JPS61275695A - Fuel assembly for boiling water reactor - 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 [Technical Field of the Invention] The present invention relates to a fuel assembly C for a boiling water reactor that improves fuel economy without impairing thermal margin.

[Technical background of the invention and seven problems] FIG. 2(a) is an example of a fuel rod arrangement diagram of a conventional boiling water reactor fuel assembly, and FIG. Diagram (a)
FIG. 3 is a diagram showing the axial enrichment distribution of each fuel rod in FIG.

In the figure, 1.2.3.4 shows fuel rods with different enrichment degrees of fissile material loaded therein, and A.

B, C, and D indicate the degree of concentration. The degree of concentration is A) B)
C) In order of D. G is a burnable poison (using gadolinia)
The enrichment of fissile material is the same as in fuel rod 2. The enrichment of fissile material in each of these fuel rods is uniform in the axial direction C. W indicates a water rod.

As shown in Figure 2 (a), in a conventional boiling water reactor fuel assembly, the enrichment is high in the center and low in the periphery in the cross section of the assembly. There is. If the average enrichment is kept constant (1) and the enrichment distribution is changed to make it lower in the center and higher in the periphery, the infinite multiplication factor increases and at the same time the local power peaking coefficient in the cross section increases. . The relationship between the local output peaking coefficient and the infinite multiplication factor is shown in FIG. 3C. Figure 3 shows the values at the burnup where the infinite multiplication factor reaches its peak. In FIG. 2C, point a shows the case of the conventional fuel assembly shown in FIGS. 2A and 2C, and point b shows the case where the enrichment of all fuel rods is the same.

As is clear from Fig. 3, in order to improve fuel economy (-) it is best to use fuel that is enriched in the peripheral areas as much as possible; An increase in the maximum linear power density as a thermal margin (- will impair the safety of the reactor).

Therefore, it has been difficult to satisfy both the improvement of fuel economy and reactor safety.

[Object of the Invention] The present invention was made in view of the above circumstances, and an object thereof is to provide a fuel assembly for a boiling water temperature nuclear reactor that is designed to improve fuel economy without impairing thermal margin. That is.

[Summary of the Invention] The present invention relates to a fuel assembly for a boiling water reactor, which is formed by arranging a plurality of fuel rods in a lattice shape in a channel box. The fissile material-containing lid of the fuel rod increases from the center to the periphery of the cross section of the aggregate in at least one of the upper and lower regions.
The fuel assemblies (with respect to It is.

[Embodiments of the invention] Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a fuel rod arrangement diagram of a fuel assembly showing one embodiment of the present invention, and FIG. 1(b) is the fissile material axial enrichment distribution of the fuel rods in FIG. In FIG.
indicates a fuel rod containing burnable poison, and W indicates a water rod. Fuel rods 1'-4' and gadolinia nG
The enrichment degree of fissile material loaded in ' is shown in Figure 1(b)c.
As shown, they differ in the axial direction. That is, each fuel rod is 1 m in the axial direction, three of them are in the lower region, and the parts of the entire length from the upper end are in the upper region.

The remaining central region is the central region, and the enrichment distribution in the central region is the same as the enrichment distribution of the fuel assembly shown in FIG. A-D respectively
, G' are B, but the enrichment distribution in the cross section in the upper and lower regions is all uniform and E.

The axial power distribution of the fuel assembly of the above embodiment is compared with that of a conventional fuel assembly, and the relationship between the improvement in fuel economy and thermal margin of the fuel assembly of the present invention will be explained below.

As mentioned above, when improving fuel economy, the problem is that the safety of the nuclear reactor is impaired due to the increase in maximum linear power density C-. This maximum linear power density becomes a problem when the infinite multiplication factor reaches its peak, which is when the fuel is in the first cycle at the end of the operating cycle. When the relative linear power density distribution in the axial direction of this l-th cycle fuel is shown for the conventional fuel C in FIG. M2 (a), it becomes a curve C in FIG. 4. Since there is no difference in the power distribution of the core between the conventional fuel and the fuel of this example, it can be considered that the linear power density at a certain point in the core is proportional to the local power peaking coefficient at that point. Therefore, curve C in FIG.
is multiplied by the ratio of the local power peaking coefficient of the uniform sting degree fuel to the local power peaking coefficient of the conventional fuel in the upper and lower regions (see wcS diagram 1. The horizontal axis in Figure 1 represents this ratio). The relative linear power density distribution in Example 12 is obtained. The curve d in Figure 4 looks like this
This is the distribution curve obtained by . In addition, in Fig. 5, the burnup differs depending on the axial position ζ2 in the first cycle fuel, and the local power peaking coefficient changes with the combustion C, so the ratio of the local power peaking coefficient changes depending on the axial position. ing.

As is clear from the curve d in FIG. 4, in this example, the linear power density at the upper and lower ends increases, but this does not cause the maximum linear power density to deteriorate.

In this embodiment 12, the boundaries of the upper and lower regions are defined as the minus and the bottom of the total length, respectively, as described above, because in this case, the maximum linear power density becomes worse than that of conventional fuel. Generally, the ratio of the local output peaking coefficient of the upper and lower regions I: the local output peaking coefficient of the applied foreshortening distribution (uniform enrichment distribution in the above example) to the local output peaking coefficient C of the central region (i.e., the conventional enrichment distribution) is set to 1. Then, if the upper region and the lower region are regions where the normal axial output is 1/j or less of the maximum allowable value, the maximum linear output density will not deteriorate in case C:. In order to increase the effective multiplication factor of the reactor core, we want to use fuel with a large local power peaking coefficient over a wide range (U), but we also want to use fuel with a large local power peaking coefficient over a wide range and without deteriorating the maximum linear power density. The condition holds if the lengths of the upper and lower regions are thus limited (-).

The increase in the effective multiplication factor of the reactor core in this embodiment can be calculated using Cl as follows. In other words, the effective multiplication factor K
The increase in off ΔKoff is approximately ΔKoff:
/ P (Z) Δ,, (z) d Z / /
It is expressed as P (Z) d Z (1).

Here, 2 represents the axial position, p (z) is the power at position [Z] of the conventional fuel, and ΔKao (Z) is the increase in the infinite multiplication factor at position 2. Integration is performed over C from the bottom end to the top end. The above formula is a good approximation when the special C △KC is small (Y is a good approximation).

Now, in the case of this example, when this is applied, the difference between the infinite multiplication factor in both the upper and lower regions with a uniform cross-sectional enrichment distribution and the infinite multiplication factor in the central region with the enrichment distribution as conventional fuel. is approximately 1.5%Δ (see Figure 3). Further, the conventional axial power distribution is represented by curve C in FIG. Therefore, when these are applied to the above equation (1), ΔKsff ”0.3%Δ is obtained, which corresponds to an approximately 2% improvement in fuel economy.

The present invention has been explained above based on the above embodiment (;; however, if the visibility of the upper and lower regions is determined based on the above idea (2), then in the case of other concentration distributions, the maximum linear power density can also be determined. Fuel economy can be improved without deterioration.Furthermore, if the maximum linear power density has a margin with respect to the limit value, it is possible to further expand the upper and lower regions by C.

Further, in the above embodiment, fuel having a large local output peaking coefficient is placed in both the upper and lower regions (-), but it may be applied only to either the upper region or the lower region (-).

In addition, in the above embodiment, the central region excluding the upper and lower regions is composed of one region, but in order to flatten the axial output distribution (the central region is further divided into 62 parts). Good too.

[Effects of the Invention] As stated above, the present invention makes the enrichment distribution of fissile material in the fuel assembly (-) different in the upper, middle, and lower regions, which may worsen the maximum linear power density. By adopting an enrichment distribution such that the cross-sectional local power peaking coefficient increases in both the upper and lower regions (U) with a lower value than in the conventional case, fuel economy can be improved without impairing the thermal margin.

[Brief explanation of the drawing]

Figure 1 (-) is a fuel expansion diagram of a fuel assembly showing an embodiment of the present invention, and Figure #I1 (b) is a diagram showing the fissile material axis of the fuel rod in Figure 1 (a). Diagram showing enrichment distribution, 2nd
Figure (-) is a fuel rod layout diagram of a conventional fuel assembly, and Figure 2 (
b) is a diagram showing the axial enrichment distribution of fissile material in the fuel rod in Figure 2(a), Figure 3 is a diagram showing the relationship between the local power peaking coefficient and the infinite multiplication factor, and Figure 4 is a diagram showing the relationship between the local power peaking coefficient and the infinite multiplication factor. Figure 5 shows the axial distribution of the relative linear power density of the fuel of the present invention.
! FIG. 2(-) is a diagram showing the axial distribution of the ratio of the conventional fuel to the local power peaking coefficient shown in FIG. 1', 2', 3', 4'...Fuel rod G'...
・Gadolinia rod W...Waterod agent Patent attorney Yoshiaki Inomata (one person unknown) No. 1
'm (α) 1234 (7W Fig. 1 (b) Fig. 2 ((Z) / l J 4 cr, ,
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t, 4 tA eyebrow/rl 2 nh'-'t soar ko (da. Figure 3 o, o a!s tD t,
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Claims (3)

[Claims] (1) In a fuel assembly for a boiling water reactor in which a plurality of fuel rods are arranged in a grid in a channel box, the fuel assembly is divided into three regions in the axial direction: an upper region, a central region, and a lower region. and the fissile material content of the fuel rods in at least one of the upper and lower regions increases or does not change from the center to the periphery of the aggregate cross-section, or A fuel assembly for a boiling water reactor, characterized in that the amount of water is also reduced at a small rate. (2) A patent claim in which the upper region and the lower region are set in regions where the axial output is less than or equal to the allowable maximum value X (local output peaking coefficient of the central region)/(local output peaking coefficient of the upper region and the lower region) A fuel assembly for a boiling water reactor according to item 1. (3) A claim in which the upper region is 6/24 of the total region, the lower region is 1/24 of the total region, and the content of fissile material in the upper region and the lower region does not change in the cross section thereof. The fuel assembly for a boiling water reactor according to item 2.