JP3159959B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly loaded in a boiling water reactor, and more particularly to a fuel assembly aiming at high burnup, which improves fuel economy and increases thermal margin. The present invention relates to a fuel assembly that contributes to improvement of reactor shutdown margin and stability.

[0002]

2. Description of the Related Art A conventional fuel assembly loaded in a boiling water reactor comprises a rectangular channel box and a fuel bundle housed inside the channel box. The fuel bundle consists of an upper tie plate and a lower tie plate which fit into the upper and lower portions of the channel box, a plurality of spacers spaced apart along the axial direction inside the channel box, and a tie plate extending through the spacers. And a plurality of fuel rods and at least one water rod arranged in a square lattice with both ends fixed.

In recent years, higher burnup of fuel has been promoted from the viewpoints of extending the continuous operation period, effectively utilizing uranium resources, and reducing the amount of spent fuel generated. To increase the burnup, it is necessary to increase the fuel enrichment. However, as the enrichment increases, the average energy of neutrons increases, which causes problems such as an increase in reactivity due to void changes and an impairment of the effective use of fissile fuel (fuel economy). Was. The increase in the reactivity change accompanying the void change increases the absolute value of the void coefficient, decreases the core stability, and reduces the reactor shutdown margin because the reactivity difference between operation and cold temperature increases. The countermeasure is to increase the moderator ratio (moderator to fuel ratio) in the fuel assembly and reduce the average neutron energy (soften the neutron spectrum).

In a boiling water reactor, a control rod and a neutron detector instrumentation tube are arranged outside a channel box. Therefore, there is a gap between the fuel assemblies so that these devices can be inserted. Is provided. Since the gap is filled with the saturated water, the effect of the saturated water in the gap differs between the fuel rods at the periphery of the fuel assembly (the area near the gap) and the fuel rods at the center of the fuel assembly. That is, the fuel rods near the fuel assembly near the gap are effectively in a region where the moderator-to-fuel ratio is large, and the moderator-to-fuel ratio, which is a factor that determines the nuclear characteristics of the fuel assembly, depends on the position. different.

Methods of increasing the moderator-to-fuel ratio include a method of reducing the fuel loading and a method of increasing the moderator area or moderator density. Specifically, there is an increase in the boiling water region (reducing the number of fuel rods or reducing the diameter of the fuel rods). There is an increase in the non-boiling water region (water rod region or gap water region). However, in all fuel assemblies employing such measures, the amount of fuel loaded is reduced,
Although fuel economy is improved in terms of increasing the moderator to fuel ratio, fuel economy is impaired in terms of fuel loading, and eventually,
It does not improve fuel economy. Further, in the case, the linear power density increases due to the decrease in the total length of the fuel rods, and the thermal margin decreases. In the case, a new problem arises that the pressure loss increases due to a decrease in the flow path area of the coolant.

In the conventional fuel assembly, fuel rods are arranged in a grid of 8 rows and 8 columns (8 × 8). However, the number of grids of fuel rods is increased to 9 × 9 and 10 × 10. If so, the average linear power density is reduced and the heat transfer area is increased, thereby increasing the thermal margin. Further, as described in JP-A-52-50498, it is known that a fuel assembly is constituted by using a fuel rod having a short fuel rod effective length, and a two-layer flow portion having a large friction pressure loss is known. By increasing the flow path area (upper core),
Pressure loss can be reduced without reducing the fuel loading.
Therefore, if these two measures are used together and the above is adopted, a fuel assembly suitable for high burnup can be obtained.

From the above viewpoints, the outer diameter of the fuel rods is reduced to form a 9 × 9, 10 × 10 grid fuel rod array with an increased number, and the cross-sectional area of the water rod is made larger than the cross-sectional area of the unit fuel grid. And a fuel assembly having a plurality of partial length fuel rods has been proposed.
No. 8292, JP-A-62-276493, JP-A-64-31089, JP-A-64-23195
There is an official gazette.

Japanese Unexamined Patent Publication No. 64-88292 discloses a fuel assembly having an increased number of grids of fuel rods, in which a plurality of partial length fuel rods are arranged adjacent to a large diameter water rod. Have been. Japanese Patent Application Laid-Open No. 62-276493 discloses that in a fuel assembly having an increased number of grids of fuel rods, a large number of water rods or large-diameter water rods are arranged, and a diagonal line including corners of the fuel rod grid array is provided. A plurality of partial length fuel rods are arranged in a row. Japanese Unexamined Patent Publication (Kokai) No. 64-31089 discloses a fuel assembly having an increased number of fuel rod arrangement grids, in which a large-diameter water rod is arranged and a plurality of partial length fuel rods are formed at corners of the fuel rod lattice arrangement. It is disclosed that it is located.
Japanese Patent Application Laid-Open No. 64-23195 discloses a fuel assembly having an increased number of fuel rod arrangement lattices, in which a large-diameter water rod is arranged and a line dividing each side of the outermost layer of the fuel rod lattice arrangement into two equal parts. Alternatively, a plurality of partial-length fuel rods are arranged in a row on a diagonal line including corner portions of a fuel rod lattice-like arrangement.

[0009]

In the development of a high burnup fuel assembly, the pressure loss, thermal margin (linear power density, critical power), etc. must be reduced by considering the backfit to the current core. Must be equivalent to that of the aggregate.

As mentioned above, it is advantageous for a high burnup fuel assembly to increase the number of grids of fuel rods by reducing the outer diameter of the fuel rods. However, simply increasing the number of fuel grid arrays increases the degree of freedom for the fuel arrays,
As the wet edge length increases and the pressure loss increases, the outer diameter of the fuel rod decreases, and the time constant of the fuel rod increases, thereby causing a new problem that the stability (channel stability and core stability) becomes severe. In order to solve this, the absolute value of the void coefficient needs to be smaller than that of the current fuel assembly. In other words, until now, measures to reduce the absolute value of the void coefficient have been discussed from the viewpoint of increasing the enrichment of the fuel. .

As described above, reducing the reactivity coefficient such as the void coefficient includes increasing the moderator-to-fuel ratio;
That is, it is necessary to increase the water rod area and reduce the fuel loading. However, a reduction in fuel loading is a measure that should be avoided because it impairs fuel economy. Therefore, in developing a high burn-up fuel assembly, a new method of reactivity control (reduction of the absolute value of the void coefficient and reduction of the reactivity difference between the operation time and the cold temperature) that does not depend on the decrease in the fuel load is developed. It becomes important.

As a reactivity control method which does not depend on a decrease in the fuel loading, Japanese Patent Application Laid-Open No. 64-88292 mentioned above discloses
JP-A-62-276493, JP-A-64-310
As described in JP-A-89-89 and JP-A-64-23195, the arrangement position of the partial length fuel rod may be selected.
That is, Japanese Patent Application Laid-Open No.
In JP-A-31089, a plurality of partial-length fuel rods are arranged in a position adjacent to a large-diameter water rod or in a corner portion of a fuel rod lattice arrangement. In Japanese Unexamined Patent Publication No. 64-23195, a plurality of partial-length fuel rods are arranged in a row on a diagonal line including a corner portion of a fuel rod lattice arrangement or on a line bisecting each side of the outermost layer of the fuel rod lattice arrangement. Have been placed. In each of these, the non-boiling water region (water rod or gap water region) and the partial length fuel rod are adjacent to each other to promote the neutron moderating effect, reduce the void coefficient, and reduce the reactivity difference between operation and cold temperature. I am aiming for.

However, in these prior art methods, the reactivity coefficient such as the void coefficient is reduced and the reactivity controllability is improved, but the reactivity itself changes depending on the position of the partial length fuel rod, or the local output peaking. Has not been given sufficient consideration. That is, in the prior art, a plurality of partial-length fuel rods are arranged so as to be at least partially adjacent to a water rod or a gap water region of a corner portion, so that the absorption of resonance neutrons is increased and the reactivity is increased. There is a problem that the loss increases and fuel economy is impaired.
Further, in the cross section above the partial length fuel rod, there is a problem that the local output peaking of the fuel rod adjacent to the partial length fuel rod increases and the thermal margin decreases. Furthermore, in the above prior art, no consideration is given to optimizing the cross sectional area of the water rod in relation to the arrangement of the partial length fuel rods in the fuel assembly aiming at high burnup.

A first object of the present invention is to provide a fuel assembly which enables high burnup, reduces the void coefficient without lowering the reactivity, and flattens the local output peaking. is there. A second object of the present invention is to provide a fuel assembly that enables high burnup, reduces the void coefficient without lowering the reactivity, and optimizes the moderator cross-sectional area of the neutron moderator rod. That is.

[0015]

According to a first aspect of the present invention, there is provided a fuel cell system comprising: a large number of fuel rods arranged in a square grid of 9 rows and 9 columns; A fuel assembly having two water rods disposed in the area of the portion, the cross-sectional area of which is greater than the cross-sectional area of the unit fuel grid;
(A) the fuel rods include a plurality of first fuel rods and a plurality of second fuel rods whose active fuel length is shorter than the first fuel rods; (b) the second fuel rods include at least the fuel rods. (C) being arranged in the outermost layer of the square grid-like fuel rod array;
Wherein said second fuel rods are arranged in the outermost layer are respectively arranged one by one in the central position of each side of the outermost layer, the outermost
The other fuel rods in the outer layer are the first fuel rods ;
(D) among the fuel rods arranged in the second layer from the outside of the square lattice-shaped fuel rod array, the fuel rod adjacent to the second fuel rod in the outermost layer is the first fuel rod; A fuel assembly is provided.

According to a second aspect of the present invention for achieving the above first and second objects, there are provided a large number of fuel rods arranged in a square grid of 9 rows and 9 columns, wherein the cross sectional area is smaller than the cross sectional area of the unit fuel grid. In a fuel assembly having at least one large water rod, (a) the fuel rod includes a plurality of first fuel rods and a plurality of second fuel rods whose active fuel length is shorter than the first fuel rod. (B) the second fuel rods are arranged at least in the outermost layer of the square lattice-shaped fuel rod array; (c) the second fuel rods arranged in the outermost layer are: One fuel rod is disposed at the center of each side of the outermost layer, and the other fuel rods of the outermost layer are the first fuel rods.
In; (d) square of the fuel rods disposed in the second layer from the outside of the lattice arrangement of fuel rods, the fuel rods adjacent to the second fuel rods of the outermost layer of the first fuel rod; lying (E) The cross-sectional area of the water rod in the water rod is 7-1.
4 cm ² ;

According to a third invention, in the first and second inventions, the second fuel rod is arranged at a position adjacent to the water rod.

In a fourth aspect of the present invention, in the first to third aspects, (d ') all of the fuel rods arranged in the second layer from the outside of the square grid-like fuel rod array are replaced with the fuel rods. First
It is a fuel rod.

According to a fifth aspect of the present invention, in the first to third aspects, the second fuel rods are also arranged at the corners of the second layer from the outside of the square grid-like fuel rod arrangement. It is.

Further, in a sixth aspect based on the first to fifth aspects, the length of the second fuel rod is set to a range of 1/2 to 3/4 of the active fuel length of the first fuel rod. Is what you do.

The operation of each of the first to sixth inventions is as follows.

According to the study by the present inventors, the following has been found. When the moderator in the non-boiling water region is localized and arranged in the fuel assembly in which the number of grids of the fuel rods is increased to 9 × 9 or more, the reactivity controllability is improved (from the change in the void ratio and the operating temperature to the cold temperature). In order to reduce the reactivity change due to the change to the state; the void coefficient is reduced), it is better to localize in the outer region of the fuel assembly facing the channel box than in the inner region of the fuel assembly facing the water rod. Great effect (high sensitivity) (see FIG. 4).

When a fuel rod having an effective fuel length shorter than that of a normal fuel rod, that is, a fuel rod having a partial length, is disposed, a cross section above the fuel rod having a partial length has the same structure as that of the non-boiling water region described above. This is effective for improving the reactivity controllability (reducing the void coefficient), and the sensitivity of the void coefficient reduction depending on the position of the partial length fuel rod is as follows (see FIG. 5). (1) Fuel outside the first layer of the fuel assembly facing the channel box (2) Fuel outside the first layer of the fuel assembly facing the channel box other than (1) (3) Adjacent to the water rod Fuel in the inner area of the fuel assembly (4) Fuel not adjacent to the channel box or water rod In addition, when the partial-length fuel rods are placed on the outermost layer of the fuel rod array, the effect of their location on control value Is
In each case, arranging the partial length fuel rods on the outermost layer facing the channel box has a greater control rod value than arranging them adjacent to the water rods (see FIG. 7).

Therefore, as described in (b) of the first and second aspects of the present invention, by arranging the second fuel rods, which are partially long fuel rods, in the outermost layer of the fuel rod arrangement in a square lattice,
The effect of reducing the void coefficient is obtained, and the reactivity controllability is improved. In addition, the effect of improving the control rod value can be expected, which contributes to the improvement of safety.

According to the study by the present inventors, in a fuel assembly in which the number of grids of fuel rods is increased to 9 × 9 or more, when a partial length fuel rod is arranged in the outermost layer of the fuel rod array, The following has been found regarding the influence of the arrangement position on the reactivity and the local output peaking (see FIG. 6). When the partial length fuel rods are arranged at the outermost corners, both the reactivity loss and the local power peaking of the partial length adjacent fuel rods are large. When the partial length fuel rods are placed in the grid position adjacent to the outermost corner, the reactivity loss is greatly improved, but the local power peaking of the partial length adjacent rods and the corner fuel rods is still relatively high. large. When the partial length fuel rods are placed at grid positions further adjacent to the outermost layer, the reactivity loss is further improved, but the local power peaking of the partial length adjacent fuel rods and the corner fuel rods is still large. When the partial length fuel rods are placed further in the grid position,
In other words, when the water rods are placed inside the outermost grid positions including the outermost grid positions, the reactivity loss is almost eliminated, and the local output peaking of the partial length adjacent fuel rods and the fuel rods at the corners is also large. To decline.

Therefore, as described in item (c) of the first and second aspects of the present invention, the second fuel rod, which is a partial-length fuel rod, is positioned at the center of each side in the outermost layer of the fuel rod array in a square lattice.
One for each, and the other fuel rods in the outermost layer
As the first fuel rod , both the reactivity loss and the local output peaking can be improved, and the fuel economy and the thermal margin can be improved.

According to the study by the present inventors, the following has been found. That is, in order to maximize the localization effect of the moderator region, it is usually necessary to localize the fuel rods. Normally, rod localization reduces the probability that resonant neutrons will be absorbed, contributing to greater fuel economy.

As described in item (d) of the first and second aspects of the present invention, the second layer from the outside of the square grid fuel rod array (the innermost layer of the square grid fuel rod array inside the outermost layer). Of the fuel rods arranged in one layer (adjacent to the outer layer), the fuel rod adjacent to the second outermost fuel rod is the first fuel rod, so that the area of the first fuel rod is the moderator area. As a result, the thermal neutrons that have been efficiently decelerated in the moderator region efficiently flow into the region of the first fuel rod, so that resonance absorption is reduced, and not only reactivity controllability but also fuel economy Is improved. This effect is further enhanced by setting the region where the first fuel rods are arranged to be the entire second layer from the outside, as in the item (d ') of the fourth aspect of the present invention. This will make it even larger.

On the other hand, as in the fifth aspect of the invention, the second fuel rod, which is a partial length fuel rod, is also arranged at the corner of the second layer from the outside of the fuel rod arrangement in a square lattice, thereby forming the channel box. This has the effect of making the distribution of the coolant flow rate and the distribution of the steam volume ratio in the inside uniform. That is, in general, the area facing the channel box, particularly in the vicinity of the corner, has a large frictional resistance, and the flow rate of the coolant tends to decrease. This can be solved by arranging the partial length fuel rods at the corners of the second layer from the outside.

Here, "adjacent" with respect to the arrangement of the fuel rods includes not only adjacent in the row and column directions but also adjacent in the oblique direction.

Further, according to the study of the present inventors, regarding the effect of the localization of the moderator region (non-boiling water region) on fuel economy, and the optimization of the water rod cross-sectional area and shape,
The following has been found: 9 × number of grids of fuel rods
In the fuel assembly increased to 9 or more, the case where the water rod region inside the fuel assembly is increased and the case where the gap water region on the side of the fuel assembly element is increased while the fuel loading amount is fixed, The water rod region is more effective (higher sensitivity) than the gap water region for improving the neutron infinite multiplication factor (improving fuel economy), and increasing the cross-sectional area inside the water rod for improving fuel economy (See FIG. 8). In this case, the cross-sectional area inside the water rod is 7-14.
cm ² is the optimal range.

Therefore, as shown in item (e) of the second invention, the cross-sectional area of the water rod as the neutron moderating rod is 7 to
14 cm ² optimizes the cross-sectional area within the water rod, increases reactivity, and further improves fuel economy.

Further, by disposing the partial length fuel rods adjacent to the water rods, it is almost equivalent to increasing the water rod area in the center of the fuel assembly. There is an effect that can be increased.

Therefore, by arranging the second fuel rod, which is a partial length fuel rod, adjacent to the water rod as in the third invention, the neutron infinite multiplication factor is further increased, and the reactivity and fuel economy are improved. The performance is improved.

According to the study by the present inventors, in order to improve the channel stability and the core stability, the upper end position of the partial length fuel rod is preferably set at the fourth to sixth spacers. It turns out that there is. The positions of these spacers correspond to 1/2 to 3/4 in terms of the ratio to the active fuel length of the full length fuel rod. Therefore, the sixth
As described in the invention, the length of the second fuel rod, which is a partial length fuel rod, is in the range of 燃料 to ／ of the active fuel length of the first fuel rod, thereby achieving channel stability and core stability. The effect of improving the performance is obtained.

The number of grids of fuel rods is reduced from 8 × 8 to 9
When the number is increased to × 9 or more, the number of layers of the fuel rods constituting the fuel assembly increases, so that the degree of freedom in distributing the fuel in the fuel assembly increases. Therefore, the fuel or the moderator in the fuel assembly can be localized and arranged, and the above configuration can be easily realized. Here, the term “localization” means that the length of the boundary line per unit volume in the fuel or moderator region surrounded by the boundary line between the fuel and the moderator is reduced.

[0037]

Embodiments of the present invention will be described below in detail with reference to the drawings. First Embodiment A first embodiment of the present invention will be described with reference to FIGS.
In FIG. 1, a fuel assembly 1 of the present embodiment is for a boiling water reactor, and has a large number of fuel rods 2 arranged in a 10 × 10 square lattice and an outermost layer of the fuel rod arrangement. It has only four partial length fuel rods 3 and three large water rods 4 having a circular cross section and arranged diagonally in a 4 × 4 lattice area at the center of the fuel rod array. The fuel rods 2 and 3 and the water rod 4 are surrounded by a channel box 5 having a rectangular cross section. Fuel rod 2 and partial length fuel rod 3
Are 90 in total.

The fuel rod 2 is a normal full-length fuel rod, and the partial-length fuel rod 3 has an active fuel length shorter than that of the fuel rod 2 and is 15/24 of the active fuel length. This is the length of which the upper end is supported by the fifth spacer, as described later. Also, by borrowing the concept of a mathematical matrix, the rows of the fuel rods arranged in a square grid are arranged in rows in the horizontal direction, and the columns in the vertical direction are arranged in columns. When the grid positions in the row are expressed as (i, j), the partial length fuel rods 3 are (1, 5), (5, 1), (6, 10),
It is arranged at each grid position of (10, 6). That is, the partial length fuel rods 3 are arranged symmetrically with respect to each of two diagonal lines of the outer peripheral shape of the fuel rod array, one on each side of the outermost layer of the square grid fuel rod array. On the inner side of the outermost layer of the square lattice fuel rod array, only the ordinary fuel rods 2 are arranged in a total of two layers, one layer adjacent to the outermost layer and one layer adjacent thereto.

The three large water rods 4 are arranged diagonally in the 4 × 4 lattice area in the center of the fuel rod array, in a region where ten fuel rods can be arranged, and the remaining large water rods 4 are arranged. Six fuel rods 2 are arranged in the area. In this case, the outer diameter of the water rod 4 is 20.7 mm, and the total cross-sectional area in the water rod is about 9 cm ² in consideration of the thickness of the water rod 4.
It has become. The grid positions at which the water rods 4 are arranged are the grid positions at which two or more of the four adjacent fuel grids are located at the water rods 4. The outermost grid positions occupied by the three large water rods 4 are (4,7) and (7,4) on a diagonal line of a 4 × 4 grid area, and the outermost grid positions are fourth in the row direction (lateral direction). -Seventh, fourth in the column direction (vertical direction)
7 and the partial length fuel rods 3 are located in the range of these rows or columns. That is, when the water rods 4 are projected in the row direction and the column direction in the outermost layer of the square grid-shaped fuel rod array, the partial length fuel rods 3 include both outermost grid positions in the projection range. Are arranged inside of the lattice position of.

FIG. 2 shows the overall structure of the fuel assembly 1. The water rod (not shown) and the fuel rod 2 are supported at the upper end by the upper tie plate 7, at the lower end by the lower tie plate 8, and at seven intermediate portions at 1 to 7 levels of spacers 9.
Is held by The partial length fuel rod 3 has a lower end supported by the lower tie plate 8, an upper end supported by the fifth spacer 9, and four intermediate portions held by the first to fourth spacers 9. The channel box 5 surrounds the entire fuel bundle thus configured.

FIG. 3 shows an arrangement of the fuel assembly 1 in the core. Four fuel assemblies 1 according to the present embodiment are arranged so as to surround the cross-shaped control rod 10, and constitute one fuel unit. A neutron detector measurement tube 11 is arranged adjacent to one corner of the fuel unit.

Next, the operation of this embodiment will be described. By changing the grid arrangement of the fuel rods from the conventional 8 × 8 grid to the 10 × 10 grid, the degree of freedom in distributing the fuel in the fuel assembly is increased. Therefore, the fuel or moderator in the fuel assembly can be localized and arranged. The term “localization” as used herein means that the length of the boundary line per unit volume in the fuel or moderator region surrounded by the boundary line between the fuel and the moderator is reduced.

The effect of the position where the moderator region (non-boiling water region) is localized on the reactivity control (reduction in the change in the reactivity accompanying a change in the void fraction and a change from the operating state to the cold state) will be described. . FIG. 4 compares the change in the void coefficient when the water rod region (moderator in the inner region) is increased and the gap water region (moderator in the outer region) is increased while the fuel loading is constant. Shown. From this figure, it can be seen that the outer region of the fuel assembly facing the channel box is more effective (higher sensitivity) than the inner region of the fuel assembly facing the water rod for improving reactivity controllability. I understand.

FIG. 5 shows a change in the void coefficient in a cross section above the partial length fuel rod depending on the position of the partial length fuel rod. From this figure, it can be seen that the sensitivity of the void coefficient reduction is in the following order. (1) Fuel outside the first layer of the fuel assembly facing the channel box (2) Fuel outside the first layer of the fuel assembly facing the channel box other than (1) (3) Adjacent to the water rod Fuel in the area inside the fuel assembly (4) Fuel not adjacent to the channel box or water rod This is similar to the localization of the non-boiling water area described in FIG. It is shown that the conversion (adjacent the part length fuel rod to the non-boiling water region) is effective for the reactivity control. This effect is remarkable in the upper core region (high void fraction region) where the hydrogen to heavy metal atomic ratio (H / U) is small and the neutron moderating effect is highly sensitive.

For the above reasons, in this embodiment, the fuel rods 2 and 3 are arranged in a 10 × 10 square lattice, and the partial length fuel rods 3 are arranged in the outermost layer of the fuel rod arrangement. Thereby, the effect of reducing the void coefficient is obtained, and the reactivity controllability is improved.

When the partial length fuel rods are arranged in the outermost layer of the fuel rod arrangement, the influence of the arrangement position on the reactivity and the local output peaking will be described. FIG. 6 shows a case where four partial-length fuel rods are symmetrically arranged on the outermost layer facing the channel box as in the above embodiment, and the arrangement position of the partial-length fuel rods is (1, 1) when viewed from the upper side in the figure. ~ (1,10)
Of the partial length fuel rod in the section above the partial length fuel rod at each position and the neutron infinite multiplication factor (A)
And the relationship between local output peaking (B and C). The fuel rod arrangement is a 10 × 10 lattice as in the above embodiment, and three water rods are arranged in the central portion of the fuel rod arrangement.
In the × 4 grid area, the fuel rods are arranged in an area where ten fuel rods can be arranged. In addition, A shows the difference in reactivity with respect to the case where the partial length fuel rod is located at (1, 5) when viewed from the upper side in the figure, and B shows the partial length fuel rod located at the corner. When the normal fuel rod next to it,
In other cases, the local output peaking coefficient of the normal fuel rod adjacent to the partial length fuel rod on the left side in the figure is shown.
4 shows a local output peaking coefficient of a normal fuel rod located at a corner.

As can be seen from FIG. 6, when the partial length fuel rod is located at the outermost corner, the reactivity loss is large,
In addition, the local output peaking of the fuel rod adjacent to the partial length is large.
When the position of the partial length fuel rod moves to the lattice position adjacent to the corner portion of the outermost layer, for example, (1, 2) when viewed from the upper side in the drawing, the reactivity loss is greatly improved, but the partial length adjacent fuel rod is reduced. And the local power peaking of the fuel rod at the corner is large.
Part length fuel rods are 3 from the outermost corner to the corner
When located at the third grid position, for example at (1,3) as viewed in the upper side of the figure, the reactivity loss is further improved, but the local output peaking of the partial length adjacent fuel rods and the corner fuel rods is still large. When the partial length fuel rod is located at the fourth or inner side including the corner portion from the outermost corner portion, for example, when it is located at (1, 4) or inner side as viewed from the upper side in the drawing, a reaction occurs. The power loss is almost eliminated, and the local power peaking of the part length adjacent fuel rod and the corner part fuel rod is also greatly reduced.

For the above reasons, in this embodiment,
The partial-length fuel rods 3 are arranged on the outermost layer of the fuel rod array, inside the lattice positions including both outermost lattice positions in the projection range of the water rods, and this arrangement causes the reactivity loss and the locality loss. Can improve both output peaking,
Fuel economy and thermal margin can be improved. If local countermeasures against local output peaking are to be taken separately, only reduction in reactivity loss should be considered.In this case, the partial length fuel rods are placed in the lattice position adjacent to the outermost layer corner, or in the outermost layer. It may be arranged at the third lattice position from the corner part to the corner part.

In the case where the partial length fuel rods are arranged in the outermost layer of the fuel rod arrangement, the effect of the arrangement position on the control value will be described. FIG. 7 shows a case where four partial-length fuel rods are symmetrically arranged on the outermost layer facing the channel box as in the above embodiment, and the arrangement position of the partial-length fuel rods is (1, 1) when viewed from the upper side in the figure. The relationship between the position of the partial-length fuel rod and the control rod value in the cross section above the partial-length fuel rod when changing to (1, 10) is shown. The control rod value is indicated by a difference from a case where the same number of partial length fuel rods are adjacent to the water rod. As can be seen from this figure, placing the partial length fuel rods in the outermost layer facing the channel box has greater control rod value in each case than placing it adjacent to the water rods. This is because the thermal neutron flux near the control rod (particularly the absorption rod) has increased.

Therefore, in the present embodiment, by arranging the partial length fuel rods 3 in the outermost layer of the fuel rod arrangement, an effect of improving the control rod value can be expected, which contributes to the improvement of safety.

In order to maximize the localization effect of the moderator region, it is usually necessary to localize the fuel rods. Normally, rod localization reduces the probability that resonant neutrons will be absorbed, contributing to greater fuel economy.
In the present embodiment, only the normal fuel rods 2 are arranged at the grid positions adjacent to the partial length fuel rods 3 in one layer adjacent to the outermost layer inside the outermost layer of the square grid fuel rod array,
The water rods 4 are concentrated in the center, and the partial length fuel rods 3 are arranged in the outer layer (outermost layer). The area of the fuel rods 2 is usually surrounded by the moderator area. I have. As a result, the thermal neutrons that have been efficiently decelerated in the moderator region efficiently flow into the region of the normal fuel rod 2, so that the resonance absorption is reduced, and not only the reactivity controllability but also the fuel economy is improved. This effect is further enhanced by setting the region where only the fuel rods 2 are usually arranged to be the entire layer adjacent to the outermost layer, and further increased by forming the layer into two layers as in this embodiment.

The effect of the localization of the moderator region (non-boiling water region) on fuel economy and the optimization of the water rod cross-sectional area and shape will be described. FIG. 8 shows a case where the water rod area (moderator in the inner area) is increased while the fuel loading is constant in the fuel assembly where the number of grids of the fuel rods is 10 × 10, and the gap outside the channel box is shown. Changes in reactivity when the water region (moderator in the outer region) is increased are shown in comparison. The horizontal axis shows two indicators of the increment of the water rod area or the gap water area (the increment of the non-boiling water area) and the ratio of the cross-sectional area in the water rod to the cross-sectional area in the channel box with respect to the increment of the water rod area. The axis is 3 cm ² (1.7, which is the current cross-sectional area inside the water rod).
%) Shows the difference of the infinite neutron multiplication factor based on the neutron intensifier. According to this figure, the inner region of the fuel assembly is more effective than the outer region of the fuel assembly facing the channel box for improving the neutron infinite multiplication factor, that is, improving the reactivity or fuel economy ( Sensitivity is high). This is because neutron absorption by the moderator is reduced by flattening the neutron flux distribution. Since flattening of thermal neutron flux distribution affects not only fuel economy but also local power distribution,
This is important from the viewpoint of securing thermal margin.

As described above, in the fuel assembly of the 10 × 10 grid, it is effective to increase the cross-sectional area in the water rod in order to improve the fuel economy. It is necessary to increase from 3 cm ² (1.7%). On the other hand, increasing the number of fuel grids in the water rod region decreases the number of fuel rods, which is against high burnup. In consideration of the symmetry of the fuel assembly, in the 10 × 10 grid fuel assembly, the number of fuel grid positions in the water rod region is currently in the range of 2 to 8 to 16 in the water rod region. The fuel unit grid area of the 10 × 10 grid fuel assembly is about 60% of the current 8 × 8 grid;
In addition, the number of fuel grid positions in the water rod area is 4 to 8 times higher than the above.
In consideration of the doubling, the cross-sectional area in the water rod is suitably in the range of 7 (4%) to 14 (8%) cm ² .

In this embodiment, the total cross-sectional area of the three water rods 4 is about 9 cm ² for the above reasons.
This can also increase reactivity and improve fuel economy.

Further, there is a viewpoint of stability in optimizing the cross-sectional area in the water rod. There are two modes of stability: channel stability and core stability. First, the channel stability is to prevent the oscillation of the flow rate distribution of the cooling water, and is evaluated for the fuel assembly having the highest output in the core. The core stability is to prevent an unstable phenomenon caused by coupling of a neutron flux and thermal hydraulic vibration in the core. Each limit value is expressed by a reduction ratio. That is, both the channel stability and the core stability are limited so that the reduction ratio is 1.0 or less.

Therefore, the present inventors have studied the stability of the 10 × 10 fuel assembly. FIG. 9 shows a stability limit line on a map of the uranium loading amount and the cross-sectional area in the water rod. The uranium loading amount is a uranium loading amount that can be loaded per fuel assembly, and an increase in the uranium loading amount means an increase in the outer diameter of the fuel rod. On the other hand, the cross-sectional area in the water rod indicates the cross-sectional area of the water flow path in the water rod located in the fuel assembly, and an increase in the cross-sectional area in the water rod means an increase in the outer diameter of the water rod. Therefore, the increase in uranium loading and cross-sectional area in the water rod is
Since the outer diameters of the fuel rods and water rods are increased and the flow path area in the fuel assembly is reduced, the flow resistance increases and the margin of channel stability is reduced. Therefore, as shown in FIG. 9, channel stability becomes severe in a region where the cross-sectional area inside the water rod is large and the uranium loading amount is large, and the limit line of the channel stability becomes lower right. On the other hand, regarding the core stability, the limit line rises to the right because an increase in the uranium loading and a decrease in the water rod inner area deteriorate the core stability. Therefore, the stability allowable region exists in the center of FIG. 9 and is a mountain-shaped region surrounded by the channel stability limit line and the core stability limit line.

Increasing the uranium loading is preferable in terms of fuel economy because it reduces fuel cycle costs. It can be seen that the fuel cycle cost is the lowest and the optimum portion near the intersection of the channel stability limit line and the core stability limit line (at the top of the mountain) in the stability allowable region of FIG. That is, the range close to the top (around 10 cm ² in the cross section of the water rod) is the optimum point for achieving both channel stability, core stability, and fuel economy. 9-11 cm ² is preferred.

In the prior art, to increase the stability, it has been proposed to provide an orifice plate immediately below the fuel assembly, to increase the resistance at the orifice plate, or to employ a low pressure drop type spacer. . Therefore, when the orifice resistance is increased or a low pressure loss type spacer is used, the stability is improved, and the stability limit line is moved, and the allowable area is expanded. However, in this case, since both limit lines move upward, the cross-sectional area in the water rod with respect to the optimum point (the top of the mountain) hardly changes. Therefore, it is desirable from the viewpoint of fuel economy that the cross-sectional area of the fuel assembly in the water rod be 9 to 11 cm ² .

For the above reasons, in this embodiment, the water rod 4
Has a total cross-sectional area of about 9 cm ² , indicating that it is located at a position asymptotic to the optimum point. Therefore, in this embodiment, channel stability, core stability, and fuel economy are improved. Further, the improved stability eliminates the need for equipment provided for improving stability.

In order to increase the cross-sectional area of the water rod, the adoption of a large water rod can reduce the number of fuel rods that must be sacrificed, and further, the coolant passage area with a small fuel rod cooling effect is reduced. Can be reduced (increase the limit output)
This is advantageous. Assuming that the distance between the water rod and the fuel rod adjacent to the water rod is constant, in the case of a circular water rod, it is most preferable to use a 2 × 2 fuel grid as a water rod in terms of effective use of the above space. ing.

Therefore, in this embodiment, the water rod region is defined as 10 cells in the central region of the fuel assembly having a small neutron moderating effect, and the fuel grid position of the water rod region is set to at least two of the four adjacent fuel grids. Three or more large circular water rods equivalent to 2x2 are arranged by making at least one adjacent to the water rod area, resulting in a configuration in which the coolant passage area with a small fuel rod cooling effect is reduced. ing. This will increase the marginal power.

Finally, the effect of the length of the partial length fuel rod 3 on the channel stability and the core stability will be described. FIG.
0 and FIG. 11 show the results of evaluating the channel stability and the core stability by changing the length of the partial length fuel rod 3 under the condition of a constant uranium loading. The horizontal axis in the figure is the partial length fuel rod 3
And the vertical axis indicates the reduction ratio. If the upper end of the partial length fuel rod is shortened from the position of the seventh spacer from the bottom (the right end in the figure), the flow path area at the upper part of the assembly is increased, reducing pressure loss and improving both channel stability and core stability. I do. However, when the length of the fuel rod is set to be equal to or less than the third spacer from the bottom, the analysis is performed under the condition that the uranium loading is constant, so the outer diameter of the fuel rod increases, and the pressure at the lower part of the fuel assembly increases. It can be seen that the loss has increased and has become unstable. Also,
When the outer diameter of the fuel rod increases, there is no gap between the fuel rods, and the fuel rod becomes thermally severe. Therefore, the uppermost position of the partial length fuel rod is optimally the fourth-stage spacer and the fifth-stage spacer, and the sixth-stage spacer can be used. If the number of stages of these spacers is expressed as a ratio to the active fuel length of the full-length fuel rod, 1
Since there is a lower tie plate below the stage spacer and an upper tie plate above the seventh spacer, the fourth stage spacer corresponds to the position where the active fuel length is 4/8, that is, 1/2. , 6-step spacer corresponds to the position where the active fuel length is 6/8, that is, 3/4. Therefore, the length of the partial-length fuel rod 3 is set to か ら to 3 times the active fuel length of the normal fuel rod 2.
If it is in the range of / 4, the effect of improving channel stability and core stability can be obtained.

For the above reason, in this embodiment, the length of the partial length fuel rod 3 is set to 15 / the active fuel length of the full length fuel rod 2.
24, that is, the length whose upper end is supported by the fifth-stage spacer, whereby the channel stability and the core stability are improved. If the upper end is set to a length supported by the fourth-stage spacer, the stability margin is further increased.

According to the present embodiment, by optimizing the fuel rod lattice arrangement and the arrangement of the water rods and the partial length fuel rods, the effect of reducing the void coefficient is obtained, the reactivity controllability is improved, and the reactivity is improved. Power loss and local output peaking can be improved, fuel economy can be improved and thermal margin can be improved. According to trial calculations, by adopting the fuel assembly configuration shown in the present embodiment, a large number of fuel rods 50 arranged in a square grid of 8 × 8 shown in FIG. The absolute value of the void coefficient showing the reactivity controllability can be reduced by about 10% as compared with the conventional fuel assembly having two water rods 51 having a circular cross section arranged. The difference in reactivity at the time can be reduced by about 1.0% Δk. Further, there is an effect of increasing the neutron infinite multiplication factor of the fuel assembly by about 0.5 Δk without increasing the local output peaking.

Further, according to the present embodiment, the effect of improving the control rod value can be expected, and the safety is improved and the core stability is also improved.

Second Embodiment A second embodiment of the present invention will be described with reference to FIGS. This embodiment is different from the first embodiment in that the total number of partial length fuel rods is eight. That is, in FIG. 13, the fuel assembly 1 </ b> A of the present embodiment has eight partial-length fuel rods 3, and these partial-length fuel rods 3 are located on the outermost layer 4 sides of the fuel rod array facing the channel box 5. Two are arranged adjacent to each other. The arrangement position of the partial-length fuel rods 3 can be expressed by (1,5), (1,6) if each grid position of the fuel rod array is represented by a matrix method as in the first embodiment.
(5,1), (6,1); (5,10), (6,1)
0); (10, 5), and (10, 6). In this embodiment, the partial-length fuel rods 3 are provided with the water rods 4 in the outermost layer of the square grid fuel rod array. When the projection is performed in the direction and the column direction, it is arranged inside the lattice positions including both outermost lattice positions of the projection range.

The effect of having two adjacent partial length fuel rods 3 adjacent to each other will be described with reference to FIG. FIG. 14 shows the reactivity controllability and the control rod value when the partial length fuel rods are adjacent to each other. As can be seen from this figure, by making the fuel rods 3 adjacent to each other, both the reactivity controllability and the control rod value are more than the effects obtained by simply adding the effects of only one fuel rod.
Even if two or more wires are arranged adjacent to each other (even if the moderator is localized), if they are arranged at positions that contribute to flattening for local output peaking as described with reference to FIG. If it is arranged within the range of the projected area of the rod 4, there is no problem in securing thermal margin. Therefore, according to the present embodiment, it is possible to obtain more than twice the effect of the first embodiment in terms of improvement in reactivity controllability and control rod value.

Third Embodiment A third embodiment of the present invention will be described with reference to FIG. This embodiment is different from the second embodiment in that the position of the partial length fuel rod and the size of the water rod are changed. That is, in FIG. 15, the fuel assembly 1C of this embodiment has eight partial-length fuel rods 3, and each of these partial-length fuel rods 3 is adjacent to each of the four outermost layers of the fuel rod array. They are distributed without being placed. In addition, three large water rods 4a are arranged at the center of the assembly. The arrangement positions of the partial length fuel rods 3 are represented by (1,4), (1,7); (4,1), (7,
1); (4,10), (7,10); (10,4),
This is the grid position of (10, 7). That is, when the water rods 4a are projected in the row direction and the column direction in the outermost layer of the square grid-shaped fuel rod array, the partial length fuel rods 3 include both outermost grid positions of the projection range. Are arranged inside of the lattice position of. In this embodiment, the reactivity control effect is reduced as compared with the second embodiment, but there is an advantage that the partial length fuel rod can be arranged at a place where the effect of increasing the control rod value is large.

The three large water rods 4a are arranged in contact with each other in a 4 × 4 lattice area at the center of the fuel rod array. In this case, the outer diameter of the water rod 4a is 21.6 m
m, considering the thickness of the water rod 4a, the total cross-sectional area in the water rod is about 10 cm ² . That is, in this embodiment, the total cross-sectional area of the water rod 4 is closer to the optimum point shown in FIG. Therefore, in the present embodiment, channel stability, core stability, and fuel economy are further improved.

Fourth Embodiment A fourth embodiment of the present invention will be described with reference to FIG. This embodiment is different from the second embodiment in that the total number of the partial length fuel rods is further increased by four, and the channel box and the partial length fuel rod adjacent to the water rod coexist. That is,
In FIG. 16, the fuel assembly 1 </ b> C of this embodiment has eight partial-length fuel rods 3 arranged on the outermost layer of the fuel rod array and four partial-length fuel rods arranged adjacent to the large water rod 4. Stick 3
a. The active fuel length of the partial-length fuel rods 3 and 3a is 15/24 of the active fuel length of the normal fuel rod 2 as in the first embodiment.

In this embodiment, arranging the partial length fuel rod 3a adjacent to the water rod 4 is almost equivalent to increasing the water rod area at the center of the fuel assembly. This is effective in flattening the thermal neutron flux distribution inside and outside 1C, and can increase the neutron infinite multiplication factor of the fuel assembly as shown in FIG. In this case, the effect of reducing the void coefficient and the effect of increasing the control rod value are greater in the outer region than in the water rod as shown in FIGS. By increasing the number of partial length fuel rods 3 adjacent to the channel box 5 more than the number of adjacent partial length fuel rods 3a, the number of partial length fuel rods required from the viewpoint of reactivity controllability can be reduced, The effect of increasing the control rod value is obtained. In addition, coexistence of the partial length fuel rods 3 and 3a adjacent to the channel box and the water rod in this manner is effective in flattening the thermal neutron flux distribution inside and outside the fuel assembly 1C.

According to a trial calculation, in this embodiment, a total of four partial-length fuel rods 3a are arranged adjacent to the large water rod 4, thereby increasing the non-boiling water area at the center of the assembly by about 3 cm ² . In calculation, the neutron infinite multiplication factor increases by about 1%. The absolute value of the void coefficient indicating the reactivity controllability is
The fuel assembly can be reduced by about 40% as compared with the conventional fuel assembly shown in FIG. 12, and the fuel loading can be secured without sacrificing core stability.

Fifth Embodiment A fifth embodiment of the present invention will be described with reference to FIG. As shown in FIG. 3, the core of the nuclear reactor has a C lattice core in which the gap width d1 of the gap water region on the side where the cruciform control rod 10 is inserted and the gap width d2 of the gap water region on the side where it is not inserted are the same. 17, as shown in FIG.
And a D lattice core wider than the gap width D2 of the gap water region on the side where the gap water is not inserted. So far, the preferred embodiment for loading into the C lattice core has been described, but this embodiment is suitable for loading into the D lattice core.

That is, in FIG. 17, the fuel assembly 1 C of this embodiment has eight part-length fuel rods 3 located on the outermost layer facing the channel box 5 and four parts adjacent to the water rod 4. The long fuel rods 3a and the partially long fuel rods 3 located in the outermost layer are unevenly distributed closer to the gap water region where the gap width is small. According to the present embodiment, by eccentrically distributing the partial length fuel rods 3 in this manner, the hydrogen to metal atomic ratio (H / U) with respect to the fuel in the outermost layer is optimized,
Therefore, local output peaking can be suppressed without giving too much fuel enrichment distribution.

Sixth Embodiment A sixth embodiment of the present invention will be described with reference to FIG. This embodiment is also suitable for loading on a D lattice core as in the fifth embodiment. That is, in FIG. 18, the fuel assembly 1D of this embodiment has two partial length fuel rods 3 located on two adjacent sides of the outermost layer facing the channel box 5, and the partial length fuel rods 3 , Are unevenly distributed closer to the gap water region where the gap width is small. Also in this embodiment, the partial length fuel rods 3 are unevenly distributed to optimize the hydrogen to metal atomic ratio (H / U) with respect to the fuel in the outermost layer,
Local output peaking can be suppressed without giving too much fuel enrichment distribution.

Seventh Embodiment A seventh embodiment of the present invention will be described with reference to FIG. The fuel assembly 1E of this embodiment employs one cross-shaped water rod 4E as a large water rod located at the center. Also, four partial length fuel rods 3A are arranged at four corners of the cross-shaped water rod 4E. According to this embodiment, the same effect as that of the fourth embodiment can be obtained.

Eighth Embodiment FIG. 20 shows an embodiment relating to the fuel assembly loading method of the present invention.
This will be described with reference to FIG. FIGS. 20 to 22 show the structure of the core loaded by the loading method of this embodiment. 20 and 21, the core of the nuclear reactor according to the present embodiment includes:
The central region OA, the outer peripheral region AB, and the outermost peripheral region BC include a fuel assembly of the present invention, for example, a fuel assembly 1C according to a fourth embodiment shown in FIG. And a conventional fuel assembly 52 having no partial length fuel rod shown in FIG. However, FIG.
2, the loading ratio of the fuel assembly 1C of the present invention is smaller in the core central region than in the core outer peripheral region.

A feature of the fuel assembly of the present invention is that the fuel load is largely different in the axial direction due to the frequent use of partial length fuel rods. Therefore, when considering the transition core from the conventional fuel, it is necessary to consider the effect of the neutron flux distribution in the axial direction due to the difference in fuel loading in the axial direction.

That is, when the fuel assembly 1C of the present invention is loaded into the conventional fuel assembly 53 having no partial-length fuel rod shown in FIG. 23, the power output of the lower part of the core increases. However, in the fuel assembly 1C of the present invention, the output of the upper part of the core tends to increase. The core is divided into regions as described above, and the fuel assembly 1C of the present invention in the center region of the core is provided.
By making the loading ratio of (new fuel) smaller than that in the core outer peripheral region, the linear power density can be set to a set value or less.

FIG. 24 shows a conventional fuel assembly.
As shown in the above, the above loading method is similarly effective for the fuel assembly 54 in which the ratio of the partial length fuel rods 55 is smaller than that of the fuel assembly 1C of the present invention. As mentioned above,
The loading method of this embodiment is effective when fuel bodies having different ratios of the partial length fuel rods are mixed in the core.

Ninth to thirteenth embodiments The ninth to thirteenth embodiments of the present invention will be described with reference to FIGS. In each of these embodiments, the cross section of the water rod as the neutron moderating rod is changed from the fourth embodiment in FIG. That is, in the fuel assembly 1F according to the ninth embodiment of FIG.
26, four water rods 4F each composed of two large-diameter water rods and two small-diameter water rods.
In the fuel assembly 1G according to the embodiment, a water rod 4G having an array-type cross section is arranged. In these examples, compared with the embodiment of FIG. 16, the water rods 4F, cross-sectional area is further increased in 4G, can approximately 11cm ^2. Therefore, the cross-sectional area inside the water rod can be set to the optimum point near 10 cm ² shown in FIG. 9, and the stability and fuel economy are improved.
Also, in this embodiment, similarly to the embodiment of FIG. 16, two partial-length fuel rods 3 and 3a are arranged adjacent to each other at a position adjacent to the outermost layer and the water rod. Therefore, as in the fourth embodiment shown in FIG. 16, the stability margin is improved due to a decrease in the apparent absolute value of the void coefficient, the fuel cycle cost is reduced, and the fuel economy is improved.

The fuel assembly 1H according to the eleventh embodiment in FIG. 27, the fuel assembly 1I according to the twelfth embodiment in FIG. 28, and the fuel assembly 1J according to the thirteenth embodiment in FIG.
In this example, the water rods 4H, 4I, and 4J having different shapes are arranged. Also in these embodiments, the cross-sectional area inside the water rod is increased, and can be reduced to about 11 cm ² . Therefore, the same effects as in the ninth and tenth embodiments can be obtained.

In the above embodiment, a water rod is used as the moderating rod means. However, instead of the water rod, a solid moderator (eg, zirconium hydride) having a high hydrogen density and a small neutron absorption cross-sectional area is sealed. Similar effects can be obtained for a fuel assembly using a solid reduction rod.
Although the description has been given of the case where uranium is used as the fuel substance, similar effects can be obtained by using plutonium extracted from spent fuel or a mixture of uranium and plutonium using recovered uranium.

Fourteenth Embodiment A fourteenth embodiment of the present invention will be described with reference to FIG. This embodiment is different from the seventh embodiment in FIG. 19 in that the cross-sectional shape of the water rod as the neutron moderating rod is changed. The fuel assembly 1K of the present embodiment employs four large circular water rods 4K as large water rods located at the center. Further, a partial length fuel rod 3a is arranged at a lattice position adjacent to the two large circular water rods 4K. According to this embodiment, effects similar to those of the seventh embodiment can be obtained.

Fifteenth Embodiment A fifteenth embodiment of the present invention will be described with reference to FIG. In this embodiment, the position of the partial length fuel rod is changed from the fourth embodiment in FIG. That is, in the fourth embodiment, the four partial-length fuel rods 3a adjacent to the water rods are replaced with ordinary full-length fuel rods 2, and instead, one layer adjacent to the outermost layer of the fuel rod array is used. 8 placed on the outermost layer
The partial length fuel rods 3b are arranged at lattice positions except for the lattice position adjacent to the partial length fuel rods 3a, that is, at the corners. The effective length of the partial length fuel rods 3, 3b is 15/24 of that of the normal fuel rod 2 as in the first embodiment.

In this embodiment, by disposing the partial-length fuel rods 3b in one corner portion adjacent to the outermost layer, there is an effect of making the coolant flow distribution and the steam volume ratio distribution in the channel box uniform. . Generally, the area facing the channel box, particularly near the corners, tends to have high frictional resistance, and the flow rate of the coolant tends to decrease. This can be solved by disposing the partial length fuel rods in one corner portion adjacent to the outermost layer. Further, by preventing the partial-length fuel rods 3b from being adjacent to the eight partial-length fuel rods arranged in the outermost layer, an increase in local output peaking of the normal fuel rods 2 adjacent to the partial-length fuel rods is suppressed. Can do it. As a result, the limit output, which is an index of the thermal margin, can be increased as compared with the fourth embodiment.

Sixteenth and Seventeenth Embodiments Sixteenth and seventeenth embodiments of the present invention will be described with reference to FIGS. Both of these embodiments are illustrated in FIGS.
Is applied to a 9 × 9 lattice fuel assembly.

In FIGS. 32 and 33, fuel assembly 1
M and 1N are a large number of full-length fuel rods 22 and partial-length fuel rods 22a (FIG. 32) or 22b (FIG. 33) arranged in a 9 × 9 square lattice, and 3 And two large water rods 24 having a circular cross section and arranged diagonally in a × 3 lattice area, and the fuel rods and the water rods are surrounded by a rectangular channel box 2 having a rectangular cross section.
It is surrounded by five. The total number of the fuel rods 2 and the partial length fuel rods is 74. In the sixteenth embodiment shown in FIG. 32, the partial length fuel rods 23, 23a are (1, 5), (5,
1), (9, 5) and (5, 9), four at the center of the outermost layer and four adjacent to the water rod 24, a total of eight. In the seventeenth embodiment shown in FIG. Part length fuel rod 23,2
3b is a total of four at the outermost layer position and the lattice position excluding the lattice positions adjacent to the four partial length fuel rods arranged at the outermost layer, that is, four at the corner portion. There are eight. The effective length of the partial length fuel rod is 15/24 of that of the normal fuel rod 2 as in the first embodiment. The total cross-sectional area in the water rod is about 9 cm ² , as in the first embodiment.

According to this embodiment, the reactivity control effect is smaller than that of the fourth or fifteenth embodiment because the ratio of the number of partial length fuel rods is reduced. However, 10 × 10
By making the grid a 9 × 9 grid, pressure loss can be reduced and stability performance can be improved, so that there is an advantage that the fuel charge can be further increased.

Eighteenth and Nineteenth Embodiments The eighteenth and nineteenth embodiments of the present invention will be described with reference to FIGS.
This will be described below. In each of these embodiments, the cross section of the water rod, which is a neutron moderating rod, is changed from the embodiment of FIGS. 32 and 33. That is, the fuel assemblies 1P and 1Q of the present embodiment employ one large square water rod 24P as the large water rod located at the center. According to this embodiment, the same effects as those of the sixteenth and seventeenth embodiments can be obtained.

Twentieth Embodiment A twentieth embodiment of the present invention will be described with reference to FIGS. In the present embodiment, as shown in FIG.
In this embodiment, the neutron moderating rod is changed to a so-called spectral shift rod 24R whose axial water level changes depending on the core flow rate.

The detailed structure of the spectrum shift rod 24R is shown in FIGS. Spectrum shift rod 2
The 4R includes an inner tube 30, an outer tube 31, and a spacer 32, as described in JP-A-63-73187. The outer tube 31 and the inner tube 30 are arranged concentrically, and the outer tube 31 surrounds the outer periphery of the inner tube 30. The upper end of the outer tube 31 is sealed by a cover part 33, and the upper part of the cover part 33 is inserted and held in the upper tie plate 12. The cover 33 narrows the upper end of the inner tube 30 so as to form a gap between the cover 33 and the upper end of the inner tube 30. The upper end of the inner tube 30 is connected to the outer tube 31 via a plate-like spacer 32 radially arranged from the axis of the spectrum shift rod 24R.
It is fixed to the inner surface of. The lower end of the outer tube 31 is closed by a closing portion 34. The lower end of the inner tube 30 penetrates the sealing portion 34 and protrudes downward therefrom. The lower end of the inner pipe 30 penetrates the fuel rod support 14 of the lower tie plate 13. Cooling water inlet 38 formed at the lower end of inner pipe 30
Is open to the space 15 of the lower tie plate 13.
The inside of the inner pipe 30 forms a cooling water ascending flow path 35 and the inner pipe 3
An annular passage formed between 0 and the outer pipe 31 forms a cooling water descending passage 36. On the tube wall at the lower end of the outer tube 31,
A plurality of cooling water discharge ports 39 are formed in the circumferential direction. These cooling water discharge ports 39 are provided at equal intervals in the circumferential direction. The cooling water discharge port 39 is open in a region above the fuel rod support 14. In this embodiment, the fuel rod support 1
4 has the function of a resistor. The cooling water ascending passage 35 and the cooling water descending passage 36 are connected to the spectrum shift rod 24.
They are connected by a reversing part 37 formed at the upper end of R. As described above, the spectrum shift rod 24 </ b> R has an inverted U-shaped cooling water flow path including the cooling water rising flow path 35, the cooling water falling flow path 36, and the reversing section 37 inside.

The fuel assembly 1R of this embodiment is loaded into the core of a boiling water reactor (all the fuel assemblies are fuel assemblies 1).
R) When operating a boiling water reactor, most of the cooling water
Space 15 of lower tie plate 13 and fuel rod support 14
Is introduced directly between the fuel rods of the fuel assembly 1R loaded in the core through the through-hole 18 provided in the core. The remaining part of the cooling water flowing into the space of the lower tie plate 13 is:
A region that flows from the cooling water inlet 38 into the cooling water ascending passage 35 of the spectrum shift rod 24R, and further from the cooling water discharge port 39 through the inverting portion 37 and the cooling water descending passage 36 above the fuel rod support portion 14. Is discharged. The cooling water discharged from the cooling water discharge port 39 becomes liquid or gas (steam) depending on the flow rate of the cooling water flowing into the spectrum shift rod 24R from the cooling water inlet 38. In the present embodiment, a state where a liquid surface is formed in the cooling water ascending flow path 35 at a core flow rate of 100% or less occurs in the spectrum shift rod 24R, and the cooling water ascending flow path 35 and the cooling water descending flow path at a core flow rate of 110%. The pressure loss of the fuel rod support portion 14 and the specifications of the inner pipe 30 and the outer pipe 31 are set in advance so that a state in which almost single-phase flow occurs in the spectrum shift rod 24 </ b> R.

The spectrum shift rod 24R can adjust the neutron moderating effect by changing the water level in the spectrum shift rod according to the core flow rate as described above, and as a result, can be used for reactivity control or power control. Details of this operation are described in JP-A-63-73187.

Meanwhile, in the BWR fuel assembly, the combustion reactivity is generally controlled by gadolinia. Therefore,
At the beginning of the operation cycle, it is necessary to consider the interaction between gadolinia and the spectrum shift rod contained in the new fuel. FIG. 39 shows that the core flow rate is gradually increased from the initial stage of combustion, and the steam volume ratio in the
It shows the change of the neutron infinite multiplication factor when changing from 0% to 0%. It can be seen that in the early stage of the life when gadolinia is present, the neutron infinite multiplication factor rises conversely as the water level in the spectrum shift rod drops. This is a result of the decrease in thermal neutron absorption by gadolinia due to the impaired neutron moderating effect. That is, in order to effectively perform the reactivity control or the output control based on the water level in the spectrum shift rod, it is understood that the gadolinia amount needs to be reduced.

In this embodiment, the reactivity control effect is improved by the arrangement of the partial length fuel rods, and the margin for stopping the furnace is improved, so that the gadolinia amount can be reduced. As a result, as shown in other embodiments, fuel economy can be improved, and the effect of the spectrum shift rod can be maximized.

According to the first and second aspects of the present invention, since the second fuel rods, which are partial-length fuel rods, are arranged in the outermost layer of the fuel rod arrangement in a square lattice, the effect of reducing the void coefficient can be obtained, and The degree of controllability is improved, and the effect of increasing the control rod value can be expected, contributing to the improvement of safety. Further, since the arrangement in a central position of the outermost sides of a certain partial length fuel rods at a position other than the position adjacent to the corner portions other than and corners of the outermost layer, it is possible to reduce the void coefficient without lowering the reactivity And fuel economy can be improved.

[0098] Further, since the second fuel rods (part length rods) arranged in a central position of each side of the outermost layer which is a position within a range of rows and columns of water rods are located, reactivity loss and the local output Both peaking can be improved, and both fuel economy and thermal margin are improved.

[0099] Of the fuel rods arranged in the second layer from the outside of the square grid-like fuel rod arrangement, the fuel rod adjacent to the outermost second fuel rod (partial length fuel rod) is the first fuel rod. Being a rod (normal fuel rod), the resonance absorption is reduced, improving fuel economy as well as reactivity controllability.

According to the second aspect of the present invention, since the cross-sectional area of the water rod in the water rod is 7-14 cm ² , the reactivity of the fuel assembly in which the number of grids of fuel rods is 9 × 9 or more is obtained. And fuel economy is improved.

According to the third aspect of the present invention, since the second fuel rod (partial length fuel rod) is arranged adjacent to the water rod, the reactivity and fuel economy are further improved.

According to the fourth aspect of the present invention, the fuel rods arranged in the second layer from the outside of the square grid-like fuel rod arrangement are all first fuel rods (normal fuel rods). Resonance absorption is reduced, and the effect of improving fuel economy as well as reactivity controllability is further enhanced.

According to the fifth aspect of the present invention, the second fuel rods (partially long fuel rods) are arranged at the corners of the second layer from the outside of the square grid-like fuel rod arrangement, so that the coolant in the channel box is formed. This has the effect of making the flow rate distribution and the steam volume ratio distribution uniform.

According to the sixth aspect of the present invention, the length of the second fuel rod (partial length fuel rod) is set to a range from １ ／ to ／ of the active fuel length of the first fuel rod which is a normal fuel rod. Therefore, an effect of improving channel stability and core stability can be obtained.

As described above, according to the present invention, in a fuel assembly aimed at high burnup, the void coefficient can be reduced without lowering the reactivity, and the local output peaking can be flattened. The water rod cross-sectional area can be optimized, improving fuel economy, increasing thermal margin,
This has the effect of improving furnace shutdown margin and improving stability.

[Brief description of the drawings]

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

FIG. 2 is a sectional view taken along line II-II of FIG.

FIG. 3 is a diagram showing an arrangement state of a fuel assembly shown in FIG. 1 in a core.

FIG. 4 is a diagram showing a void coefficient reduction effect (absolute value) by increasing a moderator area.

FIG. 5 is a diagram showing a partial length fuel rod position and a void coefficient reduction effect (absolute value).

FIG. 6 is a diagram showing a relationship between a partial length fuel rod position, a change in reactivity, and local output peaking.

FIG. 7 is a diagram showing a relationship between a partial length fuel rod position and a control rod value.

FIG. 8 is a diagram showing the effect of improving reactivity by increasing the moderator area.

FIG. 9 is a diagram showing a stability map in a fuel assembly having a 10 × 10 fuel rod array.

FIG. 10 is a diagram showing a relationship between a partial fuel rod length and channel stability.

FIG. 11 is a diagram showing a relationship between a partial fuel rod length and core stability.

FIG. 12 is a cross-sectional view of a conventional fuel assembly.

FIG. 13 is a cross-sectional view of a fuel assembly according to a second embodiment of the present invention.

FIG. 14 is a diagram showing an effect when the partial length fuel rods are adjacent to each other.

FIG. 15 is a cross-sectional view of a fuel assembly according to a third embodiment of the present invention.

FIG. 16 is a cross-sectional view of a fuel assembly according to a fourth embodiment of the present invention.

FIG. 17 is a cross-sectional view of a fuel assembly according to a fifth embodiment of the present invention.

FIG. 18 is a cross-sectional view of a fuel assembly according to a sixth embodiment of the present invention.

FIG. 19 is a cross-sectional view of a fuel assembly according to a sixth embodiment of the present invention.

FIG. 20 is a top view of a core according to a seventh embodiment of the method for loading a fuel assembly of the present invention.

FIG. 21 is a longitudinal sectional view of a core according to a seventh embodiment.

FIG. 22 is an enlarged view of the core shown in FIG.

FIG. 23 is a cross-sectional view of a conventional fuel assembly having no partial length fuel rod.

FIG. 24 is a cross-sectional view of a conventional fuel assembly having a partial length fuel rod.

FIG. 25 is a cross-sectional view of a fuel assembly according to an eighth embodiment of the present invention.

FIG. 26 is a cross-sectional view of a fuel assembly according to a ninth embodiment of the present invention.

FIG. 27 is a cross-sectional view of a fuel assembly according to a tenth embodiment of the present invention.

FIG. 28 is a cross-sectional view of a fuel assembly according to an eleventh embodiment of the present invention.

FIG. 29 is a cross-sectional view of a fuel assembly according to a twelfth embodiment of the present invention.

FIG. 30 is a cross-sectional view of a fuel assembly according to a fourteenth embodiment of the present invention.

FIG. 31 is a cross-sectional view of a fuel assembly according to a fifteenth embodiment of the present invention.

FIG. 32 is a transverse sectional view of a fuel assembly according to a sixteenth embodiment of the present invention.

FIG. 33 is a cross-sectional view of a fuel assembly according to a seventeenth embodiment of the present invention.

FIG. 34 is a cross-sectional view of a fuel assembly according to an eighteenth embodiment of the present invention.

FIG. 35 is a transverse sectional view of a fuel assembly according to a nineteenth embodiment of the present invention.

FIG. 36 is a transverse sectional view of a fuel assembly according to a twentieth embodiment of the present invention.

FIG. 37 is a partial cross-sectional front view showing the structure of a spectrum shift rod.

FIG. 38 is a sectional view taken along line ZZ of FIG. 37.

FIG. 39 shows that the vapor volume ratio in the spectrum shift rod is 1
It is a diagram which shows the relationship between the neutron infinite multiplication factor and the burnup when changing from 00% to 0%.

[Explanation of symbols]

 Reference Signs List 1 fuel assembly 2 (full length) fuel rod 3 partial long fuel rod 3a partial long fuel rod 4 water rod 5 channel box 1M fuel assembly 22 (full length type) fuel rod 23 partial long fuel rod 23a partial long fuel rod 24 water Rod 25 channel box

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoko Ishibashi 1168 Moriyama-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd.Energy Research Laboratory (72) Kunihara Kurihara 1168 Moriyama-cho, Hitachi City, Ibaraki Prefecture Energy, Hitachi, Ltd. In the laboratory (72) Inventor Osamu Yokomizo 1168 Moriyama-cho, Hitachi City, Ibaraki Prefecture Energy Research Laboratory, Hitachi, Ltd. (72) Inventor Junichi Yamashita 3-1-1, Sachimachi, Hitachi City, Hitachi, Ltd.Hitachi Ltd., Hitachi Plant (72) Inventor Yasunori Bessho 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Prefecture Inside Hitachi, Ltd.Hitachi Plant (72) Inventor Yasuhiro Masuhara 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Hitachi, Ltd. Inside the plant (72) Inventor Junjiro Nakajima 3-1-1, Sachimachi, Hitachi City, Ibaraki Prefecture Shares (56) References JP-A-64-23194 (JP, A) JP-A-64-23195 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 3/30

Claims (6)

(57) [Claims] 1. A large number of fuel rods arranged in a square grid of 9 rows and 9 columns and a central area where seven fuel rods can be arranged. In the fuel assembly having two large water rods, (a) the fuel rod includes a plurality of first fuel rods and a plurality of second fuel rods whose active fuel length is shorter than the first fuel rod. (B) the second fuel rods are arranged at least in the outermost layer of the square lattice-shaped fuel rod array; (c) the second fuel rods arranged in the outermost layer are:
The fuel rods are disposed one by one at the center of each side of the outermost layer, and the other fuel rods of the outermost layer are the first fuel rods ; (d) the square grid-like fuel rod arrangement A fuel rod adjacent to the second fuel rod in the outermost layer among the fuel rods arranged in a second layer from the outside of the fuel rod is the first fuel rod; 2. A fuel assembly having a large number of fuel rods arranged in a square grid of 9 rows and 9 columns and at least one water rod having a cross-sectional area larger than a cross-sectional area of a unit fuel grid, wherein: The fuel rods include a plurality of first fuel rods and a plurality of second fuel rods having a shorter active fuel length than the first fuel rods. (B) The second fuel rods are at least the square lattice (C) the second fuel rods arranged in the outermost layer are arranged in the outermost layer of
The fuel rods are arranged one by one at the center of each side of the outermost layer, and the other fuel rods in the outermost layer are the first fuel rods ; (d) the square grid-like fuel rod arrangement Of the fuel rods disposed in the second layer from the outside of the fuel rod, the fuel rod adjacent to the second fuel rod in the outermost layer is the first fuel rod; The area is 7-14c
m ^2. A fuel assembly comprising: 3. The fuel assembly according to claim 1, wherein the second fuel rod is also arranged at a position adjacent to the water rod. 4. The fuel assembly according to claim 1, wherein (d ') the fuel rods arranged in the second layer from the outside of the square grid-like fuel rod array are all A fuel assembly, being the first fuel rod. 5. The fuel assembly according to claim 1 , wherein the second fuel rods are also arranged in a corner portion of a second layer from the outside of the square grid-like fuel rod array. A fuel assembly, comprising: 6. The fuel assembly according to claim 1, wherein a length of said second fuel rod is in a range of ２〜 to ／ of an active fuel length of said first fuel rod. A fuel assembly, characterized in that: