JPH0375077B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a pressurized water reactor fuel assembly, and in particular to a pressurized water reactor fuel assembly that passes through a drain hole in a control rod guide tube including an upper large diameter portion and a lower small diameter dart pot portion. This invention relates to a pressurized water reactor fuel assembly that enables coolant flow rate control.

(Prior Art) As illustrated in FIG. 10, a pressurized water reactor fuel assembly has a large number of long fuel rods 1 and control rod guide tubes 3 arranged in parallel in a square lattice shape, and a plurality of support lattices. 2, and an upper nozzle 4 and a lower nozzle 5 are installed at the upper and lower ends, respectively.For example, the fuel rod 1 and control rod guide tube, etc.
They are arranged as shown in the schematic diagram of FIG. That is, cells containing single circles are control rod guide tube cells, cells containing double circles are instrumentation guide tube cells, and other cells are fuel rod cells, each containing a control rod guide tube, an instrumentation guide tube, and a fuel rod cell. is inserted.

Here, the control rod guide tube is connected to the upper fuel nozzle 4,
At the same time, it serves as a structural member that connects the lower nozzle 5, holds the support grid, and forms the skeleton of the fuel assembly. A continuous insertion passage is provided for each of the plurality of control rods.

Such a control rod guide tube 3 is usually formed of a Sn-Fe-Cr-based zirconium alloy tube that absorbs little neutrons, and has a lower part that allows a predetermined position when the control rod is rapidly inserted by free falling due to its own weight during an emergency shutdown of the reactor. After passing the insertion length, a small diameter dart pot part 3b is installed at the upper part as shown in Figs. It is provided at the lower part of the large diameter portion 3a.

A lower end plug 8 is welded to the lowest end of the control rod guide tube 3 so that it can be mechanically connected to the lower nozzle 5 by screws, etc., and the center part of this end plug is provided with a coolant that will be inside when the control rod falls. A drain hole 9 of a predetermined size is drilled to prevent mechanical damage to the control rod guide tube 3 itself or the welded portion of the guide tube 3 and the lower end plug 8 due to excessive pressure.

In addition, near the lower end of the large diameter portion 3a of the control rod guide tube 3,
Also in the region immediately above the dart pot portion 3b, drain holes 7 of a predetermined size and quantity are usually provided so that the control rod can reach the dart pot entrance as quickly as possible.

Therefore, during normal operation of the reactor, in a pressurized water reactor, the control rod 6 or an insert rod such as a burnable poison rod (BPR) has the positional relationship shown in Figure 12 with respect to the control rod guide tube. The control rods 6 are held in a state where they are pulled upward by a predetermined amount in order to adjust the reactivity of the reactor, and many of the control rods 6 are pulled out to near the upper end entrance of the control rod guide tube. It's been taken out.

At this time, the coolant flows through the water drain hole 7 into the control rod guide tube 3 due to the difference in flow resistance between the flow paths inside and outside the control rod guide tube 3, as shown by the arrows in the figure. flows into the interior of. The coolant flowing inside the control rod guide tube 3 does not contribute to cooling the fuel rods, which are the main heat source, but the coolant flows inside the control rod guide tube 3.
BPR also generates some heat due to neutron absorption reactions and gamma ray heating from the fuel rods, and it is said that it is preferable to secure a coolant flow rate just sufficient for cooling.

In contrast to the above, on the other hand, when the control rod falls, the situation is shown in Fig. 13, when the control rod 6 is on the dart pot part 3b, it cools down violently through the large diameter water drain hole 7 as shown in the figure. The material is extruded and the flow rate passing through the drain hole 9 of the lower end plug 8 is very small. However, after the control rod 6 moves further downward, only the lowest water drain hole 9 changes to have a water draining effect.

Therefore, the dimensions and arrangement of the above-mentioned drain holes 7 and 9,
The number of rods must be optimally designed in consideration of the above-mentioned coolant flow rate requirements, control rod insertion time, degree of deceleration at the final stage of control rod fall, etc.

However, even with such optimal design, it is not always possible to insert inserts such as control rods and BPR into all the fuel loaded into the core of a pressurized water reactor, and the design of the reactor itself, For the purpose of optimizing core characteristics, about half or more of the fuel in the entire assembly is used without the above-mentioned interpolation. Therefore, if the fuel assembly is loaded into the core with no inserts in the control rod guide tubes 3, the cooling inside the control rod guide tubes 3 will be reduced by the amount of coolant flow resistance caused by the inserts. The flow of material becomes easier, and the pressure balance allows more flow to flow through the guide tube.

Therefore, conventionally, for fuel assemblies in which inserts such as control rods and BPR are not inserted in order to prevent this phenomenon, plugging devices consisting of short, large diameter stainless steel rods have been used. As illustrated in Fig. 15, the plugging device plug rod 6'
A plugging device is inserted into the assembly as shown in FIG. 14, and the plugging device has a configuration in which the plate 10 is caulked, fixed by welding, screwing, etc., and elastically crimped with a hold-down bar 12 via a hold-down spring 11. The control rod guide tube 3 is inserted into the vicinity of the upper end of the control rod guide tube 3 to partially close the control rod guide tube 3 and adjust the flow rate inside the tube.

(Problems to be Solved by the Invention) However, when such a plugging device is used, there are the following drawbacks.

(1) When replacing fuel during periodic reactor inspections (referred to as shuffling), the plugging device must also be replaced each time, increasing the effort involved in loading fuel. In order to operate nuclear reactors economically, it is desirable to shorten the time required for periodic inspections as much as possible, but the work of replacing plugging devices during shuffling is a major obstacle to shortening periodic inspection periods. ing.

(2) It is expensive to manufacture the plugging device, and at the end of its life, the plugging device itself becomes radioactive waste, requiring a large amount of money to store or dispose of it.

On the other hand, when the plugging device is not used, the flow rate of the coolant flowing through the control rod guide tube 3 becomes too large as described above, and the ratio of coolant that effectively cools the fuel rods decreases, causing the reactor to deteriorate. This has the disadvantage that the output cannot be increased to a predetermined level.

Furthermore, if the cross-sectional area of the drain hole in the control rod guide tube is made smaller in order to reduce the flow rate of the control rod guide tube, the falling resistance of the control rod will increase, causing problems in terms of safe reactor shutdown in an emergency. occurs.

In view of the above-mentioned problems in the design of fuel assemblies in the prior art, it is an object of the present invention to solve the problems, and to provide a water drain hole on the outer surface of the large diameter portion of the control rod guide tube. Alternatively, a thin metal plate-like valve body may be provided that partially covers the valve body. (1) During normal operation, the valve body is closed due to the fluid force from the outside due to the difference in coolant pressure loss inside and outside the control rod guide pipe. Even when the plugging device is not used, the flow rate of coolant flowing into the control rod guide tube through the drain hole is suppressed, and more coolant than necessary flows through the control rod guide tube. (2) For fuel assemblies loaded in the control rod insertion position in the reactor core, when the control rods fall, they are compressed in the control rod guide tubes and the pressure increases. The significantly raised coolant forces the valve body to expand from the inside, providing a flow path through which the coolant can escape relatively easily to the outside of the control rod guide tube through the drain holes.

The purpose is to

(Means for Solving the Problems) The features of the present invention for achieving the above object are shown in FIGS. 1 to 9, and FIGS. The basic configuration will be explained based on the embodiment shown in FIG. At least one drainage hole 7 is provided. Then, a flow rate control sleeve 20 is attached to the outer surface thereof to cover the water drain hole 7 and to control the flow of coolant into the control rod guide tube through the water drain hole 7.

The flow rate control sleeve 20 is made of a tubular body, one half of which forms a tubular base 21, and the other half formed by a plurality of axial slits 23 cut from the other end of the base 21. On the other hand, on the base 21 side, there are a plurality of valve body parts 22 which are connected to each other. (See FIGS. 5 and 6) When the flow rate control sleeve 20 is attached to the outer circumferential portion of the control rod guide tube 3, the valve body portion 22 is attached at least to the water drain hole 7 of the control rod guide tube 3. A flow control sleeve 20 is axially positioned and held relative to the upper control rod guide tube by a positioning spacer tube 24 in such an axial positional relationship as to cover a portion of the upper control rod guide tube from the outer surface.

Here, the material of the flow control sleeve 20 is usually stainless steel or Sn-Fe-Cr-based zirconium alloy, and any of these may be used. both at the same time or intermittently on the circumference,
Bulge processing, which involves locally expanding outward plastic processing,
Although metallurgical joining means, such as joining the two by welding such as resistance welding, is common, it is also suitable to use the positioning spacer tube 24 described above.

The use of such a positioning spacer tube 24 is as follows:
A portion with a small inner diameter is provided on the base 21 side of the flow control sleeve 20, and when the sleeve 20 is attached to the outer circumference of the control rod guide tube, the portion with the small inner diameter fits the outer circumferential surface of the control rod guide tube dosspot portion 3b. This is done by installing the spacer tube 24 between the lower nozzle 5 and the flow control sleeve 20 so that the flow control sleeve 20 does not move above the outer diameter transition region of the control rod guide tube. In this case, the spacer tube 24 should have an inner diameter slightly larger than the outer diameter of the control rod guide tube dosspot portion 3b, and an outer diameter slightly larger than the inner diameter of the portion of the flow control sleeve base 21 with the small inner diameter. is necessary.

Further, in order to position and fix the flow rate control sleeve 20, it is also possible to perform expansion plastic working on the control rod guide tube 3 below the flow rate control sleeve 20 outward.

(Function) In the fuel assembly of the present invention constructed as described above, the valve body 22 is closed due to the pressure difference between the inside and outside of the control rod guide tube 3, as shown in FIG. 3, except when the control rod falls. The flow of the coolant through the drain hole 7 is suppressed.

On the other hand, when a control rod is inserted and the control rod 6 falls for an emergency shutdown of the reactor, the valve body 22 is pushed open due to the large internal pressure inside the control rod guide tube 3, as shown in FIG. The coolant can escape relatively freely to the outside as shown by the arrow.

Thus, in pressurized water reactor fuel assemblies, even for assemblies in which inserts such as control rods or burnable poison rods (BPR) are not inserted, the control rod guide tubes can be inserted without using a plugging device. In addition to controlling the coolant flow rate so that it does not become excessive, it also reduces the fluid resistance when it is necessary to quickly insert a control rod in an emergency.
The necessary control rod drop (insertion) speed can be guaranteed.

(Embodiments) Hereinafter, embodiments of the present invention will be further described based on the accompanying drawings.

Figures 1 to 6 show a first embodiment of the main parts of the fuel assembly of the present invention. In the figures, 3 is a control rod guide tube, the upper part is a large diameter part 3a, and the lower part is a small diameter dart pot part. 3b, there are normally four drain holes 7 drilled near the lower end of the large diameter portion 3a of the control rod guide tube 3, and a flow rate control sleeve 20, which is a main component in the present invention, is provided around the outer periphery of the drain hole 7. Covered.

The flow rate control sleeve 20 is a tubular body made of stainless steel or Sn-Fe-Cr zirconium alloy, and as shown in FIG. The half portion has a plurality of slits cut from the end opposite to the base to form a plurality of valve body portions 22 that are separated from each other, and the valve body portion 22 is separated from the bottom end of the control rod guide tube. It is positioned and held by interposing a positioning spacer tube 24 between it and the lower nozzle 4 in an axial positional relationship that covers a part of the water drain hole 7 of the control rod guide tube from the outer surface. .

In producing the flow rate control sleeve 20 as described above, the following steps are taken.

For example, 304 type stainless steel is machined into a tubular shape, and the base side is finished to a size that can just accept the outside diameter of the dart pot portion 3b of the control rod guide tube 3, while the other end of the base side is machined into a tubular shape. The large diameter portion 3a of the control rod guide tube is made to have an inner diameter that can accept it, and the space between the two is tapered and smoothly connected. At this time, the wall thickness on the base side is made thicker than the wall thickness on the other end side, for example, while the wall thickness on the other end side is 0.1 mm, the wall thickness on the base side is about 0.8 mm.

The tubular body made as described above is then press-worked to punch out a plurality of slits of a predetermined length from the other end of the base, so that the other end of the base is separated from each other, and the metal parts connected to each other are separated from each other at the other end of the base. A flaky valve body portion 22 is formed.

Here, the slit of the flow rate control sleeve 20 divides the valve body portion 22 into metal flakes to reduce the bending rigidity of each piece, so that the valve body can be easily bent by the flow of coolant or pressure difference. Even when the valve body seals the water drain hole 7 of the control rod guide tube 3, it is not completely sealed, but it is designed to allow coolant to flow in so that the necessary amount of coolant is secured inside the control rod guide tube. This provides fine gaps between the holes.

In the illustrated embodiment, there are 16 slits, that is, 16 valve bodies, and the slit width is determined according to the diameter of the tubular body.
It is about 0.25mm.

The flow rate control sleeve 20 having the above-mentioned configuration does not necessarily have to be fixed to the control rod guide tube in terms of rotation, but at least in its position in the axial direction. is fixed.

In this case, if the size of the inlet cross-sectional area due to the gap for coolant inflow has a particularly large effect on the amount of coolant inflow, the flow control sleeve 20
Regardless of the relationship between the direction of the slit and the direction of the drain hole 7, it is desirable to obtain a substantially constant inlet cross-sectional area.

In such a case, it is necessary to design the drain hole 7 to obtain the optimum conditions by taking into account the dimensions, arrangement, slit width, etc. Under the condition that there are four drain holes 7, it is preferable to have a shape in which the slits are not located at an angle of 90 degrees.For example, when the number of slits is 14 as shown in Fig. 2, or when the number of slits is 18, etc. can also be considered. Of course, if 180° symmetry is not considered, an odd number of slits may be used.

Next, in order to assemble a fuel assembly by applying the above configuration, when inserting the control rod guide tube into the support grid, the flow rate control sleeve 20 is fitted onto the outer periphery of the control rod guide tube 3 from below. Then, the positioning spacer tube 24 is attached from below, and the lower nozzle 4 is attached.

FIG. 1 shows such an assembled state.

As is clear from the figure, the positioning spacer tube 24 has an inner diameter sufficient to receive the outer periphery of the control rod guide tube dosspot portion 3b, and
A dimension is required so that the end faces abut against the flow rate control sleeve 20. This spacer tube 24 has strength,
The thinner the wall thickness is within the allowable range, the better in terms of fuel flow characteristics; for example, when using Sn-Fe-Cr-based zirconium alloy pipes, the wall thickness is 0.6.
Approximately mm is suitable.

Thus, in the fuel assembly in which the control rod guide tube is provided with the above configuration, the valve body portion 22 is closed due to the pressure difference between the inside and outside of the control rod guide tube, as shown in FIG. 3, except when the control rod falls. Therefore, the flow of coolant through the drain hole 7 is suppressed. Then, when the control rod 6 is inserted and the control rod falls for an emergency shutdown of the reactor, the valve body 22 is pushed open due to the large internal pressure of the control rod guide pipe, as shown in FIG. can escape to the outside relatively freely.

Note that although the above description mainly concerns insertion of control rods, the same applies to insertion of BPR and other inserts.

In addition, in the illustrated example above, the number of drain holes 7 is four, but the number of drain holes 7 is not limited to four, and the positions, dimensions, and number of the drain holes 7 are determined according to the necessary control rod guide described above. The design should be such that the optimum value can be obtained based on the requirements for the coolant flow rate in the pipes and the control rod falling speed.

Similarly, the number of valve bodies 22 of the flow rate control sleeve 20 does not necessarily need to be enough to cover all of the drain holes 7, and the number of valve bodies 22 that may be suitable is also within the design scope. .

Furthermore, it should be noted that the valve body does not have to completely cover the water drain hole and completely block the flow of coolant, and it is also important to note how much cross-sectional area of the coolant remains when the valve body is closed. , should be determined by design.

Therefore, if the above-mentioned points are taken into consideration, in the above-described configuration of the present invention, the mounting angle in the circumferential direction of the flow rate control sleeve 20 does not have such an important effect, but the position, size, slit position, and It is sufficient to appropriately design the width, etc., to obtain a configuration in which the cross-sectional area of the inlet formed by the overlap with the slit and the drain hole does not substantially differ regardless of the angular relationship.

This means that there is no need to pay special attention to adjusting the mounting angle during the assembly process of the fuel assembly, and it is sufficient to confirm that the specified dimensions and accuracy have been obtained at the component stage, and it is sufficient to assemble the assembly. This means that the work is as easy as the conventional technology.

FIGS. 7 to 9 show other embodiments of the flow control sleeve of the present invention and the fixing of the sleeve to the control rod guide tube.
In the figure, a flow control sleeve 20 similar to that shown in FIGS. 3 and 4 is used, and instead of installing a positioning spacer tube below, the control rod guide tube dart pot portion 3b is bulged in a circumferential shape. The axial position of the flow rate control sleeve 20 is determined by plastic deformation.

Here, uniform bulging plastic deformation is performed on the circumference, but the plastic deformation is not limited to this, and for example, bulging deformation can be performed locally in four directions.

These expansion plastic deformation processes are called swaging and bulging processes, which are well-known processing techniques for pressurized water reactor fuel, and can be easily applied.

On the other hand, the embodiment shown in FIG. 8 and FIG.
This is a case where a flow control sleeve 20' having a structure that is upside down from the figure is used.

This example differs from the previous example in that:
There is no portion on the inner surface of the base portion 21' of the flow rate control sleeve 20' with a small inner diameter for fitting with the control rod guide tube dosspot portion 3b.

However, for the purpose of reinforcing the strength during fixation, the outer diameter of the base 21' may be slightly increased, or the inner diameter of the base may be slightly reduced to increase the wall thickness within the range that the outer diameter of the large diameter portion of the control rod guide tube allows. do.

The optimal one is a flow control sleeve in which the outer diameter of the base is increased by 0.6 mm so that the base wall thickness is 0.4 mm and the valve body wall thickness is 0.1 mm.

Although the flow rate control sleeve can be fixed by both mechanical and metallurgical methods as explained above, in the case of FIG. 9, the flow rate control sleeve 20' is fixed in the axial direction. Its base 21' is plastically deformed together with the control rod guide tube 3 to achieve this in one bulging section.

Although this embodiment shows an example in which the valve body portion 22' is disposed below the base portion 21', it goes without saying that there is essentially no difference in the vertical relationship between them.

As another embodiment, it is also preferable to make the flow control sleeve using the same material as the control rod guide tube, a Sn-Fe-Cr zirconium alloy, for the purpose of reducing neutron absorption. Adopting the cross-sectional shape shown, the wall thickness of the flow control sleeve base 21' is 0.5 mm, and the wall thickness of the valve body part 22' is 0.08 mm.
Manufactured in mm. Since the control rod guide tube and the sleeve are made of the same material, resistance welding can be used to fix them, and assembly is easy.

Furthermore, for each of the structures of the above embodiments, it is possible to use the material of the flow control sleeve as Sn-Fe-Cr zirconium alloy as described above, taking into account the rigidity of the valve body due to the difference in material properties. The above-mentioned effects can be easily achieved mainly by appropriately designing the wall thickness of the valve body.

(Effects of the Invention) As described above, the present invention covers a flow control sleeve having a thin metal plate shaped valve body so as to cover the drain hole on the side surface of the large diameter part of the control rod guide tube. By providing the above-mentioned valve body to cover the drain hole, the size of the drain hole is kept large (1) Except when the control rod falls, water flows through the control rod guide tube regardless of the presence or absence of an insert. (2) When a control rod falls, the coolant in the control rod guide tube can easily escape to the outside through the drain hole, and the reactor The insertion of control rods into the fuel assembly during an emergency shutdown can be guaranteed.

As a result, there is no need to use a plugging device even for fuel loaded in a position where inserts such as control rods and BPR are not inserted in the reactor core. ) There is no need to replace the plugging device during fuel shuffling.

(b) Therefore, the cost of manufacturing plugging devices and the cost of storing and processing plugging devices that become radioactive waste are eliminated.

As a result, its usefulness as a pressurized water reactor fuel assembly is highly anticipated.

[Brief explanation of drawings]

Fig. 1 is a partially omitted cross-sectional explanatory diagram showing a state in which a flow control sleeve, which is a main part of the present invention, is fixed to a control rod guide tube, and Fig. 2 is another embodiment taken along the AA cross section of Fig. 1. The cross-sectional views, FIGS. 3 and 4 are schematic cross-sectional views for explaining the action of the valve body part. FIG. 3 shows the case when the control rod is not falling, and FIG. 4 shows the case when the control rod is falling. FIG. 5 is a perspective external view of the flow control sleeve according to the present invention, FIG. 6 is an axial cross-sectional view of the same sleeve, and FIGS. 7 and 8 are axial directions of the flow control sleeve in a state in which the flow control sleeve is installed, showing other embodiments. 9 is an axial sectional view of the sleeve in FIG. 8, FIG. 10 is a partially omitted front view of the fuel assembly, and FIG. 11 is a schematic diagram showing the arrangement of fuel rods, control rod guide tubes, etc. Figure, 12th
Figures 13 and 14 are schematic cross-sectional views of the control rod guide tube, and Figure 12 shows the state in which inserts such as BPR are inserted.
Figure 3 shows the state when the control rod falls, and Figure 14 shows the state when the putlugging device is inserted. FIG. 15 is a front view of the plugging device. 1... Fuel rod, 3... Control rod guide tube, 3a...
Large diameter part, 3b...Dash pot part, 4...Upper nozzle, 5...Lower nozzle, 6...Control rod, 6'
...Plugging device plug rod, 7...Drain hole, 20, 20'...Flow rate control sleeve, 21,
21'...Sleeve base, 22, 22'...Sleeve valve body part, 23...Slit, 24...Spacer tube for positioning, 25, 25'...Bulging plastic deformation part.

Claims (1)

[Scope of Claims] 1 A plurality of long fuel rods, control rod guide tubes, etc. are arranged in parallel on a square grid, held by a plurality of support grids, and upper and lower ends are provided with an upper nozzle and a lower nozzle, respectively. In the pressurized water reactor fuel assembly, the control rod guide tube includes an upper large-diameter portion and a lower small-diameter dosspot portion, and at least one drainage hole is provided on a side surface of the large-diameter portion of the control rod guide tube. At the same time, the outer part of the control rod guide tube is made of a tubular body, approximately half of which forms a tubular base, and the other half formed by a plurality of axial slits cut from the other end of the base. A flow control sleeve that is separated and forms a plurality of valve body portions is covered, and the flow control sleeve is positioned in an axial position such that the valve body portion covers at least a portion of the drain hole from an outer surface. positioned in the axial direction with respect to the control rod guide tube,
A pressurized water reactor fuel assembly characterized in that: 2. The pressurized water reactor fuel assembly according to claim 1, wherein the flow rate control sleeve is made of stainless steel. 3. The pressurized water reactor fuel assembly according to claim 1, wherein the flow rate control sleeve is made of a Sn-Fe-Cr zirconium alloy. 4. The flow control sleeve has a small inner diameter part on the base side, and when the flow control sleeve is attached to the outer periphery of the control rod guide tube, the small inner diameter part fits the outer surface of the doss pot part of the control rod guide tube. The flow control sleeve is designed to prevent the flow control sleeve from moving above the outer diameter transition area of the control rod guide tube, and the inner diameter is slightly larger than the outer diameter of the control rod guide tube dosspot between the lower nozzle and the flow control sleeve. , a positioning spacer tube having an outer diameter slightly larger than the inner diameter of the small inner diameter portion of the base of the flow control sleeve is attached, and the flow control sleeve is positioned and held in the axial direction. The pressurized water reactor fuel assembly according to item 2 or 3. 5. The flow control sleeve has a small inner diameter part on the base side, and when the flow control sleeve is attached to the outer periphery of the control rod guide tube, the small inner diameter part fits the outer surface of the doss pot part of the control rod guide tube. The flow control sleeve is configured not to move upward beyond the outer diameter transition region of the control rod guide tube, and the upper control rod guide tube is bulged outwardly on the lower side of the flow control sleeve, Claim 1: The flow control sleeve is positioned and held in the axial direction with respect to the control rod guide tube such that the flow control sleeve does not move downwardly.
The pressurized water reactor fuel assembly according to item 1, 2 or 3. 6. Positioning and holding of the flow control sleeve relative to the control rod guide tube causes the flow control sleeve base tubular portion and the control rod guide tube to bulge outward simultaneously circumferentially or intermittently and locally on the circumference. A pressurized water reactor fuel assembly according to claim 1, 2 or 3, which is produced by processing. 7. Positioning and holding of the flow control sleeve relative to the control rod guide tube is achieved by metallurgically joining the base of the flow control sleeve and the control rod guide tube by welding such as resistance welding,
The pressurized water reactor fuel assembly according to item 2 or 3.