JPH03221894A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To reduce practical resistance during the operation of the boiling water reactor and to allow urgent cooling water to easily flow into a channel at the time of generating a cooling material lossing accident by forming an upper tie plate for supporting the upper end of a fuel rod so that its center part is upward raised. CONSTITUTION:In the fuel assembly 10 provided with the fuel rod 1 charged with atomic fuel and a large aperture water channel tube 5 to obtain an efficient neutron decelerating effect in a high boiled area, the upper tie plate 2 for supporting the upper end of the fuel rod 1 is formed so that its center part is upward raised. Boiled water rising through the channel between a channel box 6 and the tube 5 along the rod 1 is guided to a large aperture on the center part along the lower spherical face of the upper tie plate 2. Consequently, the quantity of boiled water passing the large aperture of the center part is increased and the quantity of boiled water passing the narrow aperture of a peripheral part is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fuel assembly used in a boiling water nuclear reactor (BWR), and particularly to an upper tie plate structure thereof.

[Prior art] In boiling water nuclear reactors, which are used in many practical operations in Japan as power reactors, in recent years, 1ζ fuel assembly has been developed in order to more efficiently slow down neutrons and increase the burnup of nuclear fuel. A 9x9 type fuel cell system has been developed in which a large-diameter 1-Tadeyannelle tube is placed in the center and thinner fuel rods are arranged around it in a square lattice of 9 x 9 trees.

Figures 5 to 7 show an example of this 9x9 type fuel assembly. -A cross section is shown.

In FIGS. 5(a) and 5(b), a fuel rod 1 with enriched uranium dioxide pellets loaded into a Zircaloy cladding tube with a diameter of 10 mm and a length of about 4000 mm is placed in a channel box 6 with a large hole penetrating through the center. They are arranged in a square grid of 9 x 9 trees surrounding the diameter channel tube 5, and their upper end is supported by the upper type 1 notebook 2D and the lower end is supported by the lower type 1 notebook 4 (not shown). Also, between the upper and lower type 1 notes 2D14, there are 7 subesa S1~
S7 (only S7 is shown) supports the fuel rod 1 in the horizontal direction. Here, the upper and lower tie breakers 2B, 4, and spacers 81 to S7 that loosely support the fuel rod 1 are vertically supported by structural members (not shown), and the fuel assembly is carried out by a handle 3 fixed to the upper tie plate. If the entire body 10D is suspended, the fuel rod 1 may also be damaged due to temperature rise during operation.
Even when the fuel rods 1 thermally expand, no load is applied to the fuel rods 1.

As shown in FIG. 5(b), the water channel pipe 5 has a cross section with orifice holes at the top and bottom, filling a volume equivalent to 3 x 3 fuel rods in the center of the fuel assembly 10D. The tube is square in shape and stores unboiled water inside even if the surrounding area reaches a high boiling state, providing a stable neutron moderation effect to the surrounding fuel rods 1.

Here, a fuel rod 1 is surrounded by a channel pipe 5 and a channel box 6, and its cross-sectional area is narrowed at the positions of the upper and lower tie plates 2B, 4 and spacers 81 to S7.
The elongated space along the fuel rod forms a rising water channel through which boiling water exchanges heat with the fuel rod 1 during operation. This waterway is called Jinnel. Moreover, the fuel assembly 10D is
Several hundred control rods (not shown) are arranged in combination 1u to constitute the reactor core.

Figure 6 shows a part of the fuel assembly 10D type 1 throat 2D.
The structure of FIG.

As shown in Fig. 6(a), tl)), jζ, handle 3
The upper tie plate 2D, which is made up of a support portion for each fuel rod 1 and the tunnel tube 5 and a boiling water passage portion, has a flat upper and lower surface. Further, a large opening is provided in the central portion corresponding to the two-way channel tube 5 in order not to reduce the flow path resistance.

Now, when the core consisting of a large number of fuel assemblies 10D is filled with cooling water and the control rods, which are neutron absorbers, are removed, each fuel rod 1 generates heat due to the nuclear chain reaction of the enriched uranium dioxide pellets inside. The cooling water is heated and boiled, and the cooling water rises through the channels as a gas-liquid two-phase flow. The rising speed at this time is about 2 m/sec on average in the negative phase, and the steam equivalent flow rate reaches 5 to 6 m/sec.

Here, during normal operation, the pressure of the output steam is approximately 70
A boiling condition of around 290 degrees Celsius is selected while maintaining the pressure, and the extracted steam passes through a recirculation path (not shown) consisting of a steam turbine, a condenser, a feed water heater, a recirculation pump, etc.
It is returned to the lower part of the fuel assembly 10D as circulating cooling water. Here, the circulating cooling water is normally forcibly returned by a recirculation pump using a driving pressure commensurate with the pressure drop that occurs when passing through the reactor core. This driving pressure, which is called pressure drop, is composed of hydrostatic pressure drop above and below the fuel assembly 10D, acceleration pressure drop that is the accelerating reaction force of cooling water due to volume expansion due to boiling, and loss that occurs when boiling water rises through a long and narrow channel. The frictional pressure drop caused by
7 and the upper tie plate 2D, a bottleneck condition is caused, the surrounding pressure is increased, and the cooling water in the lower part of the fuel assembly 10D is compressed, resulting in a local pressure drop.

In order to explain the state of the two-phase flow rising in this channel, FIG. 7 schematically shows the area around the -L section tiebrae].2D.

The flow path at the top of the channel is as shown in Figure 7 (ζ).
Area A of the fuel rod 1 and the pipe portion of the water channel 5, Si area B of the end shaft of the water channel 5, and 1. : Separate and calm.

In the case of the conventional flat upper type 1 no h2D, the interval in region B is narrow. The flow of cooling water in the channels surrounded by the channel box 6 is predominantly axial flow, and in the case of the conventional type 1)-2D, the flow is sufficiently distributed to the central part in area B. Not done. Therefore, most of the cooling water is in the upper type 1, which has a small cross-sectional area.
The liquid flows directly through the periphery of the notebook 2D, and the pressure loss in this area becomes roughly large.

[Problems to be Solved by the Invention] As shown in Figure 6, the upper type 1 notebook 2D has a large opening in the center, and the two-phase flow that has ascended through the channel is -2 Before reaching D,
It was expected that the center would be shifted to take advantage of this opening. However, as mentioned above, in reality, two-phase flow tends to flow more through the surrounding area, so
As the actual resistance of the upper tie plate 2D increases, it is difficult to expect much improvement in local loss.

In addition, when a loss of coolant accident (LOCA) occurs, emergency cooling water is released from the upper part of the fuel assembly 10D to cool the overheated fuel rods 1, but the emergency cooling water that comes into contact with the fuel rods 1 suddenly It evaporates, expands, turns into water vapor, and blows up to the top of the fuel assembly 10D. Therefore, combined with the flushing steam generated in the lower blenum as the pressure inside the core pressure vessel decreases, subsequent cooling water accumulated on the upper tie plate 2D is restricted from falling beyond the central and peripheral orifices. (CCFL effect: Counter
Current Flow Lim1tation)
, emergency cooling of the fuel rod 1 is not performed smoothly.

The present invention has a low effective resistance during operation, and therefore a small local pressure drop, and also makes it easier for emergency cooling water to flow into the channel in the event of a coolant loss accident, which lowers the pressure drop and improves safety. The purpose of the present invention is to provide a fuel assembly for a boiling water reactor.

[Means for Solving the Problems] A fuel assembly for a boiling water nuclear reactor according to claim (1) of the present invention has fuel rods loaded with nuclear fuel and an effective neutron moderation effect particularly in a high-hoity region. This fuel assembly is equipped with a large-diameter single-channel pipe for obtaining fuel rods, and has an upper tie plate with a central portion that is high above and supports the upper ends of the fuel rods.

A fuel assembly for a boiling water nuclear reactor according to claim (2) of the present invention has an upper tie plate that is formed high in a stepped manner toward the center.

[Function] In the boiling water reactor fuel assembly according to claim (1) of the present invention, the fuel rods immersed in cooling water generate heat due to the nuclear chain reaction inside the fuel rods, and the surrounding cooling water is absorbed. Heat to boiling point. In addition, the water channel tube provides a stable neutron moderating effect to the surrounding fuel rods by allowing an appropriate amount of unboiled cooling water to flow through the tube. In addition, the cooling water that boils on the surface of the fuel rod becomes a gas-liquid two-phase state, rapidly rises along the channel along the fuel rod, and further vaporizes and expands in volume, creating a bottleneck directly below the upper type 1 note. This causes local loss.

However, the upper tie plate has a large opening in the center that accommodates a large-diameter channel tube, and its lower surface is formed high upwards, so the rising boiling water has less resistance. The amount of boiling water passing through the center opening is larger than that of the conventional example with a flat bottom surface because of the lower amount of water being kited in the center.

In addition, when cooling water is flowed into the channel from the upper part of the fuel assembly against the blown-up water vapor during LOCA, the blown-up water vapor guided to the lower surface of the upper tie plate mainly flows through the opening in the center of the upper tie plate. While the cooling water is quickly released above the fuel assembly, the cooling water guided to the upper surface of the upper tie plate falls from the opening in the periphery and flows down along the wall surface of the channel box and the fuel rods.

In the fuel assembly for a boiling water nuclear reactor according to claim (2) of the present invention, the stepped surface allows the same cooling water and steam as in the case of the fuel assembly according to claim (1). The kite is done.

[Embodiments of the invention] Embodiments of the present invention will be described with reference to FIGS. 1 to 4.

In the wooden embodiment, the upper tie plate 2D of the fuel assembly 10D described in the conventional example is replaced with an upper tie plate 1-2 whose upper and lower surfaces are formed into spherical shapes. In addition, in each figure, members having the same configuration and function as those of the conventional fuel assembly shown in FIGS. 5 to 7 are given the same reference numerals, and their explanations will be omitted. .

FIG. 1 shows the detailed structure of a fuel assembly according to an embodiment of the present invention, in which (a) is a plan view and (b) is a front view.

In FIG. 1, the upper type 1 notebook 2 has a large opening in the central part corresponding to the central 8-channel tube 5, or its upper and lower surfaces are similar to the conventional example shown in FIG. It is formed into a spherical shape that is tall upwards.

FIG. 2 schematically shows, in cross section, the area around the upper tie plate 2 in the operating state of the fuel assembly according to the embodiment of the present invention.

In FIG. 2, the boiling water rising along the fuel rods 1 through the channel between the channel box 6 and the overchannel pipe 4 flows along the spherical surface of the lower surface of the upper tie plate 2 toward the large opening in the center. You will be guided by Due to stiffness,
Compared to the conventional example, the amount of boiling water that passes through the large opening in the center increases, and the amount of boiling water that passes through the narrow openings in the periphery decreases.

FIG. 3 schematically shows the state of emergency cooling of the fuel rod 1 when a loss of coolant accident (LOCA) occurs.

When a loss of coolant accident (LOCA) occurs, emergency cooling water is released from the upper part of the fuel assembly 10 in order to urgently cool the overheated fuel rods 1. However, the emergency cooling water that comes into contact with the fuel rods 1 suddenly It evaporates into water vapor and blows up to the upper part of the fuel assembly 10. Subsequent cooling water must therefore flow down against this water vapor pressure. Here, since the upper tie plate 2 has a spherical shape with a high upper and lower surface, the blown up water vapor hits the upper tie plate 2 surface and mainly passes through the large opening in the center of the fuel assembly 10. At the same time, the cooling water on the upper tiebrae]・2 is guided by the upper tiebrae】・2 and easily falls from the opening in the peripheral area, and flows into the wall of the channel hox 6 and the salt water 1. It flows down along bar 1.

FIG. 4 schematically shows, in cross section, an upper type 1 notebook of a fuel assembly according to another embodiment of the invention.

Here, the upper tie plate 2B has a stepped upper surface and a conical lower surface, and has the same effect as the double upper tie plate 2.

In the above embodiments, an upper type plate has been described whose upper and lower surfaces become higher both vertically and horizontally toward the center.
It is also possible to make the vertical or horizontal chairs higher toward the center, for example, to form a roof shape, or to make only the upper and lower surfaces of the upper tie plate higher toward the center. . Furthermore, when constructing an upper type 1 notebook by connecting short metal pipes with ribs, it is convenient for manufacturing that the appearance is step-like on both the lower and lower surfaces.

[Effects of the Invention] In the fuel assembly for boiling water reactors of the invention, the rising boiling water is collected in the upper tie plate or in the central part where there is less flow resistance, so the upper tie plate is lower than in the conventional case. The effective resistance is reduced and the local loss is also reduced. Therefore, the stability of core operation is improved.

In addition, when cooling water is supplied from the top of the fuel assembly against the blown-up steam, the passage of the blown-up steam and the passage of the falling cooling water are divided into sections i-1 by the upper tie plate, allowing the cooling water to flow easily. . Therefore, a loss of coolant accident (L
When OCA) occurs, emergency cooling of the fuel rods can be performed more efficiently than in the conventional case.

[Brief explanation of drawings]

FIG. 1 shows the structure of a fuel assembly [upper tiebrae of a fuel assembly for a wooden type nuclear reactor] 2 according to an embodiment of the wooden invention, (a)
is a plan view, and (b) is a front view. FIG. 2 is a schematic cross-sectional view showing the area around the upper portion 911012 in the operating state of the boiling water reactor fuel assembly according to the embodiment of the present invention. FIG. 3 is a schematic diagram showing the state of emergency cooling of fuel rods when a loss of coolant accident (LOCA) occurs in a fuel rod assembly for a boiling water reactor according to an embodiment of the present invention. FIG. 4 is a schematic diagram, not shown in cross section, of the structure of an upper tie plate of a fuel assembly for a boiling water reactor according to another embodiment of the present invention. Figure 5 is a diagram for explaining the structure of a conventional fuel assembly for a boiling water reactor. -A sectional view. FIG. 6 shows the structure of a conventional upper type 1 notebook 2B fuel assembly for a boiling water reactor, in which (a) is a plan view;
b) is a front view. FIG. 7 is a schematic diagram for explaining the state of a two-phase flow rising in a channel of a conventional boiling water reactor fuel assembly around the upper tie plate. [Explanation of symbols of main parts] 1...Fuel rod 2...Upper tie plate 5
... 1-ta channel pipe 10 ... fuel assembly

Claims (2)

[Claims] (1) In a fuel assembly comprising a fuel rod loaded with nuclear fuel and a large-diameter water channel pipe for obtaining an effective neutron moderation effect particularly in a high void region, supporting the upper end of the fuel rod; A fuel assembly for a boiling water reactor, characterized by having an upper tie plate with a central portion raised upward. (2) The fuel assembly for a boiling water reactor according to claim (1), characterized in that it has an upper tie plate formed in a stepped manner toward the center.