JP2001194479A - Spacer of fuel assembly for boiling water reactor and fuel assembly using it for boiling water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase cooling capability by only attaching simple constitution parts to proper locations in spacers of fuel assembly for a boiling water reactor and removing sufficient heat from fuel rods of the fuel assembly with coolant and suppress the burnout generation on the fuel rods, even in high power operation by improving the limit power characteristics. SOLUTION: A plurality of specific number of outward projections 11, existing on the outer plate 3a of a spacer body and touching the inner wall surface 5a of a channel box 5 coupled by the outer plate are attached. According to this, the liquid film of the coolant flowing upward along the inner wall surface 5a is made to hit the outward projections 11 and dispersed into droplets. The droplets make the liquid film thicker on the outer surface 1c of the fuel rods 1, in the channel box 5 side exposed to very severe thermal conditions and attain increase in cooling effects by this.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to be incorporated in a fuel assembly for a boiling water reactor to promote heat transfer of a coolant to fuel rods and to improve the limit output characteristic of the fuel assembly. And a spacer that prevents fuel rod burnout (burnout) from occurring even during high-power operation, and a boiling water structure that is constructed by incorporating the spacer in an appropriate position so that the characteristics of the spacer can be utilized. The present invention relates to a fuel assembly for a nuclear reactor.

[0002]

2. Description of the Related Art As is known, a fuel assembly loaded in a boiling water reactor (BWR) includes a rod-shaped fuel element 1 including a fuel rod 1a and a water pipe 1b as shown in FIG. The upper end plug and the lower end plug are attached to the upper tie plate 2a and the lower tie plate 2b, respectively, at intervals, and spacers 3 are provided at a plurality of positions at intermediate height positions. The rod-shaped fuel element 1 is held at a predetermined position. The fuel assembly 4 for a boiling water reactor is shown in FIGS. 5 (A) and 5 (B).
As shown in Fig. 5A, the core R is housed in a rectangular tube channel box 5 shown in Fig. 5A made of Zircaloy so as to secure a coolant for removing heat from the fuel rods 1a in the core R. The required number of seats in the structural section are loaded, and the BWR
During the operation, coolant is supplied from the inlet orifice of the seat into the channel box 5 so as to flow upward between the fuel rods 1a of the fuel assembly 4 and the fuel rod 1
The cooling water is boiled by the reaction heat of (a) to take out the heat to the outside, and the heat removal of the fuel assembly 4 is performed as shown in FIG. 5 (B).
, And 6 indicates a control rod.

As described above, since the BWR is a direct cycle system in which steam is generated in the furnace, it allows boiling in the core R. Therefore, the cooling of the fuel rod 1a is performed in a known manner by a liquid phase and a vapor (void). 6), the flow of the coolant on the upper azimuth side of the fuel assembly 4 having a high void ratio at this time is as shown in FIG. That is, the film-like liquid phase (liquid film) a1 forms a continuous annular flow on the inner peripheral wall surface 5a of the channel box 5 and the outer peripheral surface 1c of the fuel rod 1a, and the channel box 5 and the fuel rod 1
a, and between the fuel rods 1a, 1a, an annular flow of the vapor phase b accompanied by the droplets a2 occurs. Of course, the liquid film a1 is more important for cooling the fuel rod 1a. Will play a role.

[0004] However, when the fuel assembly 4 is overpowered for some reason and placed in a severe thermal state, a boiling transition from a nucleate boiling state to a film boiling state occurs, which is understood by FIG. As described above, the liquid film a1 on the outer peripheral surface 1c of the fuel rod 1a disappears, causing a rapid deterioration of the heat removal effect, and the temperature in the disappearing region rapidly rises, and finally, the burnout of the region (drying in the BWR) occurs. Out), which will cause burnout,
The load on the entire fuel assembly 4 at this time is called a critical output.

Referring to FIG. 8, the fuel rods 1a and the like of the fuel assembly 4 are held at a predetermined distance from each other with reference to FIG. Focusing on the flow path section f, the corner sub-channel f1 existing outside the corner rod Ac, which is the fuel rod 1a, which is generally adjacent to the inner peripheral wall surface 5a of the channel box 5, and the outside of the side rod As. In the side sub-channel f5, since the friction pressure loss due to the coolant flow is large, the relative value of the coolant mass flux is shown in FIG.
, Center sub-channels f2-f4 and f6-f outside the center rod A inside.
8, which tends to be smaller. Accordingly, in the corner rod Ac in contact with the corner sub-channel f1 where the coolant mass flux value is the smallest, and then in the side rod As in contact with the side sub-channel f5, dryout due to disappearance of the liquid film a1 is likely to occur. .

Therefore, as a conventional technique for resolving the above-mentioned defects, the lattice-shaped spacer 3A shown in FIG. 10 and the ring-shaped spacer 3B shown in FIG. In the frame-shaped outer plate 3a, a projecting piece called a flow tab 7 whose upper end is bent inward is projected upward from the upper end edge of the outer plate 3a, whereby the inner peripheral wall surface 5a of the channel box 5 is formed. The flowing liquid film a1 is changed to the fuel rod 1a, whereby the liquid film a1 at the corner rod Ac or the side rod As is made thicker to improve the limit output. Here, as shown in FIG. 10, inside the outer plate 3a, the fuel rod insertion element 3d is defined by the intersection of the vertical strap 3b and the horizontal strap 3c,
3e indicates a fuel rod elastic spring for holding the fuel rod 1a. In FIG. 11, as is also known, 3f is a ring provided in the outer plate 3a in a circumscribed state, 3g is a fuel rod insertion element, and 3h is a fuel rod elastic spring. I have.

As shown in FIG. 12, the fuel rod elastic spring 3e of the lattice spacer 3A includes not only the vertical trap 3b and the horizontal trap 3c but also the outer plate 3a.
Of course, only the fixing plate 3e 'is in close contact with the outer plate 5 on the outer side of the channel box 5 side, and the fuel rod insertion element 3d on the side of the fuel rod 1a. Only when it does, it protrudes so as to contact the outer peripheral surface 1c. In the above-mentioned ring type spacer 3B, as shown in FIG. 11 (A), the outer plate 3a is not provided with the fuel rod elastic spring 3e but only provided on the ring 3f. Absent.

[0008]

In the case of the conventional spacer for a fuel assembly for a boiling water reactor having the above-described structure, the provision of the flow tab 7 is intended to improve the limit output characteristics. But uranium 235
In order to increase the utilization rate of the fuel and to improve the cycle cost of the fuel, it is necessary to increase the enrichment of the fuel rods on the outermost peripheral side where the neutron spectrum is soft, that is, the uranium 235 as the corner rod and the side rod. Therefore, in such a design, the output of the outermost fuel rods is increased. Therefore, only the protrusion of the flow tab 7 as in the conventional example alone makes the outer side of the corner rod Ac on the side of the corner sub-channel f1. The decrease in the thickness of the liquid film a1 on the surface and the surface on the side subchannel f5 side outside the side rod As cannot be compensated for, and as a result, the critical output of the fuel assembly 4 decreases.
That is, the thermal margin as fuel is reduced, and it is considered difficult to significantly reduce the cycle cost.

The present invention has been made in view of the above-mentioned problems of the prior art. According to the present invention, the outwardly projecting member provided on the outer plate of the spacer is provided inside the channel box fitted on the spacer. By making contact with the peripheral wall surface, the liquid film of the coolant is caused to strike the outwardly projecting body, thereby causing a positive turbulence with respect to the liquid film flow on the inner peripheral wall surface. Attempts are made to increase the cooling effect by attaching droplets that are generated due to turbulence to fuel rods as corner rods and side rods, thereby improving the marginal output and reducing the fuel cycle cost. But that is the purpose.

According to a second aspect of the present invention, instead of providing only the outwardly protruding body on the outer plate in the first aspect, a fuel rod elastic spring provided in a conventional lattice spacer is provided. By forming the outward projection integrally with the outer projection and, of course, forming its free end above, the rigid attachment of the outward projection to the outer plate is possible without increasing the number of manufacturing steps. Trying to be.

Further, in the case of the third aspect, the spacer according to the first aspect is used, and the spacer is disposed in the height direction of the fuel assembly for a boiling water reactor, the second highest from the top,
By arranging at least one of the third and fourth,
It is possible to effectively cool the upper part of the fuel rod, which is in the most thermally severe condition, in the upper direction, which enhances the thermal margin in that part and burns the fuel rod even when operating at high power To provide a fuel assembly in which the above does not occur.

[0012]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention is divided into a plurality of rod-like fuel elements which are held at a predetermined interval in an outer plate and is provided with a flow of coolant. In the spacer body having a large number of flow passage sections, the outer plate is in contact with the inner peripheral wall surface of a channel box mounted on the spacer body of the fuel assembly for a boiling water reactor, It is an object of the present invention to provide a spacer for a fuel assembly for a boiling water reactor, in which a required number of outwardly projecting bodies against which a liquid film of a coolant flowing along an inner peripheral wall abuts are provided.

Further, in claim 2, the above-mentioned claim 1 is provided.
, The outwardly projecting body provided on the outer plate is located above and has a free end formed therein, and in the case of a lattice spacer, the outwardly projecting body is provided on the inner side of the outer plate. It is formed in one piece with the holding spring, and the content is that the protruding portion on the lower side of the free end is configured to elastically contact the inner peripheral wall surface of the channel box.

According to a third aspect, in the first aspect,
By using the spacers of
It is another object of the present invention to provide a fuel assembly for a boiling water reactor, wherein the fuel assembly is arranged at one or more of the third, fourth and fourth height positions.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to FIGS. 1 to 4. FIGS. 1 and 2 show an embodiment in which the spacer is a lattice type spacer and a ring type spacer, respectively. The same reference numerals are used for the same members as those described in the conventional example. Here, first, the spacer body 10 supplies the coolant from below to the rod-shaped fuel element 1 such as the fuel rod 1a which is held at a predetermined interval on the outer plate 3a as described with reference to FIG. Channel sections f (f1 to f
8), the spacer body 10 according to claim 1, wherein the outer plate 3a has a required number of outwardly projecting bodies 11 on a desired number over the entire outer peripheral surface thereof. It is installed vertically with a separation interval.

FIG. 1 shows an example of the outwardly protruding body 11 in the lattice spacer 3A described above, which is integral with a fuel rod elastic spring 3e provided inside the outer plate 3a. Shows the case formed in
Of course, the fuel rod resilient spring 3e may be attached to a completely separate body and at a different location. In the case of the integral formation described above, the lower end folded portion 3k of the fuel rod elastic spring 3e is engaged with and fixed to the engaging groove 3j which is not provided at the lower end edge 3i of the outer plate 3a, and the inner protrusion is formed as is known. The curved portion 3m is in elastic contact with the outer peripheral surface 1c of the rod-shaped fuel element 1 such as the fuel rod 1a. The outwardly protruding body 11 extending upward from the lower end folded portion 3k is bent symmetrically with respect to the fuel rod elastic spring 3e, and has a curved projection formed below the free end portion 11a. Part 11b
Are formed to resiliently hold on the inner peripheral wall surface 5a of the channel box 5, and reference numeral 7 denotes the flow tab described above.

Next, referring to FIG. 2, the outwardly projecting body 11 in the case of the ring-shaped spacer 3B as shown in FIG. 3 will be described.
Conventionally, the outer plate 3a is not provided with a member corresponding to the fuel rod elastic spring 3e in the lattice spacer 3A, and the fuel rod elastic spring 3h is
It is merely provided on the ring 3f as described above. Therefore, as a matter of course, the outward projecting body 11 attached to the outer plate 3a as shown in FIG. 2 is resiliently mounted on the free end portion 11a and the inner peripheral wall surface 5a of the channel box 5 in order from the top as in the case of FIG. The contacting bent portion 11b is formed,
From the lower end folded portion 11c of the projecting body engaged with the engaging groove 3j of the lower edge 3i of the outer plate 3a, the outer plate 3a
In the illustrated example, the holding plate portion 11d to be crimped to the inner peripheral surface is extended to a height facing the free end portion 11a.

The above-mentioned outward projecting body 11 is shown in the case where it is formed in a pin shape or a band shape.
Further, as shown in the example of the ring spacer 3B,
It is also possible to adopt a plate shape. This is illustrated by the same figure (C), and the plate body is bent in a V-shape to form a protruding portion 11b at the intersection of the horizontal enhancement plate portion 11e and the horizontal lower plate portion 11f. The upper edge 11g and the lower edge 11h formed on the plate 11e and the horizontal lower plate 11f are fixed to the outer peripheral wall 3n of the outer plate 3a by welding or the like. As a result, the above-mentioned protruding portion 11b becomes horizontally long and comes into contact with the inner peripheral wall surface 5a of the channel box 5 as described above. In FIG. In the outer plate 3 a of the main body 10, each outer peripheral wall 3 of four sides
One piece is fixed at the center of n.

As shown in FIG. 4, the upward flow of the liquid film a1 on the inner peripheral wall 5a of the channel box 5 is caused by the installation of the outward projection 11 which discloses various embodiments as described above. A turbulent state is caused by the collision with the outwardly protruding body 11 that is in contact with the fuel cell 1, and this causes aggressive scattering of droplets, and the droplets are directed to the fuel rods 1 a as corner rods and side rods. Therefore, by adhering to the outer peripheral surface 1c, the liquid film a1 can be thickened. Therefore, by increasing the thickness of the liquid film, the heat removal effect of the fuel rod 1a is improved, the limit output is increased, and the thermal margin is increased. Further, it is possible to increase the enrichment of the uranium 235 related to the fuel rods arranged at the outermost periphery of the soft neutron spectrum, thereby improving the fuel cycle cost.

In order to confirm the effects of the present invention as described above,
A thermal-hydraulic test was conducted with a prototype of a 3-row, 3-column spacer.
In this test, a simulated fuel rod having a built-in electric heater was bundled into a 3 × 3 array at a predetermined interval by the spacer, and the test rod was used. The occurrence of boiling transition (dryout) was checked by temperature measurement using a thermocouple embedded in the surface of the simulated fuel rod. As a result, the limit output was smaller than when the outwardly projecting body 11 according to the present invention was not used. , A few percent improvement could be observed.

Next, claim 3 will be described. Here, a fuel assembly for a boiling water reactor obtained by appropriately arranging the spacer according to the present invention described in claim 1 is presented. I have. As is known, in a fuel assembly for a boiling water reactor such as 8 × 8, 9 × 9 and 10 × 10, etc., a plurality of spacers are arranged with upper and lower steps. It has been experimentally confirmed that the most severe thermal condition of the fuel rods is immediately below each of the spacers disposed at the highest and second or third height positions of the fuel assembly 4. Have been. Therefore, attention should be paid to the above-mentioned phenomenon, and in order to effectively use this phenomenon, a third aspect of the present invention will be described.
In this configuration, the spacers of the fuel assembly for a boiling water reactor are arranged so as to be disposed at any one or more of the second, third, and fourth positions from the top.

In the case of the above configuration, by disposing the spacer according to the present invention second in the same manner as above, the fuel rod 1a immediately below the uppermost spacer, which is in the most thermally severe state, is provided.
Has a thermal margin, and it is possible to extremely efficiently improve the limit output. Similarly, the third and fourth spacers are located at the respective locations directly below the second and third spacers, which are also subjected to severe thermal conditions, and cool the fuel rod 1a. The effect can be improved efficiently. Therefore, the cooling effect can be improved by adopting only the second, third and fourth spacers according to the present invention, but more desirable results can be obtained by further increasing the number of spacers selected. Will be done.

[0023]

Since the present invention is constructed as described above, according to the first aspect, the outward projection is provided on the outer plate of the spacer body so as to contact the channel box. The liquid film of the coolant can impinge on the body, and the droplets generated at this time can increase the thickness of the liquid film on the adjacent fuel rod. This makes it possible to effectively cool even the most difficult-to-cool corner rods and fuel rods as side rods, improving the critical power characteristics of the fuel assembly for a boiling water reactor, and improving fuel efficiency even with high power operation. Burnout of the bar can be effectively suppressed.

According to the second aspect of the present invention, in the lattice spacer, the known fuel rod elastic spring and the outwardly projecting body are integrally formed and arranged on the front outer plate. The productivity can be improved without increasing the pressure loss to the coolant and without increasing the number of manufacturing steps, and a structurally robust product can be provided.

According to the third aspect of the present invention, the fuel assembly for a boiling water reactor is constituted by disposing the spacer at an appropriate height, so that the thermal condition of the fuel rod becomes severe. It is possible to actively cool the local area, and it is possible to extremely efficiently improve the limit output characteristics.

[Brief description of the drawings]

FIG. 1 shows a spacer of a fuel assembly for a boiling water reactor which is a lattice spacer according to the present invention, wherein (A) is a partial front view thereof, and (B) is a BB line arrow of (A). FIG.

2A and 2B show a spacer of a fuel assembly for a boiling water reactor, which is a ring type spacer according to the present invention, wherein FIG. 2A is a partial front view, and FIG. 2B is a BB line of FIG. FIG.

3A and 3B are spacers of a fuel assembly for a boiling water reactor according to another embodiment of the present invention as a ring-type spacer according to the present invention, wherein FIG. 3A is a plan view, FIG. (C) is a perspective view showing the outward projection.

FIG. 4 is a view showing a state in which a liquid film is disturbed due to a relative relationship between an outwardly protruding body in a spacer of the fuel assembly for a boiling water reactor shown in FIG. FIG. 3 is an explanatory front view in vertical section showing a flow state of a droplet.

FIG. 5A is a partially cutaway perspective view showing a known fuel assembly for a boiling water reactor, and FIG. 5B is an explanatory plan view showing a reactor core constituted by the fuel assembly; is there.

FIG. 6 is a cross-sectional plan view showing a distribution state of a liquid phase and a vapor phase of a coolant at a corner of a known fuel assembly for a boiling water reactor.

FIG. 7 is a chart showing the relationship between the length (height) of the fuel rod and the temperature of the outer peripheral surface of the fuel rod for a known fuel assembly for a boiling water reactor.

FIG. 8 is a cross-sectional plan view showing a flow path section (sub-channel) divided by a fuel rod or the like in a known fuel assembly for a boiling water reactor.

FIG. 9 is a table showing relative coolant mass flux values for respective flow path sections shown in FIG. 8;

10A and 10B show a grid-type spacer provided with a conventional flow tab, wherein FIG. 10A is a plan view and FIG. 10B is a front view.

11A and 11B show a conventional ring-type spacer provided with a flow tab, wherein FIG. 11A is a plan view and FIG. 11B is a front view.

12A is a partial front view of the lattice spacer of FIG. 10, and FIG. 12B is a schematic side view of a longitudinal section of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rod-shaped fuel element 3A Grid type spacer 3a Outer plate 3e Fuel rod elastic spring 4 Boiling water reactor fuel assembly 5 Channel box 5a Inner peripheral wall surface 10 Spacer body 11 Outward projecting body 11a Free end 11b Protrusion a1 Liquid film f

Claims (3)

[Claims] 1. A spacer body having a plurality of flow passage sections which are divided for supplying coolant to each rod-shaped fuel element held at a predetermined interval in an outer plate. The outer plate abuts against the inner peripheral wall surface of the channel box mounted on the spacer body of the fuel assembly for a boiling water reactor, and the liquid film of the coolant flowing along the inner peripheral wall surface impinges on the outer peripheral plate. A spacer for a fuel assembly for a boiling water reactor, wherein a plurality of outward projections are provided. 2. The fuel according to claim 1, wherein the outward projection provided on the outer plate has a free end formed thereon, and the outward projection is provided inside the outer plate in the case of a lattice spacer. 2. The spacer for a fuel assembly for a boiling water reactor according to claim 1, wherein the spacer is formed integrally with a rod elastic spring, and a bent portion below the free end elastically contacts an inner peripheral wall surface of the channel box. 3. A spacer body having a plurality of flow passage sections divided for supplying coolant to each rod-like fuel element held at a predetermined interval in an outer plate, wherein The outer plate abuts against the inner peripheral wall surface of the channel box mounted on the spacer body of the fuel assembly for a boiling water reactor, and the liquid film of the coolant flowing along the inner peripheral wall surface impinges on the outer peripheral plate. With respect to the spacer of the fuel assembly for a boiling water reactor in which a required number of outwardly projecting members are provided, the spacers are disposed at any one or more of the second, third, and fourth positions from the top. A fuel assembly for a boiling water reactor, comprising: