JPH08233971A - Nuclear reactor core shroud and method for manufacturing and repairing it - Google Patents

Abstract

PURPOSE: To provide a nuclear reactor core of stress corrosion cracking resis tance and a method for manufacturing and repairing it. CONSTITUTION: A shield plate 6 is set at a position for covering a weld heat affect part 8 of a nuclear reactor core shroud and the circumferential edge part of the shield plate 6 is connected to the shroud 1 at a connection part 9 so that a gap between the shield plate 6 and the shroud 6 may be water-tight structure. Since the connection part 9 is the circumferential edge part of the shield plate 6, little heat penetrates by accompanying connection and stress corrosion cracking resistance can be improved without new heat affect which is given to the shroud even in the case where connection is performed by welding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reactor core shroud (hereinafter referred to as shroud) installed in a reactor pressure vessel, a manufacturing method thereof, and a repairing method thereof.

[0002]

2. Description of the Related Art It is known that, in a metal material such as austenitic stainless steel, stress corrosion cracking (hereinafter referred to as SCC) occurs in a heat-affected zone near a weld in a high temperature water environment. The occurrence of SCC is also a concern in the stainless steel shroud used in contact with the reactor cooling water.

It is known that SCC is generated by superposition of three factors of material, environment and stress. That is, the material factor is chromium deficiency (sensitization) in the vicinity of the grain boundary due to the precipitation of chromium carbide in the grain boundary of the heat-affected zone of welding, and the environmental factors are contact with the corrosive water environment and stress. The factor is the existence of tensile stress such as welding residual stress and external stress.
By removing any one of these three factors, S
It becomes possible to prevent the occurrence of CC.

Therefore, as a SCC preventive maintenance method for reactor internal equipment such as shrouds, desensitization and surface tensile stress caused by heating and cooling of the sensitized portion, as described in JP-A-5-80186. There are ways to mitigate. Further, as a method for preventing contact between the sensitized base material and cooling water, as described in JP-A-5-77082, a thin plate member having high corrosion resistance is attached to the surface of the sensitized portion of the base material, and the melt treatment is performed. A technique for obtaining a surface layer having high corrosion resistance by applying is known.

[0005]

When the SCC preventive maintenance method as described above is applied to the shroud, some problems occur. First, as a general problem, if a large amount of heat is applied to the shroud sensitized part for heat treatment or melting, new sensitization or tensile stress may occur in the heat affected part. .

Next, there is another problem during the construction on the inner surface of the shroud having a particularly high neutron irradiation amount. In stainless steel, which is the material of the shroud, helium is generated inside by transmutation when it is exposed to neutron irradiation, but this helium may precipitate at grain boundaries due to large heat input for heat treatment and melting process and cause embrittlement. There is a nature.

Further, it is known that due to the influence of neutron irradiation, chromium deficiency occurs in the grain boundary part by a mechanism different from heat sensitization. May cause sensitization. In this case, in the conventional method, since the base material sensitized part and the surface treated part are in close contact with each other, the tensile strain remaining in the base material extends to the surface treated part, and the sensitization of the surface treated part causes There is a possibility that SCC will occur again in the department. Further, since the surface-treated portion is integrated with the base material, even if the surface-treated portion becomes sensitive, it is impossible to replace only this portion.

As described above, there is a problem that the conventional method is difficult to apply, or even if the shroud can be applied to the inner surface of the shroud where the neutron irradiation amount is high, the effect is lowered in the subsequent use. The object of the present invention is to improve the SCC resistance without making a new sensitized portion or tensile stress portion, its manufacturing method, and the SCC resistance without making a new sensitized portion or tensile stress portion. To provide a shroud repair method.

Another object of the present invention is to provide a shroud having an SCC-resistant measure which is effective even in a portion having a high neutron irradiation amount, a manufacturing method thereof, and an effective SCC resistance improvement in a portion having a high neutron irradiation amount. It is to provide a shroud repair method.

[0010]

The above object is achieved by a shroud provided with a shield which covers the weld heat affected zone of the shroud and has a peripheral portion joined to the shroud so that a gap with the shroud surface is a watertight structure. To be done.

Further, the above object is to assemble a steel plate constituting a shroud by welding and then attach a shield to a position covering the heat affected zone of the weld so that a gap between the shield and the surface of the shroud has a watertight structure. This is accomplished by a shroud manufacturing method that joins the shield periphery to the shroud.

Further, the above object is to attach a shield to the repair target portion including the welding heat affected zone of the existing shroud, and to make the peripheral edge of the shield a shroud so that the gap between the shield and the shroud surface becomes a watertight structure. This is accomplished by a shroud repair method in which the shields are combined and, after a period of use, the attached shields are removed and replaced with new shields.

Here, the shield is an alloy member having corrosion resistance, which is installed in order to block the contact between the SCC generation position such as the sensitized portion of the shroud surface and the reactor cooling water. Specifically, a plate material made of stainless steel or Inconel is suitable. The watertight structure means a structure in which the reactor cooling water does not enter the gap between the shroud and the shield during use of the shroud.
Further, the joining of the peripheral portions means joining the approximately outer peripheral portion of the shield to the shroud so that the gap between the shroud and the shield has a watertight structure. That is, the joint portion does not have to be a strict outermost peripheral portion of the shield, and a part of the inside of the shield may be joined to the shroud.

[0014]

Since the shield covers the welding heat affected zone of the shroud and is coupled to the shroud and the watertight structure, the contact between the welding heat affected zone of the shroud and the cooling water is blocked, and the SCC is suppressed. Also, since the shield joins the shroud at the peripheral edge, even if the joining method is welding, the length of the welding line, that is, the amount of heat input, is minimized, and a new sensitization portion and tensile stress due to the shield attachment are added. No part is generated. Further, as described above, since the heat input amount at the time of attaching the shield is low, it is possible to perform construction even on the inner surface of the shroud having a high neutron irradiation amount. Further, since the shield and the shroud are not entirely in close contact with each other, the tensile residual stress on the surface of the shroud does not reach the shield. Therefore, even if the shield is sensitized by neutron irradiation, SCC is unlikely to occur here. Also, since the shield and the shroud are not integral, the sensitized shield can be replaced.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows a first shroud according to the present invention.
2 is a vertical cross-sectional view of the embodiment of FIG.
2 is a schematic configuration diagram of a reactor pressure vessel including the shroud of FIG. 1. FIG. The reactor core shroud 1 has an upper grid plate 2,
A core (not shown) is formed with the core support plate 3, the fuel assembly 4, etc., and is fixed to the reactor pressure vessel 5 at the lower part of the shroud. The water level 10 in the reactor pressure vessel 5 is normally located at the position shown in the figure, and the shroud 1 is used while being wholly placed in water.

The shroud 1 includes an upper body 1a and an intermediate body 1
b, and three cylindrical portions called the lower body 1c. In this embodiment, the cylindrical shield 6a is installed on the inner surface of the intermediate body 1b adjacent to the fuel assembly 4 and having the highest neutron irradiation amount. ing. The reason why the shield 6a is provided at this portion is that the present invention is suitable for application to a portion having a particularly high neutron irradiation amount, as described above.

The cylindrical shield 6a has a cylindrical shape whose outer shape is slightly smaller than the inner diameter of the shroud intermediate body 1b, and the inner surface of the intermediate body 1b including the welded portion 7 on the intermediate body 1b and the welding heat affected zone 8 around it. It is attached to cover the entire surface. The material of the cylindrical shield 6a may be the same stainless steel as the shroud 1, but it is desirable to use steel having higher corrosion resistance, for example, a carbon concentration reduced.

The tubular shield 6a is attached to the shroud 1 by welding the upper and lower circumferential portions of the tubular shield 6a to the shroud 1 at a joint portion 9 in a watertight structure. As described above, the welding accompanying the attachment of the tubular shield 6a is only for the upper and lower ends of the tubular shield 6a, so that the shroud intermediate body 1
New heat input due to the attachment of the cylindrical shield 6a hardly occurs in b. Further, the removal of the cylindrical shield 6a can be easily performed by heating only the connecting portion 9 or by machining.

Further, unlike the shroud body, the cylindrical shield 6a does not need to support the load of the core, so that the connecting portion 9
Is sufficient to hold the tubular shield 6a on the shroud 1 and maintain a watertight structure. For this reason, welding at the joint 9 can be performed with a very small heat input, and there is no fear of sensitization of this portion or embrittlement due to helium when applying to an existing shroud. The joining at the joining portion 9 can be achieved not by welding but by a method such as brazing that does not involve melting of members.

Next, a second embodiment of the shroud according to the present invention will be described with reference to FIG. FIG. 4 is a vertical sectional view of the second embodiment. In the present embodiment, the annular shield 6b is not a large one that covers the entire surface of the shroud but a small one that covers only the vicinity of the welding heat affected zone 8. Therefore, the manufacture of the annular shield 6b and the attachment to the shroud 1 are easier than the large shield. Further, in this embodiment, a shield is also attached to the outer surface of the shroud to improve the SCC resistance of these parts.

However, especially the intermediate barrel 1b of the existing shroud
When it is applied to the outer surface of the, it is difficult to use a shield that is previously formed in a cylindrical shape. In this case, the shield is divided in the circumferential direction as shown in FIG. 5, and each shield piece 6c is
The welding can be performed by attaching the joint 9 at a position covering the welding heat affected zone 8a along the circumferential welding line 7a. The method of attaching the shield shown in FIG.
b It can be applied to a place other than the outer surface, and in this case also, there is an advantage that the shield can be easily manufactured and attached to the shroud 1. Further, for example, if the shield piece 6c is attached to a position that covers the welding heat-affected zone 8b along the vertical welding line 7b in FIG. 5 as well, all the welding heat-affected zones can be protected with a small shield body area. The material required for manufacturing can be saved.

6 to 8 are cross-sectional views showing in detail the vicinity of the joint between the shroud upper body 1a and the intermediate body 1b, and show examples in which the shield 6 is joined to the shroud 1 by different methods. In these coupling examples, the shield 6 is attached to the inner surface of the intermediate barrel 1b. FIG. 6 shows an example in which the connecting portion 9 is provided on the side surface of the shroud and the shield 6 is attached. Since the structure is simple and there is no need to process the shroud, it is suitable for construction on an existing shroud.

Next, FIG. 7 shows a structure in which the upper end of the inner surface of the shroud is cut out, and the shroud is fitted with the shield 6 whose end is bent. There is less risk of falling off, and the reliability of shield attachment is improved.

Next, FIG. 8 shows a packing 11 which is a sealing means for keeping the gap between the shield 6 and the shroud in a watertight structure.
And the connecting portion 9 is constituted by the bolt 12 which is a fixing means for fixing the shield plate 6 to the shroud 1,
The shield 6 is mechanically coupled to the shroud 1. This embodiment has the advantage that no new heat input is required when attaching the shield 6. Further, removal of the shield 6 is easier than when the joint 9 is welded. Since this embodiment requires processing such as groove processing and bolt hole processing in the shroud 1, it is suitable when the shield is attached to the new shroud.

FIG. 9 is a flowchart showing a manufacturing method for manufacturing the novel shroud according to the present invention. First, the shroud body is manufactured. This is carried out by a general process of welding steel plates which are members, producing a body by rolling, and then joining the bodies by circumferential welding. Separately from this, a shield is prepared. In this embodiment, an example of manufacturing a shield body which is formed in a cylindrical shape in advance will be described. The shield may be formed by seamless integral molding or by welding to form a cylindrical shape. However, if there is a welded portion, the shield may be subjected to solution treatment or the like before being attached to the shroud. It is desirable to perform heat treatment to desensitize the weld. When the divided shield shown in FIG. 5 is used instead of the cylindrical shield, the shield piece is formed according to the curved surface of the shield attachment position and attached to the shroud in the next step.

Next, the produced shield is attached to the shroud. Specifically, first, the shield is positioned at a predetermined position that covers the weld heat affected zone of the shroud surface, and then temporarily fixed to this position by spot welding or the like. At this time, since the heat input by spot welding is very small and the range of heat influence is limited, the position of spot welding does not have to be strictly on the peripheral portion of the shield plate. The temporarily fixed shield is then welded at its peripheral edge so that the gap with the shroud has a watertight structure.
When the shroud and the shield are joined by bolting as shown in FIG. 8, spot welding for temporary fixing is not necessary, and the shroud and the shield are joined to the watertight structure by sequentially tightening the bolts.

FIG. 10 is a flow chart showing a first embodiment of the method of attaching / removing the shield to / from the existing shroud according to the present invention. This method is performed during plant shutdown, such as during periodic inspections of nuclear plants. When exchanging the shield, the shield's sensitivity is predicted by using a theoretical evaluation formula for the sensitization behavior of the material considering the effect of irradiation and an empirical prediction formula for the corrosion behavior of the material based on corrosion test data. Implement at an appropriate time, well before the time of conversion.

First, the equipment inside the shroud 1, such as the upper lattice plate 2, the core support plate 3, and the fuel assembly 4, is removed to improve workability inside the shroud. next,
If the shield 6 is already installed on the shroud 1,
Work to remove this. That is, when the joint portion 9 at the peripheral portion of the shield 6 is welded, the shield 6 is removed from the shroud 1 by cutting or heating this portion, and when bolting, the bolt is removed. This work is preferably carried out in water while the reactor pressure vessel 5 is filled with cooling water in order to reduce exposure to work equipment and workers.

Next, a new shield is attached to the shroud surface. When installing the shield plate for the first time, proceed to this work after removing the above-mentioned equipment inside the shroud. When installing the new shield, the cooling water must be installed so that the cooling water does not enter the gap between the shield and the shroud. Therefore, the cooling water must first be removed from the site where the shield plate is to be attached on the surface of the shroud. Examples of this method include a method of draining the cooling water in the reactor pressure vessel 5, or a method of providing a partition wall around the site where the shield is to be attached on the shroud surface and draining the cooling water in the partition wall.

After the cooling water is removed from the site where the shield is to be attached on the surface of the shroud by the method described above, a new shield is joined to the watertight structure by welding or bolting the peripheral edge of the shield to the shroud. After installing the new shield, the condition of the removed cooling water is restored, the device inside the shroud is installed in the original position, and the work is completed.

FIG. 11 is a flow chart showing a second embodiment of the method of attaching / removing the shield to / from the existing shroud according to the present invention. In the present embodiment, first, the core equipment such as the shroud 1, the upper lattice plate 2, the core support plate 3, and the fuel assembly 4 is removed from the reactor pressure vessel 5. After that, the existing shield is removed and a new shield is attached in the same manner as the embodiment of FIG. Then shroud 1
The core equipment including is installed again in the reactor pressure vessel 5, and the work is completed.

In this embodiment, since the work of installing a new shield can be performed outside the reactor pressure vessel, that is, in the atmosphere, means for removing the cooling water from the site where the shield is to be attached on the shroud surface. Is unnecessary. Further, in this embodiment, since the work is performed outside the reactor pressure vessel, a narrow portion is formed inside the reactor pressure vessel, and it is easy to attach and remove the shield to the outer surface of the shroud, which has poor workability.

[0033]

As described above, according to the present invention, the contact between the heat-affected zone of the shroud and the cooling water is blocked by the shielding plate, and a new sensitized portion or tensile stress portion is generated by attaching the shielding plate. Therefore, the SCC resistance of the shroud can be improved.

Further, since the heat input amount at the time of attaching the shield plate can be reduced or the heat input can be eliminated, the shroud repairing method can be applied to the inner surface of the shroud having a high neutron irradiation amount.

Further, since the shield plate and the shroud are not entirely adhered to each other and the tensile residual stress on the surface of the shroud does not reach the shield plate, the SCC resistance of the shroud is improved even under the neutron irradiation environment. be able to.

Further, since the shield plate and the shroud are not integrated and only the shield plate can be replaced when the shield plate becomes sensitive during use, the cost is greatly reduced compared to the case where the entire shroud is replaced. In addition, the amount of waste generated can be reduced.

Further, by applying the shroud having the above effects and the repair method thereof to a nuclear power plant, the reliability of the plant can be improved.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view of a first embodiment of a shroud according to the present invention.

FIG. 2 is a view on arrow I-I of FIG.

FIG. 3 is a schematic configuration diagram of a reactor pressure vessel including the shroud of FIG.

FIG. 4 is a vertical sectional view of a second embodiment of the shroud according to the present invention.

FIG. 5 is a diagram showing a method of attaching the shielding plate to the outer surface of the shroud.

FIG. 6 is a detailed cross-sectional view of a joint portion of a shroud and a shield plate.

FIG. 7 is a detailed cross-sectional view of a joint portion between the shroud and the shield plate.

FIG. 8 is a detailed cross-sectional view of a joint portion of the shroud and the shield plate.

FIG. 9 is a view showing a manufacturing method when manufacturing a novel shroud according to the present invention.

FIG. 10 is a diagram showing a first embodiment of a method of attaching / removing a shielding plate to / from an existing shroud according to the present invention.

FIG. 11 is a diagram showing a second embodiment of a method of attaching / removing a shield plate to / from an existing shroud according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor core shroud, 2 ... Upper lattice plate, 3 ... Core support plate, 4 ... Fuel assembly, 5 ... Reactor pressure vessel, 6 ... Shield plate, 7 ... Welding part, 8 ... Welding heat affected part, 9 … Coupling, 1
1 ... packing, 12 ... bolt.

Claims (16)

[Claims] 1. A reactor core shroud installed in a reactor pressure vessel, wherein a welding heat affected zone of a surface of the reactor core shroud is covered, and a gap between the reactor core shroud and the reactor core shroud has a watertight structure. A reactor core shroud comprising a shield having a peripheral portion coupled to the reactor core shroud. 2. In a reactor core shroud installed in a reactor pressure vessel, a welding heat-affected zone of the surface of the reactor core shroud is covered, and a gap with the reactor core shroud has a watertight structure, A reactor core shroud comprising a cylindrical shield having upper and lower ends coupled to the reactor core shroud. 3. The reactor core shroud according to claim 2, wherein at least an inner surface of the reactor core shroud is provided with a cylindrical shield. 4. In a reactor core shroud installed in a reactor pressure vessel, a welding heat affected zone of the surface of the reactor core shroud is covered, and a gap with the reactor core shroud has a watertight structure, A reactor core shroud characterized in that a plurality of shield pieces are provided along a circumferential welding line, and a peripheral edge portion of each shield piece is coupled to the reactor core shroud. 5. In a reactor core shroud installed in a reactor pressure vessel, a welding heat affected zone of a surface of the reactor core shroud is covered, and a gap between the reactor core shroud and the reactor core shroud has a watertight structure, and A reactor core shroud having a shield removable from the reactor core shroud. 6. A reactor core shroud according to any one of claims 1 to 5, characterized in that a shield attached to the surface of the reactor core shroud by welding is provided. 7. The reactor core shroud according to any one of claims 1 to 3, wherein the shield means holds the gap between the shield and the reactor core shroud in a watertight structure, and the shield is a reactor core shroud. A reactor core shroud characterized by comprising fixing means for fixing to a reactor core. 8. A reactor core shroud, and an in-core structure comprising an upper lattice plate and a core support plate installed inside the reactor core shroud, wherein the atom according to any one of claims 1 to 5. A core internal structure characterized by having a core shroud. 9. A method for repairing a nuclear reactor core shroud, which comprises the following steps. (1) Step of removing equipment inside the reactor core shroud (2) With the reactor core shroud installed in the reactor pressure vessel, a repair target portion including a welding heat affected zone on the surface of the reactor core shroud Step of shutting off contact with reactor cooling water (3) Coupling the peripheral portion of the shield to the reactor core shroud so as to cover the repair target part and form a watertight structure in the gap with the reactor core shroud. Steps to do 10. The reactor core shroud repair method according to claim 9, wherein a cylindrical shield is attached to at least a repair target portion of an inner surface of the reactor core shroud. Repair method. 11. A method for repairing a nuclear reactor core shroud, comprising the following steps. (1) Step of removing the reactor core including the reactor core shroud from the reactor pressure vessel (2) Covering the repair target part including the weld heat affected zone on the surface of the reactor core shroud, and the gap with the reactor core shroud So as to have a watertight structure, the step of connecting the peripheral portion of the shield to the reactor core shroud (3) the step of re-installing the reactor core shroud in the reactor pressure vessel 12. A nuclear reactor core shroud repair method comprising the following steps. (1) Step of removing the equipment inside the reactor core shroud (2) Step of removing the shield attached to the surface of the reactor core shroud with the reactor core shroud installed in the reactor pressure vessel (3) Step of cutting off contact between the repair target part including the weld heat affected zone on the surface of the reactor core shroud and the reactor cooling water (4) Covering the repair target part, and the gap between the reactor core shroud and the reactor core shroud is a watertight structure And connecting a new shield periphery to the reactor core shroud. 13. The method for repairing a reactor core shroud according to claim 12, wherein a cylindrical shield attached to the inner surface of the reactor core shroud is removed, and a welding heat affected zone of the inner surface of the reactor core shroud is removed. After cutting off the contact between the repair target part including the cooling water and the reactor cooling water, a new cylindrical shield is inserted into the reactor core shroud so as to cover the repair target part, and the reactor core shroud and A method for repairing a nuclear reactor core shroud, characterized in that the peripheral edge portion of the new cylindrical shield is connected to the nuclear reactor core shroud so that the gap between the two becomes a watertight structure. 14. A method for repairing a nuclear reactor core shroud, comprising the following steps. (1) Step of removing the reactor core including the reactor core shroud from the reactor pressure vessel (2) Step of removing the shield attached to the reactor core shroud (3) Weld heat affected zone of the surface of the reactor core shroud Covering the portion to be repaired, including the following, and joining the peripheral portion of the new shield to the reactor core shroud so that the gap between the reactor core shroud and the reactor core shroud has a watertight structure. (4) Attaching the reactor core shroud Reinstalling in the reactor pressure vessel 15. A method of manufacturing a reactor core shroud installed in a reactor pressure vessel, comprising: assembling the reactor core shroud by longitudinal welding and circumferential welding of steel plates; Circumferential welding, or covering the weld heat affected zone by both the longitudinal welding and the circumferential welding,
A method of manufacturing a nuclear reactor core shroud, characterized in that the peripheral portion of the shield is joined to the nuclear reactor core shroud so that the gap with the nuclear reactor core shroud has a watertight structure. 16. The method of manufacturing a reactor core shroud according to claim 15, wherein after the reactor core shroud is assembled by longitudinal welding and circumferential welding of steel plates, the circumferential welding or the longitudinal welding. And the circumferential heat of the welded heat-affected zone are covered, and the peripheral portion of the cylindrical shield is coupled to the reactor core shroud so that the gap with the reactor core shroud is a watertight structure. A method for manufacturing a reactor core shroud characterized by: