JPH04296694A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To obtain a fuel assembly for a boiling water reactor which is provided with a large diameter square type water channel and is economically improved by improving its hydraulic stability and dryout performance and also the local peaking coefficient of the outermost peripheral line and row. CONSTITUTION:A part of a square grid-like arranged fuel rod of 9 lines and 9 rows provided with a square water channel 41 is replaced with shorter fuel rods 40, which are dottedly arranged in the internal row adjacent to a square water channel 41 out of grid-like arrangement and in the middle row adjacent to the outside of the internal peripheral row. The clearance between the fuel rods in the outermost peripheral row and a channel box 45 coating the fuel assembly is widened to 3.8mm or more. Therefore, this can substantially improve the thermal safety of the highly economical fuel assembly, the local peaking coefficient of the outmost peripheral row of which besides the improvement of the hydraulic stability and dryout performance.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water nuclear reactor that has improved hydraulic stability and improved dryout performance.

0002

[Prior Art] Recently, in order to improve fuel economy, there has been an increasing demand for higher burnup, and in addition to increasing fuel enrichment, large-diameter water channels have been used to improve thermal neutron utilization efficiency to achieve the same enrichment. Increasing the average extraction burnup is being considered even in the case of In addition, the thermal and mechanical integrity of fuel rods can be improved by reducing the diameter of the fuel rods and incorporating many fuel rods into one band to reduce the heat load per fuel rod and increase the burnup. Efforts are being made to increase the For example, the fuel rod arrangement could be changed from the current 8 rows and 8 columns to 9 rows and 9 columns.
One example is fuel assemblies arranged in rows.

[0003] Furthermore, as a fuel assembly using a large-diameter water channel that has been put into practical use,
There is one disclosed in Japanese Patent No. 2-118297. FIG. 7 is a cross-sectional view of a 9-row, 9-column fuel assembly using a conventional large-diameter square water channel 11, and FIG. 8 shows an upper part of the fuel assembly using the water channel 11 and fuel rods 12 shown in FIG. FIG. 2 is a schematic diagram showing the entire structure held in a bundle shape together with a tie plate 14 and a spacer 13. Furthermore, 15 is a channel box, as shown in Fig. 8.
shows only the upper part of the assembly and omits the lower structure including the lower tie plate.

As shown in FIG. 7, in accordance with the objective of making the non-boiling water area as large as possible, the fuel rod insertion cell is arranged in a square lattice with 3 rows and 3 columns in the center, and has a square cross-sectional shape with dimensions that occupy all the squares. A water channel 11 is used.

FIG. 9 shows the outer peripheral row A and middle row B of the fuel rods in FIG.
, the results of evaluating the dryout performance of the inner row C are CPR
It is a line diagram shown in . Here, CPR (Critical
Power Ratio) is defined as the ratio PC/P of the output power PC (MW) at which a certain fuel rod dries out to the output power P (MW) of the fuel assembly during nuclear reactor operation, and is This is an indicator of margin.

Although the fuel assembly shown in FIG. 7 has a large non-boiling water region, as shown in FIG.
It can be seen that the CPR of the inner row C and the middle row B are low. Since the actual operating margin is determined by the minimum CPR, in the fuel assembly shown in Fig. 7, the CPR of the inner row and middle row is
It was hoped that further improvements would be made to improve dryout performance.

[0007] Therefore, in order to further improve the dryout performance of the above-mentioned large-diameter rectangular water channel,
JP-A-64-91088 discloses a fuel assembly having a fuel assembly cross section in which the area of the water channel is reduced relative to the area of a 3-by-3 square lattice. Figure 10
is a cross-sectional view of a fuel assembly equipped with this reduced large-diameter rectangular water channel, and FIG. 11 evaluates the dryout performance of the outer row A, middle row B, and inner row C of the fuel rods in FIG. 10. This is a diagram showing the results in CPR, where a, b, and c have a reduction rate of 100%, and a85, b85, and c85 have a reduction rate of 85.
%, a81, b81, and c81 indicate a reduction rate of 81%. FIG. 12 is a diagram showing changes in the size reduction rate of the rectangular water channel 21 and the average extraction burnup.

The example shown in FIG. 10 includes a large-diameter rectangular water channel 21 whose area is reduced to 95 to 75% of the area of a 3-by-3 square lattice. Note that 25 is a channel box. In addition, as shown in FIG. 11, in this reduced type water channel 21, as in the example when reduced to 85% or 80%, the dryout performance is different for each fuel rod 22: inner row C, middle row B, outer row A. It can be seen that there is an effect that the difference in the values becomes smaller and the minimum CPR is improved.

However, the fuel assembly shown in FIG. 10 is inconsistent with the objective of increasing the area of the non-boiling water region by maximizing the area of the water channel 21 and improving fuel economy. This is explained in the above-mentioned Japanese Patent Application Laid-open No. 64-
Since the decrease in the average extraction burnup shown in FIG. 12 cited from Japanese Patent No. 91088 becomes significant as the cross-sectional area of the water channel is reduced, it can be evaluated quantitatively. In other words, in order to stop the decline in the average extraction burnup within 1%, it is impossible to reduce it to 85% or less from Fig. 12.
1, it can be seen that the CPR is low at the inner circumferential portion C and the intermediate portion B, and high at the outer circumferential portion A, which is not completely resolved.

On the other hand, the high burnup 9-by-9 type fuel assembly, which combines the two features of increasing the number of fuel rods and increasing the volume of the neutron moderator inside the assembly, has an increased pressure drop with respect to the coolant flow. There was a new problem. That is, an increase in frictional pressure drop due to an increase in the surface area of the fuel rods, and an increase in pressure drop due to an increase in the coolant flow rate due to a reduction in the in-channel coolant flow area by the amount occupied by the large-diameter water rod or water channel. It is.

[0011] In a boiling water reactor, as is well known, the coolant is a two-phase flow of water and steam, so special consideration must be given to hydraulic instability. Generally, the coolant flow velocity in two-phase flow sections is higher than in single-phase flow sections, so the increase in pressure drop described above has a greater effect on two-phase flow sections, which is known to promote hydraulic instability. It was getting worse.

[0012] In this way, the 9-by-9 fuel assembly has a higher pressure loss than the 8-by-8 fuel assembly, so it has the disadvantage that channel stability and core stability deteriorate hydraulically. arise. When hydraulic instability worsens, the flow rate of the coolant oscillates, and in the worst case, when the amplitude of this oscillation increases, fuel damage may occur due to insufficient heat removal. Furthermore, the flow rate fluctuation causes fluctuations in the coolant vapor volume fraction (void fraction) and the resulting fluctuations in the nuclear reaction rate, causing fluctuations in the entire reactor core or local neutron flux, eventually leading to a reactor scram.

[0013] Therefore, it has been an important issue to ensure hydraulic channel stability in boiling water nuclear reactors. To solve this problem, some fuel rods in a fuel assembly are shortened to about 2/3 of the effective length of a normal full-length fuel rod, and the two-phase flow pressure drop at the top of the effective length is reduced. Japanese Unexamined Patent Publication No. 2-12087 proposed an attempt to suppress hydraulic instability by making the diameter smaller.

FIG. 13 is a sectional view of a fuel assembly equipped with this short fuel rod, and FIG. 14 is an explanatory diagram showing a comparison between the short fuel rod shown in FIG. 13 and a normal full-length fuel rod. In this case, a plurality of short fuel rods 30 are arranged in 9 rows and 9 columns as shown in FIG.
That is, the feature is that it is arranged in the middle row B). This is effective when used in a fuel assembly equipped with a relatively small diameter water rod 31. In the figure, 35 is a channel box.

[0015]

[Problem to be solved by the invention] However, FIG.
When the short fuel rods shown in Figure 13 are used in a fuel assembly with large-diameter square water channels, i.e., when the second column of the 9-by-9 array is used as shown in Figure 13, the dry-out There was a drawback that not only the performance was not improved, but also the dryout performance was degraded. This is because the dryout performance is significantly different thermal-hydraulicly between the combination of a conventional relatively small-diameter round water rod and short fuel rods and the combination of a large-diameter square water channel and short fuel rods. It is. That is, in a large-diameter water channel, the wetted length is longer than in a small-diameter round water rod, and the flow rate of the coolant near the water channel is insufficient due to frictional resistance.

The present invention is a fuel assembly equipped with a large-diameter square water channel, which improves hydraulic stability, improves dryout performance, and improves economy by increasing the local peaking coefficient of the outermost row. The purpose of this study is to obtain a fuel assembly for boiling water reactors with improved performance.

[0017]

[Means for Solving the Problems] In the fuel assembly for a boiling water nuclear reactor according to the present invention, fuel rods are held in a bundle shape in a square lattice arrangement of 9 rows and 9 columns, and a plurality of fuel rods are arranged in the center of the arrangement. In a fuel assembly for a boiling water nuclear reactor in which one of the fuel rods is replaced with one rectangular water channel, a part of the fuel rod is replaced with a short fuel rod that is shorter than the fuel rod, and the lattice-like The short fuel rods are interspersed in an inner peripheral row adjacent to the square water channel and an intermediate row adjacent to the outside of the inner peripheral row of the array, and the fuel rods in the outermost peripheral row,
The gap between the channel box covering the fuel assembly and the
．． It is expanded to 8mm or more.

Specifically, the length of the short fuel rod does not exceed about 2/3 of the effective length of the fuel rod,
The area of the square water channel is 85% or more of the area occupying the square lattice array of 3 rows and 3 columns, and the nuclear fuel material of the fuel rod or short fuel rod is uranium dioxide, or uranium dioxide and uranium dioxide. A mixed oxide with plutonium is disclosed.

[0019]

[Operation] In the present invention, a part of the fuel rods in a square lattice arrangement of 9 rows and 9 columns equipped with rectangular water channels are replaced with short fuel rods that are shorter than the fuel rods; The short fuel rods are interspersed and arranged in an inner row adjacent to the square water channel and an intermediate row adjacent to the outside of the inner row; the fuel rods in the outermost row and the fuel assembly. The gap between the channel box and the covering channel box has been widened to 3.8 mm or more, which improves hydraulic stability and dryout performance, and is highly economical by increasing the local peaking coefficient of the outermost row. It is possible to significantly improve the thermal safety of the fuel assembly.

That is, when short fuel rods are used as a means for improving hydraulic stability, there are two cases: a combination of a round water rod with a relatively small diameter and a short fuel rod, and a case where a square water channel is used. Because the dryout performance when combining short fuel rods and short fuel rods is thermally hydraulically different, we simply use a 9-by-9 array of fuel assemblies in a 9-by-9 square lattice arrangement with square water channels. It was found that the dry-out performance was not improved if it was placed in the second row from the outermost circumference. That is, in a rectangular water channel, the wetted length is longer than in a round water rod with a small diameter, and the flow rate of the coolant near the rectangular water channel is insufficient due to frictional resistance.

By the way, in order to improve fuel economy, it is necessary to increase the local peaking coefficient at a position where the thermal neutron flux is high so as to increase the thermal neutron utilization rate. is desirable. In a boiling water reactor, the highest thermal neutron flux is in the water region outside the channel box (ie, the gap water). In a normal boiling water reactor, the gap between channel boxes is about 15 mm, and because this gap is filled with non-boiling water, a larger amount of neutron moderator exists than in the channels. . For this reason, increasing the local peaking coefficient of the outermost row of the fuel assembly will increase fuel economy.

However, when the local peaking coefficient at the outermost periphery is increased in this way, the dryout performance of the fuel rod is significantly reduced. On the other hand, by widening the gap between the fuel rods in the outermost row and the channel box covering the fuel assembly, the dryout performance of the fuel rods in the outermost row and the fuel rods in the middle row are equal, and the The minimum dryout performance will be increased.

Therefore, in the present invention, since short fuel rods are used, the two-phase flow pressure loss in the upper part of the effective length can be reduced, and hydraulic instability can be suppressed. In addition, the decrease in dryout performance, which is a problem when using short fuel rods, is caused by the inner row adjacent to the square water channel and the middle row adjacent to the outside of the inner row in the lattice arrangement. This can be greatly improved by arranging the short fuel rods in a scattered manner. Furthermore, since the gap between the outermost row of fuel rods and the channel box covering the fuel assembly is increased to 3.8 mm or more, the dryout performance of the outermost row of fuel rods and the middle row of fuel rods is improved. are the same, and even if the local peaking coefficient at the outermost periphery, where the thermal neutron flux is high, is increased, the drop in dryout performance can be kept to a minimum. Hereinafter, the structure and operation will be explained based on examples.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view of a fuel assembly showing the arrangement of one embodiment of the present invention, and FIG. 2 is a sectional view of a fuel assembly showing the arrangement of another embodiment of the invention. Figure 1 shows eight short fuel rods 4
0, FIG. 2 shows a case where four short fuel rods 40 are used;
0 is adjacent to the center of each of the four sides of the water channel 41. Also, the water channel 41 in the figure is 3
This is an example in which the area is 85% of the area of a square lattice array with three rows and three columns.

In this embodiment, the dryout performance for each fuel rod can be expected to be made more homogeneous by arranging the short fuel rods 40 in the same way as when the water channel is reduced, so the area of the water channel 41 can be reduced to an extent that does not reduce economic efficiency. It is possible to make it sufficiently large. That is, a water channel 41 having an area of 85% or more can be used.

The reference fuel assembly of the present invention is an assembly having 72 full-length fuel rods as shown in FIG. 10, and the fuel rods have a diameter of about 11 mm. In the figure, 45 is a channel box. The fuel assembly in FIG. 1 or 2 uses eight or four short fuel rods 40;
The length of the fuel rod 42 was approximately 2/3 of the effective length of the full length fuel rod 42. In order to compensate for the reduction in the amount of fuel material loaded per assembly due to the adoption of the short fuel rods 40, the diameter of the fuel rods is 8 for both the full-length fuel rods 42 and the short fuel rods 40, compared to the standard approximately 11 mm. When using short fuel rods 40 of
Approximately 2%, approximately 1% when using four short fuel rods 40,
Each had a large diameter.

Regarding the effects of the present invention, first, the effect of improving channel stability will be described. FIG. 3 is a diagram showing analytical values of the channel stability reduction width ratio due to the short fuel rods shown in FIGS. 1 and 2. As shown in the figure, in a nuclear reactor with a rated thermal output of approximately 3300 MW loaded with 764 fuel assemblies,
This is an analysis of the channel stability of a hot channel when the reactor core coolant flow rate is approximately 30%. The hot channel is the fuel assembly with the highest output among the 764 fuel assemblies, and this analysis assumes a peaking coefficient of 1.5.

Note that the width reduction ratio in FIG. 3 is an index indicating channel stability, and represents the degree of attenuation of vibration amplitude, and becomes unstable when the width reduction ratio is 1.0 or more. The limit power of the channel stability of the fuel rod assembly of the present invention using short fuel rods, that is, the reactor thermal power at which the width reduction ratio is 1.0 in FIG. 3 is approximately 60%, and when using four short fuel rods, it is about 55%, which is about 53% of the standard.
This is an improvement of 7% and 2%, respectively.

Next, the effect of improving dryout performance by arranging short fuel rod assemblies will be described. FIG. 4 is a diagram showing the results of evaluating the dryout performance of the outer peripheral row A, intermediate row B, and inner peripheral row C of the fuel rods in FIGS. 1 and 2 in terms of CPR. In Fig. 4, a, b, and c show the CPR dryout performance of fuel rods in a conventional fuel assembly without reducing the water channel, where A is the outer row fuel rod, B is the middle row fuel rod, and C is the inner row fuel rod. CPR of circumferential fuel rods. This is shown similarly to FIG. 9 or FIG. 11 described above.

On the other hand, when eight short fuel rods are used, a', b', C', and when four short fuel rods are used, a'', b''
, c” indicates the CPR of each fuel rod. This analysis shows that CPR when the conventional water channel is reduced.
Similar to FIG. 11 showing the PR, it can be seen that the CPR of the fuel rods in the middle row and the inner row is improved, and the minimum value of the CPR of the entire assembly is improved. It has also been found that when short fuel rods are arranged as in the conventional arrangement shown in FIG. 13, that is, when they are arranged only in the middle row, the CPR of the outer row is significantly reduced and the dryout performance is deteriorated.

By the way, in order to improve fuel economy, it is necessary to increase the local peaking coefficient at a position where the thermal neutron flux is high so as to increase the thermal neutron utilization rate. is desirable. In a boiling water reactor, the highest thermal neutron flux is in the water region outside the channel box (ie, the gap water). In a normal boiling water reactor, the gap between channel boxes is about 15 mm, and because this gap is filled with non-boiling water, a larger amount of neutron moderator exists than in the channels. . For this reason, increasing the local peaking coefficient of the outermost row of the fuel assembly will increase fuel economy.

However, when the local peaking coefficient at the outermost periphery is increased in this manner, the dryout performance of the fuel rod is significantly reduced. On the other hand, by widening the gap between the fuel rods in the outermost row and the channel box covering the fuel assembly, the dryout performance of the fuel rods in the outermost row and the fuel rods in the middle row are equal, and the The minimum dryout performance will be increased.

FIG. 5 is a diagram showing the calculated value of the average extracted burnup of the fuel assembly with respect to the average value of the local peaking coefficient of the fuel rods in the outermost row. From this, it has been found that if the average local peaking coefficient of the outermost row is set to 1.10, a significant improvement in the average extraction burnup of about 1% can be obtained.

However, when the local peaking coefficient at the outermost periphery is increased in this manner, the dryout performance of the fuel rod is significantly reduced. FIG. 4 is a diagram showing calculated values of CPR when the output distribution is flat, that is, the local peaking coefficient at the outermost circumference is set to 1.0. Further, the diameter of the fuel rod used at this time was 11.2 mm, and the gap d between the fuel rod and the channel box was 3.5 mm.

The above local peaking coefficient at the outermost circumference is set to 1.
10, the results of CPR calculation using the gap distance d between the fuel rod and the channel box as a parameter are shown in FIG. As you can see, the distance d is approximately 3.8
It can be seen that by setting the diameter to be equal to or larger than mm, the dryout performance expressed in CPR of the fuel rods in the outermost row and the fuel rods in the middle row becomes equivalent, and the CPR of the assembly, that is, the minimum CPR increases.

Note that the CPR of the middle row and the CPR of the inner row are the gap distance d between the fuel rods of the outer row and the channel box.
The reason why it is not affected so much is because the coolant flow area is increased compared to the outermost row by using four short fuel rods each.

As described above, it is possible to provide a fuel assembly for a boiling water reactor with improved channel stability and dryout performance without reducing the water channel by arranging short fuel rods. , it is possible to significantly improve the thermal safety of a highly economical fuel assembly in which the local peaking coefficient of the outermost row of the fuel assembly is increased.

[0038]

As explained above, some of the fuel rods arranged in a square lattice of 9 rows and 9 columns equipped with square water channels are replaced with short fuel rods that are shorter in length than the fuel rods;
Of the lattice arrangement, the short fuel rods are interspersed in an inner row adjacent to the square water channel and an intermediate row adjacent to the outside of the inner row; fuel rods in the outermost row; The gap between the fuel assembly and the channel box covering the fuel assembly has been widened to more than 3.8mm, which improves hydraulic stability and dryout performance, as well as increasing the local peaking coefficient of the outermost row. The thermal safety of the highly economical fuel assembly can be greatly improved, and compared to conventional fuel assemblies for boiling water reactors, the characteristics of the large-diameter square water channel can be maximized. Therefore, it is possible to provide a fuel assembly that is highly economical, has operational margin regarding stability and dryout performance, and is therefore highly safe.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a fuel assembly showing the arrangement of an embodiment of the present invention.

FIG. 2 is a sectional view of a fuel assembly showing the arrangement of another embodiment of the present invention.

FIG. 3 is a diagram showing an analytical value of the channel stability reduction ratio due to the short fuel rods shown in FIGS. 1 and 2;

4 is a diagram showing the dryout performance of the fuel assembly shown in FIGS. 1 and 2. FIG.

FIG. 5 is a diagram showing the calculated value of the average extraction burnup of the fuel assembly with respect to the average value of the local peaking coefficient of the fuel rods in the outermost row.

FIG. 6 is a diagram showing calculated values of CPR when the local peaking coefficient at the outermost circumference is 1.10.

FIG. 7 is a cross-sectional view of a fuel assembly with a conventional large-diameter rectangular water channel.

8 is a schematic diagram showing the overall structure of FIG. 7. FIG.

9 is a diagram showing the dryout performance of the fuel assembly shown in FIG. 7. FIG.

FIG. 10 is a cross-sectional view of a conventional fuel assembly with a reduced large-diameter rectangular water channel.

11 is a diagram showing the dryout performance of the fuel assembly shown in FIG. 10. FIG.

FIG. 12 is a diagram showing changes in size reduction rate and average extraction burnup of a large-diameter rectangular water channel.

FIG. 13 is a sectional view of a fuel assembly including conventional short fuel rods.

FIG. 14 is an explanatory diagram showing a comparison between the short fuel rod shown in FIG. 13 and a normal fuel rod.

[Explanation of symbols] 11, 21, 41...Water channels 12, 22, 3
2, 42... Full length fuel rod 13... Spacer 14... Tie plate 15, 25, 35, 45... Channel box 30, 4
0...Short fuel rod 31...Water rod

Claims (4)

[Claims] 1. A boiling water nuclear reactor in which fuel rods are held in a bundle shape in a square lattice arrangement of 9 rows and 9 columns, and a plurality of fuel rods in the center of the arrangement are replaced with a single rectangular water channel. In the fuel assembly for use, some of the fuel rods are replaced with short fuel rods that are shorter than the fuel rods, and the inner circumferential row adjacent to the square water channel in the lattice arrangement and the inner circumferential The short fuel rods are arranged in an interspersed manner in the middle row adjacent to the outside of the row, and the gap between the fuel rods in the outermost row and the channel box covering the fuel assembly is 3.8 m.
A fuel assembly for a boiling water reactor, characterized in that the fuel assembly is expanded over m or more. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein the length of the short fuel rod does not exceed about 2/3 of the effective length of the fuel rod. . 3. The boiling water nuclear reactor according to claim 1, wherein the area of the square water channel is 85% or more of the area occupying the square lattice of the 3 rows and 3 columns of square lattice arrays. Fuel assembly for use. 4. The boiling water according to claim 1, wherein the nuclear fuel material of the fuel rod or short fuel rod is uranium dioxide or a mixed oxide of uranium dioxide and plutonium dioxide. Fuel assembly for type nuclear reactor.