JP2003130980A - Fuel assembly for nuclear reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve the safe operation of a nuclear reactor by surely suppressing the shift to the film boiling from the nuclear boiling of cooling water. SOLUTION: A beltlike body 50 which is twisted to be spiral by using an axis parallel to fuel rods 10 as the center is arranged between the fuel rods 10. The cooling water flowing along the beltlike body 50 is changed to a swiveling flow, the cooling water flowing on surfaces of the fuel rods 10 around the beltlike body 50 is stirred by the swiveling flow, and the shift to the film boiling from the nuclear boiling is suppressed.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a fuel assembly which constitutes a core of a nuclear reactor.

[0002]

2. Description of the Related Art A fuel assembly of a nuclear reactor has a structure in which a large number of fuel rods in which a large number of uranium pellets are enclosed in elongated tubes are assembled in a square lattice shape in parallel with each other. It is arranged in an orderly manner by a plurality of support grids arranged at intervals in the longitudinal direction of the fuel rods. In the core, the fuel assembly is installed with the fuel rods extending in the vertical direction, and by flowing cooling water upward from the lower end side of the fuel rods,
It cools the fuel rods that are heated by the fission reaction. The cooling water circulates with respect to the core, and the cooling water heated by the fuel rods is used to drive the power generation turbine in the middle of the circulation path outside the core.

By the way, the cooling water flowing through the core is heated by the fuel rods and boils as described above. If the state of boiling is so-called nucleate boiling, in which fine bubbles are dispersed and adhered to the fuel rod, the cooling water is kept in contact with the fuel rod at a high rate, so that the cooling ability is maintained and there is no problem. However, when heating progresses further from the state of nucleate boiling and the cooling capacity becomes insufficient, a state called film boiling in which the bubbles adhering to the fuel rod become coarse or the density becomes high is reached. When the film boiling occurs, the rate of contact of the cooling water with the fuel rods decreases, and the fuel rods are overheated, which may damage the fuel rods. Therefore, the film boiling must be avoided. Therefore, in the past, the transition from nucleate boiling to film boiling (this was called DNB: Departure from Nucleate Boilin
In order to suppress (g), a mixing vane that generates a swirling flow in the cooling water is provided at the downstream edge of the cooling water in the support grid. The cooling water turned into a swirling flow by the mixing vane stirs the cooling water flowing on the surface of the fuel rod, and thereby the transition from nucleate boiling to film boiling is suppressed.

[0004]

However, since the location of the mixing vanes is limited to the end of the support grid, it may be insufficient to suppress the transition from nucleate boiling to film boiling. It has been desired to develop a means for obtaining the effect more reliably.

Therefore, the present invention provides a fuel assembly for a nuclear reactor, which can surely suppress the transition from the nucleate boiling of cooling water to the film boiling, that is, DNB, and thus achieve safe reactor operation. It is an object.

[0006]

SUMMARY OF THE INVENTION A fuel assembly for a nuclear reactor of the present invention has a plurality of support grids arranged at intervals and a support which is inserted into each grid space of the support grid and which each support grid has. A plurality of fuel rods arranged in a lattice shape in parallel with each other by being supported by the portions, and the cooling water is flowed from one end side to the other end side in the axial direction of the plurality of fuel rods. A fuel assembly of a furnace, characterized in that the fuel rod has a swirl flow generating means for generating a swirl flow in the cooling water.

According to the above-described fuel assembly of a nuclear reactor, a swirling flow is generated in the cooling water by the swirling flow generating means of the fuel rod, and DNB is suppressed by the stirring action of the cooling water generated by the swirling flow. When the supporting grid is provided with the mixing vanes, the stirring action due to the swirling flow can be increased in combination with the action of the mixing vanes, and the DNB suppressing effect can be obtained.

As a concrete structure of the fuel assembly of the above-mentioned nuclear reactor, there is a spiral groove or ridge formed on the outer peripheral surface of the fuel rod. According to this, the cooling water flowing along the fuel rod becomes a swirl flow due to the groove or the ridge,
The swirling flow stirs the surrounding fuel rods or the cooling water flowing on the surface of the fuel rods on the downstream side of the cooling water,
The effect of suppressing DNB is exhibited. In this case, the swirling flow is generated around the fuel rod, and in the groove or protrusion forming portion, the cooling water is swirled outward while swirling due to the difference in specific gravity, while the bubbles are the fuel rods. Tend to collect on the surface of the. Therefore, if the groove or the protrusion is formed over the entire length of the fuel rod, the effect of suppressing the DNB cannot be effectively obtained. Therefore, it is preferable that the groove or the protrusion be partially formed so as to cancel out the bubbles. Further, when the supporting grid is provided with the mixing vanes, it is desirable that the groove or the protrusion be formed so as to generate a swirling flow in the same direction as the swirling flow generated by the mixing vanes.

Next, according to the present invention, a plurality of support grids arranged at intervals and inserted into each square space of the support grid,
And a plurality of fuel rods arranged in a lattice shape in parallel with each other by being supported by a support portion of each support lattice, and a swirl for generating a swirling flow in the cooling water between the fuel rods. Provided is a fuel assembly of a nuclear reactor characterized in that a flow generating means is arranged.

According to the fuel assembly of the nuclear reactor, a swirling flow is generated in the cooling water by the swirling flow generating means arranged between the fuel rods, and the DN is generated by the stirring action of the cooling water generated by the swirling flow.
B is suppressed. When the supporting grid is provided with a mixing vane, the stirring action due to the swirling flow can be increased in combination with the action of the mixing vane.
A suppression effect can be obtained.

As a concrete structure of the fuel assembly of the nuclear reactor, there is a band-like body spirally twisted about an axis parallel to the fuel rod. According to this, the cooling water becomes a swirl flow by flowing along the strip, and the swirling flow agitates the surrounding fuel rods or the cooling water flowing on the surface of the fuel rods on the downstream side of the cooling water, thereby suppressing the DNB. Is demonstrated. Such strips can be arranged within the support grid or between adjacent support grids. Further, when the supporting grid is provided with the mixing vanes, it is preferable that the band-shaped body produces a swirling flow in the same direction as the swirling flow generated by the mixing vanes.

Further, according to the present invention, a plurality of support grids arranged at intervals and a support portion which is inserted through each square space of the support grid and which is supported by each support grid are parallel to each other. A plurality of fuel rods arranged in a lattice shape in a state, the supporting portion is a strap formed of a leaf spring formed in an intermediate portion in the thickness direction of the supporting lattice, and formed at both end portions in the thickness direction. A fuel assembly for a nuclear reactor, characterized in that the straps and / or the dimples extend obliquely to the flow direction of the cooling water in order to generate a swirling flow in the cooling water. Provide the body.

In the fuel assembly of the nuclear reactor, at least one of a leaf spring strap for supporting the fuel rod and a dimple extends obliquely with respect to the flow direction of the cooling water. If such a configuration is given to the cooling water, the cooling water flowing along the strap becomes a swirling flow, and the swirling flow stirs the cooling water flowing on the surface of the fuel rod on the downstream side of the strap, thereby exerting the DNB suppressing effect. To be done. The same effect occurs in the case of dimples, and when both the strap and the dimples are formed obliquely, it is desirable that the swirling flows generated by both are in the same direction. Further, when the supporting grid is provided with the mixing vanes, it is desirable to form the mixing vanes in an oblique direction in which the swirling flow in the same direction as the swirling flow generated by the mixing vanes is generated.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. (1) First Embodiment FIG. 1 is a partially cutaway perspective view showing one unit of a fuel assembly of a nuclear reactor. In the figure, reference numeral 10 is a fuel rod in which a large number of uranium pellets are enclosed in an elongated tube, and the large number of fuel rods 10 are inserted into the square spaces of a plurality of support grids 20 so that they are square in parallel with each other. It is organized in a grid. The fuel rods 10 are arranged in, for example, 17 × 17 rows, while the support grids 20 are plurally arranged at the lower end portion, the upper end portion, and the intermediate portion of the fuel rod 10. Each support grid 20
An instrumentation guide tube 11 is inserted in the center of the fuel cell, and a control rod guide tube 12 is inserted in an appropriate place in place of the fuel rod 10.

An upper nozzle 30 and a lower nozzle 31 are arranged above the support grid 20 at the upper end and below the support grid 20 at the lower end, respectively. The control rods 13 to be inserted into the control rod guide tubes 12 are inserted through the upper nozzles 30, and the control rod assemblies 14 connected to the control rods 13 and extending upward are arranged at the centers of the control rods 13. Has been done.

In the fuel assembly, cooling water is made to flow from the lower nozzle 31 toward the upper nozzle 30 while being installed in the core of the nuclear reactor, and the fuel rod 10 is cooled by this cooling water.

As shown in FIG. 2, the support grid 20 is formed by assembling a plurality of elastic thin plate-like support plates 21 vertically and horizontally orthogonally to each other. A square space having a square cross section through which 10 is inserted is formed. A strap (support portion) 22 and a dimple (support portion) 23 that elastically press and support the fuel rod 10 are formed on the inner surface of each support plate 21. These are leaf springs formed by forming a slit in the support plate 21 and press-working the strap 22, and the strap 22 is a support plate 2
1 is formed in the middle portion in the thickness direction (vertical direction in FIG. 2),
The dimples 23 are formed on the upper and lower ends of the support plate 21. Both the strap 22 and the dimple 23 have a rectangular shape, but the strap 22 extends in the vertical direction and the dimple 23 extends in the horizontal direction.

As shown in FIG. 2, at the upper end of the support plate 21, a mixing vane 24 for generating a swirling flow of the cooling water is provided.
Is provided. The mixing vane 24 is a support plate 21.
In this case, a pair of them is alternately formed on the vertical and horizontal support plates 21 with the intersection of the support plates 21 interposed therebetween. The pair of mixing vanes 24 are slightly curved in opposite directions so that a swirl flow is generated in the cooling water.

In the first embodiment, as shown in FIG. 3A, a spiral groove (swirl flow generating means) 40 is formed on the outer peripheral surface of each fuel rod 10. The groove 40 is formed in a portion where the fuel rod 10 passes through the support grid 20, and a swirl flow is generated in the cooling water by the groove 40. The groove 40 is formed so that the swirling flow generated swirls in the same direction as the swirling flow generated by the mixing vane 24.

The structure of the fuel assembly according to the first embodiment has been described above. Next, the operation of the fuel assembly installed in the core will be described. The fuel assembly is the lower nozzle 3
The cooling rod is operated in a state where the cooling water is flowed from 1 to the upper nozzle 30 at a predetermined pressure, and the fuel rod 10 heated by the nuclear fission reaction is cooled by the cooling water. The cooling water is heated and boiled by the fuel rods 10, but it is necessary to avoid the transition from nucleate boiling to film boiling, that is, generation of DNB, as described above. In the present embodiment, when the cooling water reaches the portion of the spiral groove 40 formed in the fuel rod 10,
The cooling water flows along the groove 40 and becomes a swirling flow. When this swirling flow is generated, the cooling water flowing on the peripheral fuel rods 10 or on the surface of the fuel rods 10 on the downstream side is agitated, and DNB is suppressed. Furthermore, the swirling flow is
The collision effect is amplified by the collision with the mixing vane 24, and the DNB suppressing effect is promoted. Therefore, D
NB is suppressed, and thus safe reactor operation is achieved.

In the above embodiment, the groove 40 is formed as the swirl flow generating means formed in the fuel rod 10.
As shown in FIG. 3 (b), the same effect can be obtained by forming a ridge (swirl flow generating means) 41 instead of the groove 40.

(2) Second Embodiment In the second embodiment, as a swirling flow generating means of the cooling water, as shown in FIG. 4, a space between the fuel rods 10 is arranged in a set of four squares. A band member (swirl flow generating means) 50 twisted in a spiral shape is arranged at the center of the. Band 50
As shown in FIG. 5, a thin metal plate is formed by twisting a plurality of times around the longitudinal direction as an axis. Band 50
Are arranged between the upper and middle supporting grids 20 and between the middle and lower supporting grids 20 so as to extend parallel to the fuel rods 10, and both ends are fixed to the supporting grid 20. ing. By passing the cooling water through the strip 50,
A swirling flow occurs in the cooling water. The strip 50 is formed so that the swirling flow generated swirls in the same direction as the swirling flow generated by the mixing vane 24.

According to the second embodiment, the cooling water is strip 5
By flowing along 0, it becomes a swirling flow, and the swirling flow stirs the cooling water flowing on the surface of the surrounding fuel rods 10 and suppresses DNB. Further, the swirling flow collides with the mixing vane 24, whereby the stirring action is amplified and the DNB suppressing effect is promoted.

(3) Third Embodiment In the third embodiment, as shown in FIG. 6, the strap 22 and the dimple 23 formed on the support plate 21 are formed obliquely so as to extend in the same direction. These are the means for generating the swirling flow of the cooling water. That is, when the cooling water collides with the strap 22 and the dimples 23, the flow of the cooling water is guided obliquely upward by the strap 22 and the dimples 23, whereby a swirling flow is generated. As shown in FIG. 7, the strap 22 and the dimple 23 in this case have a closed portion 25 formed on the lower side thereof so as not to allow the straight passage of the cooling water. The strap 22 and the dimples 23 are formed so that the swirling flow generated swirls in the same direction as the swirling flow generated by the mixing vane 24.

According to the third embodiment, the swirling flow is generated by the cooling water flowing along the strap 22 and the dimples 23, and the swirling flow causes the surrounding fuel rods 10 to flow.
The cooling water flowing on the surface of the is stirred and DNB is suppressed. Further, the swirling flow collides with the mixing vane 24, whereby the stirring action is amplified and the DNB suppressing effect is promoted.

According to FIG. 6, both the strap 22 and the dimple 23 are formed obliquely so as to generate a swirling flow in the cooling water, but only one of them may be formed obliquely. 8 shows an example in which only the strap 22 is formed obliquely, and FIG. 9 shows an example in which only the dimple 23 is formed obliquely.

[0027]

As described above, according to the fuel assembly of the reactor of the present invention, the swirling flow generating means different from the existing mixing vane is provided, so that the nucleate boiling of the cooling water to the film boiling is changed. It is possible to suppress the shift and to achieve a safe reactor operation.

[Brief description of drawings]

FIG. 1 is a perspective view showing an entire fuel assembly according to an embodiment of the present invention.

FIG. 2 is a partial perspective view of a support grid according to an embodiment of the present invention.

3A is a partial perspective view of a fuel rod according to the first embodiment of the present invention, and FIG. 3B is a perspective view of a fuel rod to which a modification of the first embodiment is applied.

FIG. 4 is a cross-sectional view showing an arrangement of strips according to a second embodiment of the present invention.

FIG. 5 is a perspective view of a strip according to a second embodiment of the present invention.

FIG. 6 is a partial perspective view of a support grid according to a third embodiment of the present invention.

FIG. 7 is a vertical sectional view of a strap and a dimple according to a third embodiment of the present invention.

FIG. 8 is a partial perspective view of a support grid to which a modification of the third embodiment of the present invention is applied.

FIG. 9 is a partial perspective view of a support grid to which another modification of the third embodiment of the present invention is applied.

[Explanation of symbols]

10 ... Fuel rod, 20 ... Support grid, 22 ... Strap (supporting part), 23 ... Dimple (supporting part), 40 ... Groove (swirl flow generating means), 41 ... Projection (swirl flow generating means), 50 ...
Band (swirl flow generation means)

Claims (5)

[Claims] 1. A plurality of support grids arranged at intervals, and a grid which is inserted into each square space of the support grid and is supported by a support portion of each support grid so as to be parallel to each other. A plurality of fuel rods arranged in a line, and a fuel assembly of a nuclear reactor in which cooling water is caused to flow from one end side to the other end side in the axial direction of the plurality of fuel rods, wherein the fuel rods are A fuel assembly of a nuclear reactor comprising a swirl flow generating means for generating a swirl flow in the cooling water. 2. The fuel assembly for a nuclear reactor according to claim 1, wherein the swirl flow generating means is a spiral groove or a ridge formed on an outer peripheral surface of the fuel rod. 3. A plurality of support grids arranged at intervals, and a grid that is inserted into each square space of the support grid and supported by a support portion of each support grid, so that the grids are parallel to each other. A plurality of fuel rods arranged in a line, wherein a cooling water is flown from one end side in the axial direction of the plurality of fuel rods toward the other end side, A fuel assembly for a nuclear reactor, characterized in that a swirl flow generating means for generating a swirl flow in the cooling water is disposed therebetween. 4. The fuel assembly for a nuclear reactor according to claim 3, wherein the swirl flow generating means is a strip-shaped body that is helically twisted about an axis parallel to the fuel rod. 5. A plurality of support grids arranged at intervals, and a grid that is inserted into each square space of the support grid and supported by a support portion of each support grid so that the grids are parallel to each other. A plurality of fuel rods arranged in a line, and a fuel assembly of a nuclear reactor in which cooling water is flowed from one end side to the other end side in the axial direction of the plurality of fuel rods, wherein the support portion is A dimple consisting of a leaf spring formed at an intermediate portion in the thickness direction of the support grid and a leaf spring formed at both ends in the thickness direction, wherein the strap and / or the dimple is A fuel assembly for a nuclear reactor, wherein the fuel assembly extends obliquely with respect to the flow direction of the cooling water to generate a swirling flow in the cooling water.