JP3599835B2 - Fuel assemblies for boiling water reactors - Google Patents

Description

[0001]
[Industrial applications]
TECHNICAL FIELD The present invention relates to a fuel assembly for a boiling water reactor provided with a coolant leak control mechanism.
[0002]
[Prior art]
In general, a fuel assembly used in a boiling water reactor (BWR) has a plurality of fuel rods bundled in a square lattice by a plurality of spacers arranged in the axial direction thereof, and a handle for suspension. The upper tie plate is provided with an upper tie plate and a lower tie plate that forms a coolant inflow path from the lower part of the core, and a channel box is fixed to the upper tie plate to cover the entire fuel assembly. I have.
[0003]
In such a fuel assembly, during operation of the reactor, the internal pressure of the channel box becomes higher than its external pressure. Due to the relative pressure difference between the inside and outside of the channel box, the channel box expands and deforms outward, and the thermal effect and neutron irradiation effect of the high temperature and long-time operation of the reactor are added. Become.
[0004]
The expansion deformation of the channel box as described above not only hinders the insertion of the control rod during the operation of the reactor, but also causes the coolant to leak more than necessary from the gap between the channel box and the lower tie plate. A sufficient cooling effect cannot be obtained.
[0005]
Therefore, at present, a leak control device is provided for controlling the outflow of the coolant from the inside of the channel box due to the pressure difference between the inside and outside of the channel box. For example, a leak control plate 13 as shown in FIG. 4A is provided on each of four side surfaces of the lower tie plate 12, and a leak flow rate is controlled by adjusting an interval between the channel box 11 and the lower tie plate 12. ing.
[0006]
These leak control plates 13 are provided with a plurality of windows 14 for coolant distribution and a plurality of slits 15 provided between the windows 14, and as shown in the cross-sectional view of FIG. After the lower edge is inserted into the lower pocket 18b provided on the lower surface of the tie plate 12, the upper edge is inserted into the upper pocket 18a.
[0007]
Further, the upper edge is held by the legs 16 a of the stopper plate 16 being loosely inserted into the windows 14, and the stopper plate 16 is fixed by bolts 17. At this time, a predetermined interval (clearance) between the ceiling in the upper pocket 18a and the upper end of the leak control plate 13 is maintained. The leak control plate 13 attached in this manner can freely move between the upper and lower pockets (18a, 18b) of the lower tie plate 12 by the clearance.
[0008]
In the actual furnace, a pressure difference (internal pressure> external pressure) occurs between the inside and outside of the channel box 11 and bulge deformation occurs in the channel box 11 as shown in FIG. 4C, and the average burnup (horizontal) of the fuel body shown in FIG. As shown by the relationship of the channel box deformation (vertical axis: mm) with respect to the axis: GWd / t), the bulge deformation increases due to the creep deformation with the progress of the fuel burnup. Here, the channel box deformation (basil deformation) is a deformation of the distance H between the opposing sides of the inner surface of the channel box 11.
[0009]
When the channel box 11 swells due to such creep deformation, the main one of the coolant leak flows which is going to flow out of the channel box 11 from the gap between the channel box 11 and the lower tie plate 12 is leak control. The leak control plate 13 is urged toward the inner surface side of the channel box 11 by being taken in from the respective windows 14 of the plate 13 to the rear side thereof.
[0010]
As a result, the leak control plate 13 is deformed toward the inner surface of the channel box 11 following the deformation of the channel box 11 and is pressed, so that even if the space between the lower tie plate 12 and the channel box 11 gradually increases, the leak control plate 13 does not leak. With the interposition of the control plate, the flow rate of the coolant leaking out of the channel box 11 is suppressed to an appropriate amount and is controlled to be constant.
[0011]
[Problems to be solved by the invention]
At present, studies have been made to increase the burnup of the boiling water fuel assembly, and further study to further increase the average burnout taken out of the assembly from 45 GWd / t. As the burnup of the fuel assembly further increases, the amount of deformation of the channel box also increases, and the gap with the lower tie plate further increases. Therefore, it is also required to improve the deformation followability of the leak control plate for controlling the coolant leak flow rate by adjusting the gap between the channel box and the lower tie plate.
[0012]
In the past, the main focus was only on improving the deformation followability of such a leak control plate, and it was considered to simply increase the size of a window for taking in the coolant of the leak control plate. This makes it easy for the coolant to pass through the window, thereby ensuring the urging from the back side of the leak control plate due to the coolant leak flow taken in from the window, holding the pressing force on the inner side of the channel box, An object of the present invention is to improve the followability of the leak control plate to the creep deformation of the channel box.
[0013]
However, on the other hand, it is necessary to provide a slit (15 in FIG. 4) in order to make the leak control plate follow deformation well. Usually, slits are alternately arranged between the windows. Therefore, if the size of the window is too large, there is a problem that the width of the portion between the windows (window frame portion) where the slit is formed becomes narrow, and the strength of this portion is reduced.
[0014]
If the strength of the window frame decreases, plastic deformation occurs during prolonged use in the operation of the reactor, which not only makes it difficult to control the coolant leak flow rate, but also makes the leak control plate a channel box. If the channel box remains deformed inward, the channel box may be attached or detached in contact with the inner surface of the channel box, hindering the attaching / detaching operation, or the leak control plate itself may be damaged when attaching or detaching the channel box. There is also fear.
[0015]
Conversely, if the width of the window frame portion where the slit is formed is widened to increase the strength of this portion, the window becomes small, it becomes difficult to take in the coolant leak flow, and the rake control plate has Deformation followability is impaired.
[0016]
As described above, in the conventional fuel assembly for a boiling water reactor, with the increase in burnup, the leak control plate that has sufficient strength to withstand long-term use but can control the coolant leak flow rate well. Those with have not yet been obtained.
[0017]
SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a fuel for a boiling water reactor, which has a strength that can withstand long-term use, but has a leak control device capable of controlling a coolant leak flow rate in accordance with high burnup. The purpose is to obtain an aggregate.
[0018]
[Means for Solving the Problems]
In order to achieve the above object, in a fuel assembly for a boiling water reactor according to the present invention, a fuel bundle including a plurality of fuel rods and water rods held in a square lattice by a support lattice. An upper tie plate that suspends the upper end of the fuel bundle, a lower tie plate that supports the lower end of the fuel bundle, and a channel box that is suspended by the upper tie plate and surrounds the periphery of the fuel bundle. A leak control device provided on the outer peripheral surface of the lower tie plate so as to control the amount of coolant leaking from a gap in an overlap portion between the lower end portion of the channel box and the outer peripheral surface of the lower tie plate. equipped with the boiling water reactor fuel assemblies, an aggregate extraction average burnup 45 GWd / t or more boiling water reactor fuel assembly, the leakage control device The has a leakage control plate mounted with a width dimension substantially across the flat sides on the outer peripheral surface of the lower tie plate, the leakage control plate is located in the Ni-base alloy having a thickness of 0.2mm the a window for taking the leakage coolant flow to the back side from the channel box, and a slit portion formed between the respective windows are arranged alternately in the width direction of the side portions, wherein The ratio of the total opening width of the plurality of windows to the entire width of the leak control plate is in the range of 0.3 to 0.7 , and the window frame width is substantially 2 mm or more .
[0019]
[Action]
According to the present invention, in the coolant leak control device for a fuel assembly for a boiling water reactor, the coolant leak control device is mounted with a width dimension substantially covering a flat side portion on an outer peripheral surface of a lower tie plate, and a plurality of windows are arranged in the width direction. And, the leak control plate in which the slit portions formed between the windows are alternately arranged, the ratio of the total opening width of the plurality of windows to the total width of the leak control plate is within a specific range. It was done.
[0020]
First, as shown in FIG. 2, the relationship between the ratio of the total opening width of the plurality of windows to the entire width of the leak control plate and the relative coolant leak flow rate is shown based on the relationship in FIG. In the case of a bulge deformation of the channel box corresponding to that occurring under a high burnup condition assuming an average burnup of 45 GWd / t or more taken out of the aggregate, that is, a deformation amount of 4.5 mm (bulge deformation is large) and a normal burnup condition In the case of the bulge deformation of the channel box corresponding to the lower part, when the analysis is performed with the deformation amount of 3.0 mm (small bulge deformation), when the ratio of the window is 0.3 or less, the bulge deformation of the channel box is reduced. Under a large situation, it was clarified that the deformation control performance of the leak control plate with respect to the deformation of the channel box was poor, and the increase in the leak flow rate from inside the channel box became large.
[0021]
Therefore, in order to perform good control of the coolant leak flow rate under the high burnup condition of the fuel having a large bulge deformation, the leak control plate provided in the lower tie plate of the fuel assembly needs to have a size corresponding to the entire size of the leak control plate. Therefore, the ratio of the total opening width of the windows must be 0.3 or more.
[0022]
During operation of the reactor, bulge deformation occurs in the channel box, and when the flow rate of the coolant leak is controlled by the leak control plate, as shown in FIG. The bending stress is generated in the direction shown by the arrow, but when the bulge deformation is large, the bending stress is also large, and particularly the large bending stress is concentrated on the window frame near the lower side of the window X. Therefore, analysis was performed assuming that the leak control plate was pressed against the inner surface side of the channel box at a constant pressure in the case of large bulge deformation and the case of small bulge deformation. The result shown in FIG. 3B was obtained with respect to the relationship with the plastic deformation in the portion.
[0023]
This is because the thickness of a 0.2 mm-thick Ni-based alloy leak control plate provided in a boiling water reactor fuel assembly is reduced by corrosion under conditions equivalent to long-term use in a real reactor. The analysis was performed assuming a state. As is clear from FIG. 3B, in the case of large bulge deformation of the channel box, plastic deformation occurred at the stress concentrated portion in the window box width d of 2 mm or less.
[0024]
Therefore, in order to secure the strength of the window frame portion of the leak control plate under the situation of large bulge deformation of the channel box, the window frame width d needs to be 2 mm or more. Normally, since the leak control plate is about 110 mm wide and has at least 7 slits, that is, 8 windows, when the window frame width d is 2 mm from the equation (110−2 × 16) / 110, the leak control plate The ratio occupied by the total opening width of the windows to the entire width of the control plate is about 0.7.
[0025]
That is, in order to secure the strength of the window frame portion in the leak control plate used under the high burnup condition of the fuel, the ratio of the total opening width of the window to the entire width of the leak control plate is set to 0.7 or less. That would be good. In the present invention, the ratio of the total opening width of the windows to the total width of the leak control plate is set in the range of 0.3 to 0.7. And the leak control plate has a high deformation followability to the deformation of the channel box, and the coolant leak flow rate can be controlled well.
[0026]
Examples of the material used for the leak control plate include Inconel X-750 and Inconel 718 as the above-mentioned Ni-based alloys. 3 and the same results as those shown in FIG. 3 are obtained, so that in each case the ratio of the total opening width of the windows to the total width of the leak control plate is equally effective within the scope of the present invention.
[0027]
Further, the leak control plate used in the present invention is not limited to a configuration in which all the windows have a uniform size as shown in FIG. 3A. Even with a different configuration, the ratio of the total opening width of the windows to the entire width of the leak control plate may be in the range of 0.3 to 0.7.
[0028]
【Example】
Hereinafter, the present invention will be described in detail with reference to examples. FIG. 1A is a perspective view showing a schematic configuration of a lower tie plate portion of a fuel assembly for a boiling water reactor according to one embodiment of the present invention.
[0029]
The fuel assembly of the present embodiment has a plurality of fuel rods bundled in a square lattice by a plurality of spacers (not shown) arranged in the axial direction, and has a handle for suspension, as in the prior art. The upper tie plate (not shown) and the lower tie plate 2 that forms a coolant inflow path from the lower part of the core hold the upper tie plate, and further fix the channel box 1 to the upper tie plate to cover the entire fuel assembly. It has a configuration.
[0030]
The flat sides (four side surfaces) of the lower tie plate 2 are provided with leak control plates 3 for controlling the leak flow rate of the coolant flowing out of the channel box 1, respectively. After the lower edge is inserted into a lower pocket (not shown) provided on the lower side surface of the lower tie plate 2 in the same manner as the conventional one shown in the sectional view of FIG. An upper edge is inserted into a pocket (not shown), and the upper edge is held by loosely inserting the legs of the stopper plate 6 into the window 4, and the stopper plate 6 is fixed by bolts 7. I have.
[0031]
At this time, a predetermined interval (clearance) between the ceiling of the upper pocket of the lower tie plate 2 and the upper end of the leak control plate 3 is maintained. The leak control plate 3 attached in this way can freely move between the upper and lower pockets of the lower tie plate 2 by the clearance.
[0032]
Each of the leak control plates 3 is made of a Ni-based alloy having a thickness of 0.2 mm and a total width of 110 mm. As shown in FIG. 1B, nine windows 4 of the same size are arranged in the width direction. And eight slits 5 arranged alternately with the window 4. In this embodiment, the design is such that the ratio of the total opening width of all the windows (9) to the entire width of the leak control plate 3 is 0.5. Therefore, the window width L is (110 × 0.5) /9=6.1 mm, and the window frame width D is 3.06 mm.
[0033]
With respect to the fuel assembly having the leak control plate 3 on the outer peripheral surface of the lower tie plate 2, the control state of the coolant leak flow rate by the leak control plate 3 with respect to the increase in bulge deformation of the channel box 1 is analyzed. 1, the tendency of the increase of the coolant leak flow rate to the outside is small, and even if a large bulge deformation occurs in the channel box 1 corresponding to that occurring under the high burn-up condition assuming the aggregate take-out average burnup of 45 GWd / t or more, The deformation controllability of the leak control plate 3 is good, and the leak flow rate is well controlled.
[0034]
At this time, the plastic deformation of the stress concentration portion of the leak control plate 3 is slight. Therefore, when the channel box 1 is attached or detached in the long-term use of the leak control plate 3, a portion deformed outward by plastic deformation comes into contact with the inner surface of the channel box 1, making it difficult to attach or detach the channel box 1. However, there is no possibility that the contact portion may be damaged.
[0035]
In the above-described embodiment, the case is described in which nine windows (eight slits) of a uniform shape are formed and a leak control plate is provided in which the ratio of the total opening width of the windows to the entire width is 0.5. The present invention is not limited to such a design. For example, if the ratio of the total opening width of the windows to the entire width dimension is in the range of 0.3 to 0.7, for example, the opening widths of the individual windows are different from each other. It may be of a configuration.
[0036]
【The invention's effect】
INDUSTRIAL APPLICABILITY As described above, the present invention can ensure the strength of the leak control plate and the leak control in accordance with the increase in the basil deformation of the channel box in the long-term use of the fuel assembly for a boiling water reactor associated with high burnup. There is an effect that the deformation followability of the plate is improved, good control of the coolant leak flow rate can be maintained, and the reliability of the leak flow rate control function of the leak control plate is improved even under high burn-up conditions of the fuel.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a fuel assembly for a boiling water reactor according to one embodiment of the present invention, (a) is a perspective view of a lower tie plate portion, and (b) is a leak shown in (a). FIG. 4 is an enlarged plan view of the control plate 3.
FIG. 2 is a graph showing the relationship between the ratio of the total opening width of the window to the entire width of the leak control plate (horizontal axis) and the relative leak flow rate (vertical axis) for explaining the operation of the present invention.
FIGS. 3A and 3B are explanatory diagrams for explaining the operation of the present invention, in which FIG. 3A is a diagram illustrating a position where stress concentration of the leak control plate occurs, and FIG. FIG. 6 is a diagram illustrating a state of plastic deformation (vertical axis) of a stress concentration portion with respect to (horizontal axis: mm).
FIG. 4 is an explanatory view showing an example of a conventional fuel assembly for a boiling water reactor, where (a) is a schematic perspective view of a lower tie plate portion, and (b) is shown in (a). Sectional view of the leak control plate 13, (c) is a schematic view showing bulge deformation occurring in the channel box, (d) is the amount of channel box deformation (vertical axis: mm) with respect to the average fuel burnup (horizontal axis: GWd / t) It is explanatory drawing which shows the relationship.
[Explanation of symbols]
1, 11: channel box 2, 12: lower tie plate 3, 13: leak control plate 4, 14, X: windows 5, 15, Y: slit 6, 16: stop plate 7, 17: bolt 16a: stop plate 16 Leg 18a: upper pocket 18b: lower pocket d, D: window frame width L: window opening width H: distance between opposing sides (on the inner surface of the channel box)

Claims (1)

A fuel bundle including a plurality of fuel rods and water rods held in a square lattice array by a support lattice, an upper tie plate for suspending an upper end of the fuel bundle, and a lower portion for supporting a lower end of the fuel bundle A tie plate, a channel box suspended around the upper tie plate and surrounding the fuel bundle, and a coolant from a gap in an overlapping portion between a lower end portion of the channel box and an outer peripheral surface of the lower tie plate. A boiling water reactor fuel assembly comprising: a leak control device provided on an outer peripheral surface of the lower tie plate so as to control a leak amount;
A fuel assembly for a boiling water reactor having an average burnup of 45 GWd / t or more,
The leak control device has a leak control plate mounted with a width dimension substantially covering a flat side portion on an outer peripheral surface of the lower tie plate,
The leak control plate is made of a Ni-based alloy having a thickness of 0.2 mm, and is formed between a window for introducing a leak flow of the coolant from the inside of the channel box to a back side thereof and each of the windows. Slits are alternately arranged in the width direction of the side portion, and the ratio of the total opening width of the plurality of windows to the entire width of the leak control plate is in the range of 0.3 to 0.7. And a window frame width of substantially 2 mm or more .

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