JPH08248162A - Fuel assembly - Google Patents

Abstract

PURPOSE: To achieve the target of high burn-up by disposing fuel rods adjacent to both of a plurality of water rods having the maximum average enrichment. CONSTITUTION: Two water rods 3a disposed in the direction of a central diagonal line in a channel box 1a have large diameters which occupy the area of approximately 3 or 5 fuel rods 2a provided in the 9×9 fuel rods and non-boiling water is allowed to flow therein. Here, if a degree of enrichment of the other fuel rods 4a adjacent to both of the rods 3a is set in another degree of the maximum enrichment, local output peaking should be large at the time point when burning advances, and the output of the fuel rods facing the water area of the rods 3a will increase. A margin for 1% plastic strain of gadolinia content fuel rods can be made that or more for the 1% plastic strain of the fuel rods 4a at the time point when the minimum gadolinia of the margin for the 1% plastic strain of the gadolinia content fuel rods burns up. Thereby the degree of the enrichment of the gadolinia content fuel rods can be increased and other degree of fuel average enrichment is easily increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for use in a boiling water nuclear reactor (hereinafter referred to as BWR).

[0002]

2. Description of the Related Art In order to improve the economical efficiency of a nuclear power plant, it is effective to reduce the fuel cycle cost by increasing the burnup of fuel.

In order to increase the burnup of the fuel, it is necessary to increase the reactivity of the fuel, so that it is necessary to increase the fuel enrichment. For this reason, a conventional method of using a fuel and simply increasing the enrichment can be considered, but the neutron spectrum is hardened and the following phenomenon occurs due to the core characteristics. That is, when the neutron spectrum is hardened, (1) increase in the absolute value of void coefficient, (2) increase in core reactivity at cold temperature, (3) decrease in reactivity control ability due to combustible poisons such as gadolinia, etc. Occurs. As a result, the thermal margin and the furnace shutdown margin may be reduced. Further, when the burnup is increased by simply increasing the enrichment, the cost of natural uranium per fuel, the enrichment cost, etc., increase, so that the reduction in the fuel cycle cost due to the increase in the burnup becomes small. BWR fuels solve this problem by increasing the water to fuel ratio and softening the neutral spectrum. Specifically, the water region of the water rod (hereinafter referred to as the water rod) in the fuel assembly is increased.

In order to increase the water area of the water rod provided in the central portion of the conventional fuel assembly, for example, with the fuel rod array of 8 rows and 8 columns (hereinafter referred to as 8 × 8 fuel assembly), The number of fuel rods decreases and the thermal margin decreases. Further, since the number of fuel rods per fuel assembly is reduced, there is a demerit that the degree of freedom in arrangement of the fuel rods with gadolinia is reduced and the degree of freedom in nuclear design is reduced. For this reason, the fuel rod array is changed to 9 rows and 9 columns (hereinafter referred to as 9 × 9 fuel assembly) to reduce the average linear power density to secure a thermal margin and to freely design the fuel rods. It is considered to increase the degree.

In the fuel assembly in which the water rod is arranged in the central portion as described above, the fuel enrichment of each fuel rod is designed in consideration of the reactivity of the fuel assembly and the magnitude of local output peaking and the change accompanying combustion. It

That is, the central bundle distribution of the fuel assembly tends to be high in the region facing the channel due to the existence of the water gap outside the channel. Therefore, the neutron utilization rate is improved and the reactivity of the fuel assembly is increased by appropriately setting the enrichment of the fuel rod facing the channel so that the local output peaking does not become too large. Specifically, the fuel rods located at the corners of the fuel assembly are set to the minimum enrichment, and the fuel rods having the enrichment region higher than the average enrichment of the fuel assembly are It is placed on the outermost circumference.

Further, since the neutron flux distribution tends to be high even in the water region of the water rod located in the central portion of the fuel assembly, the enrichment of the fuel rod facing the water region of the water rod is also locally output. The size is set appropriately so that the peaking does not become too large. Specifically, in a 9 × 9 fuel assembly, two large-diameter water rods, each of which occupies an area for 3.5 fuel rods, are arranged in the center of the fuel assembly, but at least two When uranium fuel rods of different enrichment contact the water rod on the side, by setting the enrichment of the fuel rod contacting the water region of this water rod on two sides lower than the maximum enrichment, combustion of the local power peaking coefficient The trend was kept low.

On the other hand, the high burnup fuel increases the reactivity of the fuel, so that the number of gadolinia-containing fuel rods used in the fuel assembly increases. One of the thermal margins during fuel rod operation is a margin for 1% plastic strain, but BW
In the R fuel design, the margin for the 1% plastic strain of the fuel rod with gadolinia is designed to be larger than the margin for the 1% plastic strain of the uranium fuel rod. Therefore, the fuel enrichment of the fuel rod with gadolinia is set lower than that of the uranium fuel rod. When the number of gadolinia-containing fuel rods increases in high burnup fuel, if the upper limit of enrichment of uranium fuel rods is set in the manufacturing process, the above-mentioned restriction of fuel enrichment in gadolinia-containing fuel rods causes the fuel assembly. It becomes difficult to increase the average enrichment.

As conventional techniques for improving the fuel economy as described above, Japanese Patent Laid-Open Nos. 61-240193 and 2-296
Some are described in Japanese Patent No.192.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned difficulties in fuel design and to easily increase the fuel enrichment to achieve the target high burnup. Type fuel assembly.

[0011]

SUMMARY OF THE INVENTION The above object is to provide a plurality of fuel rods arranged in 9 rows and 9 columns and having different average enrichments, and a diagonal line of a fuel assembly in a central region where 7 fuel rods can be placed. Two large diameter water rods disposed adjacent to each other in a direction, wherein at least two fuel rods having different average enrichments are adjacent to the water rod, both of the two water rods being provided. This is achieved by the fuel rod adjacent to the fuel rod having the highest average enrichment.

[0012]

In order to solve the difficulty in the conventional technique, B
WR fuel design standard “1 of fuel rod with gadolinia
%, The margin for 1% plastic strain of the uranium fuel rod is larger than the margin for 1% plastic strain of the uranium fuel rod. " It is effective to design so that the former becomes relatively large at the time when the gadolinia becomes the smallest compared to the above. For this purpose, it is effective to increase the output of the fuel rods facing the water region of the water rod, where the local power peaking becomes relatively large when the combustion progresses. Can be achieved by increasing In particular, in a fuel assembly in which at least two fuel rods having different average enrichments are adjacent to two large diameter water rods, the enrichment of the fuel rods adjacent to both of the two water rods is compared to that of the fuel assembly. The highest concentration is effective.

However, simply increasing the enrichment of the fuel rods adjacent to the water rods tends to make the thermal margin itself strict. Therefore, the number of fuel rods should be increased like a 9 × 9 fuel assembly. This is especially effective when the thermal margin is increased. In addition, since increasing the number of fuel rods alone causes problems such as increased core pressure loss and worsened thermal-hydraulic stability, using fuel rods with different axial lengths increases pressure loss. A design that suppresses is effective. Therefore, a 9 × 9 combination of a large diameter water rod with an increased water area and a short fuel rod with a short axial length is used.
By adopting the fuel rod array of, the safety margin for the core characteristics such as the thermal margin and the reactor shutdown margin is secured, and the average linear power density is reduced, so the output peaking such as the local power peaking coefficient of the fuel assembly It is possible to improve the fuel recyclability by increasing.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

A horizontal sectional view of a first embodiment of the fuel assembly according to the present invention is shown in FIG.

In this embodiment, the fuel rods 2a are arranged in 9 rows and 9 columns in the channel box 1a, and two water rods 3a are arranged diagonally adjacent to each other in the center of the fuel assembly. Prepare The two water rods 3a have a large diameter occupying an area corresponding to 3.5 of the fuel rods 2a, and water that does not boil flows in the water rods.

According to the fuel assembly of this embodiment, since the water-to-fuel ratio is larger than that of the conventional fuel, the neutron spectrum is softened and (1) the absolute value of the void coefficient increases (2). ) Increase in core reactivity at cold temperature,
(3) The effect of reducing the reactivity controllability of combustible poisons such as gadolinia can be canceled and the deterioration of the core characteristics of the thermal margin and the reactor shutdown margin can be improved. At the same time, since the water-to-fuel ratio is higher than in the past, the reactivity of the fuel can be improved.

The neutron flux distribution in the fuel assembly tends to be higher in the fuel region facing them due to the water gap outside the channel box 1a and the water rod 3a in the center of the channel box 1a. In the fuel assembly of this embodiment, at least two fuel rods 4a and 5a having different degrees of enrichment are adjacent to the water rod, and the fuel rod 4a adjacent to both of the two water rods 3a is concentrated. By setting the degree of condensation to the maximum enrichment, the output of the fuel rod facing the water region of the water rod 3a is increased, which causes a relatively large local output peaking at the time when the combustion advances, and % When the gadolinia burns out with the smallest margin for plastic strain, the margin for 1% plastic strain of the fuel rod with gadolinia can be made relatively large compared to the margin for 1% plastic strain of the fuel rod. it can.

Due to this effect, the output margin of the fuel rod with gadolinia becomes large, so that the fuel enrichment of the fuel rod with gadolinia can be increased, and the average enrichment of fuel in the fuel assembly can be easily increased.

In FIG. 5, the fuel rod arrangement is 9 rows and 9 columns, and 7
In a fuel assembly in which two water rods occupying an area in which the fuel rods can be arranged are arranged adjacent to each other in a central portion of a cross section of the fuel assembly, at least two fuel rods having different enrichments are provided. Of the fuel rods adjacent to both of the two water rods as a parameter, the maximum local power peaking coefficient in the fuel assembly and the fuel rods with gadolinia It shows the change of the local output peaking coefficient with respect to the burnup. Further, FIG. 6 shows changes in the fuel rod output ratio (local output peaking of fuel rod with gadolinia / maximum local output peaking), which is the ratio of the two local output peakings, with respect to the burnup. From these figures, by increasing the enrichment of the fuel rods adjacent to both of the two water rods (case a → b → c), the local output peaking of the fuel rod with gadolinia is compared with the maximum local output peaking. It can be seen that it can be reduced.

Next, a second embodiment of the fuel assembly according to the present invention will be described with reference to FIGS. The fuel assembly 16 of this embodiment includes a fuel rod 15 and a channel box 1.
2, water rod 13, lower tie plate 19, upper tie plate 18, fuel spacer 20, and the like. The upper and lower ends of the fuel rods 15 and the water rods 13 are held by the lower tie plate and the upper tie plate (however, of the fuel rods, only the lower ends of the fuel rods are held by the lower tie plate). . Several spacers 20 are arranged in the axial direction of the fuel rods 15 and maintain the gap between the fuel rods 15 and the water rods 13 in an appropriate state. The channel box 12 is attached to the upper tie plate 18 and surrounds the outer circumference of the bundle of fuel rods 15 held by the spacers 20. Channel fasteners (not shown) are attached to the upper tie plate 18. 14 is a control rod.

Although not shown, the fuel rod 15 has a large number of fuel pellets loaded in a cladding tube whose both ends are sealed by a lower end plug and an upper end plug. Fuel pellets
It is composed of UO ₂ which is a fuel material, and contains U ²³⁵ which is a nuclear array material. A spring is located within the cladding tube in the gas plenum region and presses the fuel pellet downward.

The water rod 13 uses the same cladding tube as the fuel rod 15, but it is not loaded with a fuel substance, and although not shown, holes are formed in the upper and lower side surfaces of the cladding tube to prevent boiling inside. Are supposed to pass through.

In the BWR core, cruciform control rods 14 are inserted into one of four fuel assemblies. This core has
The width of the water gap formed on the side wall side of the fuel assembly 16 facing the control rod 14 to be inserted is the water formed on the side wall side of the fuel assembly 16 on the opposite side and not facing the control rod 14. The core (D lattice core) wider than the width of the gap and the width of the water gap formed on the side wall side of the fuel assembly 16 facing the control rod 14 are on the opposite side to the control rod 14. There is a core (C lattice core) having a width equal to the width of the water gap formed on the side wall side of the fuel assembly 16 that does not face.

The fuel assembly 16 of this embodiment is a fuel assembly loaded in the C-lattice core. As shown in FIG. 3, the fuel rods 15 that compose the fuel assembly 16 are the fuel rods 1-8.
There are different types. These fuel rods 1 to 8 are arranged in the cross section of the fuel assembly in the channel box 12 as shown in FIG. The fuel rod 6 is a short fuel rod, and the fuel rod with the number of one piece is a long fuel rod.

As the water rods 13 of this embodiment, as shown in FIG. 3, two large diameter rods are arranged so as to occupy the area of seven fuel rods 15, and they are located at the center of the assembly. As a result, the water-to-fuel ratio becomes larger than the conventional fuel, and the deterioration of the core characteristics due to the higher burnup is solved, and at the same time the reactivity is improved.

The long fuel rods 1 to 5, 7 and 8 have regions (natural uranium blanket regions) in which fuel pellets made of natural uranium are filled at the upper and lower ends of the fuel substance filling region. The natural uranium blanket occupies 1/24 of the active fuel length H from the lower end and 2/24 of the active fuel length H from the upper end of the fuel substance filling region. The short fuel rod 6 is not provided with a natural uranium blanket region.

In the long fuel rods 1 to 5, 7 and 8, 1/24 to 22/2 of the effective fuel length H is measured from the lower end of the effective fuel portion.
The range of 4 is the enriched uranium region filled with enriched uranium. As shown in FIG. 4, in the fuel rods 1, 3 to 5, 7 and 8, the enriched uranium region has a uniform enrichment in the axial direction, and in the fuel rod 2, the enriched uranium region has the enriched concentration in the axial direction. It has three different areas. Further, the short fuel rods 6 have a uniform enrichment in the axial direction. The enrichment in the enriched uranium region of each fuel rod is A wt% for fuel rod 1,
The fuel rod 3 has C weight%, the fuel rod 4 has D weight%, the fuel rod 5 has E weight%, the fuel rod 6 has B weight%, and the fuel rods 7 and 8 have F weight%. In the enriched uranium region, the fuel rod 2 has a fuel effective length h of 1 with the lower end of the fuel effective portion as a base point.
The enrichment in the range of / 24 to 8/24 is B% by weight, and the enrichment in the range of 8/24 to 20/24 of the active fuel length h is A% by weight.
And the enrichment degree in the range of 20/24 to 20/24 of the active fuel length h is C% by weight. Fuel rods 7 and 8 contain gadolinia, a combustible poison, in the fuel pellets in the enriched uranium region. The gadolinia concentration in the axial direction of the enriched uranium region is in the range of 1/24 to 8/24 of the fuel effective length h with the lower end of the fuel effective portion of the fuel rod 7 as the base point, and is H weight% and 8 of the fuel effective length h. In the range of / 24 to 22/24,
G% by weight. The fuel rods 8 have H weight% and 20/24 to 22/22 of the fuel effective length h in the range of 1/24 to 20/24 of the fuel effective length h with the lower end of the fuel effective portion as a base point.
In the range of 24, it is G% by weight. Fuel rods 1-6 do not contain gadolinia.

Here, the magnitude relationships of enrichment in the enriched uranium region are A, B, F, C, D, and E in descending order of enrichment.
The order is. In addition, the relationship between the gadolinia concentrations of the fuel rods with gadolinia is H, D
The order is.

The natural uranium blanket region formed at the upper and lower ends of the fuel effective portion of the fuel assembly 16 is 0.
It contains 71% by weight of U ²³⁵ .

In the BWR, since there are many vapor bubbles (voids) toward the upper end of the core, the density of water as a neutron moderator decreases in the upper part of the core. For this reason, when a fuel assembly having a uniform axial enrichment distribution is loaded in the core, there is a tendency for the output distribution of the lower strain to have an output peak below the fuel assembly. Further, in the fuel assembly of the present embodiment, the short fuel rods are used, and the fuel loading amount in the lower part of the fuel is larger than that in the upper part of the fuel, which causes the output distribution of the lower strain. Therefore, as described above, the output distribution in the axial direction of the fuel assembly is flattened by making the enrichment of the upper part of the fuel assembly higher than that of the lower part of the fuel assembly.

Further, as described above, in the BWR, since there are many vapor bubbles (voids) toward the upper end of the core, the density of water which is a neutron moderator is low in the upper part of the core and the density of water is high in the lower part of the core. For this reason, if the gadolinia average weight% in the axial direction is the same, the gadolinia combustion proceeds faster because the neutron spectrum is softer in the lower part of the core than in the upper part. As a result, the rate of increase in the reactivity of the lower part of the core when the gadolinia combustion progresses becomes faster, and the axial power distribution is likely to have a downward peak. In this embodiment, in order to control this appropriately, the gadolinia density in the lowermost region is made relatively large.

The fuel assembly 16 has the uppermost region (the range of 20/24 to 22/24 of the active fuel length h) of the enriched uranium region. The average cross-sectional enrichment of this uppermost region is the central region of the enriched uranium region (8 / the active fuel length h).
24 to 20/24) and the lowermost region (range of 1/24 to 8/24 of the fuel collecting length h). The uppermost region of the enriched uranium region has a low average cross-sectional enrichment and corresponds to the low concentration region of the burnable poison of the fuel rods 7 and 8, and compensates for the reduction in the reactor shutdown margin.

In the fuel assembly 16, two uranium fuel rods 1 and 2 having different enrichment are in contact with the water rod at the side surface, and the fuel rod is in contact with the water rod at two side surfaces orthogonal to each other (from the corner). , Fuel rods 1 of the highest enrichment in the fuel assembly are held as the fuel rods of the 4th and 6th fuel rods in each row. As a result, the fuel rod output ratio is reduced, and the margin for the 1% plastic strain of the fuel rod with gadolinia is relatively increased as compared with the margin for the 1% plastic strain of the uranium fuel rod.

Further, in the fuel assembly 16, the fuel rods 2 and 3 each having a region having an enrichment higher than the average enrichment of the fuel assembly are arranged at the outermost periphery of the cross section of the fuel assembly.

The fuel assembly 16 described above increases the average enrichment of the fuel assembly under the condition that the upper limit of the maximum enrichment is set, while ensuring a sufficient safety margin of the reactor, and increases the burnup. Can be achieved. As a result, the cost required for the fuel cycle can be reduced, and the amount of spent fuel assemblies generated can be reduced.

[0037]

According to the present invention, the margin for 1% plastic strain of a fuel rod containing gadolinia is relatively increased as compared with the margin for 1% plastic strain of a uranium fuel rod, so that the maximum fuel rod Under the condition that the upper limit of enrichment is set, the average enrichment of the fuel assembly can be increased and the burnup can be increased, which can be used for improving the fuel economy and increasing the extraction energy.

[Brief description of drawings]

FIG. 1 is a horizontal sectional view of a first embodiment of a fuel assembly according to the present invention.

FIG. 2 is an axial sectional view of a second embodiment of the fuel assembly according to the present invention.

FIG. 3 is a horizontal sectional view of the second embodiment.

FIG. 4 is an explanatory view of the enrichment and gadolinia distribution of the fuel rod of the second embodiment.

FIG. 5 is a diagram showing the relationship between the burnup and the local output peaking coefficient in the first embodiment.

FIG. 6 is a relationship diagram of burnup and fuel rod output ratio in the first embodiment.

[Explanation of symbols]

1a, 12 ... Channel box, 3a, 13 ... Water rod, 1-8, 2a, 4a, 5a, 15 ... Fuel rod, 14 ...
Control rod, 16 ... Fuel assembly, 18 ... Upper tie plate,
19 ... Lower tie plate, 20 ... Fuel spacer.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G21C 3/30 GDBT

Claims (6)

[Claims] 1. A plurality of fuel rods arranged in 9 rows and 9 columns and having different average enrichments and a fuel assembly in a central region where 7 fuel rods can be arranged are arranged adjacent to each other in a diagonal direction. A fuel assembly comprising two large diameter water rods, wherein at least two fuel rods having different average enrichments are adjacent to the water rods, wherein the fuel rods adjacent to both of the two water rods are averaged. A fuel assembly characterized in that the fuel rod has the highest enrichment. 2. The nuclear fuel material filling region of the fuel rod according to claim 1, wherein an upper end portion and a lower end portion of natural uranium are loaded in the axial direction of the fuel assembly, and enriched uranium is loaded and an average enrichment is increased. It is divided into different upper region, central region and lower region, the average concentration of the central region is higher than the average concentration of the upper region and the lower pond region, of the average concentration of the central region and the lower region The difference is smaller than the difference between the average enrichment of the central region and the upper region, the content of burnable poison per axial unit length in the upper region is in the central region and the lower region A fuel assembly characterized by being less than the content of combustible poisons per axial unit length. 3. The fuel rod according to claim 1, wherein the fuel rod comprises a first fuel rod having a longer axial length and a second fuel rod having a shorter axial length than the first fuel rod. An upper end of the nuclear fuel material filling region of the second fuel rod is located within the central region, and an average enrichment of the central first region located above the upper end of the second fuel rod in the central region, A fuel assembly characterized in that it is lower than the average enrichment of the central second region located below the upper end of the second fuel rod in the central region. 4. The difference in average enrichment between the central second region and the lower region is smaller than the difference in average enrichment between the central first region and the upper region. Fuel assembly. 5. The content of burnable poison per unit length in the axial direction in the enriched uranium loading region increases in the order of the upper region, the central region, and the lower region. And the fuel assembly. 6. The fuel assembly according to claim 1, wherein the fuel rods having a region having an enrichment higher than the average enrichment of the fuel assembly are arranged in the outermost layer of the cross section of the fuel assembly.