JP3761290B2 - Fuel assembly - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel assembly used in a boiling water reactor.
[0002]
[Prior art]
In general, as shown in FIG. 7 , a fuel assembly 1 for a boiling water reactor is composed of a plurality of fuel rods 2 and 3 containing nuclear fuel in a square tube channel box 7 and a coolant therein. A plurality of spacers 8a are arranged with the upper and lower ends fixed by the upper tie plate 4 and the lower tie plate 5, respectively, and the intermediate portions thereof are arranged at appropriate intervals in the axial direction. , 8b are arranged and supported. In the 9 × 9 fuel shown here, so-called short fuel rods 3 whose upper ends do not reach the upper tie plate are used in order to reduce the pressure loss of the fuel assembly.
[0003]
When designing the spacer 8, in addition to the requirements on the mechanical structure of the fuel assembly, such as maintaining the distance between the fuel rods 2 and 3, the spacer flow is controlled by the spacer to increase the margin for boiling transition. In addition, it is also important to reduce the pressure loss in the spacer portion as much as possible from the viewpoints of pump power for circulating the coolant and thermohydrodynamic stability of the fuel.
[0004]
For example, in the case of a fuel assembly using short fuel rods, two types of spacers designed according to such requirements are used depending on the position where they are arranged. Here, the lower spacer 8a spacers disposed below the length fuel rods upper end, the one arranged above the upper end and the upper spacer 8b, respectively 8, 9.
[0005]
8A is a top view of the spacer, and FIG. 8B is a side view. Here, the spacers 8a are arranged adjacent to each other by arranging cylindrical spacer cells 10 (hereinafter referred to as cells) joined to each other by spot welding so as to form a fuel rod insertion passage in a band 15 formed in a rectangular shape. A cell spring 13 is arranged on two side surfaces of the cells 10. The fuel rod is supported from three directions by the cell spring 13 and the two cell stops 12 provided in the cell 10. Further, a water rod support fitting 11 and a water rod spring for supporting the two large diameter water rods 6 are provided in the central portion.
[0006]
Further, a protrusion (lobe) 16 that protrudes outward is provided on the side surface of the band 15, whereby an appropriate interval can be provided between the band 15 and the channel box 7 surrounding the band 15. In addition, a plurality of flow tabs 18 are provided on the upper portion (downstream side) of the band 15 for the purpose of improving the limit output characteristics.
[0007]
Further, as shown in FIG. 9 , the spacer 8b has substantially the same configuration as the spacer 8a shown in FIG. 8 , except that the eight cells 10 at positions where the short fuel rods are arranged are deleted.
[0008]
[Problems to be solved by the invention]
In recent years, in a nuclear power plant, for the purpose of improving the nuclear fuel economy and reducing the number of spent fuel bodies, higher burnup has been promoted to increase the energy that can be extracted from one fuel assembly 1. In order to realize this high burnup, it is necessary to increase the average enrichment of the fuel assembly, which causes some undesirable effects on the core characteristics.
[0009]
One of these is the effect on the thermal properties of the fuel. That is, when the enrichment of the fuel is increased, the heat output generated by one fuel assembly at the early stage of the fuel life increases, and the thermal margin for the boiling transition decreases. In particular, since the fuel rods arranged on the outer periphery in the fuel assembly are adjacent to a large water region outside the channel box 7, the supply of thermal neutrons is large and the output is high. For this reason, the thermal tolerance at this position tends to be particularly low.
[0010]
On the other hand, since the channel box 7 is non-heat-generating and has non-boiling water having a low temperature on the outside, a coolant flow having a low vapor quality (a lot of liquid phase) exists along the channel box 7. Therefore, as a measure for improving the thermal margin, the effective outer diameter D (the distance between the opposite sides) of the spacer is increased as much as possible, and this coolant flow is stripped off by the band 15, and the flow tab 18 described above is used. Means are used to improve the thermal tolerance of the outer peripheral fuel rod by guiding it to the outer peripheral fuel rod.
[0011]
However, when the effective outer diameter of the spacer 8 is increased or the flow tab 18 is installed, a new problem arises that obstacles to the coolant flow increase and the pressure loss of the fuel assembly 1 increases. .
[0012]
Next, another adverse effect of increasing fuel enrichment is the effect on nuclear characteristics such as an increase in void coefficient and a decrease in reactor shutdown margin. The former is a problem related to the nuclear safety of the nuclear reactor, which increases the overpower of the core at the time of a transient change, and the latter deteriorates the ability of the control rod to shut down the reactor. One countermeasure against this is the use of the above-mentioned short fuel rods, which is an effective means for improving these nuclear characteristics. In particular, when the short fuel rod 3 is disposed adjacent to the channel box 7, it is known that the effect is very large, but there have been almost no examples actually employed so far. The reason is briefly explained as follows.
[0013]
Originally, the flow path is locally wider above the upper end (downstream side) of the short fuel rod 3, so that the coolant tends to concentrate, and there is a tendency that the coolant supplied to the fuel rod at a position away from it tends to decrease. there were. However, as described above, when the short fuel rod 3 is disposed adjacent to the channel box, the liquid phase tends to flow along the surface of the channel box 7, so that there is a concern that this tendency is further strengthened and the thermal margin is impaired. . Therefore, there is a problem that even if the short fuel rod 3 is introduced, the effect cannot be fully utilized.
[0014]
The present invention has been made to solve the above-mentioned problems, and in particular, improves the cooling efficiency of the fuel rods on the outer periphery of the fuel assembly, and improves the nuclear characteristics by arranging short fuel rods adjacent to the channel box. In addition, an object of the present invention is to provide a fuel assembly that does not deteriorate the thermal characteristics of the fuel.
[0015]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the invention according to claim 1 is a channel box in which a plurality of fuel rods and a water rod in which a part of the coolant flows are bundled by spacers arranged in the axial direction. The one or more spacers are formed by projecting an inclined flow tab that directs the coolant flow toward the fuel rods on the downstream side of the band that is the outer frame of the one or more spacers. It includes two or more types of spacers having different numbers.
[0016]
With this configuration, an increase in pressure loss due to the flow tab can be reduced, and an increase in pressure loss of the fuel assembly can be prevented. The invention according to claim 2 is characterized in that a flow tab is not formed on a plurality of spacers continuous from the lowermost end of the fuel assembly.
[0017]
The invention according to claim 3 is characterized in that a flow tab is not formed in a spacer positioned below the upper end of the longest of the short fuel rods. The invention according to claim 4 is characterized in that a flow tab is not formed in the spacer at the uppermost end of the fuel assembly.
[0018]
With the configuration of the invention according to claims 2 to 4, the pressure loss of the fuel assembly can be reduced while increasing the thermal margin of the fuel assembly. The invention according to claim 5 is a fuel assembly formed by bundling a plurality of fuel rods and a water rod in which a part of a coolant flows inside by a plurality of spacers arranged in the axial direction and surrounded by a channel box. in a continuous, possess two or more spacers are effective outer diameter is the outer diameter different spacer excluding a protrusion may contact the channel box provided on the outside of the spacer, and the lowermost end of at least the fuel assembly The effective outer diameter of the plurality of spacers is the smallest .
[0019]
The invention according to claim 6 is characterized in that the effective outer diameter of the spacer located below the upper end of the longest of the short fuel rods is the smallest.
[0020]
The invention according to claim 7 is characterized in that the effective outer diameter of the spacer at the uppermost end of the fuel assembly is the smallest. According to the configuration of the invention according to claims 5 to 7 , the amount of the fuel spacer can be minimized, so that the pressure loss is reduced, the liquid phase flowing in the vicinity of the fuel channel is increased, and the fuel spacer is increased. The thermal margin increases because the band is directed to the outermost fuel rod.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
FIG. 1 shows a fuel assembly according to a first embodiment of the present invention. FIG. 1 (A) is a partial longitudinal sectional view of the fuel assembly, and FIGS. It is a BB, CC arrow cross-sectional view. In the figure, the same parts as those in the prior art are denoted by the same reference numerals, and redundant description is omitted.
[0022]
In FIG. 1, the fuel assembly 1 has four different types of spacers arranged at seven locations. The first to fourth counters from the bottom are the first spacer 8c, the fifth is the second spacer 8d, the sixth is the third spacer 8e, and the seventh fourth spacer 8f. 2 to 5 are shown in FIGS. 2 to 5 and will be described while comparing with each other.
[0023]
The first spacer 8c shown in FIG. 2 and the second spacer 8d shown in FIG. 3 and the third spacer 8e shown in FIG. 4 and the fourth spacer 8f shown in FIG. Compared with the first and second spacers 8c and 8d disposed below the upper end of the short fuel rod 3, it corresponds to the number of fuel rods 2 and 3 held at each spacer position. In the third and fourth spacers 8e and 8f arranged above, the cell 10 at the short fuel rod arrangement position is omitted.
[0024]
On the other hand, regarding the structure of the band 15, the first spacer 8c and the fourth spacer 8f, and the second spacer 8d and the third spacer 8e are the same. Hereinafter, these two types of band structures will be described.
[0025]
In FIG. 3, a flow tab 18 is formed on each side of the band 15b in the second spacer 8d. The basic structure is the same as that of the conventional example, but the band 15b is as close to the channel box 7 as possible. ing. That is, a part of the upper and lower ends of the band 15b is punched inward to form a cell support protrusion 19, which is welded and held to the cell 10 inside the spacer 8. Details of this structure are described in Japanese Patent Application No. 9-52657, which is a prior application of the present applicant. The band 15 of the third spacer 8e shown in FIG. 4 has the same configuration.
[0026]
On the other hand, in FIGS. 2 and 5, the flow tab 18 is not formed on the band 15a of the first spacer 8c and the fourth spacer 8f. In addition, since the band 15a is also welded directly to the round cell 10 without the inward protruding portion, the effective outer diameter D1 is smaller than the effective outer diameter D2 of the second spacer 8d and the third spacer 8e. . For this reason, the distance between the inner surface of the channel box 7 and the band 15 is larger than that of the second spacer 8d and the third spacer 8e, and the height (hereinafter referred to as lobe) 16 of the protrusion toward the outer side of the band 15 corresponds to the second. It is higher than the spacer 8d and the third spacer 8e.
[0027]
The first spacer 8c and the fourth spacer 8f are provided with lead-in tabs 17 in place of the flow tabs. This is provided with a slope as a guide when assembling a fuel assembly. Since the lead-in tab 17 is smaller than the flow tab, there is almost no thermal hydraulic influence such as pressure loss.
[0028]
FIG. 6 schematically shows the flow of the coolant by each spacer. FIG. 6A shows the flow of the coolant in the second and third spacer portions. The coolant liquid phase portion F flowing on the inner surface of the channel box 7 is peeled off by the flow tab 18 and the fuel rods 2 and 2 are peeled off. 3 is supplied. For this reason, cooling of the fuel rods 2 and 3 can be promoted. FIG. 6B shows the flow of the coolant in the first spacer portion that does not have the flow tab 18, and the spacer 15a has no flow tab 18 so that there is little pressure loss, and there is steam near the inner surface of the channel box. Low quality coolant is flowing. Further, the projected area viewed from the axial direction of the first spacer is smaller than that of the second spacer 8d and the third spacer 8e, that is, the effective outer diameter of the spacer is small, and the distance between the band 15a and the inner surface of the channel box 7 is increased. , Pressure loss can be reduced.
[0029]
Next, from the viewpoint of thermal margin (limit output), each spacer arrangement position has the following characteristics. That is, in the upper part of the fuel assembly, there is little margin for boiling transition. Further, the lower part of the fuel assembly has a low vapor quality, and the surface heat flux is low near the upper end of the effective fuel length.
[0030]
Accordingly, by disposing the first spacer 8c having a small effective outer diameter and no flow tab as in the present embodiment, the pressure loss is reduced without reducing the thermal margin, and the channel A coolant having a low vapor quality can flow near the inner surface of the box 7. For this reason, the coolant flow with low vapor quality in the vicinity of the inner surface of the channel box 7 at the upper part of the fuel assembly is preserved, and the outer peripheral fuel rods can be efficiently cooled at the upper part of the fuel assembly.
[0031]
That is, in the conventional spacer arrangement, the spacer 8a installed at the lower part of the fuel assembly and the spacer 8b installed at the upper part have substantially the same structure, so the flow along the inner surface of the channel box is disturbed and cooling with low steam quality is performed. Although the rate at which the material reaches the upper part of the fuel assembly was low, this problem can be solved by using a plurality of types of spacers with a small effective outer diameter or no flow tab that can reduce pressure loss as in this embodiment. Will be solved.
[0032]
Further, in the present embodiment, the pressure loss of the entire coolant flow is also reduced by arranging the fourth spacer with the low pressure loss near the upper end of the fuel effective length having a very large margin for the boiling transition.
[0033]
In the present embodiment, four types of spacers of the first to fourth spacers are used. For example, the first spacer is used below the upper end of the short fuel rod 3, and the third spacer is used above it. Thus, the number of types of spacers can be reduced. It is also clear that a similar effect can be obtained by not removing the flow tabs of the first and fourth spacers completely, but by making the number smaller than that of the second and third spacers.
[0034]
【The invention's effect】
According to the present invention, it is possible to obtain a fuel assembly having good soundness while reducing the pressure loss of fuel and having a large thermal margin.
[Brief description of the drawings]
FIG. 1A is an elevation view showing a first embodiment of a fuel assembly according to the present invention in a partial cross section, FIG. 1B is a cross-sectional view taken along the line BB in FIG. ) Is a cross-sectional view taken along the line CC of (A).
2A is a top view of a first spacer in the first embodiment of a fuel assembly according to the present invention, and FIG. 2B is a side view of FIG.
3A is a top view of a second spacer in the first embodiment of the fuel assembly according to the present invention, and FIG. 3B is a side view of FIG. 3A.
4A is a top view of a third spacer in the first embodiment of the fuel assembly according to the present invention, and FIG. 4B is a side view of FIG. 4A.
5A is a top view of a fourth spacer in the first embodiment of the fuel assembly according to the present invention, and FIG. 5B is a side view of FIG. 5A.
6A is a schematic diagram illustrating the function of the first spacer in the first embodiment of the fuel assembly according to the present invention, and FIG. 6B is a schematic diagram illustrating the function of the second and third spacers.
7 (A) is an elevational view of a conventional fuel assembly in partial cross section, (B) is taken along line B-B sectional view of (A), C-C of (C) is (A) FIG.
8A is a top view of an upper spacer in a conventional fuel assembly, and FIG. 8B is a side view of FIG. 8A.
9 (A) is a top view of the lower spacer in the conventional fuel assembly, a side view (B) in (A).
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Long fuel rod, 3 ... Short fuel rod, 4 ... Upper tie plate, 5 ... Lower tie plate, 6 ... Water rod, 7 ... Channel box, 8 ... Spacer, 9 ... Fuel rod effective , 10 ... Spacer cell, 11 ... Water rod support bracket, 12 ... Cell stop, 13 ... Cell spring, 14 ... Water rod spring, 15 ... Band, 16 ... Robe, 17 ... Lead-in tab, 18 ... Flow tab, 19 ... Cell Support protrusion, 20 ... opening at the lower end of the lobe

Claims (7)

  In a fuel assembly in which a plurality of fuel rods and a water rod in which a part of the coolant flows are bound by a plurality of spacers arranged in the axial direction and surrounded by a channel box, at least one of the spacers is A fuel assembly comprising: two or more types of spacers having different numbers of the flow tabs, wherein an inclined flow tab for directing a coolant flow toward the fuel rod side is formed on the downstream side of the band which is an outer frame.   The fuel assembly according to claim 1, wherein a flow tab is not formed on the plurality of spacers continuous from the lowermost end of the fuel assembly.   3. The fuel rod has a plurality of types having different lengths, and a flow tab is not formed on a spacer positioned below the upper end of the longest fuel rod other than the longest type fuel rod. The fuel assembly according to 1.   The fuel assembly according to any one of claims 1 to 3, wherein a flow tab is not formed in a spacer at an uppermost end of the fuel assembly. A fuel assembly formed by bundling a plurality of fuel rods and a water rod in which a part of the coolant flows inside by a plurality of spacers arranged in the axial direction and surrounded by a channel box is provided outside the spacers. is the effective outer diameter is the outer diameter of the spacer excluding a protrusion may contact the channel box have a different two or more kinds of spacers with, and effective outside the plurality of spacers to be continuous from the lowermost end of at least the fuel assembly A fuel assembly having the smallest diameter . The plurality of types of fuel rods having different lengths, wherein an effective outer diameter of a spacer positioned below the upper end of the longest fuel rod other than the longest type of fuel rod is the smallest. 5. The fuel assembly according to 5 . The fuel assembly according to claims 5 to 6 effective outer diameter of the uppermost end of the spacer and wherein the smallest of the fuel assembly.

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