JPH022977A - Boiling water reactor fuel assembly - Google Patents

Abstract

PURPOSE:To contrive the adjustment of an axial enrichment distribution by making fissionable material concentration in which one is lower than the average concentration of a fuel assembly and the other is higher than that and simultaneously allowing them to differ from the average fissionable material concentration. CONSTITUTION:Natural uranium corresponding enrichment N is used on the upper part and the lower part of full length fuel rods 21, 22, 23, their enrichments H, M, L are low in order or the fuel rods 21, 22, 23 respectively and the enrichment M is the same degree as the average enrichment of a fuel assembly. The fuel rod 23 is arranged on four corners and the fuel rod 22 on eight peripheral positions adjacent to the fuel rod 23. The long partial length fuel rod 24 uses the same maximum enrichment H as the fuel rod 21 and the short partial length fuel rod 25 uses the same minimum enrichment as the fuel rod 23. These fuel rods 24, 25 do not use any blanket, their lower ends are cladding tubes alone and fissionable producing gas adsorption getter is inserted in place of fuel pellets. The combustible toxicant containing full length fuel rod 26 is the enrichment M, the concentration of the combustible toxicant is G and the enrichment N is used in its upper part and the lower part.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly for a boiling water reactor that is constructed by combining three types of fuel rods of different lengths.

(Prior art) Conventional 'aII! In fuel assemblies for water reactors, the length of the fuel rods is constant, so in order to set the required reactivity difference between the upper and lower sides in advance, the upper and lower enrichment distributions for each fuel rod are calculated.
A burnable poison distribution is established.

For example, a typical boiling water reactor fuel assembly (8 x 8
An example of a conventional design for fuel) is shown in Fig. 8. The same figure (a
) is an example of the arrangement of fuel rods 11 to 17 constituting one fuel assembly, and in FIG. 16
is e3+G2, also fire. For the feed rod 17, there is an e on the upper half of the rod.
3. e3+Q1 is used in the lower half. W is a water rod.

Therefore, in this design example, the degree of contraction a is constant for one fuel rod, and the difference in reactivity between the upper and lower sides is realized by the distribution of burnable poison. In other words, the number of fuel containing burnable poisons is one more at the bottom of the assembly, and the concentration of burnable poisons at the bottom is higher than at the top, corresponding to the difference in burning speed between the top and bottom. .

FIG. 9 shows another conventional example, in which FIG. 9(a) is a layout diagram of fuel rods 61 to 68 constituting a fuel assembly, and FIG. 9(b) is a layout diagram of fuel rods 61 to 68 of the fuel rods 61 to 66. 63.65.66
have enrichment degrees e1, e3, ea, and e, respectively.
A constant of 5 is used, with fuel rods 62 and 64 having higher enrichment in the upper half than in the lower half. Further, the number of fuel rods 67 and 68 containing burnable poison is the same on the upper and lower sides. Among these fuel rods, the fuel rod 67 containing burnable poison has a concentration of e3+Gz over its entire length, and the fuel rod 68 containing burnable poison has a concentration of G2 at the top.

Since it is 01 at the bottom, the bottom is higher than the top.

In all of the conventional examples described above, the difference in reactivity in the vertical direction due to the void distribution characteristic of boiling water reactors is offset by enrichment and burnable poison over a certain operating period, and the power distribution in the vertical direction is It is designed to be flat and suppress the maximum linear power density, a design commonly used in current boiling water reactors.

(Problem to be Solved by the Invention) By the way, the conventional design method described above is effective for suppressing thermal peaking in the vertical and axial directions of the fuel assembly, and contributes to improving the safety of the nuclear reactor. I've done it. However, when designing for high burnup or long-term operation according to the conventional design method shown in Figure 8 or Figure 9 based on the current 8X8 fuel, various methods as explained below are required. It has become clear that there are problems.

In other words, in high-burnup fuel, the concentration of fissile material is higher than in conventional fuel because the fuel burns in the core for a long period of time, but this increases the excess reactivity of the fuel, which reduces the necessary flammability. The amount of toxic substances will have to be increased.

However, since the relative output of burnable poison is only about 1/2 of the output of fuel rods without burnable poison at the initial stage of combustion, an increase in the number of burnable poison greatly increases the local power peaking of the lattice. let In order to suppress the void reactivity from increasing due to higher burnup, the 8×8 fuel has a reduced number of fuel rods, which tends to further increase local power peaking.

In addition, in order to suppress this local output peaking, it is necessary to design the enrichment level by dividing it more roughly than in the conventional example.

The present invention has been made in view of the above circumstances, and its purpose is to provide a fuel assembly for a boiling water nuclear reactor that can adjust the enrichment distribution in the axial direction by using fuel rods of different lengths. It's about doing.

[Structure of the invention] (Means for solving the problem) The present invention has been made to achieve the above object,
A full-length fuel rod whose length is about the same as the length of the fuel assembly, a water rod whose cross-sectional area is large at the top and small at the bottom, and a water rod whose cross-sectional area is about the same length as the part of the water rod whose cross-sectional area is small. a first part-length fuel rod having a length shorter than that, and a second part-length fuel rod that is longer than the first part-length fuel rod and shorter than the full-length fuel rod; In a fuel assembly for a boiling water reactor configured in the same manner, the fissile material concentration of each partial length fuel rod is set to a substantially constant value in the vertical direction, and the fissile material concentration of the first partial length fuel rod is set to a substantially constant value in the vertical direction. is a fissile material concentration that is the same as or lower than the average concentration of the fuel assembly, and the fissile material concentration of the second part-length fuel rod is the same as or lower than the average concentration of the fuel assembly. The concentration of fissile material in the first and second part-length fuel rods is not simultaneously equal to the average concentration of fissile material in the fuel assembly, and the concentration of fissile material in the first and second partial-length fuel rods is set to be high. It is characterized by containing a burnable poison in part.

(Function) In the present invention, the characteristics of the 9×9 high burnup assembly are as follows:
Since the number of rods is large at the bottom, the ratio of hydrogen atoms to uranium atoms (H
/U ratio) is flattened, and the burnup ratio I[ is flattened.

In other words, the vertical fissionable material concentration distribution is
From the point of view of flattening, approximately 1/3 of the central part should be made higher and the lower 1/3 part should be made higher.
3 is lower than that, and the remaining upper part is set lower than the central part from the perspective of improving reactor shutdown margin, or if the fissile material concentration distribution is kept constant, it is recommended to increase the number of burnable poisons in the lower part to avoid the thermal peak. However, in order to achieve such a distribution using a 9x9 high burnup assembly, the fissile material concentration of the first part-length fuel rod must be the average concentration of its cross section. or below, and by making the fissile material concentration in the second partial length fuel rod greater than or equal to the average concentration of its cross section, or by creating the required vertical reactivity distribution in the first
．． Creating an appropriate distribution of the number of burnable poison-containing fuels by setting the second fissionable material concentration at about the average enrichment level and including combustible waste in a portion of the full-length fuel rods and a portion of the partial-length fuel rods. An appropriate vertical reactivity distribution can be obtained.

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

FIG. 1(e) is a schematic side view of an embodiment of the present invention,
Figures (a), (b), and (C) are from the same figure (
Figure (d) shows the fissionability of the various fuel rods used in this example in the vertical direction. FIG. 2 is a diagram showing the substance concentration and the distribution of burnable poisons.

The fuel bundle 2 inside the fuel assembly 1 of this embodiment is the first
As shown in figure (e), fuel rods 3, 4.5 with different lengths
The water rods 6 (indicated by 5LCWR in the figure below), which are thick at the top and thin at the bottom, are assembled in a lattice shape with the center as the boundary, bundled with spacers 7, the upper part is fixed with the upper tie plate 8, and the lower part is Lower tie plate 9
It is fixed at The longer partial length fuel rods 4, which are about 273 of the full length fuel rods, are arranged in the second row from the outside. A short long-burn fuel rod 5, which is about 1/3 of the total length of the fuel rod, is arranged with the water rod at the center in a space created by a thinner water rod 6 located at the center of the fuel assembly.

Further, the fuel assembly is composed of fuel rods as shown in FIG. 2(d). That is, the full length fuel rod 21.22
．． 23 uses natural uranium equivalent enrichment N at the ratio of 2/24 in the upper part and 1/24 in the lower part, as shown in the cross-sectional view in ~(d), and except for this natural uranium N, the upper and lower parts are −
The enrichment of the full-length fuel rods 21, 22, and 23 decreases in the order of H, M, and L. The enrichment M is approximately the same as the average enrichment of the aggregate. Full length fuel rod 23
As shown in FIGS. 3A to 4C, the full-length fuel rods 22 are arranged at four corners, and the full-length fuel rods 22 are arranged at eight peripheral positions adjacent to the full-length fuel rods 23. There are 40 full-length fuel rods 21, 8 full-length fuel rods 22, and 4 full-length fuel rods 23.

Long part length fuel rods 24 use the same highest enrichment H as full length fuel rods 21 and short part length fuel rods 25 use the same lowest enrichment as full length fuel rods 23. None of these burning rods 24.25 use a blanket, and the 1/24 length of the lower end is only a fuel cladding tube.
A getter for adsorbing fission product gases is inserted in place of the fuel pellets. The long partial length fuel rod 24 is shown in FIG.
As shown in (C), eight pieces are arranged in symmetrical positions in the second row from the outside.

The full-length fuel rod 26 containing combustible waste has an enrichment level M and a concentration of combustible poison G, and 2/24, 2/24,
Natural uranium equivalent enrichment N is used in the lower part at a ratio of 1/24. The full-length fuel rod 26 containing burnable poison is shown in the same figure (a).
As shown in ~(C), they are arranged at symmetrical positions that are not adjacent to each other.

This example has an average extraction burnup of 38 GWd/l to 45 GWd/l and an operation period of 1 using a three-dimensional analysis program.
As a result of the study assuming a period of 5 to 18 months, it has been confirmed that it has the effects described in the law.

In other words, by realizing an appropriate vertical enrichment distribution, a flat vertical power distribution is achieved over most of the operating period as shown in Figure 4, and local power peaking is shown in Figure 7. As a result, as shown in FIG. 5, the maximum linear power density is sufficiently low compared to the conventional design. Further, as shown in FIG. 6, although the concentration of flammable green matter is made uniform in the upper and lower portions, appropriate combustion can be achieved and a sufficiently flat surplus reactivity can be achieved.

There are only three types of enrichment to obtain this unique characteristic, and by adopting a uniform concentration over the entire length, the number of types of fuel rods is also limited to partial length fuel rods, fuel rods containing burnable poison, etc. This can be achieved with only six types in total, and therefore only a small number of enrichments and fuel rods need to be prepared on the production line, allowing for efficient production and cost reduction.

The small number of enrichment types means that the difference between the maximum enrichment and the average enrichment can be reduced, and the extraction burnup is 45 GW.
d/l, even if the average concentration is 4.0W10, it is possible to design within the range of 5W10 or less, which is subject to the maximum concentration limit, and even in this design, the maximum concentration is 4.9
W10 or less.

FIG. 2 is a diagram showing the arrangement of fuel rods and the distribution of fissile material concentration and burnable poison in the vertical direction, according to a second embodiment of the present invention. Note that Figures 2 (a) to (C) are similar to Figure 1 (a) above.
) to (C) are layout diagrams of fuel rods corresponding to FIG.

In this embodiment, as shown in FIG. 2(d), natural uranium equivalent enrichment N is used at the ratio of 2/24 in the upper part of the full-length fuel rod and 1/24 in the lower part, and the full-length fuel rod is 31.32°. Except for this natural uranium N, 33 has a vertically-like enrichment, with H, M, and L having the lowest enrichment in this order, and the enrichment M is almost the same as the average enrichment of the aggregate. ing. Further, as shown in FIGS. 3A to 3C, the full-length fuel rods 33 are arranged at four corners, and the full-length fuel rods 32 are arranged at eight peripheral positions adjacent to the full-length fuel rods 33. long part length fuel rod 3
Part length fuel rods 35 as short as 4 use the same intermediate enrichment M as fuel rods 32.

The burnable poison is contained in the full length fuel rod 36 and a part of the short partial length fuel rod 37, and the burnable poison concentration in the fuel rod 37 is 1.0w101<
It has become. In the fuel assembly of this embodiment, there are 40 full-length fuel rods 31, 8 full-length twisted 11 rods 32, and 4 full-length fuel rods 33.
Book, fuel rod 36 with combustible filth is 12 trees.

Two part-length fuel rods 37 containing combustible filth are comprised.

This example is an example in which the enrichment is constant within the fuel rod like in the conventional example, and unlike the conventional example in which the concentration of burnable poison was varied in the fuel rod, it is possible to use a simpler structure. Therefore, excellent characteristics similar to those of the first embodiment can be obtained.

FIG. 3 is a diagram showing the arrangement of fuel rods and the distribution of fissile material concentration and burnable poison in the vertical direction, according to a third embodiment of the present invention. Note that Figures 3 (a) to (C) are similar to Figure 1 (a) above.
) to (C) are layout diagrams of fuel rods corresponding to FIG.

In this embodiment, as shown in Fig. 3(d), natural uranium equivalent enrichment N is used at the ratio of 2724 in the upper part of the full-length fuel rod and 1/24 in the lower part, and the full-length fuel rod is 41.42°43. Except for this natural uranium N, the enrichment levels are similar to those above and below, with the lowest enrichment levels being H, M, and L in this order, and the enrichment level M is the average of the aggregate! ! It is almost the same as the degree of contraction. Also,
As shown in (a) to (C) in the same figure, the total length fuel rods 43 are 4
Two corner, full-length fuel rods 42 are located at eight peripheral locations adjacent to full-length fuel rods 43.

The second part-length fuel rod 44 has an average enrichment of I! An intermediate concentration M corresponding to the degree of condensation is used. Further, the enrichment degree of the first partial length fuel rod 45 is set to L, which is the lowest condensation degree. The burnable poison is contained in the full length fuel rod 46 and the second part length fuel rod 47, and the burnable poison concentration in the part length fuel rod 47 is the same as that in the full length fuel rod 46.

The fuel assembly of this embodiment includes 40 full-length fuel rods 41, 8 full-length fuel rods 42, 4 partial-length fuel rods 43, 8 partial-length fuel rods 44, and 4 partial-length fuel rods 45. , 12 fuel rods 46 filled with burnable poison, 4 part-length fuel rods filled with burnable poison
7 consists of two pieces.

This example is an example in which the concentration distribution according to the first example is divided into two regions, and the burnable poison is roughly marked with "I", and the same characteristics as the above-mentioned first and second examples are obtained. It will be done.

In each of the above embodiments, a fuel assembly with 9 rows and 9 columns has been described, but the present invention can also be applied to a fuel assembly constructed by combining a plurality of fuel rods into one subbundle. Of course.

[Effects of the Invention] As explained above, according to the present invention, it is possible to create an enrichment distribution in the axial direction by using three or more types of fuel rods of different lengths. It is possible to provide a fuel assembly for a boiling water reactor that does not require fuel rods, has sufficiently low local power peaking, has a sufficiently flat excess reactivity, and can be manufactured efficiently.

[Brief explanation of the drawing]

Figures 1 (a), (b), and (C) are arrangement diagrams of fuel rods in the fuel assembly taken along lines H-A, H-8, and C-C in Figure (e), respectively; (d) is a diagram showing the fissile material concentration and burnable poison distribution in the vertical direction of various fuel rods used in this example, and (e) is a schematic side view of one example of the present invention. Figure 2 shows the arrangement of fuel rods according to the second embodiment of the present invention, and the distribution diagram of fissile material concentration and combustible waste in the vertical direction of various fuel rods, and Figure 3 shows the fuel rods according to the third embodiment of the present invention. Fig. 4 is a diagram showing the relationship between the operating period of the fuel assembly of the present invention and the output peaking in the vertical direction; Figure 6 shows the relationship between the operating period and maximum linear power density of the fuel assembly of the present invention, Figure 6 shows the relationship between the operating period and excess reactivity of the fuel assembly of the present invention, and Figure 7 is a diagram showing the local power peaking at the time when the maximum linear power density occurs in the fuel assembly of the present invention, FIGS. Diagram for explaining the hydrogen to uranium atomic ratio of the invention, 1st
FIG. 1 is a vertical burnup distribution diagram of the present invention. 35, 45...Short partial length fuel rods 36, 46...Full length fuel rods containing burnable poison 47...
Part-length fuel rods containing burnable poison 1...Entire fuel assembly 2...Fuel bundle 3...Full-length fuel rods 4...Long part-length fuel rods 5...Short part-length fuel rods 6...・Water rod 7...Spacer 8...Upper tie plate 9...Lower tie plate

Claims (2)

[Claims] (1) A full-length fuel rod whose length is approximately the same as the length of the fuel assembly, a water rod whose cross-sectional area is large at the top and small at the bottom, and the length of the portion of the water rod where the cross-sectional area is small. a first part-length fuel rod having a length of the same or less length, and a second part-length fuel rod that is longer than the first part-length fuel rod and shorter than the full-length fuel rod; In a fuel assembly for a boiling water reactor configured with the lower end positions aligned, the fissile material concentration of each partial length fuel rod is set to a substantially constant value in the vertical direction,
The first part-length fuel rod has a fissile material concentration that is equal to or lower than the average concentration of the fuel assembly, and the second part-length fuel rod has a fissile material concentration that is equal to or lower than the average concentration of the fuel assembly. The fissile material concentration is the same as or higher than the average concentration of the fuel assembly, and the fissile material concentration of the first and second part-length fuel rods is simultaneously the average fissile material concentration of the fuel assembly. 1. A fuel assembly for a boiling water reactor, characterized in that the fuel rods contain a burnable poison in a part of the full-length fuel rods. (2) At least some of the first part-length fuel rods and the second part-length fuel rods contain a burnable poison. Fuel assembly for boiling water reactors.