JPH02147889A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To improve stability to fluid vibration by forming fuel spacer abutting parts at channel box thin-thickness parts. CONSTITUTION:Channel abutting parts 6 are formed on both sides of the outer peripheral part of fuel spacers 2, which are all in the same structure; and the internal surfaces 41, 42, and 43 of channel box 4 decrease in thickness toward the downstream side of a cooling material. Here, the thin internal surfaces 42 and 43 are dimpled to form the spacer abutting parts 5 and a fuel bundle 3 is supported from inside. No spacer abutting part 5, however, is provided at a position next to where the thick part becomes the thin part, and gaps 71 and 72 are formed between the channel box 4 and spacers 2 as shown in figures (C) and (D). Consequently, the bundle is prevented from vibrating owing to a cooling water flow and vibrating laterally owing to an earthquake, thereby improving the mechanical soundness.

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 nuclear reactor, and particularly to a fuel assembly with improved thermal margin. This invention relates to a fuel assembly for a boiling water reactor that has improved seismic integrity and stability against fluid vibration.

(Prior Art) A fuel assembly for a boiling water reactor (BWR) consists of a metal cladding tube in which a large number of fuel rods filled with nuclear fuel material are regularly arranged (fuel bundle) in a rectangular channel box. It is configured by being stored inside.
In the core of a boiling water reactor, cells each consisting of one cross-shaped control rod and four fuel assemblies surrounding it are regularly arranged. In other words, the fuel assemblies and i control rods in the BWR core are arranged so that their axes are perpendicular and parallel to each other, and the cooling water, which functions as a moderator, flows from the bottom of the core to the top. It is structured so that it flows in the opposite direction. The fuel rods generate heat due to the nuclear fission reaction, and the cooling water removes that heat from the fuel rods while flowing upward (
flowing downstream). A portion of the cooling water evaporates and generates bubbles (voids). Near the effective lower end of the core, that is, the lower end of heat generation! However, bubbles are generated everywhere in the core inside the channel box except near the bottom end, and from the axial center of the core to the top end, the proportion of air bubbles occupying the cooling water passage, that is, voids. The ratio increases significantly and exceeds 70% near the top of the core. When the void ratio increases, the cross-sectional area of the cooling water passage in the direction perpendicular to the axis is constant in the height direction within the channel box, so the flow rate of the cooling water inevitably increases. Since the frictional resistance of the flow path increases approximately in proportion to the square of the flow velocity, the pressure loss of the cooling water increases from the center to the top of the core where the flow velocity is high (the void ratio is high).

The force required to flow the coolant is mainly provided by the discharge pressure at the outlet of the recirculation pump, so a large pressure loss means that a large amount of power must be applied to the pump, which increases the size of the equipment and reduces power generation efficiency. This will cause a decrease. Therefore, if this pressure loss can be reduced, the pump power can be reduced.

Incidentally, various studies have been conducted in recent years from the perspective of improving the economic efficiency of nuclear power generation. For example, it has been found that by varying the spacing between fuel rods within the fuel handle, it is possible to improve the neutron multiplication factor during reactor operation, thereby achieving longer operating cycles and extending burnup. I got it.

In this way, the reason why the neutron multiplication factor improves during reactor operation is that the probability of escaping resonance can be improved due to the mutual effect of the fuel rods, and although the thermal neutron utilization rate is slightly disadvantageous,
This is based on the phenomenon that the degree of disadvantage is suppressed when a nuclear reactor is operated at high temperature and output. Part of this research is
This method was disclosed by the present inventors in Japanese Patent Application Laid-Open No. 62-75378@.

However, changing the gap between the fuel rods within the fuel bundle corresponds to configuring areas where the coolant flows easily and areas where it does not flow easily, so new measures are required. This is because when the coolant flow deteriorates, the thermal margin decreases, and the margin for ensuring fuel integrity decreases.

In order to improve this, the inventors of the present invention applied for a patent application filed in
He made the invention shown in 6191@. That is, this invention has an upper tie plate at the upper end.

A lower tie plate is arranged at the lower end, a plurality of spacers are arranged at predetermined intervals between the upper tie plate and the lower tie plate, and the upper tie plate is arranged at a lower end thereof.

Using a lower tie plate and multiple spacers, a large number of fuel rods are regularly arranged to form a fuel bundle, and a metal channel box is placed around the outer periphery of the fuel bundle to allow flow around the fuel rods in the axial direction of the fuel rods. In a fuel assembly configured such that the flow of cooling water for removing heat generated in the fuel rods is non-uniform within the fuel bundle at least downstream of the coolant, the tunnel box has an external shape similar to that of a channel box. is uniform in the axial direction of
The wall thickness of the channel box material is thicker upstream of the flow of cooling water, and except for the downstream end where the upper tie plate comes into contact, the wall thickness of the channel box material becomes gradually thinner toward the downstream, increasing the internal cross-sectional area of the channel box. It is structured as follows.

By the way, the fuel assembly disclosed in the above-mentioned Japanese Patent Application No. 62-256191@ has a water flow inside and outside the channel box, that is, a water pressure difference caused by the difference between the inch channel flow and the out channel flow, and the stress distribution in the channel box and the earthquake. By focusing on the combined value of the generated stress and the channel creep phenomenon caused by the stress caused by neutron irradiation and the water pressure difference, we clarified the distribution of possible thinning in the axial direction of the channel box, and determined that the parts that can be thinned are correctly assembled into the fuel assembly. The team discovered that this corresponds to the part that is particularly effective in improving the thermal margin of the channel box, and the downstream side of the channel box was shaved to expand the area inside the channel box.

(Problems to be Solved by the Invention) However, while considering the fuel assembly of the above-mentioned Japanese Patent Application No. 62-256191, it was discovered that the fuel spaces are spaced apart in the axial direction due to the stress generated during earthquakes. As a result, local stress was generated, and it became clear that there was room for improvement in improving the integrity of the fuel assembly.

The present invention has been made in view of the above circumstances, and its purpose is to further improve the soundness of a fuel assembly in the event of an earthquake, and to keep the structure of the fuel spacer the same in the axial direction while keeping the structure of the fuel spacer the same in the axial direction. It is an object of the present invention to provide a fuel assembly for a boiling water reactor that has good contact with a spacer and has improved stability against fluid vibrations.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel handle in which a large number of fuel rods are regularly arranged using fuel spacers, and the fuel rods are arranged regularly using fuel spacers. A channel box is arranged to surround the bundle, and the wall thickness of the channel box is made gradually thinner toward the downstream side so that the flow area of the cooling water flowing inside the channel box increases. In fuel assemblies for nuclear reactors, the next fuel spacer whose wall thickness changes stepwise from the thicker side to the thinner side is placed on the inner surface of the channel box when the fuel assembly is standing vertically and stationary. At other channel contact parts, gain pulling is performed from the outside to the inside to form a convex part toward the inside of the channel box corresponding to the fuel spacer position, and the fuel spacer contact part It is characterized by comprising the following.

(Function) In the boiling water reactor fuel assembly of the present invention, the wall thickness of the channel box is thick on the upstream side of the cooling water, and thinner in a stepwise manner toward the downstream side. The fuel spacer located on the downstream side, which changes from the thick wall side to the thin wall side, is spaced apart so that it does not come into contact with the inside of the channel box when the fuel assembly is vertically stationary, thereby preventing the occurrence of an earthquake. During horizontal photography, the local stress exerted by the fuel spacer on the channel box is significantly alleviated, and the stress at the thick stepped portion where stress tends to be concentrated is significantly reduced, improving the integrity of the fuel assembly. .

In addition, in other thin-walled portions of the channel, inward heading was performed from the outside to form a convex portion toward the inside of the channel box corresponding to the fuel spacer neck, which served as the fuel spacer contact portion, so that the fuel bundle did not come into contact with the channel. The channel box makes contact with the contact part (the convex part on the outside of the spacer), and the channel box positions the fuel assembly in a direction perpendicular to its axis, preventing vibrations of the bundle caused by the flow of cooling water and imaging caused by seismic lateral shaking. This improves the mechanical integrity of the fuel.

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

FIG. 1(A) is a partial vertical sectional view of an embodiment of the present invention, and FIG. It is a diagram showing a half.

The fuel assembly of this embodiment has a large number of fuel rods 1 arranged regularly using fuel spacers 2 to form a fuel handle 3,
A channel box 4 is arranged to surround this fuel bundle 3.

By the way, as shown in the plan view of (8) and the front view of (B) of FIG. 2, the fuel spacer 2 has channel contact portions 6 formed on both sides of its outer periphery, and they all have the same structure. However, the inside of the channel box 4 has been shaved off in order to enlarge the upper (coolant downstream) flow path.
It is written as 42.43. Therefore, if left as is, the bundle 3, except for the thick portion 41, will not be supported by the inner surface of the channel box when it is vertical. In order to avoid this, in the thin wall portions 42 and 43, dimples are performed from the outside of the channel box 4 toward the inside to form the convex portion 5 on the inside. In the present invention, this convex portion 5 will be referred to as a spacer contact portion. However, instead of making all the spacer contact parts 5 have the same structure, the next spacer part moves from the thick part to the thin part (that is, from the thick part to the middle part and from the middle part to the thin part). In this case, the channel box 4 is configured so that the spacer contact portion 5 is not provided on the inner surface thereof. Therefore, gaps 71 and 72 are formed between the channel box 4 and the spacer 2. Therefore, even when the bundle 3 is shaken horizontally due to an earthquake or the like, stress does not concentrate on the thin-walled side step portion where stress tends to be concentrated. That is, since the bundle 3 does not come into contact with the channel box 4, the health of the channel box 4 is improved. The thickness step position is determined so that the spacer position that contacts the thick side of the step is as far away from the step portion as possible. This allows further control of the stress concentration.

Next, the effect of the present invention will be explained from the perspective of the stress surface moving in the channel box.

As shown in Fig. 3 (^), the cooling water flows inside and outside the channel box, but the stress △P generated by the water pressure difference within the channel box is much greater upstream, as shown in Fig. 3 (B). In addition, the stress σ generated during an earthquake is considered to be taken horizontally, but
The bottom end of the channel box is almost a fixed end,
The top edge is somewhat variable. Therefore, the maximum stress σ generated during an earthquake is shifted slightly upward from the center, as shown in FIG. 3(C). Therefore, when the resultant stress acting on the channel box is determined, the resultant stress ΔP+σ is as shown in FIG. 3(D).

FIG. 4 is a diagram for explaining the stress acting on the channel box of the present invention by roughly adding the channel creep phenomenon to the stress shown in FIG. 3.

Figure 4 (8) is a schematic configuration diagram of a normal fuel assembly,
In the illustrated fuel assembly, fuel rods 10 are connected to Svena SP1 to SP1.
7, the regularly arranged fuel bundles are stored in the channel box 11, and a lower tie plate 12 is placed at the lower end of the channel box 11.
, and also the top tie-brake on its top edge!・13 is installed.

By the way, the channel creep phenomenon occurs due to the pressure difference between the inside and outside and neutron irradiation, but the range where this phenomenon is likely to occur is in the fourth region.
As shown in Figure (B), since this is the range (A) on the upstream side, in the present invention, the wall thickness 41 of the channel box 4 on the upstream side of this area is made thicker. After this, the resultant stress on the downstream side remains almost the same, and the resultant stress on the upstream side of the channel box total length 1'' becomes even smaller after about 173 days, so the wall thickness of channel box 4 will change as the resultant stress decreases. It is gradually thinned to 41.42.

Therefore, the amount of cooling water as a moderator in the upper half of the core increases, so the power in the upper part of the core increases, the power distribution becomes flat, and the fuel integrity improves. Furthermore, if any spacer (spl~5i)7) is brought into contact with the channel box, local stress as shown in the dashed line will occur in the spacers sp3 and sp6, but as in the present invention, the spacers sp3 and sp6 are in contact with the channel box. Since the structure is configured so that it does not come into contact with , local stress as indicated by the broken line C does not occur. For spacer Sp3, spacers Sp2 and S
In p4, spacer (JS
Stress increases slightly at p5 and Sp7, but since they are far from the thick stepped portion, an increase in stress at the stepped portion is avoided, and fuel integrity is greatly improved.

[Effects of the Invention] As explained above, according to the fuel assembly for a boiling water reactor of the present invention, the wall thickness of the channel box is made thicker on the upstream side of the cooling water, and stepped in a stepped manner toward the downstream side. The fuel spacer, which is located on the downstream side and whose wall thickness changes stepwise from the thick side to the thin side, is spaced apart so that it does not come into contact with the inside of the channel box when the fuel assembly is vertically stationary. Because of this structure, the local stress exerted on the horizontal (fuel base) channel box in the event of an earthquake (in the event of a commotion) is significantly alleviated. The integrity of the assembly is improved.Also, in the other thin channel parts, protrusions are formed toward the inside of the channel box corresponding to the fuel spacer position to serve as fuel spacer abutting parts, so the fuel bundle does not fit into the channel. Since the channel box makes contact with the abutting part and positions the fuel-flour assembly in a direction perpendicular to the axis of the fuel-flour assembly, vibration of the bundle due to the flow of cooling water and imaging due to seismic lateral shaking are eliminated, and the mechanical integrity of the fuel is improved. will improve.

[Brief Description of the Drawings] Figure 1 (^) is a vertical cross-sectional view of a channel box that constitutes the main components of an embodiment of the present invention, and Figure CB) to ([) are 2 (A) and (8) are plan views and front views of the spacer in FIG. A diagram for explaining the resultant stress moving in such a channel box,
FIGS. 4(A) to 4(C) are diagrams for explaining the channel box of the present invention in terms of thermal hydraulics. 1.10 Fuel rod 2 Spacer 3 Fuel bundle 4.11 Channel box 5 Spacer contact portion 6 Channel V channel contact portion 12 Lower part Tie plate 13... Upper tie plate (8733) Agent: Yoshiaki Inomata, patent attorney (and one other person) CD) Fig.

Claims (1)

[Claims] (1) A large number of fuel rods are regularly arranged using fuel spacers to form a fuel bundle, a channel box is arranged to surround the fuel bundle, and the wall thickness of the channel box is determined by the cooling that flows inside the channel box. In fuel assemblies for boiling water reactors whose walls are gradually thinner toward the downstream side so that the flow path area expands downstream, the wall thickness changes from thicker to thinner in a stepwise manner. The next fuel spacer is spaced apart enough that it does not come into contact with the inner surface of the channel box when the fuel assembly is standing still vertically, and in the other thin channel parts, the channels correspond to the fuel spacer positions. 1. A fuel assembly for a boiling water reactor, characterized in that a fuel spacer abutting portion is formed by dimpling from the outside to the inside so as to form a convex portion toward the inside of the box.