JPS6184591A - 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 [Field of Application of the Invention] The present invention relates to a fuel assembly, and particularly to a fuel assembly suitable for use in a boiling water nuclear reactor.

[Background of the invention]

Figure 3 shows the appearance of the fuel assembly. The fuel assembly is placed in a channel box 10. It consists of a lower tie plate 11, an upper tie plate 12, a spacer 15, and a fuel rod 16. The upper and lower ends of the fuel rods 16 are connected to the lower tie plate 11.
and is held by the upper tie plate 12. Spacer 1
5 are arranged in the morning direction of the fuel rod 16, and the fuel rod 1
6. The gap between them is maintained in an appropriate state. Channel box 10 is attached to upper tie plate 12 and surrounds a bundle of fuel rods 16 held by spacers 15 . A channel 7 assemble 13 is attached to the upper tie plate 12.

FIG. 4 shows the detailed structure of the fuel rod 16. The fuel rod 16 is sealed at both ends with a lower end plug 17 and an upper end plug 18*
A large number of fuel pellets 21 are loaded into a casing 20. A spring 22 is located within the gas plenum within the cladding tube 20 and urges the fuel pellets 21 .

FIG. 5 shows a cross section of the fuel assembly of FIG. 3.

Inside the channel box 10, fuel rods 16 are arranged in a grid pattern, and two water rods 14 are arranged in the center. Some of the fuel rods 16 contain Gado I, which is a burnable poison.
Contains J=A. A water gap is formed between adjacent fuel assemblies, and a control rod 19 is inserted into this water gap.

On the other hand, in a boiling water reactor, there is a void distribution in the vertical direction (or axial direction) of the reactor core, so the power distribution has a characteristic that the power distribution is distorted downward due to the axial difference in void reactivity. As a fuel assembly that corrects this downward peak power distribution in the axial direction and flattens the axial power distribution, there is a fuel assembly that has an enrichment distribution in upper and lower regions. This fuel assembly is
It is detailed in Japanese Patent Publication No. 58-29878. In such a fuel assembly with two upper and lower enrichment regions, the enrichment of several fuel rods arranged on the outer periphery is about 15 inches higher in the upper region than in the lower region.

The fuel assembly disclosed in the above publication can flatten the power distribution in the axial direction. However, in the fuel assembly of a boiling water reactor, voids occur inside the channel box 10 and no voids occur outside it, so the density distribution of water (moderator) in the cross section taken by the dashed-dotted line in Fig. 5 is It's not uniform. That is, the density distribution of water is low within the channel 10. Therefore, the thermal neutron flux φ along the dashed line in FIG. 5 has a distribution that is low at the center of the fuel assembly and high outside the channel box 10, as shown in FIG.

The output power P of each fuel rod in the fuel assembly is expressed by the following equation.

P=φ・σr・N ・・・・・・・・・(
1) Here, φ is the thermal neutron flux at the position of the fuel rod 16, σy is the fission cross section of the fissile material, and N is the atomic number density of the fissile material in the fuel rod 16.

In conventional fuel assemblies, the power distribution of each fuel rod (this is called the local power distribution) is flattened to minimize the local power peaking, which is the ratio of the maximum power of the fuel rod to the average power of the fuel assembly. In addition, the fN (proportional to the enrichment e) of the fissile material in the fuel rods arranged at the outer periphery where the thermal neutron flux φ is large is reduced by several atomic orders of magnitude, as shown in FIG. In the fuel assembly shown in the above publication, the enrichment of uranium-235 in the fuel rods adjacent to the channel box is 25 to 50 degrees lower than that in the central fuel rod.

When a new fuel assembly is loaded into the reactor core, the excess reactivity of the core increases, and it is difficult to suppress the excess reactivity using the control rods 19 alone. In order to suppress this excess reactivity in the initial stage of combustion, gadolinia is added to several fuel rods of the fuel assembly as described above. Since such combustible active materials have a very large neutron absorption cross section, their neutron absorption cross section rapidly decreases as the reactor operates. After a certain amount of operating time, the burnable poison completely disappears, and has the beneficial effect of not having any adverse effect on the reactivity in the later stages of combustion.

Figure 8 shows the effect of suppressing reactivity by using cadrinia.

The pointed edge indicates the infinite multiplication factor of the gadolinia-filled fuel assembly;
The broken edges indicate the infinite multiplication factor of the fuel assembly without gadolinia. The difference between the two represents the reactivity suppressing effect of gadolinia.

In addition, the infinite multiplication factor of the gadolinia-containing υ fuel assembly is the 8th
As shown by the solid line in the figure, it takes about 100 minutes for gadolinia to burn out.
Wd/l′! Then it increases monotonically, and after that it decreases monotonically. The first stage of combustion is the period up to the point when the infinite multiplication factor reaches its peak.
The period after that is called the late stage of combustion.

In recent years, increasing the burnup of a fuel assembly, that is, increasing the burnup, has been studied. In order to achieve this, fuel rods with a high content of fissile solids, that is, with a high degree of contraction, should be placed near the channel box where the thermal neutron flux is high. As a fuel assembly for achieving high burnup using the principle shown in Japanese Patent Publication No. 58-29878, the one shown in FIG. 4 of Japanese Patent Application Laid-Open No. 58-26292 is known.

[Purpose of the invention]

An object of the present invention is to provide a fuel assembly that can reduce the difference in cross-sectional power distribution at each position in the axial direction.

[Summary of the invention]

A first feature of the present invention is to reduce the average enrichment in most of the upper region of the first fuel rod having burnable poison in most of its axial region to the average enrichment in most of the lower region of the first fuel rod. The reason is that the am degree of the second fuel rod, which does not contain burnable poison, is uniformly distributed over most of its axial region.

The second feature is that the upper region of the first fuel rod (above-mentioned first fuel rod) is further divided into thinner areas according to the fuel rod containing burnable poison, and the burnable poison concentration in the fuel rod portion near the top end of the fuel rod is reduced. This is because it is lower than the average value in the upper region of the fuel rod.

The present invention is disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 58-29878, and was achieved through detailed study of the characteristics of conventional fuel assemblies. The results of the study are described below.

Fuel assembly 2 shown in Japanese Patent Application Laid-Open No. 58-29878
A cross section of No. 3 is shown in FIG. FIG. 10 shows the enrichment distribution and gadolinia concentration distribution of the fuel rods 1 to 6, G1 and G2, which constitute the fuel assembly 23. Fuel rods G1 and G2 are
Contains gadolinia uniformly in the axial direction. Fuel rod G1
and G2's Gado IJ near concentration are equal. Fuel rods 1-6
does not contain gadolinia. Fuel rods 1-6, G1
and G3 are made by pelletizing U02, which is a nuclear fuel material, and filling it in the tube. The magnitude of the enrichment e1 to e6 of the back/235 of each fuel rod is el)
The relationship is 6z > as ) G4 ) G5 > es. In the fuel assembly 23, many fuel rods having an average enrichment higher than the average enrichment of the fuel assembly are arranged in the outer circumference, and fuel rods having a lower average enrichment are arranged in the center.

In such a fuel assembly 23, the output of the fuel rods at the outer periphery increases throughout the operating period of the reactor, and the local output at the outer periphery increases. As shown in FIG. 11, the infinite multiplication factor of the fuel assembly 23 increases almost linearly in proportion to an increase in the local output at the outer circumference. Therefore, in order to maximize the increase in the infinite multiplication factor of the fuel assembly, it is sufficient to increase the local power peaking. Since the maximum value of local power peaking is determined by the thermal limitations of the fuel rods, the amount of increase in the infinite multiplication factor is regulated. Furthermore, in order to increase the infinite multiplication factor by increasing the local output of the fuel rods on the outer periphery, it is necessary to increase the average output of the fuel rods on the outer periphery.

Therefore, once the average enrichment of the fuel assembly 23 and the maximum value of local power peaking are determined, the local power distribution and enrichment distribution that will maximize the reactivity will be determined. FIG. 12 shows an example of the optimal local power distribution at the outer periphery, especially at the outermost periphery, when the local output peaking is set to 1.30. One square meter of the grid corresponds to one fuel rod.

The fuel assembly 23 uses fuel rods with high enrichment that increases local power peaking and fuel rods with two enrichment regions, upper and lower, that lowers axial power peaking, in the outer circumference.

Since the fuel assembly 23 can increase the local output at the outer circumference by the amount that the axial output peaking has decreased, the reactivity gain can be increased.

However, it has been found that this fuel assembly 23 has the following problems. That is, since the fuel rods 2 and 3 having a difference in enrichment between the upper and lower sides are arranged on the outer periphery of the fuel assembly 23 as shown in FIG. The local output distribution deviates from the optimal local output distribution shown in FIG. For this reason, it becomes impossible to realize a local power distribution that maximizes the reactivity in both the upper and lower regions of the fuel assembly 23. FIG. 13 shows the local power distribution in the upper region of the fuel assembly 23 when it is not combusted.

FIG. 14 shows the local power distribution in the lower region of the fuel assembly 23 when it is not combusted. These local power distributions cannot make the power of the fuel rods at the outer periphery, particularly at the outermost periphery, uniform (for example, 1.30) like the local power distribution shown in FIG. 12. Moreover, a difference occurs in the local power distribution at the outermost periphery of the upper region and the lower region of the fuel assembly 23.

As a result of studying the characteristics of such a conventional fuel assembly 23, the inventors believe that if fuel rods containing burnable poison in most of the axial region are given a difference in enrichment between the upper and lower parts, the above-mentioned fuel It has been found that the problems of aggregate 23 can be solved. That is, the output P of the fuel rod is proportional to the product of the enrichment e and the thermal neutron flux φ (pe·φ). Therefore, if the enrichment in the upper region of a fuel rod with a large thermal neutron flux φ or power P is made larger than that in the lower region, the change in the output P with respect to the change in the enrichment e becomes large. This is because the output difference in the axial direction of the fuel rods with different degrees of attenuation is not large.
This results in an increase in the power distribution difference between the upper and lower regions of the fuel rod. On the other hand, in most areas in the axial direction, the GAD IJ
In the fuel rod containing Ni, the thermal neutron flux φ is small in the presence of Gadolinia, so the change in the output P with respect to the change in the enrichment e is small. Therefore, if a fuel rod containing gadolinia in most of the axial region and a fuel rod containing gadolinia in most of the axial region are given a difference in upper and lower crimp degrees, This makes it possible to make the local power distribution at the top and bottom of the fuel rod almost the same.

On the other hand, the effective multiplication factor of the core at cold temperature (therefore the reactor shutdown margin)
Note that if the enrichment at the upper end of the fuel rod is low, the infinite multiplication factor at cold temperatures can be kept low, which has the advantage of increasing reactor shutdown margin. Here, it is known that the number of fuel rods containing gadolinia greatly contributes to the influence of gadolinia on the infinite multiplication factor at cold temperatures, and the influence of the gadolinia quality itself is small. Therefore, by lowering the enrichment level at the upper end of the fuel rod and, for fuel rods containing gadolinia, reducing the concentration of gadolinia at the upper end of the fuel rod, we were able to increase the reactor shutdown margin and promote the combustion of gadolinia. It was found that a reactivity gain was obtained during power operation, and economical efficiency was improved.

The present invention has been made based on the results of such studies. Examples of the present invention will be described below.

[Embodiments of the invention]

A preferred embodiment of the invention is shown in FIGS. 1 and 2. The fuel assembly 30 has the same configuration as shown in FIG. Why is it different?The fuel rods 31, 32, and 16 have the enrichment distribution and Gad IJ near concentration distribution shown in FIG.
33, 34. There are six fuel rods, G4 and G, and their fuel rods are arranged as shown in FIG.

The fuel rods 31 to 34 use UOz as a nuclear fuel material, and the fissile material contained therein is uranium-23.
It is 5. These fuel rods do not contain gadolinia, a burnable poison. Fuel rods G3 and G4 contain gadolinia at the same time as UO2.

The magnitudes of the enrichments e1 to e4 shown in FIG. 2 are el>G2
>63) There are 64 relationships. Fuel rods 31-34 and G4
The degree of a contraction is uniformly distributed over the entire length in the axial direction. The fuel rod G4 uniformly contains gadolinia in the upper region above the position 11/24 of the effective fuel length from the lower end of the effective fuel length, and gadolinia in the lower region below that position. Not included. The effective fuel length is the height within the fuel rod that is filled with nuclear fuel material. The fuel rod G3 has three regions, upper and lower in the axial direction, and the division points are at a position of 11/24 of the total length from the lower fuel end and at a position of 1/6 of the total length from the upper fuel end. The combination of (uranium enrichment and gadolinia concentration) for fuel rod G3 is (e4+C)dt)-(ez+Gdt)→(e4+Gd) from the bottom of the fuel rod to the top.
z) (however, G2>G4, Gdt) is Gdz).

Since the fuel assembly 30 has fuel rods G3 and G4, it is divided into upper and lower binary regions at a position of 11/24 from the lower end of the effective fuel length in the axial direction, and the upper region is divided into upper and lower binary regions in a plane perpendicular to the axis. The average enrichment in the plane perpendicular to the axis of the lower region −
It is higher than the degree of contraction. Moreover, the fuel assembly 30 is
The amount of gadolinia mixed in the upper region is greater than the amount mixed in in the lower region. Looking at the infinite multiplication factor of the fuel fruit assembly 30, the infinite multiplication factor in the upper region is larger than that in the lower region by υ. Also, when viewed from a plane perpendicular to the axial axis of the fuel assembly 30, the outer periphery of the plane (in this embodiment, an annular shape in which two layers of fuel rods exist outward from the channel box 10 side) , i.e., the area outside the dashed line), the average degree of contraction in the central part of the plane (in this example, three and four layers of fuel rods exist from the channel box 10 side inward) (in other words, the area inside the dashed-dotted line).

In this example, since fuel rod G3, which contains gadolinia in most of the axial region, has a difference in enrichment between the upper and lower parts, the power distribution between the upper and lower parts of fuel rod G3 is almost the same as described above. Become the same. Therefore, the difference in local output between the outer periphery of the upper region and the outer periphery of the lower region of the fuel assembly 30 is small, and their values are almost the same. In particular, since the fuel rods G3 are disposed in the outer circumferential portion excluding the outermost circumferential portion, the effect is greatest. This increases reactivity and improves fuel economy. Further, in this embodiment, the number of types of fuel rods is two fewer than that of the conventional fuel assembly 23, and the manufacturing of the fuel assembly is significantly simplified.

Since the average daughter m degrees in the outer circumference is larger than the average enrichment in the center, it is possible to obtain the same effect as the fuel assembly shown in FIG. 4 of JP-A-58-26292. That is, the combustion period of the fuel assembly 30 is long. Further, in this example, since the average enrichment in the upper region is higher than the average concentration in the lower region, the fuel assembly is assembled as in the fuel assembly shown in FIG. 4 of Japanese Patent Publication No. 58-29878. It is possible to flatten the output distribution in the axial direction. This effect is due to the fact that the fuel assembly 30
It can be used after the Gadolinia within disappears. Nuclear reactors do not require shallowly inserted control rods, and output can be controlled using only deeply inserted control rods.

Control rod operation of the nuclear reactor is thus significantly simplified. The location of the boundary between the upper area and the lower area is determined by
As shown in Japanese Patent Application No. 878, it is desirable to range from 1/3 to 7/12 of the effective fuel length from the lower end of the effective fuel length. Furthermore, in this example, as described above, the enrichment degree in the upper region is higher than that in the lower region, and at the same time, the amount of gadolinia mixed in the former region is larger than that in the latter region.

For this reason, the spectral shift effect shown in page 8, line 5 to page 9, line 2 of the specification of Japanese Patent Application No. 57-194300 and FIGS. 3 and 5 occurs.

Due to this spectral shift effect, patent application No. 57-1
As shown in No. 94300, the extraction burnup can be increased and the combustion period of the fuel assembly can be extended accordingly.

Furthermore, in this embodiment, the fuel rod G3 has three regions in the axial direction, and the uranium enrichment and the Gad IJ concentration are lowered particularly at the upper end of the fuel rod, so that the reactor shutdown margin and the extraction burnup are increased.

Another embodiment of the present invention will be described using FIG. 15 and FIG. 16. In the fuel assembly 40 of this embodiment, fuel rods 41 to 44, G5 and G6 shown in FIG. 16 are arranged as shown in FIG. 15. The fuel rods 41 to 44, Gl, and G6 are obtained by disposing natural uranium C5 at the ends of the fuel rods 31 to 34, G3, and G4 shown in FIG. 2, as shown in FIG. 16. Since the output of the fuel end is small, even if refrigerated uranium is used, it will not burn much, so the uranium will not be used effectively. For this reason, natural uranium, which does not require enrichment, is placed at the end to make effective use of uranium.

Fuel rods G5 and G6 do not have natural uranium e5 placed at their upper ends. Enrichment e4 contains more uranium-235 than natural uranium e5. This contains gadolinia, and a large amount of gas is generated within the fuel rods during reactor operation.
This is to increase the volume of the gas plenum. The effective fuel length of the fuel assembly 40 is the same as the effective fuel length of the fuel rods 41 to 44. The length of the region filled with natural uranium is 1/24 of the effective length of the fuel.

In this example, the ends are filled with natural uranium.
It has the same configuration as the fuel assembly 30. Fuel rods 41-44.
The @condensation degrees and gadolinia concentrations of G and G6 are shown in Table 1.

17 shows the local power distribution at the outermost periphery of the upper region of the fuel assembly 40 when it is not combusted. FIG. 18 shows the local power distribution at the outermost periphery of the lower region of the fuel assembly 40 when it is not combusted. As is clear from these figures, the local outputs at the outermost periphery of the upper region and the outermost periphery of the lower region of the fuel assembly 40 are almost the same. This embodiment can obtain the same effect as the fuel assembly 30.

In the above example, the uranium enrichment and gadolinia concentration were reduced only at the upper end of the fuel rod, which contains gadolinia in most of the axial region, but if it is desired to further increase the reactor shutdown margin, The uranium concentration or gadolinia concentration of other fuel rods may also be reduced.

On the other hand, if there is sufficient margin for reactor shutdown, high burnup can be achieved by reducing only the gadolinia enrichment without changing the uranium enrichment.

〔Effect of the invention〕

According to the present invention, it is possible to significantly reduce the difference in power distribution in the cross section at each position in the axial direction of the fuel assembly, and improve fuel efficiency. Therefore, the burnup of the fuel assembly can be increased.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a fuel assembly that is a preferred embodiment of the present invention, FIG. 2 is an explanatory diagram of the enrichment distribution of each fuel rod used in the fuel assembly of FIG. 1, and FIG. Fig. 4 is a structural diagram of a fuel assembly, Fig. 4 is a structural diagram of a fuel rod, Fig. 5 is an explanatory diagram of the V-V section in Fig. 3, and Fig. 6 is a thermal neutron flux distribution in the A IA 2 cross section of Fig. 5. The characteristic diagram shown in Figure 7 is At-A in Figure 5.
A characteristic diagram showing the enrichment distribution in two cross sections, Figure 8 is a characteristic diagram showing the relationship between burnup and infinite multiplication factor, Figure 9 is an explanatory diagram of a conventional fuel assembly, and Figure 10 is a diagram showing the relationship between burnup and infinite multiplication factor. An explanatory diagram of the enrichment distribution of each fuel rod used in a fuel assembly. Figure 11 is a characteristic diagram showing the relationship between the increase in the average local power at the outer circumference and the increase in the infinite multiplication factor. Figure 12 is the diagram of Figure 9. An explanatory diagram showing the optimal local power distribution in the cross section of the fuel assembly, FIG. 13 is an explanatory diagram showing the actual local power distribution in the cross section in the upper region of the fuel assembly in FIG. 9, and FIG. FIG. 9 is an explanatory diagram showing the actual local power distribution in the cross section in the lower region of the fuel assembly, FIG. 15 is an explanatory diagram of the fuel assembly which is an embodiment of the pond of the present invention, and FIG. Explanatory diagram of the concentration and accuracy distribution of each fuel rod used in the fuel assembly in Figure 15, No. 17
The figure is an explanatory diagram showing the actual local power distribution in the cross section in the upper region of the fuel assembly in Figure 15, and Figure 18 is an explanatory diagram showing the actual local power distribution in the cross section in the upper region of the fuel assembly in Figure 15.
FIG. 3 is an explanatory diagram showing an actual local power distribution in a cross section in a lower region of the fuel assembly shown in the figure. DESCRIPTION OF SYMBOLS 10... Channel box, 11... Lower tie plate, 12... Upper tie plate, 15... Spacer, 21... Fuel pellet, 30, 40... Fuel assembly.

Claims (1)

[Claims] 1. In a fuel assembly having a plurality of fuel rods containing fuel pellets, most of the upper region of the first fuel rod has burnable poison in most of the region in the axial direction. the average uranium enrichment in the lower region of the first fuel rod is higher than the average uranium enrichment in most of the lower region of the first fuel rod; the average enrichment of uranium in a region of 1/24 to 1/4 of the total length of at least one of the first fuel rods or the second fuel rod from the upper end of the first fuel rod or the second fuel rod; A fuel assembly characterized in that the average burnable poison concentration is lower than the average uranium enrichment or the average burnable poison temperature in the upper region. 2. The fuel assembly according to claim 1, wherein the second fuel rod is arranged at a portion other than the outermost periphery. 3. The boundary between the upper region and the lower region of the first fuel rod is set at 1/1/2 of the total length from the lower end of the first fuel rod.
3. The fuel assembly according to claim 1 or 2, wherein the fuel assembly is located in a range of 3 to 7/12. 4. The enrichment of the second fuel rod is made uniform over most of the region in the axial direction, and the average enrichment at the outer circumference in a plane perpendicular to the axial axis is equal to the average enrichment at the center in the plane. 2. The fuel assembly according to claim 1, wherein the fuel assembly is larger than 100%. 5. The fuel assembly according to claim 4, wherein the second fuel rod is disposed in a portion of the outer circumference excluding the outermost circumference. 6. The boundary between the upper region and the lower region of the first fuel rod is set at 1/1/2 of the total length from the lower end of the first fuel rod.
The fuel assembly according to claim 4 or 5, characterized in that the fuel assembly is located in a range of 3 to 7/12.