JPH01253693A - Fuel assembly - Google Patents

Abstract

PURPOSE:To increase a control rod value at a cold temperature, by decreasing a fissile material enrichment of fuel rods mainly facing water gaps at the side where control rods are not inserted, rather than a fissile material enrichment of the other region in a reactor core. CONSTITUTION:An average enrichment of the first layer fuel rods 6, 7, 8, 9, 10, 11, 12, 13 and 14 which face water gaps of the subject fuel assembly, is less than an average enrichment of whole fuel rods, however, in the first layer fuel rods, an average enrichment of fuel rods 6, 7, 8 and 9 which are locate at the side where fuel rods are inserted, is set to be greater than an average enrichment of fuel rods 10, 11, 12 and 14 which are located at the side where fuel rods are not inserted. When a case in which a enrichment distribution in a fuel assembly is set, is considered to be a reference, control rod values at a cold temperature in the cases 1 and 2, are intended to be increased than the reference one.

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) In recent years, in order to reduce the fuel cycle cost 1-, there has been a demand for increasing the burnup at the time of fuel assembly removal. In order to increase the fuel ML at the time of fuel assembly removal, it is necessary to increase the average enrichment of the fuel assembly, but as the enrichment increases, the temperature-temperature reactivity difference during output operation increases due to hardening of the neutron spectrum. As the value of the control rods increases, the value of the control rods also decreases, making it difficult to secure reactor shutdown margin that satisfies the refurbishment standards.

FIG. 3(a) shows a typical example of a conventional fuel assembly, and the numbers δ·1 to 9 in the figure represent uranium ronds with different enrichment degrees. The enrichment of each fuel rod is shown in FIG. 3(b). Further, the symbol W represents a water rod. In the fuel assembly that is actually loaded, gadolinia is added to several fuel rods as burner pull poison, but since burner pull poison is not directly related to the present invention, burner pull poison will be omitted in the following explanation. do.

In the above fuel assembly, considering that the fuel rods facing the water gap produce more output than the fuel rods not facing the water gap, the average enrichment of the fuel rods adjacent to the water gear knob is 2.
．． This is set to be 6% by weight, which is slightly smaller than the average enrichment of all fuel rods, which is 3.0% by weight.

(Problems to be Solved by the Invention) 1. In conventional fuel assemblies, in order to even out the power distribution within the fuel assembly, a water rod is placed in the center of the fuel assembly, and a water rod is placed adjacent to the water gap. The enrichment of the first layer fuel rods is slightly lower.

During reactor operation, the critical eigenvalue is maintained at 1.0, so from the perspective of increasing the degree of subcriticality at reactor shutdown and making it easier to secure reactor shutdown margin, the temperature response during power operation is It is effective to reduce the temperature difference (-infinite multiplication factor at cold temperature - infinite multiplication factor at output operation), and for this purpose it is effective to reduce the enrichment of the first layer fuel rods. An invention utilizing this point has been proposed, for example, in Japanese Patent Publication No. 62-19946.

However, since reactor shutdown is performed by inserting control rods, it is important to increase the value of control rods at cold temperatures in order to increase the subcriticality of reactor shutdown.
-'' Publication No. 2 has a flaw in that this point is not taken into consideration.

The present invention was made in consideration of such a situation, and is intended to reduce the difference in reactivity at temperature during output operation by using the same average enrichment as the conventional fuel assembly (not to reduce the difference in reactivity at temperature at cold temperature) Its purpose is to provide fuel assemblies of increasing value.

[Structure of the invention]

(Means and Effects for Solving the Problems) In order to achieve the above object 1, the present invention provides a fuel assembly in which a large number of fuel rods are regularly arranged. The feature is that the number density of fissile material centered on the fuel rod facing the gigang is smaller than the number density of fissile material in the fuel rods in other areas.

Therefore, in the fuel assembly of the present invention, the number density of fissile material centered on the fuel rods facing the water gear on the side where no control rods are inserted is made smaller than the number density of fissile material in the fuel rods in other areas. , the relative output of the fuel rod facing the control rod increases, increasing the value of the control rod, and the average relative output of the fuel rods adjacent to the water gap decreases, reducing the temperature-dependent reactivity difference during output operation. This increases the reactor shutdown margin.

(Example) Embodiments of the present invention will be described with reference to the drawings.

FIG. 1(a) is a plan view of one embodiment of the present invention.

Numbers 1 to 14 in the figure represent uranium lots with different enrichments, and the enrichment of each fuel rod is shown in FIG. 1(b). As can be seen from the enrichment of each fuel rod in FIG. 1(b), the first layer fuel rods (6, 6,
7, 8, 9, 10,], 1.12.13゜14) is smaller than the average enrichment of all fuel rods, but among the first layer fuel rods, the fuel on the side where the control rod is inserted Stick (6, 7, 8
, 9, ) is set to be larger than the average enrichment of the fuel rods (10, 11, 1, 2° 14) on the side where no control rod is inserted.

Next, in order to show the magnitude of the effect of this example, Figure 3 (
Using the case where the enrichment distribution within the fuel assembly is set as shown in (b) of the same figure in a) as a reference, the control rod is inserted among the first layer fuel rods while the average enrichment of the fuel assembly remains constant. The enrichment in the fuel rod on the side where the control rod is inserted is the same or lower than the reference, and the enrichment in the fuel rod on the side where the control rod is not inserted is smaller than the reference. The temperature distribution was set, and the control rod value at cold temperature and the difference in reactivity during power operation and cooling kit were compared. Also, for reference, we have also included the fourth version of
In Figure (a), a comparison was also made with the case where the enrichment distribution within the fuel assembly (cases 1 to 4) of Figure (b) was set.

(In the same cases shown in FIG. 1(b) and FIG. 4(b), the average enrichment of the fuel rods in the first layer and the fuel enrichment distribution in layers other than the first layer are the same).

Figure 5 shows a comparison of cold control rod values. From this, in the embodiment of the present invention, as shown by the solid line, in case 2, the value of the control rod in cold and hot conditions is increased compared to the reference. Cases 3 and 4 are slightly reduced compared to the reference, but this is due to the case number and the enrichment of the fuel rods facing the control rods, which is considerably smaller than the reference (60% in case 3 and about 40% in case 4). ) is set. On the other hand,
In No. 946, the case number decreases monotonically as the reference case number increases, as shown by the dotted line.

FIG. 6 shows a comparison of the reactivity difference at cold temperature during power operation. Both this example and the conventional example (Japanese Patent Publication No. 62-49946) are smaller than the reference, indicating that both examples are effective from the viewpoint of increasing reactor shutdown margin. However, the solid line is systematically smaller than the dotted line, and the present example is better than the conventional example (Special Publications Publication No. 1983-1986
No. 49946).

FIG. 7 is a comparison of the reactivity difference during power operation and cold shutdown.

It is thought that the increase in this amount from the reference leads to an increase in reactor shutdown margin.

Therefore, the present invention, the conventional example (Japanese Patent Publication No. 62-4994G)
It can be seen that both methods capture an effective method for increasing reactor shutdown margin, but the present invention is better than the conventional example
2-49946)).

Coupling of fuel assemblies is performed through a water gap that exists between fuel assemblies, and the method shown in the present invention reduces the relative power around the water gap without significantly reducing the relative power around the control rods. By reducing this, the degree of separation between fuel assemblies at cold temperatures is increased, thereby increasing the margin for reactor shutdown. An invention based on the same idea can be found, for example, in Japanese Patent Application No. 72990/1983. Special request 1986
In No. 72990, a small portion without fuel is provided in the axial direction of the fuel assembly to reduce the cup link between the fuels above and below the gap where there is no fuel at cold temperatures, but the present invention It can be said that the same idea is applied to the horizontal direction in between.

Figure 8 shows the core average axial distribution during cold shutdown, and the power distribution is extremely biased toward the second part of the core. Therefore, if the present invention is implemented at a portion including at least the core height at which the core average axial power peaking is considered to occur at the point when the reactor shutdown margin becomes the most severe during reactor operation, for example, at A or B in FIG. A sufficient effect can be obtained.

Note that the same effect as in the case of uranium rond can be obtained even if it is carried out for the enrichment levels of Plu 1 to Ni 1.239.241.

FIG. 2 is a plan view of another embodiment of the present invention, in which the hatched fuel rods indicate the stack density (
An example of a fuel rod (hollow fuel rod) with a small stack density, ty) is shown.

In order to explain the effect of this embodiment, the case where the enrichment distribution within the fuel assembly of FIG. 3(a) is set as shown in FIG. 3(b) is used as a reference, and the shaded fuel rods in FIG. ,
That is, a hole is made in the fuel rod centering on the fuel facing the water gap on the side where the control rod is not inserted, making it hollow, and AR20,
Alternatively, they are filled with chemically stable materials such as ZrO2 that have a small neutron reaction cross section, making the number density of fissile material smaller than in fuel rods in other areas. FIGS. 9 to 11 show changes in the control rod value at cold temperature, the difference in reactivity at temperature during power operation, and the difference in reactivity at cold stop during power operation when the degree of hollowness is used as a parameter.

This embodiment can also produce sufficient effects if it is implemented in a range that includes the core height at which the core average axial power peaking is considered to occur at the point in time during reactor operation when the reactor shutdown margin becomes the most severe.

From these figures, it can be seen that the present example also provides the same effects as those of the previous example.

Furthermore, in the fuel assembly, the enrichment of the fuel rods is made smaller than the enrichment of the fuel rods in other areas, at least in most of the fuel rods that face the water gap without facing the control rods, or Drill a hole in the frame to make it hollow, and fill it with a chemically stable substance with a small neutron reaction cross section, such as 80.203 or ZrO2, to make the number density of fissile material lower than that of the fuel rods in other areas. Even if it is made smaller, the same effects as those of the above embodiments can be obtained.

〔Effect of the invention〕

- As explained above, the present invention can provide a fuel assembly with a large reactor shutdown margin when compared with conventional fuel assemblies at the same average enrichment, and by satisfying the design criteria, it is possible to provide a fuel assembly with a high enrichment level. It becomes possible to create a fuel assembly, and it is possible to reduce the fuel cycle cost I-.

[Brief explanation of the drawing]

Figure 1 (a) is a plan view of one embodiment of the present invention;
b) is a diagram showing the enrichment of each fuel rod in Fig. 2(a), Fig. 2 is a plan view of another embodiment of the present invention, Fig. 3(a) is a plan view of a conventional fuel assembly, Figure 3(b) is a diagram showing the enrichment of each fuel rod in Figure 4(a), Figure 4(a) is a plan view of another conventional fuel assembly, and Figure 4(b) is the same figure. The figure showing the enrichment of each fuel rod in (a), and Figures 5 to 7 are the control rod value at cold temperature, the reactivity difference at temperature during power operation, and the cold shutdown during power operation, respectively, of the present invention and the conventional example. A graph comparing the time reactivity difference,
Figure 8 is a core average axial distribution diagram during cold shutdown, Figures 9~
FIG. 11 is a graph showing the cold control rod value, the reactivity difference during power operation at temperature, and the reactivity difference during power operation and cold shutdown of the present invention shown in FIG. 2, respectively. 1~14・Fuel rod number W...Wataro y1~L
・Hollow fuel rod (8733) Agent Patent attorney Inomata Yoshiaki (boka) - first name) (a) Full text γ and 1 finger ≧ Throat cost L: 3.0
(t-t°/, )) ＾0＠Mega 11 now 4 ya A Ranji grang sword 3 = 2.6 (Mlllon) (b
) Figure 3 (a) (b) Figure 4

Claims (1)

[Claims] (1) In a fuel assembly formed by regularly arranging a large number of fuel rods, the number density of fissile material in other regions is determined by the number density of fissile material centered on the fuel rod facing the water gap on the side where the control rods are not inserted. A fuel assembly characterized by having a number density smaller than that of the fissile material of the rod.