JPH0441796B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a core of a boiling water nuclear reactor.

[Background of the invention]

The core of a boiling water reactor, as shown in its horizontal cross section in FIG. It is made up of. 6 indicates the neutron detector instrumentation tube, and 7 indicates the gap water. FIGS. 2, 3, and 4 are respectively an external view, a vertical cross section, and a plan view in the X direction of FIG. 2 of a conventional fuel assembly. 3 and a fuel handle 8 housed inside the fuel handle 8. The fuel handle 8 includes an upper tie plate 9 and a lower tie plate 10 fitted into the upper and lower parts of the channel box 3, and a plurality of spacers 11 installed at intervals along the axial direction inside the channel box 3. It is composed of a plurality of fuel rods 12 that pass through this spacer 11 and have both ends fixed to upper and lower tie plates 9 and 10. The fuel rods 12 are aligned and supported by spacers 11 in a square grid.

Inside the channel box 3 of the fuel assembly 4, the coolant flows upward into the core, entering the fuel assembly in the form of subcooled water of around 10 Kcal/Kg from the lower inlet, and about 60% of the coolant at the upper exit. It has become a void rate. On the other hand, the fuel assembly 4 is surrounded by gap water 7, which is in a state of saturated water. In such a core structure, the coolant that enters the channel box 3 from the lower part of the core flows through the same fuel assembly until it exits from the core outlet, so that the coolant does not mix between adjacent fuel assemblies. There isn't. As a result, the coolant throughout the core is distributed such that the pressure drop across all fuel assemblies is equal. Generally, in a fuel assembly with a high output, the amount of voids generated is large and the two-phase flow pressure loss increases, resulting in a small coolant flow rate, and conversely, in a fuel assembly with a low output, the coolant flow rate becomes large.

On the other hand, in boiling water reactors, about 1 in 4 reactors is replaced with new fuel when the fuel is replaced. Therefore, fuels with different core residence periods coexist in the reactor core. In other words, there are fuels with different burn-ups, resulting in variations (mismatch) in the output of each fuel assembly. The output mismatch becomes larger as the enrichment of the fuel is increased in order to increase the extraction burnup. In conventional core configurations, there is a problem in that the thermal margin is reduced because it is difficult for coolant to flow into fuel assemblies with high output.

Furthermore, in the conventional core configuration, a two-phase flow portion (inside the channel box) and a saturated water portion (gap region) are unevenly distributed in the core. This not only increases the local power peaking of the fuel rod, but also deteriorates fuel economy because the gap water does not effectively contribute to the moderation of neutrons and absorbs neutrons in vain. However, the coolant flow path becomes narrower and the pressure loss of the coolant increases, which causes a deterioration in the stability of the furnace.

[Purpose of the invention]

An object of the present invention is to eliminate these problems and provide a core for a boiling water reactor that can increase thermal margin and improve fuel economy.

[Summary of the invention]

The present invention provides for a boiling water reactor core in which four fuel assemblies are arranged in each space partitioned by four adjacent cross-shaped control rods. A thin plate for guiding a control rod is attached only to at least the portion facing the control rod of the two surfaces facing the control rod on the outer periphery of each body, and the other outer periphery portions respectively constitute the fuel assembly. When the fuel rods of the fuel assembly are exposed and the surfaces of the fuel assembly that do not face the control rods are arranged to face each other,
The fuel assembly is characterized in that a coolant flow path is formed between the opposing surfaces of each of the four fuel assemblies.

In the present invention, in order to allow mixing of coolant between fuel assemblies in each core region surrounded by four control rods, the channel box on the side not facing the control rods is eliminated and a water gap is installed. By eliminating the area, using the space that used to be the water gap area as a coolant flow path, and widening the spacing between fuel rods, the fuel rods are arranged more uniformly in the reactor core.
This aims to improve the neutron utilization rate.

In such a core structure, when fuel assemblies with different outputs are placed next to each other, the coolant flows so that the pressure loss is equal in each height direction of the core. In other words, in the fuel assembly section with high output, the two-phase flow pressure loss due to void generation is large, so the mass flow rate of coolant decreases, and conversely, in the fuel assembly section with low output, the mass flow rate of coolant increases. . The void fraction also has a large distribution in the high-power fuel assembly portion and a small distribution in the low-power fuel assembly portion. In such a situation, a flow occurs in the lateral direction between the fuel assemblies due to the turbulent mixing phenomenon and the void drift phenomenon. Turbulent mixing phenomena generally flatten the void distribution. That is, it functions to flatten the void ratio in the high output fuel assembly region and the low output fuel assembly region. On the other hand, the void drift phenomenon is a phenomenon in which steam collects in areas where the coolant flow rate is high, that is, in areas where the mass flow rate is high, and as a reaction, water flows in the opposite direction.This also occurs due to the void distribution in this core structure. This contributes to flattening the area. Therefore, the void fraction in the upper part of the fuel assembly becomes smaller than in the conventional case where one fuel assembly is partitioned by channel boxes, and the coolant flow rate increases. This increases the thermal margin in the high power fuel assembly section. Furthermore, the high-power fuel assembly portion is a region where the infinite neutron multiplication factor is large, and the void fraction is small in that region, so the effective multiplication factor of the entire core is increased compared to the case of the conventional core structure.

A boiling water reactor has cross-shaped control rods arranged at a pitch of 304.8 mm, and four fuel assemblies are arranged in a square grid in the space surrounded by the four control rods. . In a typical reactor, there is 13.26 mm of gap water between each fuel assembly. Due to the gap water, the water-to-uranium ratio differs greatly between the periphery and center of the fuel assembly. Figure 5 shows the relationship between the infinite neutron multiplication factor and the water to uranium ratio. In this figure, the horizontal axis is H/U (relative ratio) (here H, U
indicate the number of atoms of hydrogen and uranium, respectively), and the vertical axis shows the neutron immovable multiplication rate (relative ratio).
When the water-to-uranium ratio is increased, the neutron flux reduction effect by water becomes effective, and the infinite neutron multiplication factor generally increases. However, when the water-to-uranium ratio exceeds a certain value, the increase in the infinite neutron multiplication factor due to the moderating effect and the decrease due to the neutron absorption effect of water are approximately equal, and the infinite neutron multiplication factor no longer increases. Fifth
Points A, B, and C shown in the figure indicate the neutron infinite multiplication factors of the aggregate average, the periphery of the aggregate, and the center of the aggregate, respectively. In conventional fuel assemblies, the number of fuel rods at the periphery of the assembly facing the gap water is approximately the same as the number of fuel rods at the center of the assembly.
The fuel assembly average infinite neutron multiplication factor shown at point A is the average value of points B and C.

It can be seen from FIG. 5 that in order to keep the local power peaking coefficient low and improve fuel economy, the water-to-uranium ratio in the center of the assembly can be increased. Therefore, we removed the channel box on the side where the control rods do not face each other, expanded the fuel spacing just enough to eliminate the water gap, and created a large fuel assembly in which the fuel rods of four adjacent fuel assemblies were arranged in a 16x16 square lattice. Think about the structures that make up the body. With such a structure, the water-to-uranium ratio of the unit cell increases, and as a result, the neutron infinite multiplication factors at the periphery and center of the aggregate change from point B to point B' and from point C to point C in Figure 5. ′. Since the center of the aggregate is an insufficiently decelerated region, the infinite neutron multiplication factor increases greatly as the water-to-uranium ratio increases, and becomes almost equal to the infinite neutron multiplication factor at the periphery of the aggregate. As a result, even though the proportion of fuel rods in the periphery of the assembly decreased from 1/2 to 1/4, the average neutron infinite multiplication factor of the fuel assembly shown at point A' was higher than the conventional point A. Become.

In addition, since channel box and gap water are reduced, wasteful absorption of thermal neutrons is eliminated.
Neutron utilization rate improves. Furthermore, by increasing the cross-sectional area of the coolant flow path, the spectrum shift width due to flow rate control increases, making it possible to save uranium.

Taking advantage of the increased water-to-uranium ratio in the unit cell, it is possible to increase the uranium loading. This makes it possible to extend the life of the fuel assembly with a constant burnup, thereby improving fuel economy.

On the other hand, the cross-sectional area of the coolant flow path increases and the pressure drop decreases, resulting in improved stability.

[Embodiments of the invention]

Examples will be described below.

FIG. 6 is a cross-sectional view showing the configuration of the core of one embodiment;
FIG. 7 is a perspective view showing the structure of a fuel assembly loaded in the core of this embodiment, and the same parts as in FIGS. 1 to 4 are given the same reference numerals. When the fuel assembly is placed in the reactor core, thin plates 13 made of Zircaloy-4 for guiding control rods are attached only on the two sides facing the control rods, and the other two sides are open laterally. It has a structure. This fuel assembly consists of 8x8 fuel rods or water rods. By arranging the fuel assemblies so that the four sides on which the thin plates 13 for control rod guides are not attached face each other, the reactor core is constructed as if 16×16 type fuel assemblies were loaded into the reactor core. Ru. That is,
The fuel rod spacing in a conventional 8x8 fuel assembly is 16.26
mm, and the spacing between fuel rods facing each other in adjacent fuel assemblies when loaded into the core is 38.6 mm, whereas in this example, the spacing between fuel rods was 16.26 mm.
By configuring the core with fuel assemblies expanded from mm to 17.65 mm, the distance between the centers of fuel rods facing each other in adjacent fuel assemblies is also 17.65 mm.
This is equivalent to a core loaded with 16 x 16 large fuel assemblies in a square lattice shape with a fuel rod spacing of 17.65 mm.

The ratio of the cross-sectional area of the coolant flow path and the cross-sectional area of the uranium dioxide material in the horizontal cross section of the unit fuel rod grid is 1.75 to 2.32.
, by a factor of approximately 1.32, resulting in an increase in the water-to-uranium ratio. By lowering the average energy of neutrons at the center of the assembly, the fuel assembly becomes more homogeneous, improving fuel economy. Figure 8 shows the relationship between the water-to-uranium cross-sectional area ratio and the core average neutron infinite multiplication factor. It has a certain multiplication factor. Since the fuel assembly has become nearly homogeneous, there is no need to provide an enrichment distribution except for the fuel rods at the corners of the fuel assembly facing the control rods. In addition, since the neutron moderating effect in the center of the aggregate has increased, water rods are no longer necessary, and the uranium loading amount can be increased by approximately 2%, resulting in an extension of the continuous operation period without increasing the burnup.

Also, by increasing the cross-sectional area of the coolant flow path by 32%, when the coolant flow rate is constant, the flow rate is 75% of the conventional one.
The pressure drop is reduced to 57% of the conventional value.

As a result, the uranium saving is reduced by 7%, and the value of the stability damping coefficient is reduced to approximately 1/2.

Furthermore, in the core structure of this embodiment, when replacing fuel, the fuel assemblies can be replaced one by one as in the past, so the internal Since there is no large change in the power distribution in the reactor, the problem with a core loaded with 16 x 16 large fuel assemblies can also be solved.

Such an effect is applicable not only to the structure of the 8×8 fuel assembly but also to fuel assemblies of 7×7, 9×9, etc.

In addition, in this invention, in order to allow mixing of coolant between fuel assemblies in each core region surrounded by four cross-shaped control rods, the channel box on the side where control rods are not inserted is eliminated. In addition to eliminating the water gap area, it became possible to use the space that was the water gap area as a coolant flow path, increasing thermal margin and improving fuel economy. When the surfaces are placed opposite each other,
The distance between the centers of the fuel rods constituting the facing surfaces of each of the four fuel assemblies (hereinafter referred to as the distance between adjacent fuel rods) is preferably 15 to 33 mm. In other words, there is a relationship between the distance between adjacent fuel rods and the flow path area as shown in FIG. 9 (the horizontal and vertical axes represent the distance between adjacent fuel rods (mm) and flow path area (relative ratio), respectively) be. D and E show the case of 8x8 lattice and 9x9 lattice, respectively, and F shows the case of conventional fuel assembly. Increasing the flow path area not only increases the thermal margin and reduces pressure loss, but also increases the degree of freedom in assembly design, such as the arrangement of multiple water rods, and improves fuel economy.

If the water gap area is reduced and the spacing between adjacent fuel rods is reduced, the flow path area increases, but if the water gap area is eliminated, the flow path area will not increase even if adjacent fuel rods are brought closer together.

Next, there is a first difference between the spacing between adjacent fuel rods and the reactivity.
Figure 0 (the horizontal and vertical axes show the distance between adjacent fuel rods (mm) and reactivity difference (%Δk), respectively, and the standard value of reactivity is the value of the current assembly). There is a relationship to show. G and H are each 8x8 grid,
The case of a 9×9 grid is shown. This figure shows, as an example, when the flow path area is increased to 1.1 times the current size.As the number of adjacent fuel rods decreases, it becomes possible to homogenize the water-to-fuel ratio, reduce neutron absorption by gap water, and increase the number of water rods. Reactivity increases. However, if the water gap region disappears, bringing adjacent fuel rods closer together will reduce the water-to-fuel ratio near the adjacent fuel rods, which will adversely affect fuel economy. Furthermore, the thermal margin near adjacent fuel rods is also reduced.

On the other hand, the distance between adjacent fuel rods that eliminates the water gap region is 1.76 mm for an 8×8 grid and 15.7 mm for a 9×9 grid, so the minimum distance is approximately 15 mm, which is the maximum distance between adjacent fuel rods. The desired purpose can be achieved by keeping the conventional minimum value of approximately 33 mm or less, so by setting the maximum value to approximately 30 mm, the distance between adjacent fuel rods that can achieve the desired purpose is approximately 15 ~ 30
mm.

As is clear from the above explanation, the water gap on the side where the control rods are not inserted is eliminated, the space is used as a coolant flow path, and the spacing between the fuel rods is widened to allow more fuel rods to be placed in the reactor core. By arranging them homogeneously, it is possible to improve the thermal margin and fuel economy by mixing the coolant between the fuel assemblies.

〔Effect of the invention〕

The present invention makes it possible to provide a boiling water reactor core that can increase thermal margin and improve fuel economy, and has great industrial effects.

[Brief explanation of drawings]

Figure 1 is a horizontal sectional view of the core of a conventional boiling water reactor, Figure 2 is a perspective view showing the external appearance of a fuel assembly of a conventional boiling water reactor, and Figure 3 is a longitudinal sectional view of the core of a conventional boiling water reactor. Figure 4 is a plan view in the X direction of Figure 2, Figure 5 is a diagram showing the relationship between the water-to-uranium ratio and the infinite neutron multiplication factor, and Figure 6 is an implementation of the core of the boiling water reactor of the present invention. A horizontal sectional view of the example, FIG. 7 is a perspective view showing the appearance of the fuel assembly, and FIG. 8 is a diagram showing the relationship between the water to uranium cross-sectional area ratio and the infinite neutron multiplication factor in the example of FIG. 6. 9 is a diagram showing the relationship between the distance between adjacent fuel rods and the flow path area, and FIG. 10 is a diagram showing the relationship between the distance between adjacent fuel rods and the reactivity. 1...Fuel rod, 5...Control rod, 6...Neutron detector instrumentation tube, 7...Gap water, 9...Upper tie plate,
10...Lower tie plate, 11...Spacer, 12
...fuel rod, 13...thin plate for control rod guide.

Claims (1)

[Scope of Claims] 1. In the core of a boiling water reactor in which four fuel assemblies are arranged in each space partitioned by four adjacent cross-shaped control rods, the four fuel assemblies A thin plate for guiding control rods is provided only on at least the portion facing the control rod of the two surfaces facing the control rod on the outer periphery of each fuel assembly, and the other outer periphery portions are provided with a thin plate for guiding the control rod. When the fuel rods constituting the four fuel assemblies are exposed, and the surfaces of the fuel assemblies that do not face the control rods are arranged to face each other, coolant flows between the opposing faces of each of the four fuel assemblies. A reactor core of a boiling water reactor characterized by being configured so as to form a channel. 2. The boiling water reactor core according to claim 1, wherein the fuel rods constituting the opposing surfaces serving as the coolant flow paths have a center-to-center spacing of about 15 to 30 mm. 3. The boiling water nuclear reactor core according to claim 2, wherein the center distance between the fuel rods is equal to the center distance between the fuel rods constituting the fuel assembly.