JP2547066B2 - Fuel assembly for boiling water reactor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a fuel assembly for a boiling water reactor (BWR), and more specifically, a 9 × 9 type fuel equipped with a large diameter water channel. The present invention relates to a fuel assembly in which the moderator distribution is made as uniform as possible for a BWR in which the fuel arrangement in the core is asymmetrical (D lattice) and the pressure loss is reduced.

[Prior Art] I.8 × 8 type fuel assembly Currently, most of the fuel assemblies practically used in BWRs in Japan are 62 rows together with two water rods in a square lattice arrangement of 8 rows and 8 columns. The fuel rods are bundled (8 × 8-2).

This conventional 8 × 8 type fuel assembly is shown in FIGS.

Fig. 5 shows the conventional 8x8 fuel assembly in the BWR core 1
/ 4, and the remaining 3/4 cores are arranged symmetrically about the I axis and J axis of the 1/4 core in the figure.

In the figure, 100 is a cross-shaped control rod including a neutron absorber, and 101d is a fuel assembly.

As shown in FIG. 6, the fuel assembly 101d has a total of NO.
A channel box made of Zircaloy, which is configured by arranging 62 fuel rods 102b and two water rods 108 which do not contain a fuel substance inside and which circulates cooling water in a hollow manner in a square lattice of 8 rows and 8 columns. The outer periphery is surrounded by 103b and is loaded into the reactor adjacent to the control rod 100. During the output operation of the nuclear reactor, the cooling water flows from the lower part of the fuel toward the upper part to remove the heat generated by the fuel rods 102b.

FIG. 7 shows the structure of the 8 × 8 type fuel assembly 101d. In the figure, the fuel rod 102b and the water rod 108 are an upper tie plate 109 and a lower tie plate.
Fixed by 110. Further, spacers 111b are arranged at regular intervals along the fuel assembly 101d so that the distance between the fuel rods is kept constant.

By the way, in recent years, in order to improve the economical efficiency of nuclear fuel, high burnup of fuel has been demanded.

However, in order to increase the burnup of the fuel, it is basically necessary to increase the fuel enrichment, and as a result, the neutron absorption of ²³⁵ U increases and the neutron deceleration is insufficient, resulting in the reactivity of the fuel. The characteristics (void reactivity characteristics, reactor shutdown margin) will deteriorate.

Therefore, it is necessary to increase the non-boiling water region in the assembly in response to the increase in fuel enrichment.

On the other hand, of the fission products (FP) accumulated in the high burnup fuel material, the gas components are retained by dissolving in the crystals and grain boundaries that make up the fuel material, but when the temperature rises, the fuel material Is released to the outside to increase the internal pressure of the fuel rod.

Therefore, when increasing the burnup, it is necessary to have a design margin that can withstand an increase in the internal pressure of the fuel rod in order to maintain the mechanical integrity of the fuel rod cladding tube.

II. 9 × 9 type fuel assembly In order to satisfy the above requirements, the fuel is highly enriched, and at the same time, the output per fuel rod is reduced to reduce the temperature and the square grid array of 9 rows and 9 columns. In addition to adopting the above fuel rod, a large-diameter water channel was arranged in the center of the fuel assembly in order to further increase the area of the non-boiling water region in the center of the assembly in response to higher fuel enrichment. A × 9 type fuel assembly is under consideration.

As a 9 × 9 type fuel assembly having a large diameter water channel known in the related art, there is a fuel assembly disclosed in JP-A-62-118297. In this structure, as shown in FIG. 8, according to the purpose of making the non-boiling water region as large as possible, a square-shaped cross section having a size occupying all the grids of the fuel rod insertion cell of the 3 × 3 square lattice array. The water channel 106b is arranged at the center of the fuel assembly. Around the water channel 106b, a total of 72 (9 of No. 1 to 72)
× 9-3 × 3) fuel rods 102a are arranged in a square lattice with 9 rows and 9 columns.

In addition, the grid of each cell is a broken line that defines a cross-sectional area surrounded by the channel box 103a of the fuel assembly 101c.
Indicated by 104. As is clear from the figure, the cell area of each cell is the same except for the corners.

According to the 9 × 9 type fuel assembly 101c provided with the large diameter water channel 106b, the fuel rods 102a are arranged in 9 rows and 9 columns. Therefore, compared with the 8 × 8 type fuel assembly 101d, the same fuel assembly is used. The output per fuel rod can be reduced depending on the output conditions. Therefore, the pellet temperature becomes low, the emission of FP is small, and the design margin for increasing the burnup is increased.

Furthermore, by adopting a water channel 106b that covers the central 3 × 3 grids, all the fuel rods 102a are enclosed except for four corner rods (NO. 1, 9, 64, 72 fuel rods 102a). Since the areas of the cooling water are the same, the flow of the cooling water becomes uniform and the thermal operation performance (burnout margin) is improved.

However, the fuel assembly arrangement in the core is asymmetric, that is, the water gap (the space between the adjacent fuel assemblies,
In a so-called D-lattice BWR core in which the non-boiling cooling water flows during operation) is asymmetrical, even if the water channel 106b is provided at the center of the fuel assembly 101c, the cooling water distribution as a moderator is symmetrical. I won't.

Here, consider the case where the fuel assembly 101c shown in FIG. 8 is loaded on the BWR of the D lattice.

In FIG. 8, the center line 105 indicates the center of the water gap in the B lattice core of the D lattice, and the cross-shaped control rod (not shown) is to be inserted on the wide gap A side. Also,
The narrow gap B indicates the gap on the side opposite to the control rod, and the width thereof is about 1/2 of the gap A.

In the output operation state of the BWR in which the water gap is asymmetric, the boiling water containing a large amount of steam voids flows around the fuel rod 102a, and the water gap and the water channel 106b.
The non-boiling water flows through, and the distribution of the entire moderator surrounded by the center line 105 is biased to the wide gap A side. As a result, fuel rods
The output of 102a increases at the portion facing the wide gap A, decreases at the portion facing the narrow gap B on the opposite side, and causes a problem that the local peaking coefficient increases.

In order to reduce the local peaking coefficient, it is usual to lower the fuel enrichment of the fuel rods on the wide gap A side and increase the fuel enrichment on the narrow gap B side. As is well known, the aggregate has a disadvantage that the reactivity decreases as the local peaking coefficient decreases.

III.9 × 9 type fuel assembly (for D-lattice) BW of D-lattice with asymmetrical fuel assembly arrangement in the core
As a means for solving the problem in the R core, the large diameter water channel 106b is arranged so as to be offset in the narrow gap B side direction with respect to the center of the fuel assembly to improve the uniformity of the moderator distribution. Have been proposed in the following documents.

(Technical magazine "Kerntechnik No. 50/1987" page 222; D. Be
nder et al., “Boiling Water reactor reload fuel
for high burnup: 9 × 9 with internal water
channel ") Fig. 9 shows a cross-sectional view of a 9x9 type fuel assembly in which the water channels described in this document are arranged asymmetrically.

In the figure, reference numerals 102a to 106b denote the same constituent members as those in FIG. However, the arrangement of the water channels 106b is an asymmetrical arrangement in which the water channels 106b are offset toward the narrow gap B by an amount corresponding to the fuel rod arrangement of one row and one column with respect to the center of the fuel assembly 101b. If the position of the cell at the corner on the control rod insertion side, that is, the position of the grid where the NO.1 fuel rod is arranged is the 1st row and 1st column, the grid at the center position of the water channel 106b is 6th row and 6th column. Is represented.

The fuel assembly in which the water channels 106b are asymmetrically arranged is economical because the reactivity can be increased without increasing the local peaking coefficient due to the relatively flat enrichment distribution.

[Problems to be Solved by the Invention] However, the conventional 9 × 9 type fuel assembly 101b in which the large diameter water channels are asymmetrically arranged has the following problems.

First, as compared with the water rod 108 of the conventional 8 × 8 type fuel assembly 101d shown in FIG. 6, the water channel 106b having a remarkably large area is inevitably adopted. Along with the reduction in the number of seats on the road surface, it causes an increase in fuel assembly pressure loss, and if the capacity of the coolant circulation pump does not change, it causes a decrease in the cooling water flow rate in the core and the heat removal capacity of the fuel assembly 101b decreases. There is a problem of doing.

Secondly, since the water channel 106b has a large area, the degree of freedom of arrangement in the fuel assembly is small, and it is difficult to make the optimum arrangement for the D lattice. For example, in the arrangement shown in FIG. 9, since the water channel 106b has a large area, the gap B tends to be offset too much, and the local peaking coefficient of the corner rod NO.72 and its periphery increases,
It is necessary to adjust the concentration distribution, and there is a problem that the reactivity does not reach the optimum value again.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a 9 × 9 type fuel provided with a large diameter water channel suitable for increasing the economical efficiency of nuclear fuel. In the assembly, the moderator distribution is made as uniform as possible and the low pressure loss is achieved, so that the fuel assembly in the core can be applied to the asymmetric B lattice BWR.

[Means for Solving the Problems] In order to achieve the above object, a fuel assembly for a BWR of the present invention is a fuel assembly loaded in a core of a D lattice in which the fuel assembly arrangement is asymmetric, 6 out of a plurality of fuel rods forming a 9 × 9 square lattice array with the fuel rod insertion cell at the corner on the control rod insertion side as the first row and the first column
In a fuel assembly for a boiling water reactor in which one square large-diameter water channel is arranged in a region corresponding to a 3 × 3 square lattice array centered on the fuel rod insertion cell in the sixth row The water channel is arranged such that its cross-sectional area is smaller than the area of the 3 × 3 square lattice array and its central axis is displaced from the central position of the 3 × 3 square lattice array to the control rod side. It has been done.

In this case, the cross-sectional area of the reduced water channel is preferably 80% with respect to the square area occupied by all the cells of the fuel rod insertion cell of the 3 × 3 square lattice array.
To 90%. Along with the contraction and deviation of the water channel, a gap is formed between the water channel and the square occupying all the cells of the fuel rod insertion cell of the 3 × 3 square lattice array. In order to guide the circulating cooling water to the fuel rod side, cooling water guide means for guiding the cooling water in a direction away from the water channel may be provided at the upper end of the spacer lattice plate along the gap.

[Operation] In the fuel assembly configured as described above, the cross-sectional area of the large-diameter water channel is the same as that of the conventional one (the square area occupied by all the grids of the fuel rod insertion cell of the 3 × 3 square lattice array). (Corresponding to the above), the cooling water flow passage area in the boiling region increases, so that low pressure loss is achieved. Furthermore, by arranging this water channel in a grid of a 3 × 3 square lattice array so as to be offset to the control rod insertion side with a wide water gap, the distribution of cooling water as a moderator becomes symmetrical, so the reactivity is optimal. Turn into.

Further, the cooling water guide means provided on the upper end of the spacer lattice plate along the gap with the water channel guides the cooling water in the outer circumferential direction of the water channel, so that the cooling water flowing in the gap is directed to the fuel rod side. It guides and works to contribute to the heat removal of the fuel rods.

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

FIG. 1 shows a sectional view of an assembly of 9 × 9 type fuel of the present invention. In the figure, reference numerals 102a to 105 are the same as those in FIGS. 8 to 9 of the above-mentioned prior art.

The features of the 9 × 9 type fuel assembly 101a of the present invention are as follows:
The area of the water channel 106a is reduced to 80 to 90% with respect to the conventional 9 × 9 type fuel assembly for D lattice shown in the figure, and the center of the water channel 106a is composed of 3 × 3 grids. Wide gap A that does not match the center of the square
That is to be biased towards.

FIG. 2 shows a characteristic graph of the fuel assembly shown in FIG.

In Fig. 2, the horizontal axis of the graph is the water channel.
The cross-sectional area of 106a is shown by the reduction rate (%) with respect to the area of a square composed of 3 × 3 cells (the cross-sectional area of the conventional water channel 106b), and the vertical axis shows the reduction rate (%) of the pressure loss of the fuel assembly and the removal. Indicates the rate of decrease (%) in burnup.

In the graph, the characteristics of the present invention are shown by a one-dot chain line 200 (reduction of pressure loss) and a solid line 201 (reduction of burnup degree).

The preferred cross-sectional area of the water channel 106a of the present invention is in the range of 80% to 90% with respect to the area of the grid of 3 rows and 3 columns, and in this range, as is apparent from FIG. About 1.5% to 2.5% reduction. This low pressure loss increases the cooling water flow rate of the fuel assembly, thus improving the thermal margin.

If the water channel is simply reduced, that is, the center of the water channel is fixed to the center of a square composed of 3 × 3 grids, the take-out burnup decreases as indicated by the dotted line 202. However, when the water channel 106a is biased toward the control rod insertion side as in the present invention, the decrease in the burn-out degree as shown by the solid line 201 is slight. The reason is that the symmetry of the moderator distribution is improved in the entire D lattice.

By the way, according to the configuration of the present invention shown in FIG. 1, a gap 107 is formed between the water channel 106a and the square occupied by all the cells of the fuel rod insertion cell of the 3 × 3 square lattice array. The cooling water flowing in this gap 107 is useless because it does not contribute to the heat removal of the fuel rods. In order to improve this, as shown in FIG. 3 (a), the cooling water guide tab 301 is provided at the position facing the gap 107 at the upper end of the spacer lattice plate (corresponding to the broken line 104 in FIG. 1). Should be provided. The cooling water guide tab 301 has a role of directing the cooling water flowing in the gap 107 toward the fuel rod 102a.

To explain the cooling water guide tab 301 in more detail,
As shown in FIG. 3B, the cooling water guide tab 301 is bent from the upper end of the lattice plate 300 of the spacer toward the water channel 106a, and is formed by folding back upward just before coming into contact with the water channel 106a. . The cooling water that has flowed through the gap 107 between the grid plate 300 of the spacer and the water channel 106a has an opening 30 provided at the center of the bend, as indicated by an arrow in FIG.
2 and flows toward the fuel rod side by the cooling water guide tab 301. The spring member 303 is provided to support the water channel 106a, and presses the water channel 106a against the control rod side of the virtual lattice of 3 rows and 3 columns.

In the above embodiment, the cooling water guide tab 301 is added to the upper end of the lattice plate 300 of the spacer, but as shown in FIG. 4 which also serves as the cooling water guide tab and the spring member for supporting the water channel. A member may be used.

The tab / spring member 400 has three holding claws 401 in the lower half portion, and the upper half portion forms a curved bent portion 402 so as to perform a spring function.
Is provided with an opening 403 so as to also have a tab function.

The tab and spring member 400 configured in this way is
It is attached to the lattice plate 300 of the spacer by the holding claw 401, while pressing the water channel 106a at the bent portion 402,
The cooling water flowing out from the opening 403 is made to flow to the fuel rod side.

[Advantages of the Invention] Since the present invention is configured as described below,
The following effects are achieved.

By making the cross-sectional area of the water channel smaller than that of the conventional one to secure the cooling water flow passage area in the boiling region, low pressure loss can be achieved.

Furthermore, by arranging this reduced water channel on the side of the control rod insertion side where the water gap is wide in the D lattice, the moderator distribution is made symmetrical.

Therefore, when loaded on the BWR of the D-lattice, pressure loss does not increase, and the local peaking coefficient can be reduced without reducing the reactivity, and a large diameter water channel suitable for improving the economic efficiency of nuclear fuel can be provided. The 9 × 9 type fuel assembly provided can be applied to the BWR of the D lattice.

[Brief description of drawings]

FIG. 1 is a cross sectional view of a 9 × 9 type fuel assembly of the present invention, FIG.
FIG. 3 is a diagram showing the characteristics of a 9 × 9 type fuel assembly, and FIGS. 3 (a) and 3 (b) are perspective views of a main part of a cooling water guide tab used for the 9 × 9 type fuel assembly of the present invention and its Fig. 4 is a longitudinal sectional view of an essential part, Fig. 4 is a perspective view of an essential part of a spring member for supporting a cooling water guide tab and a water channel, and Fig. 5 is a schematic view showing 1/4 of a core of a boiling water reactor, Fig. 6 And FIG. 7 shows the conventional 8
A cross-sectional view of a × 8 type fuel assembly and a perspective view thereof,
8 and 9 are cross-sectional views of a conventional 9 × 9 type fuel assembly. 101a …… 9 × 9 type fuel assembly 106a …… Water channel 301 …… Cooling water guide tab 400 …… Cooling water guide tab and spring member for supporting water channel

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Guyunter Ril Herzogen Auratsuha, Federal Republic of Germany Handelshyutraße 10 (72) Inventor Peter Urban, Trevor, Republic of the Federal Republic of Doats, Jansi Utraße 24 (56) References Sho 60-13284 (JP, A) JP 61-212788 (JP, A)

Claims (3)

(57) [Claims] 1. A fuel assembly to be loaded in a core of a D-lattice having an asymmetric fuel assembly arrangement, wherein the fuel rod insertion cell at the corner on the control rod insertion side is the first row and the first column and is 9 × 9 square. One square large-diameter water channel is arranged in a region corresponding to a 3 × 3 square lattice arrangement centered on the fuel rod insertion cell at the 6th row and the 6th column among the plurality of fuel rods forming the lattice arrangement. In the fuel assembly for a boiling water reactor, the water channel has a cross-sectional area smaller than the area of 3 × 3 square lattice array and the central axis of the water channel is the central position of the 3 × 3 square lattice array. A fuel assembly for a boiling water nuclear reactor, wherein the fuel assembly is arranged at a position displaced toward the control rod side with respect to the above. 2. The boiling water reactor according to claim 1, wherein the cross-sectional area of the water channel is within a range of 80% to 90% of the area of the 3 × 3 square lattice array. Fuel assembly. 3. A spacer grid for cooling water guiding means for guiding cooling water flowing along a gap between a water channel and a square having an area corresponding to the 3 × 3 square grid array, in a direction away from the water channel. The fuel assembly for a boiling water reactor according to claim 1 or 2, which is provided at the upper end of the plate.