JPH0486596A - Fuel assembly - Google Patents

Abstract

PURPOSE:To achieve a higher limit output characteristic with higher cooling efficiency of fuel rods by mounting a coolant guide member on a plurality of cut parts provided on an internal wall of a channel box CB in such a manner that the center thereof protrudes to the center of a passage. CONSTITUTION:During the operating of a nuclear reactor, a coolant 14 flows into a CB 4 at a lower opening of a lower tie plate 3 while the coolant 14 tends to flow along a lower cylinder peripheral part a the peripheral part in the CB4 is at a low temperature. As the coolant 14 rising along the internal surface of the CB4 reaches the position of a coolant guide member 7 mounted on cut parts 6 provided on the internal wall, it moves toward the center of a fuel assembly being guided by the member 7 protruded to the center of a passage and converges with a main flow of the coolant rising among fuel rods 1 to enhance a cooling effect with an increase in a flow rate of the coolant flowing in the perimeter of the fuel rod 1. With the convergence, the main flow of the coolant 14 is disturbed to promote the mixing of stream bubbles generated on the surface of the fuel rod 1 as caused by heating thereby preventing the covering of the surface of the fuel rod 1 with a vapor phase.

Description

Detailed Description of the Invention [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly loaded in the core of a boiling water nuclear reactor.

(Prior Art) A conventional fuel assembly used in a boiling water nuclear reactor will be explained with reference to FIG. A large number of fuel rods 21 are arranged in a square grid, and their upper and lower ends are supported by an upper tie plate 22 and a lower tie plate 23, respectively. Upper tie plate 22 and lower tie plate 2
3 is provided with a flow path through which the coolant flows in the vertical direction. The entire fuel rods arranged in a square grid are covered with a channel box 24 in the shape of a rectangular tube. A plurality of lattice-shaped fuel spacers 25 are provided between the upper tie plate 22 and the lower tie plate 23 to maintain the distance between the fuel rods 21 and between the fuel rods 21 and the inner surface of the channel box 24. It is designed to ensure a vertical flow path.

When this fuel assembly is loaded into a boiling water reactor (not shown) and operated, the coolant flows in a single liquid phase through the opening at the bottom of the lower tie plate 23, and the coolant flows into the lower tie plate 23. and flows into the channel box 24. Here, the heat of the fuel rods 21 is transferred to the coolant, and the coolant increases in temperature and further boils. This heat transfer from the fuel rods 21 to the coolant prevents the fuel rods 21 from overheating and maintains the integrity of the fuel rods 21.

The coolant whose temperature rises and boils to become a gas-liquid two-phase flow passes through the upper tie plate 22 and flows out of the channel box 24 . A boiling water reactor separates and utilizes steam from the high-temperature gas-liquid two-phase flow thus obtained.

(Problem to be Solved by the Invention) In order to transfer as much heat as possible to the coolant without overheating the fuel rods 21, the coolant supplied into the fuel channel 24 must be cooled by effectively contacting the nuclear fuel rods 21. It is necessary to increase the efficiency and increase the critical power, which is the fuel assembly power when boiling transition occurs and cooling efficiency deteriorates.

By the way, the distribution state of the coolant flowing into the channel box 24 tends to be concentrated near the periphery of the channel box 24 rather than the center. This has two fuel rods in the center.
This is because the temperature is high due to the nuclear reaction in No. 1, whereas the surrounding area is at a low temperature, so the coolant flows to the low temperature surrounding area where flow resistance is low. Therefore, in the channel box 24, a large amount of the coolant flows along the inner peripheral surface of the channel box 24, and the amount of coolant flowing between each fuel rod becomes small.

In order to improve the cooling efficiency of each fuel rod 21, the flow rate of the coolant along the inner peripheral surface of the channel box 24 may be suppressed so that a large amount of coolant can be guided into the arrangement of the fuel rods 21.

However, since the conventional channel box 24 has a smooth inner peripheral surface, the coolant along the inner peripheral surface of the channel box 24 cannot rise smoothly and be guided between the fuel rods 21. There wasn't.

Therefore, the coolant cannot be effectively and actively used to cool each fuel rod 21, and therefore the cooling efficiency of the fuel rods 21 is not necessarily high.

On the other hand, it is conceivable to provide a guide member in the channel box 24 to guide the coolant between the fuel rods 21 along the channel box 24.

However, in this case, when assembling this fuel assembly, the fuel rods 21, the upper tie plate 22, the lower tie plate 23
In order to avoid collision between the guide member and the outside of the fuel spacer 25 during the process of covering the channel box 24 with the fuel spacer 25 and the like assembled, only an extremely small guide member could be provided.

The present invention was made in view of the above problems, and it actively guides the coolant rising along the inner surface of the channel box between each fuel rod, thereby increasing the cooling efficiency of each fuel rod. The purpose is to improve the limit output characteristics and also to prevent problems from occurring when assembling the fuel assembly.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides an upper tie plate and a lower tie plate that connect a large number of fuel rods and the upper and lower portions of these fuel rods, respectively, and have an upward flow path. a cylindrical channel box that covers the sides of the entire fuel rod; and a plurality of lattice-shaped fuel spacers for maintaining spacing between the fuel rods and between the fuel rods and the channel box. In this method, a plurality of notches are provided in the inner wall surface of the channel box, and a coolant guide member made of a leaf spring is attached to the notch in a dogleg shape, and the center portion of the coolant guide member is The present invention relates to a fuel assembly characterized in that it has a shape that projects toward the center of a flow path.

(Function) In the fuel assembly of the present invention, a plurality of notches are provided on the inner wall surface of the channel box, and a dogleg-shaped coolant guide member is attached to the cutouts, so that the coolant guide member can flow easily. The coolant protruding toward the center of the channel and rising along the inner surface of the channel box is directed toward the center of the fuel assembly, improving the cooling effect of the fuel rods. Particularly in areas on the surface of the fuel rod where liquid film is likely to peel off, this change in coolant flow can prevent liquid film separation, contributing to an increase in critical output.

Since the coolant guide member is pushed down into the notch by the spacer outer member during assembly or disassembly of the fuel assembly, there is no risk of it becoming an obstacle during assembly or disassembly of the fuel assembly.

(Example) An example of a fuel assembly according to the present invention will be described with reference to FIGS. 1 to 4.

FIG. 1 is a sectional view of a fuel assembly that is an embodiment of the present invention. In this fuel assembly, a large number of fuel rods 1 are arranged in a square lattice as in the past, and their upper and lower ends are supported by an upper tie plate 2 and a lower tie plate 3, respectively. 3 is provided with a flow path through which coolant flows in the vertical direction, and the entire fuel rod is covered with a rectangular tube-shaped channel box 4. A plurality of lattice-shaped fuel spacers 5 are provided between the upper tie plate 2 and the lower tie plate 3 to maintain the distance between the fuel rods 1 and between the fuel rods 1 and the inner surface of the channel box 4. It is designed to ensure a flow path in the vertical direction.

The above structure is the same as that of a conventional fuel assembly, but in this embodiment, as shown in FIG. 2, a plurality of notches 6 are provided in the inner wall surface of the channel box 4, and a coolant guide member 7 is installed.

Figures 2(a) and 2(b) are enlarged views of section A in Figure 1, and (a) shows the spacer pushed down into the notch by the outer spacer member during assembly or disassembly of the fuel assembly. (b) is a diagram during normal operation.

It is appropriate that the notch 6 be provided at a position between the fuel spacers 5. For example, when the space between the fuel spacers 5.5 is divided into three equal parts, it is disposed at a position ⅓ from above. Further, as shown in FIG. 2(a), the coolant guide member 7 is attached to the channel pock 4 by welding at its upper and lower portions in a "<" shape. This coolant guide member 7 is made of a plate spring.

Therefore, the coolant guide member 7 attached to the notch 6 on the inner wall of the channel box deforms when the fuel spacer passes through the notch 6 during fuel assembly or disassembly. fits in. On the other hand, when the assembly is completed and the coolant guide member 7 is installed inside the reactor core, the center portion of the coolant guide member 7 protrudes toward the flow path in the channel box 4 .

As shown in FIG. 3, the fuel spacer 5 is made up of plate-like members 8 assembled in a lattice shape, and a fuel rod 1 is accommodated in each lattice hole 9, and the fuel rod 1 is a guide member IQ.

11, held in place by +2. Plate member 8
There are a plurality of protrusions 13 on the outside of the outer periphery of the plate-like member 8.
The distance between the channel box 4 and the channel box 4 is maintained. The coolant guide member 7 is located in such a position that it does not hit the protrusion 13 when the channel box is moved up and down.
The height of the protrusion of the guide member 7 during operation is greater than the height of the protrusion 13. The protrusion of the guide member 7 at room temperature is the protrusion 13
greater than the height of. Note that FIG. 3 is a sectional view taken along the line II in FIG. 1, and some of the fuel rods 1, 1, . . . are omitted.

Next, the operation of this embodiment will be explained. As mentioned above, when a fuel assembly is loaded into a boiling water reactor and operated, the coolant flows in a single phase liquid through the opening at the bottom of the lower tie plate 3, passes through the lower tie plate 3, and enters the channel box. 4. Here, the heat of the fuel rods], ■... is transferred to the coolant, which rises in temperature and boils further, becoming a gas-liquid two-phase flow that passes through the upper tie plate 2 and exits the channel box 4. leaks to.

At this time, the "<"-shaped coolant guide member 7 has a shape that protrudes toward the center of the flow path of the channel box 4, as shown in FIG. 2(b).

The coolant inside the channel box 4 tends to flow more along the periphery, that is, along the inner surface of the channel box 4 because the surrounding region is at a lower temperature. When the rising coolant 14 reaches the position of the coolant guide member 7, it is guided by the coolant guide member 7 and directed toward the center of the fuel assembly, and the coolant rises between each fuel rod 1. It is made to merge with the main flow of coolant. This merging increases the flow rate of the coolant flowing around the fuel rods 1, increasing the cooling effect of the fuel rods 1. Further, as the merging occurs, turbulence occurs in the mainstream of the coolant 14, promoting the mixing of the coolant 14 with vapor bubbles generated on the surface of the fuel rod 1 due to heat generation, and the surface of the fuel rod 1 is covered with a vapor phase. is prevented.

In addition, in conventional fuel assemblies without the coolant guide member 7, the coolant liquid film on the surface of the fuel rods tends to break at a position slightly upstream of the first fuel spacer from the top end of the fuel rods. It is particularly effective to provide the coolant guide member 10.

During assembly or disassembly of this fuel assembly, when the coolant guide member 7 made of a leaf spring comes into contact with the spacer outer member, it deforms into the notch 6 on the inner surface of the channel box 4 as shown in FIG. 2(a). It fits.

Therefore, the channel box 4 can be covered and removed without the coolant guide member 7 becoming an obstacle.

[Effects of the Invention] In the fuel assembly according to the present invention, during nuclear reactor operation, the coolant rising along the inner peripheral surface of the channel box is guided toward the inside of the fuel assembly, and the coolant reaches the surface of the fuel rods. Since the contact is improved, the cooling efficiency of the fuel rods can be increased and the critical power output can be improved. Moreover, since the coolant guide member is composed of a leaf spring, it is pushed and deformed by the spacer outer member during assembly and disassembly of the fuel assembly, and fits inside the notch, causing no adverse effects during assembly and disassembly of the fuel assembly. It has no effect.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway vertical sectional view of an embodiment of a fuel assembly according to the present invention, and FIGS. 2(a) and 2(b) are enlarged views of part A in FIG. When assembling or disassembling the body, (b
) is a diagram showing the state during normal operation, and Figure 3 is the I- in Figure 1.
FIG. 4 is a sectional view taken along line I, FIG. 4 is a diagram for explaining the effect of the coolant guide member of the present invention during nuclear reactor operation, and FIG. 5 is a partially cutaway vertical sectional view of a conventional fuel assembly. 1...Fuel rod 2...Upper tie plate 3...Lower tie plate 4...Channel box 5...Fuel space 6...Notch 7...Coolant guide member ( 8733) Agent: Yoshiaki Inomata, patent attorney (and others)
1 person) Rod I diagram

Claims (1)

[Claims] (1) A large number of fuel rods, an upper tie plate and a lower tie plate that connect the upper and lower parts of these fuel rods, each having an upward flow path, and a cylindrical channel that covers the sides of the entire fuel rod. In a fuel assembly comprising a box and a plurality of lattice-shaped fuel spacers for maintaining spacing between the fuel rods and between the fuel rods and the channel box, a plurality of notches are formed in the inner wall surface of the channel box. and a coolant guide member made of a plate spring is mounted in the notch in a dogleg shape, and the coolant guide member has a shape such that its central portion protrudes toward the center of the flow path. A fuel assembly featuring: