JPH01269085A - Fuel aggregate for boiling water reactor - Google Patents

Abstract

PURPOSE:To sharply reduce the fuel cycle cost for the title reactor by making the thickness of the parts of a channel box other than the corner sections thinner and respectively providing recessed sections on the inner surface of the channel box on the top side and on the outer surface on the bottom side. CONSTITUTION:The thickness of the parts of a channel box except its corner sections 6 are reduced in such a way that the inside surface is machined so that the outside surface is flushed with the surface of the corner sections 6 in the upper section 7 and the outside surface is machined so that the inside surface is flushed with the corners 6 in the lower section 8. Accordingly, the allowance degree in furnace core stability and channel stability can be increased and a fuel rod can be increased in thickness. Therefore, the quantity of uranium charged to a fuel aggregate can be increased and, as a result, the fuel cycle cost can be reduced sharply and the economic efficiency of the fuel can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a fuel assembly for a boiling water reactor that has a water region on its outer periphery, a channel box as its boundary, and a plurality of fuel rods inside. In particular, the present invention relates to a fuel assembly for a boiling water reactor that has improved fuel economy and stability.

[Conventional technology]

FIG. 2 is a perspective view of a fuel assembly of a conventional boiling water reactor, and FIG. 3 is a sectional view of the upper part of the fuel assembly. A conventional fuel assembly 1 includes fuel rods 2 arranged in a lattice pattern. water rod,
Consisting of channel boxes 3, etc., there is a water area (hereinafter referred to as "water area") in the space surrounding the aggregate.

A bypass area (referred to as a bypass area) is provided. Further, inside the fuel rod 2, as shown in FIG. 3, fuel pellets 4 are arranged in a stacked manner in the axial direction. Among the fuel pellets 4, natural uranium fuel is used for the fuel pellets 4 from the top stage to the second stage. The coolant flows between the fuel rods in the channel box 3 and the fuel pellets 4 within the fuel rods.
transports the heat energy generated by

Incidentally, in recent years, from the viewpoint of high economy, there has been an idea of a 9x9 fuel assembly (see Fig. 4) in which the outer diameter of the fuel rods is reduced and the number of fuel rods is increased. The 9x9 lattice type increases the heat transfer area in contact with the coolant and reduces the thermal load per fuel rod, increasing the degree of freedom in operation. By utilizing this degree of freedom in driving, fuel can be burnt more effectively, increasing fuel economy. However, on the other hand, if the 9×9 lattice type is used, the length of the wetted edge in contact with the coolant increases, which increases friction. In addition, the response of the heat flux when the amount of heat generated within the fuel rod changes becomes acute, increasing the two-phase flow pressure loss and worsening safety.

[Problem to be solved by the invention]

In the conventional technology, an orifice plate is provided directly below the fuel assembly in order to increase the safety margin, and there is a method of increasing the stability margin by increasing the resistance at this orifice plate. Quantitative evaluation of this method is shown in FIG. The horizontal axis of the figure represents the resistance value at the orifice, and the vertical axis represents the attenuation ratio, which is an index of safety. This reduction ratio is defined by the ratio of adjacent amplitudes, as shown in FIG. Therefore, when the reduction ratio is 1 or more, the amplitude increases over time, making the flow unstable. Conversely, when the reduction ratio is less than 1, the amplitude attenuates over time, making the flow unstable. Become. Therefore, the smaller this reduction ratio, the more stability margin there is. Therefore, increasing the resistance at the orifice plate is considered effective for stability. However, increasing the resistance at the orifice plate also increases the resistance throughout the core, reducing the flow rate. Therefore, in order to ensure the flow rate, a large-capacity pump is required, which has the drawback of increasing costs.

Furthermore, as a patent of this kind, Japanese Patent Application No. 58-24136
6. Japanese Patent Application No. 55-127247 discloses this, but these conventional examples have a structure in which the orifice diameter is changed, resulting in high costs.

Another way to increase the stability margin is to reduce the outer diameter of the fuel rods and water rods, increase the flow path area, and lower the two-phase flow pressure drop. . However, in this case, since the outer diameter of the fuel rod is reduced, the amount of uranium loaded as fuel is reduced, which shortens the fuel replacement cycle, resulting in an increase in fuel cycle cost. FIG. 7 shows the relationship between the fuel cycle cost and the cross-sectional area of the flow passage in the water rod, using the amount of uranium loaded per fuel assembly as a parameter. It can be seen that the fuel cycle cost decreases as the water area in the water rod increases and as the uranium loading increases. That is, it can be seen that when the fuel rods are made thinner, the uranium loading amount decreases, which increases the fuel cycle cost, and as mentioned above, the fuel economy deteriorates. For this reason, patents have been filed to improve the fuel economy of the 9x9 fuel assembly.

However, in these examples, stability limitations force fuel rods to be made smaller, resulting in lower uranium loadings and lower fuel rod diameters.
No improvement in economic efficiency can be expected.

In this way, the stability of the 9x9 fuel assembly becomes a bottleneck in core design. Methods to increase the stability margin include increasing the orifice resistance at the core inlet and reducing the outer diameter of the fuel rods and water rods, but in both cases, improving stability increases cost. On the other hand, lowering the cost will worsen the stability.

The purpose of the present invention is to solve the problem of stability and also to improve economic efficiency. In other words, sufficient stability margin is secured without increasing the resistance at the orifice plate or reducing the outer diameter of the fuel rod.

Another object of the present invention is to provide a 9x9 lattice fuel assembly with improved fuel economy.

[Means to solve the problem]

The wall thickness of the channel box forming the outer surface of the fuel assembly is reduced except for the corner portions. However, the upper part of the channel box has a thinner structure by cutting the inner surface, while the lower and center parts have a thinner structure by cutting the outer surface.

With this structure, the above objectives are achieved.

[Effect]

There are two stability modes (channel stability and core stability), each with a design limit value. Channel stability is aimed at preventing oscillations in flow distribution, and is evaluated for the fuel assembly with the highest output in the reactor core. On the other hand, core stability is aimed at preventing unstable phenomena caused by coupling of neutron flux and thermal-hydraulic vibrations within the core. Each limit value is calculated by the reduction ratio defined in FIG. That is, both channel stability and core stability are limited so that the width reduction ratio is 1.0 or less.

Therefore, we will discuss channel stability and core stability in the fourth section.
The analysis was performed on the 9x9 fuel assembly shown in the figure.

Stability changes depending on the area of the water rod or the amount of uranium loaded. Therefore, we analyzed the area of the water rond or the amount of uranium loaded as parameters, and the limit values are shown at the top of Figure 7. The results are shown in FIG. There is a restricted area due to channel instability on the lower right side of the figure (area where the water flow path area is large and the uranium loading is two people).
In addition, there is a restricted area due to core instability in the lower left side of the figure (area where the water passage area in the rond is small and the uranium loading is two people). The tolerance zone for stability exists in the center of the diagram and forms a valley shape. It can be seen that within the allowable range shown in the figure, the fuel cycle cost is the lowest and is optimal in this valley. The fuel cycle cost used here is the 4th fuel cycle cost.
It is based on the structure of the figure. Also, from the figure, 9X
It can be seen that for the 9 fuel assembly, increasing the margin for both stability moves both stability boundaries downward and reduces the fuel cycle cost.

The structure of the present invention is shown in FIG. 1. In the present invention, the wall of the channel box 3 except for the corner portions 6 is made thinner. However, the upper part 7 of the channel box is thinned by cutting only the inner surface, and the outer surface and the corner 6 remain the same surface. Further, the other portions 8 are made thinner by cutting only the outer surface, so that the inner surface is flush with the corner. With this structure, the following effects can be expected. That is, in the upper 7 area of the channel box,
Cutting the inner surface increases the flow path area.

If the wall thickness of the channel box currently in use is reduced by about 40%, the flow path area will increase by about 5 d.

Furthermore, since the vapor volume ratio (void fraction) is high in the channel upper part 7, the two-phase flow pressure loss is also large.

Therefore, by increasing the flow path area by 5d, the pressure loss is significantly reduced, and as a result, the margin of channel stability is improved, and it can be expected that the width can be reduced by about 0.03. On the other hand, the center and lower area 8 of the channel box
In this case, by cutting the outer surface, the area of the bypass region around the fuel assembly can be increased. The size of the bypass region affects the void coefficient. That is, as the bypass area increases, the water-to-uranium ratio increases and the absolute value of the void coefficient decreases. As a result, the response of the neutron flux to void fraction fluctuations becomes slower, the margin for core stability increases, and it is expected that the attenuation ratio will decrease by about 0.12.

Due to this reduction in the width reduction ratio, the limit value shown in FIG. 8 moves, and the result is shown in FIG. 9. Limit values for core stability and channel stability will be moved downward, the permissible range will be expanded, and fuel cycle costs will also be reduced. That is, the cycle cost was reduced by 1.3% at the core stability boundary, 0.4% at the channel stability boundary, and 0.8% at the stability valley, and the fuel economy was improved. I understand.

〔Example〕

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

FIG. 1 shows an embodiment of the present invention. In this embodiment, the channel box has a structure in which the portions excluding the corner portions 6 are made thinner. However, in the upper part 7 of the channel box, the inner surface is cut so that the outer surface and the corner 6 are flush with each other, and in the other parts 8, the outer surface is cut so that the inner surface is flush with the corner 6. . With this structure, the margin for core stability and channel stability increases.

Fuel rods can be made thicker. As a result, the amount of uranium that can be loaded into the assembly increases, significantly reducing fuel cycle costs and improving fuel economy.

In addition, backfooting is possible even with conventional plants, and there are no backfooting problems. Therefore, if this embodiment is adopted even in a conventional plant, the margin for stability will increase, and a significant improvement in fuel economy can be expected.

Further, regarding the production of this embodiment, since it can be handled only by machining the inner surface, there is almost no increase in cost for modification.

FIG. 10 shows yet another embodiment. In this example, the maximum total length L + / L from the top of the channel box
It has a structure in which the inner surface of the area up to o is cut, and the outer surface of the other lower part is cut. Lengthening the internally milled top region increases the channel stability margin increase and conversely reduces the core stability increase. Therefore, we analyzed the length of the upper region where the inner surface is cut as a parameter. The results are shown in FIG. The horizontal axis of the figure is the length of the upper enlarged channel (inner cutting channel) divided by the total length of the channel box, and the vertical axis is the amount of reduction in fuel cycle cost (the shift of the boundary line in Figure 9). quantity). It can be seen that when the enlarged upper channel (inner cut channel) is lengthened, the increase in core stability decreases, while the increase in channel stability increases. By the way, as described in FIG. 9, if the fuel assembly is designed so that it falls within the stability valley (8 points in the diagram), fuel economy can be significantly improved. Therefore, one possible method is to make the water rod thicker so that the area of the water rod is at the trough of stability. However, the upper limit of the dimensions of the water rod is determined by the outer diameter, pitch, etc. of the fuel rods. That is, in the water rod 5 of the type shown in FIG. 4, the upper limit of the internal flow passage area of the water rod is 0.045 relative to the cross-sectional area of the fuel assembly. Then, the relative value of the upper limit area is 0.063,
Therefore, it turns out that it is difficult to make the water rondo thicker and position it in the valley of stability. Furthermore, fuel assemblies with wide inner widths are also used in nuclear reactors currently in operation, but with this type, the stability valley shifts to the right, as shown in Figure 13, so the water rods are made thicker. However, it is almost impossible to place the water rondo area at the trough of stability.

By the way, in this method, as mentioned above, changing the length of the upper region where the inner surface is cut improves core stability and
The amount of movement of the channel stability changes, and the position of the stability valley also moves. Therefore, by adjusting the length of the channel to be cut inside, the stability valley can be moved to the position of the upper limit area of the water rod. An example thereof is shown in FIG. The broken line in the figure shows the result of the conventional example, the - dotted chain line shows the result when the length Ll of the upper region to be internally cut is short, and the two-dot chain line shows the result when L+ is long. That is, when the length of LA is increased, the position of the stability valley shifts to the right, and at a certain point, the stability valley becomes the upper limit area of the water rondo (relative area 0.0
63), and as a result, the fuel cycle cost can be reduced by about 0.8%.

FIG. 15 shows a second embodiment of the present invention. This embodiment is characterized in that the cut portion on the outer surface of the channel box is made of a bellows surface 9 and shaped to protrude inside. With such a structure, margins for core stability and channel stability are increased, and fuel cycle costs are significantly reduced, as in the embodiment. Furthermore, there is a problem in this embodiment that the channel box creeps at high burn-up.

The elongation due to creep of the bellows can be absorbed by the bellows surface 9.

Furthermore, since the bellows surface protrudes into the interior of the assembly, the size of the bypass area increases, and it is expected that the margin of core stability will further increase.

FIG. 16 shows a third embodiment of the present invention. This embodiment is characterized in that the upper and lower parts of the channel box are made singly and are welded into an integral structure. With this structure, as in the second embodiment, margins for core stability and channel stability are increased, and fuel cycle costs can be significantly reduced. Furthermore, manufacturing is easy and costs are reduced.

Figure 17 shows the effect of the stability boundary line on the wall thickness of the channel box. When the wall thickness of the channel box is made thinner, the stability trough moves downward.

Fuel cycle costs can further be reduced. Therefore, the wall thickness of the channel box shown in the embodiment is made even thinner. With conventional technology, due to corrosion resistance and pressure differences, the wall thickness can be increased up to the current wall thickness of 60%.

However, if a metal with high corrosion resistance and good neutron economy is developed, it will be possible to make the channel box thinner, further reducing fuel cycle costs, and it is expected that fuel economy will be significantly improved.

〔Effect of the invention〕

According to the present invention, the stability margin of the fuel assembly can be increased, and the fuel rods can be made thicker by the increased thickness, so that the fuel cycle cost can be significantly reduced and high fuel economy can be achieved.

[Brief explanation of the drawing]

1 and 10 are perspective views of one embodiment of the present invention, and FIG.
Figure 3 is a perspective view of a conventional fuel assembly, Figure 3 is a cross-sectional view of the upper part of a conventional fuel assembly, Figure 4 is a cross-sectional view of a 9X9 lattice fuel assembly, and Figure 5 is the effect of orifice resistance on stability. Figure 6 is an explanatory diagram of the reduction ratio, which is an index of stability, Figure 7 is a diagram showing the effect on fuel cycle cost, and Figure 8 is a diagram showing the effect on fuel cycle cost.
The figure is a stability map of a 9x9 lattice type fuel assembly, Figure 9. 11 and 14 are diagrams showing the effects of the present invention, FIG. 12 is a sectional view of an alternative 9X9 lattice type fuel assembly, and FIG. 13 is a characteristic diagram of an assembly with a large inner width of the channel box. 15 and 16 are a plan view and a perspective view showing another embodiment of the present invention, and FIG. 17 is a diagram showing the effect of the wall thickness of the channel box. DESCRIPTION OF SYMBOLS 1... Fuel assembly, 2... Fuel rod, 3... Channel box, 4... Fuel pellet, 5... Water rod, 6... Corner part, 7... Upper part of channel box , 8... lower part of the channel box, 9... bellows surface. Fig. 1 Thousand wholesale rice tea Zt￥J 3rd mouth $4 5th mouth 6th mouth (Sec) Momme Poki Cycle Cos La floor straight σ not t'4 + cycle cost quasi θ dimension value (gradient 10th Trouble 11th Figure 13 Figure 14 Mizuguchi ・) ° Inner Machine w14R/Nephew 91 Gathering Stop Chume Han (-) Tea 15 [¥1 Detection ! 6 Figure

Claims (1)

[Claims] 1. A fuel assembly for a boiling water reactor in which the outer periphery is a water region, the boundary is formed by a channel box, and a plurality of fuel rods are provided inside, except for the corner portion of the channel box. 1. A fuel assembly for a boiling water nuclear reactor, characterized in that the channel box has a thinner wall structure in the upper part of the channel box, and a recess is provided in the inner surface of the upper part of the channel box, and a recess part is provided in the outer surface of the lower part of the channel box. 2. A fuel assembly for a boiling water reactor according to claim 1, characterized in that a cut portion of the outer surface of the channel box is formed into a bellows surface and has a shape that projects inside. 3. The fuel assembly for a boiling water nuclear reactor according to claim 1, wherein the upper and lower portions of the channel box are made singly and are welded into an integral structure.