JPH02284095A - Fuel assembly and core of nuclear reactor - Google Patents

Abstract

PURPOSE:To suppress the deformation of a channel box by disposing a Venturi means delineated with flow passages by the inside surfaces of the channel box and the side faces on the outer side of a lower tie plate between the box and the plate. CONSTITUTION:Recesses 12 of a depth (b) are provided on the respective side faces 4A on the four sides of the lower tie plate 4. The recesses 12 are rectangu lar in shape and open toward the outer side. Slopes 13 inclining at an angle are formed atop the recesses. Clearances 14, 15 of widths G1, G2 are formed between the plate 4 and the inside surfaces of the box 8 and are directly opened into the coolant flow passages. The coolant flow passages 17 are small in the sectional area of the flow passages in the part of the clearance 14, is large in the part of the clearance 15 and is large in the flow passage area in the part upper than the clearance 14 part. The side faces 4A, the slopes 13 and a base 12A as well as the inside surfaces of the box 8, therefore, constitute the Venturi means which internally includes the flow passages 17. The pressure on the inner side of the box 8 is consequently smaller than the pressure on the outer side and the box 8 is pressed to the side face 4A side, by which the deformation toward the outer side is suppressed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a fuel assembly and a nuclear reactor core, and in particular,
The present invention relates to a fuel assembly suitable for application to a boiling water nuclear reactor and a reactor core.

[Conventional technology]

A conventional fuel assembly used in a boiling water reactor will be explained.

This fuel assembly includes an upper tie plate, a lower tie plate, a plurality of fuel rods whose ends are held by the upper and lower tie plates, a channel box surrounding the bundled fuel rods, and a finger attached to the lower tie plate. Has a spring.

The finger spring has a function of suppressing leakage of cooling water from between the channel box and the lower tie plate. However, the finger springs exert an outward force on the channel box. This force promotes creep deformation of the lower end of the channel box and pushes the lower part of the channel box outward. In order to suppress the creep at the lower end of the channel box, Japanese Patent Application Laid-open No. 170692/1983 discloses a structure that does not use finger springs. In this structure, as shown in FIG. 7 of the above-mentioned publication, a step having an inward slope is provided on the outer periphery of the lower tie plate, and a channel box is placed on the step. The channel box's own weight is used to keep the channel box in close contact with the lower tie plate at all times to prevent leakage of cooling water inside the channel box. Furthermore, since the channel box is placed on a step having an inward slope as described above, an inward component force is applied to the lower end of the channel box 2, thereby suppressing outward deformation of the lower end.

In the conventional example described above, the outward force of the finger spring promotes bulge due to creep at the lower end of the channel box. In addition, in a structure that does not use finger springs, the lower end of the channel box and the lower tie plate are always in close contact with each other, so the pressure difference between the inside and outside of the channel box causes a force to act on the lower end of the channel box from the inside to the outside, potentially promoting creep. be. When the lower end of the channel box is pushed out, the amount of leaked cooling water naturally increases.

An object of the present invention is to provide a fuel assembly and a nuclear reactor core that can suppress deformation of a channel box with a simple structure that utilizes the driving force of a coolant.

[Means to solve the problem]

The above object can be achieved by disposing a venturi means between the channel box and the lower tie plate, the flow path being defined by the inner surface of the channel box and the outer side of the lower tie plate.

[Effect]

By allowing the coolant to flow through the venturi means, the force pushing the channel box outward can be significantly reduced, and deformation of the channel box can be significantly suppressed. Further, the structure of the venturi means itself is simple.

〔Example〕

A fuel assembly according to an embodiment of the present invention suitable for use in a boiling water reactor will be described with reference to FIGS. 1 and 2.

The fuel assembly 1 of this embodiment includes an upper tie plate 2, a lower tie plate 4, a fuel rod 5, a water rod 6, and a fuel spacer 7. Both ends of the fuel rod 5 and the water rod 6 are held by the upper tie plate 2 and the lower tie plate 4. The upper and lower tie plates are connected by connecting fuel rods, not shown. Each fuel rod 5 is supported horizontally by a fuel spacer 7. Therefore, a coolant passage having a predetermined width is formed between each fuel rod 5.

The channel box 8 is a cylindrical body with a square cross section, and is attached to the corner post 3 of the upper tie plate 2 by a cap screw 10 provided on a channel fastener 9. The lower end of the channel box 8 is a free end. Such a channel box 8 surrounds the fuel rod bundle bound by the fuel spacer 7. The channel fastener 9 holds each fuel assembly 1 loaded in the core so that the upper end of the fuel assembly 1 is pressed against an upper core support plate (not shown). channel box 8
The area between the fuel rods 5 within is a coolant passage 11.

A recess 12 having a depth b is provided on each of the four outer side surfaces 4A of the lower tie plate 4. The recess 12 is
It has a rectangular shape and opens outward. Recess 12
The upper surface thereof is an inclined surface 13 inclined at an angle θ, as shown in FIG. 3(A). The upper end of the inclined surface 13 is located at a distance a from the upper surface of the lower tie plate 4.

As shown in FIG. 3(A), there is a gap 14 having a width Gl between the lower tie plate 4 and the inner surface of the channel box 8.
and width G2, rs15 is formed. That is, gap 1
4 is formed between the outer side surface 4A of the lower tie plate 4 and the inner surface of the channel box 8. Also, the gap 14
is formed between the bottom surface 12A of the recess 12 and the inner surface of the channel box 8. The inclined surface 13 and the bottom surface 12A are also
It can also be said to be a part of the side surface 4A. The gap 14 is located above the gap 15 and opens directly into the coolant passage 11.

Gap 15 is located below one gap 14 and communicates with gap 14. Since the inclined surface 13 is located between the gap 14 and the gap 15, a gap 16 whose cross-sectional area gradually increases downward is formed between the gap 14 and the gap 15.

Cooling water is supplied into a fuel assembly 1 loaded in the core of a boiling water reactor. This cooling water is led into the channel box 8 via the lower tie plate 4. Most of the cooling water rises within the coolant passage 11 and flows upward from the upper tie plate 2. Some of the cooling water is in the gap 14.1
6 and 15 to the outside of the fuel assembly 1. However, the amount of leakage of this cooling water is
As described in No. 6, the cooling water is suppressed by a jet flow of cooling water that is ejected from the outer circumference of the lower tie plate 4 toward the inner surface of the channel box 8.

The mechanism that generates this jet stream is not shown in this embodiment.

Side surface 4A of lower tie plate 4 and channel box 8
cooling water passage defined by the inner surface of (between vX14,1
ti') 17i;t,, the flow passage cross-sectional area is small in the gap 14 part, the flow passage area in the gap 16 part gradually increases toward the gap IS part, and the flow passage area in the S gap 15 part The flow path area is large. Above the gap 14, the flow path area is large. Therefore, it can be said that the side surface 4A, the inclined surface 13, the bottom surface 12A, and the inner surface of the channel box 8 that faces these constitute a venturi means that includes therein the cooling water passage 17 having the above-described flow passage cross section.

In the portion 1114 where the flow path area is small, the cooling water flows at high speed and the static pressure is lower than that in the coolant path 1.1. especially,
The static pressure is lowest at the lower end of the gap 14 (starting point of the inclined surface 13). Therefore, the pressure P1 inside the channel box 8 becomes smaller than the pressure P2 outside the channel box 8 in the gap 14, and the pressure difference ΔP (=Pt-Pz) between them becomes negative pressure. In other words, the pressure gradually recovers at a portion of the gap 16 where the pressure pushing the channel box 8 inward is greater than the pressure pushing the channel box 8 outward (the force that spreads the channel box 8). At the gap 15, the pressure difference between the inside and outside of the channel box 8 becomes zero. When the pressure difference ΔP becomes a negative pressure, the channel box 8 is pressed toward the side surface 4A of the lower tie plate 4. Therefore, outward deformation at the lower end of the channel box 8 is significantly suppressed, and the amount of deformation is significantly reduced. The widths Gi and G2 of the gaps 14 and 15 remain approximately constant from the beginning to the end of the operating cycle.

Figure 3 (A) shows the state after manufacturing the fuel assembly 1 and before loading it into the reactor core (state at burnup Q k w d / t), and Figure 3 (B) shows the state during reactor operation. The state of the fuel assembly 1 is shown. While the reactor is shut down, the fuel assembly 1 is in the state shown in FIG. 3(A). The fuel assembly 1 before being loaded into the core has a considerably long length Lo overlapping the side surface 4A of the lower tie plate 8 of the channel box 8.

However, during nuclear reactor operation, the channel box 8 also moves upward because the upper tie plate 2 is lifted upward due to the axial extension due to thermal expansion of the fuel rods 5 and the like. Therefore, during reactor operation, the length Ll of the channel box 8 of the portion that overlaps the side surface 4A of the lower tie plate 4 becomes shorter than the length Lo.

The length 1 must be determined so that the lower end of the channel box 8 faces the bottom surface 12A of the recess 12 even during reactor operation. That is, (Ll> a
+c) must be satisfied. Here, C is the height of the inclined surface 13 in the axial direction of the lower tie plate. In the new fuel assembly 1 before core loading, the length Ll
The length Lo is set so that .

The negative pressure difference ΔP as described above is obtained by utilizing the Venturi effect shown by the following equation (1).

Ps=Po-pvz/2g-
(1) Here, Pa is the total pressure of the cooling water flowing in the cooling water passage 17, Ps is the static pressure of the cooling water, and ρvz/2g is the dynamic pressure of the cooling water.

FIG. 4 shows the effect of the venturi means described above.

FIG. 4 shows the total pressure Pa and static pressure PS in the cooling water passage 17 at a position below the upper end of the lower tie plate 4, and the amount of deformation of the channel box. In FIG. 4, the broken line indicates the characteristics of this embodiment shown in FIG. Moreover, the characteristics of the solid line are the side surface 4 that overlaps the channel box 8 without providing the recess 12 on the outer side surface 4A of the lower tie plate.
A is for a conventional fuel assembly 1 having a straight shape. Note that the vertical axes in FIGS. 4(A) and 4(B) indicate the distance from the upper end of the lower tie plate 8.

In the conventional example, as shown in FIG. 4(A), a force (positive static pressure P5) that deforms the channel box 8 outward acts on a portion of the channel box 8 below the 4° upper end of the lower tie plate. ing. According to this embodiment, due to the function of the venturi means, the force that presses the channel box 8 toward the lower tie plate 4 side is greater than the force that presses the channel box 8 outward, as shown in FIG. 4(A). Become. Therefore, in this embodiment, as shown in FIG. 4(B), the amount of outward deformation of the channel box 8 is smaller than in the conventional example.

The channel box 8 is constantly subjected to pressure from the inside to the outside due to the static pressure of the cooling water in the coolant passage 11 during operation of the nuclear reactor, so that the channel box 8 undergoes creep deformation toward the outside. In the conventional example, due to the creep deformation of the channel box, the flow rate of cooling water leaking from between the lower tie plate 4 and the channel box 8 as the burnup increases, as shown by the solid line in FIG. leakage flow rate) increases. However, in this embodiment, as described above, the force that presses the channel box 8 toward the lower tie plate 4 side is large, so an increase in the amount of creep deformation of the channel box 8 can be suppressed.

Therefore, in this embodiment, as shown in FIG. 5, the leakage flow rate of the cooling water does not change much even when the burnup increases.

In this way, this embodiment can maintain the leakage flow rate of cooling water almost constant throughout the operation cycle. This also leads to the ability to further stabilize the thermal output of the fuel assembly 1.

Each of the above-mentioned functions in this embodiment can be achieved by a simple structure such as the cooling water passage 17 formed between the 5-channel box 8 and the lower tie plate 4, in which the venturi effect is exerted by changing the area area of the flow passage. be able to.

In this embodiment, in order to more effectively produce the function of the venturi means, it is desirable to consider the following.

That is, in the new fuel assembly 1 before core loading, a
/ Lo preferably satisfies the following conditions.

0.3≦a/Lo<a-(
2) Here, α is such that even if the lower end of the channel box 8 moves upward as shown in FIG. This is a limit value set to prevent this from occurring. This limit value depends on the setting conditions of the burnup of the fuel assembly 1.

When the lower end of the inclined surface 13 is located below the channel box 8, the venturi effect by the venturi means disappears. In this case, the leakage flow rate of cooling water increases rapidly.

Increasing the distance a increases the abrasion pressure loss between the channel box 8 and the lower tie plate 4, thereby suppressing an increase in the leakage flow rate of the coolant. However, since the starting point (upper end) of the inclined surface 13 approaches the lower end of the channel box 8, a negative pressure and zero pressure region of the pressure difference ΔP that can suppress outward deformation of the channel box 8 is located at the lower end of the channel box 8. As the distance 1a becomes closer, the creep deformation of the channel box 8 becomes larger than when the distance 1a is small.

FIG. 6 shows the dependence of the leakage flow rate of cooling water on the distance 1ia when creep deformation of the channel box 8 is considered. From the characteristics shown in FIG. 7, the leakage flow rate of cooling water is significantly reduced in the region where a/Lo≧0.3. For this reason, a
/Lo≧0.3 is preferably satisfied. In particular, a
/Lo≧0.6, the leakage flow rate of cooling water is minimized, so
It is even better if a/Lo≧066 is added by 1121.

Further, the angle θ of the inclined surface 13 is preferably set to 5 to 6 degrees.
The part of the gap 16 (the inclined surface 13
pressure loss is minimized. Therefore, the Venturi effect can be maximized.

The depth b is preferably in the range of 1 to 2 m. The depth b of the recess 12 needs to be determined so that the pressure difference ΔP, which has become a negative pressure in the gap 14, can be restored to zero. That is, it is necessary to reduce the dynamic pressure ρv''72g to almost zero in the gap 15. Therefore, the depth b must be 11 m or more.

However, if the depth b is increased, the wall thickness of the side wall of the lower tie plate 4 becomes too thin, which poses a problem in terms of strength. Taking this into consideration, the appropriate range for the depth b is 1 to 2 m.

In order to utilize the gaps 14 to 16 as venturi means, no protrusion that would narrow the width G2 of the gap F$15 should be provided in the recess 12.

In the embodiment shown in FIG. 1, the bottom surface 12A and sloped surface 13 of the recess 12 provided in the lower tie plate 4, and the portion of the side surface 4A located above the sloped surface 13 are located between these and the channel box 8. It can also be said that this is a means for generating a force in the channel box 8 toward the lower tie plate 4 when cooling water flows.

A fuel assembly according to another embodiment of the present invention is shown in FIG.

FIG. 7 shows the vicinity of the lower tie plate of the fuel assembly IB of this embodiment. This embodiment differs from the embodiment shown in FIG. 1 in the structure of the lower tie plate. The lower tie plate 4B used in this embodiment has a stepped portion 18 instead of the recessed portion 12 on the side surface 4A. The stepped portion 18 is formed by continuing the recessed portion 12 shown in FIG. 2 over the entire circumference of the side surface of the lower tie plate 4. The stepped portion 18 is located closer to the central axis of the lower tie plate 4B than the side surface 4A. The stepped portion 18 and the side surface 4A are smoothly connected by the inclined surface 13.

This embodiment also produces effects similar to those of the embodiment described above. Machining of the side surface 4A of the lower tie plate 4B is easier than that of the lower tie plate 4.

A fuel assembly according to another embodiment of the present invention is shown in FIG.

FIG. 8 shows the structure of this embodiment corresponding to FIG. 3(A). The fuel assembly 1c of this embodiment is the same as the fuel assembly 1 except for the structure of the channel box and lower tie plate. The fuel assembly 1c has a lower tie plate 4c and a channel box 8A shown in FIG. The lower tie plate 4c is a lower tie plate 4 that does not have the recess 12 on the side surface 4A. Channel box 8A is attached to upper tie plate 2 similarly to channel box 8. Similarly to the fuel assembly 1, the fuel assembly 1c also has a side surface 4A of the lower tie plate 4c and a channel box 8.
A is provided with venturi means defined by the inner surface of A.

However, the venturi means provided in this embodiment is
It is constructed by providing recesses 1 and 2 in the channel box 8A. That is, a protrusion 8C that protrudes outward is formed at the lower end of the channel box 8A. The protrusion 8C is also a tube with a square cross section, similar to the main body 8B of the channel box 8A. The main body portion 8B and the protruding portion 8C are connected by an inclined portion.

The inner surface 8E of the protruding portion 8C is located outside the inner surface 8D of the main body portion 8B by a depth b. An inclined surface 13A inclined at 0 degrees is formed inside the inclined portion. The upper end of the inclined surface 13A is connected to the inner surface 8D, and the lower end of the inclined surface 13A is connected to the inner surface 8E.

Also in this embodiment, there is a gap 14 with a width G1 between the lower tie plate 4 and the channel box 8A.
It has a cooling water passage 17 including a gap 15 having a width G2 and a gap 16 at a portion of the inclined surface 13A.

Therefore, in this embodiment, the venturi means including the cooling water passage 17 is connected to the lower tie plate 4 and the channel box 8A.
placed between.

In this embodiment as well, effects similar to those of the fuel assembly 1 can be obtained. However, manufacturing for providing the venturi means, that is, machining the lower end of the channel box 8A is more troublesome than machining the recess 12.

In this embodiment, an inner surface 8E, an inclined surface 13A, and a portion of the inner surface 8B located above the inclined surface 13A and facing the outer surface 4A are located between these and the lower tie plate 4C. It can also be said that this is a means for generating a force in the channel box 8A toward the lower tie plate 4C when cooling water flows.

FIG. 8 shows the structure of the new fuel assembly IC before loading into the core. It is desirable that the values of a/Lo, θ, and b in this embodiment are set similarly to the fuel assembly 1.

An example of a core of a boiling water reactor will be described below. A reactor core is provided within a reactor pressure vessel. This core is constructed by loading a large number of fuel assemblies 1. The lower tie plate 4 of the fuel assembly 1 is supported by a fuel support fitting provided on the lower core support plate. Four fuel assemblies are supported by one fuel support fitting. Control rods can be inserted between the fuel assemblies 1 through fuel support fittings. Cooling water is filled into the reactor pressure vessel,
Cooling water flows into the reactor core. The flow of cooling water will be explained in detail. Cooling water supplied by driving the recirculation pump is supplied into the fuel assembly 1 from the lower tie plate 4 through a passage provided in the fuel support fitting. Most of the cooling water flows out of the fuel assembly 1 from above through the coolant passage 11, as described above. A part of the cooling water leaks to the outside from the cooling water passage 17 and rises through the gap formed between the fuel assemblies 1. In this embodiment, the cooling water passage 17 reduces the static pressure of the coolant between the channel box 8 and the lower tie plate 4 compared to the static pressure of the coolant between the fuel assemblies 1 at that level. It is a means. The core of this embodiment naturally produces the effect obtained by the five fuel assemblies l.

[Effects of the Invention] According to the present invention, deformation of the channel box can be significantly suppressed with a simple structure that can utilize the driving force of the coolant.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of a fuel assembly that is a preferred embodiment of the present invention, FIG. 2 is a perspective view of the vicinity of the lower tie plate of the embodiment of FIG. 1, and FIG. A vertical cross-sectional view of the vicinity of the lower tie plate of the embodiment shown in Figure 1 before core loading, Figure 3.
FIG. 4(B) is a longitudinal cross-sectional view of the vicinity of the lower tie plate during the reactor operation of the embodiment of FIG. 1, and FIG. 4(A) is a characteristic diagram showing the pressure distribution at each level from the upper end of the lower tie plate. FIG. 4(B) is a characteristic diagram showing the distribution of the amount of deformation of the channel box at each level from the upper end of the lower tie plate, FIG. 5 is a characteristic diagram showing the change in cooling water leakage flow rate with respect to burnup during the operation cycle, FIG. 6 is a characteristic diagram showing the relationship between a/Lo and the cooling water leakage flow rate, FIG. 7 is a perspective view of the vicinity of the lower tie plate of a fuel assembly according to another embodiment of the present invention, and FIG. FIG. 7 is a vertical cross-sectional view of the vicinity of the lower tie plate of a fuel assembly according to another embodiment of the invention. 1...Fuel assembly, 2...Upper tie plate, 4...
...Lower tie plate, 4A...Side surface, 5...Fuel rod, 8...Channel box, 11...Coolant channel, 12...Recess, 12A...Bottom surface, 13...Slope surface ,
14th to 16th concave and concave portions (A) (B) Hou (A) Pressure distribution The cat in the 1st turn of the sword, the cutting degree (5 pieces / 2A, the 2nd picture). The furnace, the bottom of the group assembly is P type L4 f Stomach Tsu 6j Shinaya Nonel to Nkunu

Claims (1)

[Claims] 1. A plurality of fuel rods, a lower tie plate that supports the lower ends of the fuel rods, and a channel box that surrounds the bundled fuel rods and into which the upper ends of the lower tie plates are inserted. In the fuel assembly, ventilary means having a flow path defined by an inner surface of the channel box and an outer side surface of the lower tie plate is disposed between the channel box and the lower tie plate. A fuel assembly featuring: 2. A fuel assembly comprising a plurality of fuel rods, a lower tie plate that supports the lower ends of the fuel rods, and a channel box that surrounds the bundled fuel rods and into which the upper ends of the lower tie plates are inserted. In the body, means is provided on the lower tie plate for generating a force in the channel box toward the lower tie plate when the coolant guided from the fuel assembly passes between the opposing channel boxes. A fuel assembly characterized by: 3. A fuel assembly comprising a plurality of fuel rods, a lower tie plate that supports the lower ends of the fuel rods, and a channel box that surrounds the bundled fuel rods and into which the upper ends of the lower tie plates are inserted. In the body, the channel box is provided with means for generating a force in the channel box toward the lower tie plate when the coolant guided from the fuel assembly passes between the opposing lower tie plates. A fuel assembly characterized by: 4. A plurality of fuel assemblies including a plurality of fuel rods, a lower tie plate that supports the lower ends of the fuel rods, and a channel box that surrounds the bundled fuel rods and into which the upper ends of the lower tie plates are inserted. In a nuclear reactor core loaded with fuel cells and through which coolant flows, the fuel assemblies reduce the static pressure of coolant between the channel box and the lower tie plate at that level. A core of a nuclear reactor, characterized in that it is equipped with means for reducing the static pressure of the reactor.