JPH0499991A - Fuel assembly - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a fuel assembly for a boiling water nuclear reactor.

(Prior Art) As shown in FIG. 6, the core of a boiling water reactor is mainly composed of a large number of fuel assemblies 1, control rods 2, and a local power range monitor 3, which is one of the nuclear instrumentation needles. ing.

The fuel assemblies 1 are arranged in a set of four around a control rod 2 having blades combined in a cross shape. The local power range monitor 3 basically surrounds four control rods 2 in the radial direction of the reactor core, that is, the 16 fuel assemblies 1
It is placed in a rectangular position surrounding the area.

As shown in FIG. 7, the fuel assembly 1 has a large number of fuel rods 4 and water rods 5 bundled together in a square lattice shape by spacers 6, and fixed at the top and bottom by tie plates 7.8.

Further, the outside thereof is covered with a channel box 9.

In this way, in the conventional fuel assembly 1, the fuel rods 4 having the same length are arranged in the channel box 9, and the upper and lower ends of the fuel rods 4 are fixed at the positions of the upper and lower tie plates 7.8. has been done. However, with the recent increase in fuel burnup, some fuel rods may be designed to be shorter than others in order to prevent deterioration of the nuclear or thermo-hydraulic properties of the core. ing. This short fuel rod is hereinafter referred to as short fuel rod 4s. FIG. 8 shows an example of a fuel assembly 1a employing such a structure. The one indicated by neutral number 4s in Figure 8 is a short fuel rod.

The arrangement positions of the short fuel rods 4s in the fuel assembly 1a in this example are shown in FIG. 9(a). FIG. 9 schematically shows a cross section of the fuel assembly shown in FIG. 8. This example is an example of a fuel assembly in which fuel rods are arranged in a 9x9 lattice. There are 8 of them placed in different locations.

Therefore, as shown in FIG. 9(b), there are no short fuel rods at these eight positions in the upper part of the fuel assembly.

In this manner, in the fuel assembly 1a employing the short fuel rods 4s, the amount of nuclear fuel material, for example the amount of uranium, in the fuel rods 4, 4s differs in the axial direction. That is, in the axially upper part where some fuel rods are not present, the amount of uranium is relatively smaller than in the lower part where all the fuel rods are packed.

Figure 10 shows a long fuel rod 4 of normal length and a short fuel rod 41J.
4s vertical section and its effective length are shown.

These long fuel rods 4 and short fuel rods 4s have the same structure, but differ only in length.

That is, fuel pellets 11 are loaded into the cladding tube 10, end plugs 12 and 13 are provided at both upper and lower ends of the cladding tube 11 for sealing, and a coil is placed between the upper fuel pellets 11 and the upper end plugs 12. A spring 14 is interposed.

Next, the effect will be explained based on the results of quantitative evaluation.

First, an overview of the arrangement of local output area monitors will be described. The local power range monitors are regularly arranged in the radial direction of the reactor core as shown in Figure 6. At each position, four local power range monitors are arranged along the axial direction of the core. has been done. These four sets are called strings. The structure of the local output area monitor 3 in this string is as shown in FIG. That is, as shown in FIG. 11, in the local output area monitor 3, detectors 17A, 17B, 17G, and 17D are arranged in a cover tube 15 at predetermined intervals in the axial direction, and each detector is connected to a signal cable 16. are connected and the signal is derived. In this specification, each detector in the cover tube 15 is called 17A, 17B, 17G, and 17D from the bottom.

Generally, when discussing various quantities in the axial direction, in a boiling water reactor, the axial direction of the reactor core is divided into 24 parts. Each division unit is called a node. In this case, the detectors 17A, 17B, 17G, and 17D (hereinafter simply A) of the local output area monitor are used.
, B, C, and D) are 3 nodes, 4 nodes, 9 nodes, 10 nodes, 15 nodes, 16 nodes, and 1 node, respectively.
It is generally located between the 2nd node and the 22nd node.

Now, FIG. 12 shows an example of the axial power distribution when a conventional fuel assembly, that is, a fuel assembly not equipped with short fuel rods, is loaded into the reactor core. This axial power distribution is the average axial power distribution of the four fuel assemblies surrounding the local power area monitor string. In FIG. 12, arrows are shown at locations corresponding to the positions of detectors A, B, C, and D. As is clear from FIG. 12, each detector A, B, C,
Changes in the output distribution near D are small. That is, the difference in output between adjacent axial nodes is small. The local output area monitor 3 does not directly measure the output, but measures the neutron flux generated according to the output, especially thermal neutron flux, but if the output distribution is smooth, the thermal neutron flux distribution It's also smoother. Therefore, even if there is some axial positional deviation of the detector, the reading will not change significantly, and the output at that position can be measured with very high accuracy.

Next, we will discuss a fuel assembly that is equipped with short fuel rods and is compatible with high burnup. What will be described here is, for example, a fuel assembly that includes a normal long fuel rod 4 shown in FIG. 10 and shorter fuel rods 4s.

FIG. 13 shows an example of the axial power distribution when such a fuel assembly is loaded into the reactor core. In this example, since the upper end of the axial effective length of the short fuel rod is between the 15th node and the 16th node, it can be seen that the difference in output between these nodes is large. Furthermore, if the amount of nuclear fuel material varies discontinuously in the axial direction as described above, the change in thermal neutron flux distribution becomes larger than the amount of change in power distribution. The thermal neutron flux distribution in the axial direction in this case is shown in FIG. A peak of thermal neutron flux appears between the 15th and 16th nodes.

This is called a power spike. This power spike is a phenomenon that occurs because if there is a part where no nuclear fuel material is loaded, the rate at which thermal neutrons flowing from below that part are absorbed by the nuclear fuel is significantly reduced.

(Problem to be solved by the invention) As mentioned above, since the local power range monitor measures thermal neutrons, the readings of the local power range monitor will fluctuate significantly in places where the thermal neutron flux distribution changes drastically. It turns out. As a result, this will cause a significant problem in core performance monitoring.

In other words, in order to cope with higher burnup, short fuel rods 4s
In the fuel assembly 1a employing the above, the neutron flux distribution and power distribution change greatly near the boundary where the uranium content distribution differs in the axial direction. If the detector of the local power range monitor 3 is located in the vicinity where the neutron flux distribution etc. are changing significantly, there is a possibility that the readings of the local power range monitor 3 will vary greatly, and the heat of the reactor core may increase. The accuracy of monitoring the target status is reduced.

Therefore, there is an undesirable problem in that the thermal neutron detection position where the local power range monitor 3 is disposed is close to the upper end of the effective length of the short fuel rod.

The present invention was made in order to solve the above-mentioned problems, and an object of the present invention is to provide a fuel assembly that can handle high combustion changes without causing problems in core performance monitoring.

[Structure of the invention]

(Means for Solving the Problems) The present invention provides a fuel assembly in which a plurality of fuel rods having different effective lengths in the axial direction are arranged in a square lattice shape in a channel box through spacers, in which the fuel rods have effective lengths in the axial direction. It is characterized in that the upper end of the long portion is configured not to be close to a detection position of a local power range monitor disposed in the reactor core loaded with the fuel assembly.

In addition, in a fuel assembly in which a plurality of fuel rods with different effective lengths in the axial direction are arranged in a positive lattice shape in a channel box via spacers, a local power range monitor in the core in which the fuel assemblies are loaded is used. 4x4 with the fuel rod position located at the corner on the side where it is arranged as one corner of the square
The fuel rods arranged in the fuel rod lattice are configured such that the upper ends of their axially effective lengths are not close to the detection position of the local power range monitor arranged in the reactor core. It is characterized by

Note that the axial effective length of a fuel rod does not refer to the length of the fuel rod itself, but refers to the length of the portion in which nuclear fuel material is actually loaded in the cladding tube.

(Operation) According to the present invention, it is possible to make changes in the neutron flux distribution and the output distribution very small near the detector arrangement position of the local output area monitor. This eliminates variations and fluctuations in the local power range monitor readings, further improving the ability to monitor thermal performance within the core.

(Example) A first example of a fuel assembly according to the present invention will be described with reference to FIGS. 1 and 2.

The basic configuration of the fuel assembly according to the present invention is the same as that of the conventional example shown in FIGS. The reason is that the position of the thermal neutron detector of the local output area monitor is configured so as not to coincide with the position of the detector that detects thermal neutrons. Therefore, in each case, the same parts as in the conventional example are given the same reference numerals, and the explanation of the overlapping parts will be omitted.

That is, in this embodiment, the number of long fuel rods 4 shown in FIG. 3 is 66, and the number of short fuel rods 4s is 8. Note that two water rods 5 having a larger diameter than the fuel rods 4 are arranged in the center.

In this embodiment, the axial effective lengths of all the short fuel rods 4s are the same.

FIG. 1 shows the relationship between the axial effective length of these fuel rods and the axial position of the detector of the local power range monitor. Figure 2 shows the relationship between axial nodes and relative thermal neutron flux. The upper end position of the short fuel rod 4s is located one node above the position of the detector C of the local power range monitor 3. That is, the upper end is at the boundary between the 16th node and the 17th node. Normally, the effective length of the core of a boiling water reactor is approximately 3.7 m.
The width of one node is approximately 15 cm. The power spike of the thermal neutron flux described above occurs within a range of several square meters from this boundary, and by shifting by one node, the influence of the power spike on the local output range monitor can be significantly reduced. In this embodiment, the effective length of the short fuel rod may be shifted downward by one node. Of course, there is no need to be particular about one node, and it is possible to be further ahead than that.

Next, a second embodiment of the fuel assembly according to the present invention will be described with reference to FIGS. 3 to 5.

In a boiling water reactor, the average distance traveled by thermal neutrons is several meters, so the effect of the power spike that occurs at the top of a short fuel rod becomes smaller when it is more than a few meters in front of the fuel rod. .

Therefore, even if the upper end of the effective length of the short fuel rod is sufficiently far away from the position of the detector of the local power range monitor, it does not pose a big problem even if the upper end of the effective length of the short fuel rod is close to the height of the local power range monitor.

On the other hand, the influence of power spikes cannot be ignored for short fuel rods placed toward the corner on the side where the local power range monitor is installed, so it is necessary to shift the upper end of the effective length from the position of the local power range monitor. be.

FIG. 3 shows a combination of fuel rods in a fuel assembly of such an embodiment. The relationship between the effective length of these fuel rods and the position of the local power range monitor is shown in FIG. Among the short fuel rods shown in Fig. 3, 4s is a short fuel rod whose effective length is at a position shifted by one node from the height of the local power range monitor, and 4sa is at the same height position as the local power range monitor. It is a short fuel rod having one length shorter than its effective length.

The lattice arrangement of these fuel rods is the same as in Figure 9, but the short fuel rod S located at the position indicated by 48 in Figure 9(a) is shifted by one node from the height of the local power range monitor. This corresponds to having an effective length at a certain position.

In this way, it is effective at least for short fuel rods arranged in a 4x4 fuel rod lattice in which one corner of a square is the fuel rod located at the corner on the side where the local power range monitor is arranged. The upper end of the long part should not be close to the height position of the local output area monitor. This can significantly reduce the influence of power spikes on the local power range monitor.

Further, a structure in which short fuel rods are not arranged within this 4×4 lattice is also effective.

Note that the example of a fuel assembly equipped with short fuel rods described here is just an example, and other fuel assembly designs equipped with short fuel rods as shown in FIGS. 5(a) and (b) may be used. However, it is possible to apply the present invention in the same way.

It is also necessary to devise ways to reduce power spikes as much as possible at the tips of the short fuel rods themselves. For this reason, the content of fissile material in the fuel pellets loaded near the upper end of the active part of short fuel rods should be reduced as much as possible from below, and the tips should be loaded with natural uranium, reduced uranium, depleted uranium, etc. It is also effective.

Therefore, when the above modification is applied to the present invention, the influence of the local output area monitor can be further reduced.

〔Effect of the invention〕

According to the present invention, it is possible to make changes in the neutron flux distribution and the power distribution extremely small near the installation height of the local power area monitor. This eliminates variations and fluctuations in local power range monitor readings and provides greater ability to monitor thermal performance within the core. This makes it possible to provide a fuel assembly compatible with higher burnup while maintaining sufficient reliability.

[Brief explanation of the drawing]

Fig. 1 is a layout diagram showing the positional relationship between the effective length of the fuel rod and the local power range monitor in the first embodiment of the fuel assembly according to the present invention, and Fig. 2 also shows the effect of the first embodiment. FIG. 3 is a vertical cross-sectional view showing the interior of each fuel rod in comparison with the second embodiment of the present invention, FIG. 4 is a characteristic diagram showing the second embodiment, and FIG. (a) and FIG. 5(b) are cross-sectional views showing the fuel assembly of the present invention, FIGS. 6 to 14 are for explaining the conventional example, and FIG. 6 is a schematic diagram of the core. FIGS. 7 and 8 are a vertical sectional view and a vertical sectional view showing the fuel assembly, respectively.
Figures (a) and 9(b) are cross-sectional views showing the fuel assembly, Figure 10 is a vertical cross-sectional view showing the interior of each fuel rod, and Figure 11 is a local power range monitor. The arrangement diagram showing the axial direction, Figures 12 and 13 are power distribution diagrams in the axial direction when fuel assemblies are loaded into the core, respectively.
FIG. 14 is also a thermal neutron flux distribution diagram in the axial direction. DESCRIPTION OF SYMBOLS 1... Fuel assembly, 2... Control rod, 3... Local power range monitor, 4... Fuel rod, 4s... Short fuel rod, 5... Water rod, 6... Spacer,
7, 8... tie plate, 9 - channel box, 10... cladding tube, 11... fuel pellet, 12... upper end plug, 13... lower end plug,
14...Coil spring, 15...Cover tube, 1
6...Signal cable, 17A/17B/17C/17
D...Detector. Agent Patent attorney Akira Inomata Yoshiaki (and 1 other person) 0.5 1.0 1.5 Mel°Tojohaba 0.5 1.0 1.5 Sashibiri Doka Akuta/3 Vice

Claims (2)

[Claims] (1) In a fuel assembly in which a plurality of fuel rods with different axial effective lengths are arranged in a square lattice shape through spacers in a channel box, the upper ends of the axial effective lengths of the fuel rods are connected to the fuel assembly. 1. A fuel assembly characterized in that the fuel assembly is configured so as not to be close to a detection position of a local power range monitor disposed in a reactor core in which the fuel assembly is loaded. (2) In a fuel assembly in which a plurality of fuel rods with different effective lengths in the axial direction are arranged in a square grid in a channel box via spacers, a local power range monitor in the core loaded with the fuel assemblies is used. The upper end of the effective axial length of the fuel rods arranged in a 4x4 fuel rod lattice, in which the fuel rod position located at the corner on the side where the fuel rods are arranged is one corner of the square, is located in the core. A fuel assembly characterized in that the fuel assembly is configured so as not to be close to a detection position of a local output area monitor disposed within the fuel assembly.