourth spacer position from the bottom in the fuel assembly of the present embodiment. 3 is changed, and the four corners of the second layer in the cross section of the fuel assembly are made to be long fuel rods 2.

FIGS. 4A and 4B show the enrichment and the distribution of burnable poisons in the fuel assembly according to the present embodiment. The number of each fuel rod in the fuel assembly of FIG.
This corresponds to the fuel rod number shown in FIG. Fuel rod number 6
Is a short fuel rod 3.

In FIG. 4B, the enrichment of the fuel rod is A
>B>C> D, and natural uranium containing no burnable poison is used at the upper and lower ends of the long fuel rod. Some long fuel rods contain gadolinia, which is a burnable poison, and its concentration is in the order of HG> LG.

According to the present embodiment, most of the long fuel rods at the four corners of the second layer contain gadolinia except for the upper and lower ends, and the output of the fuel rods at the outermost four corners is reduced. ing. In addition, low-concentration gadolinia is contained only in the lower part of some fuel rods to flatten the axial power distribution.
Further, by lowering the enrichment of the short fuel rods, the cross-sectional average enrichment of the lower part of the fuel assembly is made lower than that of the upper part,
The axial power distribution is flattened. As a result, the maximum linear output density can be sufficiently reduced.

(Third Embodiment) (Corresponding to Claims 1, 2, 3, 4, 6, 7, 8, and 9) A third embodiment of the present invention will be described with reference to FIGS.
FIG. 5 is a 1/4 cross-sectional view of the core of the nuclear reactor according to the third embodiment. The basic fuel arrangement is the same as that of the first embodiment shown in FIG.
In other words, the different types of fuel assemblies are arranged, that is, loaded.

Each of the two types of fuel assemblies arranged in this embodiment is a 9-row / 9-column type fuel assembly shown in FIG. 3, but has different gadolinia contents. Since the difference in the gadolinia content does not affect the fuel assembly characteristics after the second cycle fuel, FIG. 5 shows only the first cycle fuel.

The enrichment of the fuel rods and the distribution of burnable poisons in each fuel assembly are shown in FIG.
(B). The axial distribution of each fuel rod is the same as in the second embodiment shown in FIG. FIG.
(A) is a first group of fuel assemblies, in which most of the fuel rods containing gadolinia (fuel rod number 4) except for the upper and lower ends have 16 fuel rods.
It is a book. FIG. 6B shows a second group of fuel assemblies, in which most of the fuel rods except for the upper and lower ends contain 12 gadolinia-containing fuel rods (fuel rod number 4), and only the lower part contains low-concentration gadolinia. There are two rods (fuel rod number 5).

In FIG. 5, the fuel in the first cycle loaded in the third layer area is all the fuel rods containing gadolinia and is a second group of fuel assemblies having a small number. The neutron absorption capacity of gadolinia has disappeared in the fuel assemblies after the second cycle, but during the first cycle, the reactivity is higher as the number of gadolinia-containing fuel rods is smaller. Therefore, in the present embodiment, the output in the third layer region is increased, and thus there is an effect that the radial output distribution is flattened.

[0051]

According to the present invention, a sufficient thermal margin can be secured, neutron leakage from the core can be reduced, and fuel combustion inside the core can be suppressed. Is increased. Therefore, it is possible to extend the operation period of the nuclear reactor, reduce the number of fuel replacement bodies, and reduce the fuel enrichment, thereby improving fuel economy.

[Brief description of the drawings]

FIG. 1 is a 1/4 sectional view of a core of a boiling water reactor according to a first embodiment of the present invention.

FIG. 2A is an upper cross-sectional view of a 10-row, 10-column fuel assembly used in the core of the boiling water reactor according to the first embodiment of the present invention, and FIG. Area view.

FIG. 3A is a longitudinal sectional view of a 9-row / 9-column fuel assembly used in a boiling water reactor core according to a second embodiment of the present invention, and FIG. BB arrow direction cutaway view,
(C) is a sectional view taken along the line CC of (a).

FIG. 4A is a cross-sectional view showing enrichment and burnable poison distribution of a 9-row / 9-column fuel assembly used in a core of a boiling water reactor according to a second embodiment of the present invention. (B) is an axial distribution diagram of each fuel rod in (a).

FIG. 5 is a quarter sectional view of a core of a boiling water reactor according to a third embodiment of the present invention.

FIG. 6 is a cross-sectional view showing the enrichment and burnable poison distribution of a first-row 9-by-9-column fuel assembly used in a boiling water reactor core according to a third embodiment of the present invention. And (b) is a cross-sectional view showing a second group of fuel assemblies.

FIG. 7 is a quarter sectional view of a core of a conventional boiling water reactor.

8 (a) is a longitudinal sectional view showing an overview of a conventional 8-row / 8-column fuel assembly, FIG. 8 (b) is a sectional view taken along the line BB of FIG. 8 (a), and FIG. FIG.

FIG. 9 is a characteristic diagram showing a relationship between a thermal margin and an effective multiplication factor by a conventional fuel arrangement.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Long fuel rod, 3 ... Short fuel rod, 4
... upper tie plate, 5 ... lower tie plate, 6 ... water rod, 7 ... channel box, 8 ... spacer.

Claims (9)

[Claims] 1. A reactor core comprising a plurality of fuel assemblies bundled with a plurality of fuel rods bundled by a plurality of spacers and surrounded by a channel box, wherein a plurality of fuels arranged on the outermost periphery of the core are arranged. An outermost peripheral region comprising an aggregate;
A second layer region composed of a fuel assembly disposed in the second layer from the outermost peripheral region, and a control rod provided toward the axis from the second layer region and inserted during operation of the reactor. A control region composed of a plurality of fuel assemblies and a plurality of internal regions divided from the control region toward the axis and having the longest in-furnace residence period and the second longest fuel assembly A reactor core, wherein the ratio of the total to the outermost peripheral region and the second layer region is greater than the ratio to the internal region. 2. The control apparatus according to claim 1, wherein the second layer area and the control area are separated from each other by an internal area different from the control area and the internal area installed in the axial direction. Reactor core. 3. The inner region is divided into a third layer region composed of a fuel assembly disposed at a third layer from the outermost peripheral region and a central region other than the third layer region, 3. The reactor according to claim 1, wherein the sum of the shortest fuel assembly and the second shortest fuel assembly occupies the third layer region in a ratio greater than the ratio occupying the central region. 4. Core. 4. A nuclear reactor having a first group of fuel assemblies and a plurality of second group fuel assemblies having a smaller number of burnable poison-containing fuel rods than the first group of fuel assemblies. In the core, a ratio of the second group of fuel assemblies and the first group of fuel assemblies having the shortest stay period in the core is greater in the third layer region than in the central region. Item 3. The reactor core according to item 1 or 2. 5. The fuel assembly loaded in the internal region, wherein a plurality of fuel rods are bundled in a square grid of 10 rows and 10 columns and are surrounded by the channel box. 4. The reactor core of claim 4. 6. The fuel assembly loaded in the internal region is surrounded by the channel box by bundling a plurality of fuel rods in a square grid of 9 rows and 9 columns or 10 rows and 10 columns with eight or more spacers. 5. The reactor core according to claim 1, wherein the spacer interval is smaller at an upper portion than at a lower portion. 7. The fuel assembly disposed in the internal region is configured such that a long fuel rod and a short fuel rod having an effective length shorter than the long fuel rod are bundled and surrounded by the channel box. The reactor core of a nuclear reactor according to claim 5 or 6, wherein: 8. The fuel assembly disposed in the internal region has the long fuel rods disposed at four corners of a second layer from the outermost periphery in a cross section, and at four corners of a second layer from the outermost periphery. 8. The reactor core according to claim 5, wherein the disposed long fuel rod contains a burnable poison at least in most parts except upper and lower ends. 9. The fuel assembly disposed in the internal region, wherein some of the fuel rods contain burnable poisons only in the lower portion, or the lower section average enrichment is lower than the upper section average enrichment. 9. The reactor core of a nuclear reactor according to claim 5, wherein said core is also small.