JP2013092482A - Nuclear reactor control rod - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nuclear reactor control rod which is resistant not only to crevice corrosion, to which a crevice between a tie rod and a sheath is susceptible, but also to corrosion that can occur when a surface coating is cracked due to residual stress and the like.SOLUTION: A control rod of the present invention includes a tie rod having a semi-cruciform cross-section, four sheaths having a semi-U-shaped cross-section and extending from the semi-cross shape of the tie rod in four directions, and a neutron absorbing hafnium member located inside each of the sheaths. One end of each sheath is welded to the tie rod, and each crevice section formed between each sheath and the tie rod in the vicinity of the weld is provided with a coating containing a metal which promotes passivation of the sheath or the tie rod and a metal diffusion layer.

Description

  The present invention relates to a nuclear reactor control rod, and more particularly to a nuclear reactor control rod that is used in a boiling water nuclear reactor and is suitable for one using hafnium metal.

  In the part where the structural materials existing in the water are close to or in contact with each other (hereinafter referred to as the gap), oxygen contained in the water that contacts the structural member is consumed, and oxygen outside the gap and the oxygen concentration cell are present in abundance. It is formed. For this reason, it is thought that it becomes an environment where a corrosion reaction is easy to continue.

  In addition, in the gap portion between adjacent structural members made of stainless steel disposed in the nuclear reactor, the three factors of radiation irradiation to the structural member, welding residual stress of the structural member, and gap structure overlap, so in the structural member An environment where the corrosion reaction is more likely to proceed.

  For example, since the reactor control rod for controlling the reactor power is installed in the reactor and inserted into the reactor core, the tie rod of the reactor control rod is included in the fuel assembly loaded in the reactor core. Material degradation and oxidation environment due to neutron and gamma irradiation generated by nuclear fission reaction of nuclear fuel material, tensile residual stress generated in the sheath by welding the tie rod and sheath in the manufacturing process, crevice corrosion environment between tie rod and sheath Since the factors overlap, there is a concern that the crevice corrosion is likely to occur.

  One of the shapes used as reactor control rods for boiling water reactors is to form one blade by encapsulating two tubular hafnium elliptical tubes as neutron absorbers in a U-shaped sheath. There is a combination of four blades in a cross shape. For example, there are reactor control rods as described in Patent Documents 1 to 3, and these reactor control rods usually have a structure as shown in FIGS.

  A conventional reactor control rod of a boiling water reactor having a hafnium elliptic tube described in Patent Documents 1 to 3 shown in FIGS. 1 and 2 has a handle 2 at the upper end of a tie rod 1 having a cross-sectional shape. Is attached, and the lower support member 3 (or the drop speed limiter) is attached to the lower end portion to form a skeleton structure called a frame.

  On this frame, a hafnium elliptic tube 5 having an approximately half length of the tie rod 1 and having an elliptical cross-sectional shape is attached to the tongue-like portion 2a of the handle 2 as an upper hafnium 5a and fixed by a pin 6 Is done. Similarly, the lower hafnium 5 b is attached to the tongue-like portion 3 a of the lower support member 3 (or the drop speed limiter) and fixed by the pin 6.

  Further, the upper hafnium 5a and the lower hafnium 5b are encapsulated by a sheath 4 made of stainless steel and having a substantially U-shaped cross-sectional shape. A single blade is formed by being fixed to a drop speed limiter. Similarly, the other three blades are formed so that the cross-sectional shape is substantially a cross shape, thereby forming a reactor control rod.

  As one of the features of the shape of the conventional nuclear reactor control rod as described above, focusing on the gap structure formed by the sheath 4 and the hafnium elliptic tube 5, the structural design that can reduce the possibility of corrosion, Measures to increase the gap interval as much as possible are being studied. Also, for example, as described in Patent Documents 4 to 6, a method of applying a corrosion-resistant film to the material surface has been studied. In particular, Patent Document 6 describes that a film is formed on the inner surface of the sheath facing the gap formed between the hafnium member and the sheath.

Japanese Patent Laid-Open No. 2-10299 JP 2002-71868 A JP-A-9-61576 JP 2010-144192 A JP 2010-216881 A JP 2010-164508 A

  However, in the above-described prior art, there is no description of a measure against corrosion at a gap portion in the welded portion between the tie rod and the sheath.

  Furthermore, due to the characteristics of the reactor control rod, tensile and compressive stresses are repeatedly applied to the tie rods by repeating the vertical movement, the temperature fluctuates from room temperature to 300 ° C or higher at the time of startup, and irradiation is performed. Therefore, there is a concern about cracking or peeling of the surface film. When cracks occur on the surface film, the base material is exposed at the location, so corrosion is likely to occur, and the surface film and the base material have superior electrochemical characteristics, so that no film is applied. Compared to the above, there is a possibility of further accelerating the dissolution of the base material.

  Further, when a corrosion-resistant metal such as zirconium is used for the sheath or tie rod, since the density is higher than that of stainless steel, there is a concern that the weight of the entire reactor control rod increases and the equipment specifications are not satisfied.

  The present invention has been made in view of the above-mentioned points. The object of the present invention is to suppress the occurrence of crevice corrosion, which is feared to occur at the gap portion between the tie rod and the sheath, and of course, the surface film is formed by residual stress. An object of the present invention is to provide a nuclear reactor control rod that can suppress the occurrence of corrosion even when cracked.

  In order to achieve the above object, a nuclear reactor control rod according to the present invention includes a tie rod having a substantially cross-sectional cross section, a handle attached to the upper end of the tie rod, and a lower support attached to the lower end of the tie rod. A member, an upper end portion is attached to the handle, and a lower end portion is attached to the lower support member, is attached to the tie rod, has a substantially U-shaped cross section, and extends from the cruciform shape of the tie rod in all directions. In a nuclear reactor control rod comprising four sheaths and a hafnium member that is a neutron absorber disposed inside each of the sheaths, the end of the sheath is connected to the end of the sheath and the tie rod by welding In the gap formed by the sheath and the tie rod in the vicinity of the welded portion of the tie rod and the tie rod, the passivation of the sheath or tie rod is promoted Wherein the diffusion layer of the coating and the metal include genus is formed.

  According to the present invention, the gap formed in the vicinity of the welded portion between the sheath and the tie rod includes a film containing a metal that promotes passivation and a diffusion layer of the metal. In addition to being able to suppress the occurrence, a diffusion layer is also given to the inside of the base material together with a film having a sufficient thickness, so even if the surface film cracks due to residual stress etc., a component that promotes passivation Is present in the diffusion layer, the occurrence of corrosion can be suppressed by passivation without exposing the base material itself.

It is a block diagram which shows the conventional nuclear reactor control rod. It is sectional drawing which follows the AA line of FIG. It is explanatory drawing which shows distribution of the neutron irradiation amount which the sheath of a reactor control rod receives. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a reactor control rod according to a first embodiment of the present invention, and is a cross-sectional view of a tie rod and one blade in a cross section perpendicular to the longitudinal direction of the nuclear reactor control rod, and an enlarged view of a sheath and a tie rod weld. It is a figure which shows the manufacture procedure of the reactor control rod in Example 1. FIG. It is an electron micrograph of the test piece cross section after the corrosion test with respect to the test piece made from stainless steel. It is a figure which shows the Cr density | concentration analysis in the test piece after the corrosion test with respect to the test piece made from stainless steel. 1 is a schematic diagram of a film formed in Example 1. FIG. It is explanatory drawing which shows the measurement result of the initial depth of the crack which generate | occur | produces after the corrosion test with respect to the test piece made from stainless steel. FIG. 3 shows a reactor control rod according to a second embodiment of the present invention, and is a cross-sectional view of a tie rod and one blade and an enlarged view of a sheath and a tie rod welded portion in a cross section perpendicular to the longitudinal direction of the nuclear reactor control rod. FIG. 5 is a diagram illustrating a procedure for manufacturing a reactor control rod in the second embodiment. It is a block diagram which shows Example 3 of the reactor control rod of this invention.

Crevice corrosion can be prevented by removing any one factor of radiation exposure, residual stress and crevice structure. However, since the nuclear reactor control rod is disposed in the nuclear reactor, it is difficult to remove the environment for radiation irradiation. In particular, as shown in FIG. 3, the nuclear reactor control rod has an upper portion of the sheath 4 in which a hafnium member, which is a neutron absorber, is arranged, that is, the total length (L _{0) of the} sheath 4 from the upper end to the lower end of the sheath 4. ) Receive high neutron irradiation in the region up to about 1/2 position.

  Further, in a nuclear reactor control rod using a conventional hafnium member (for example, a hafnium elliptic tube 5), tensile residual stress is generated on the surface of the sheath 4 as the sheath 4 and the handle 2 and the sheath 4 and the tie rod 1 are welded. To do. The heat treatment after welding as a method for reducing the residual stress of the structural member used in the prior art is difficult to apply to the nuclear reactor control rod due to restrictions on the structure and dimensional accuracy of the nuclear reactor control rod.

  Further, the corrosive environment in the gap is considered to contribute to the occurrence of corrosion by depositing impurities, corrosion products, and the like contained in the core coolant in the gap between the sheath 4 and the tie rod 1. Further, when a corrosion product or the like accumulates in the gap portion between the sheath 4 and the tie rod 1, the gap width is narrowed and an environment in which corrosion is more likely to occur occurs.

  From the above, as a measure for suppressing the crevice corrosion, the present inventors are effective in suppressing the deterioration of the corrosive environment over time in a portion where the gap structure cannot be avoided, and the sheath is left with a tensile residual. It was considered necessary that the base material was difficult to be exposed even when stress was applied.

  As a result of various studies, the present inventors have found that a sheath containing a coating that can suppress corrosion and a component that promotes passivation of stainless steel only in a gap portion that is a region where a tie rod and a sheath are welded. Alternatively, the present inventors have come up with the idea that a tie rod surface coating and a sheath matrix or a tie rod matrix may be provided with a coating containing a component that promotes passivation of stainless steel and a metal diffusion layer.

  The metal that promotes passivation of stainless steel is a component that promotes passivation of stainless steel, and preferably contains at least one of chromium, nickel, titanium, zinc, niobium, and manganese.

  Examples of the present invention reflecting the above examination results will be described below.

  FIG. 4 shows a first embodiment of the reactor control rod according to the present invention. Since the schematic configuration is substantially the same as the configuration described in FIGS. 1 and 2, detailed description thereof is omitted here, and only the portion related to the present invention is described.

  In the present embodiment shown in FIG. 4, the passive state of the sheath 4 is promoted on the sheath inner surface 4 a in the gap formed by the end portion of the sheath 4 and the sheath 4 near the welded portion 10 of the tie rod 1 and the tie rod 1. A film component 7a containing metal and a metal diffusion layer 9a are formed. That is, a coating component 7a containing a metal that promotes passivation of the sheath 4 is formed on the surface of the sheath 4 facing the tie rod 1 in the gap formed by the sheath 4 and the tie rod 1, and the coating component 7a A diffusion layer 9 a is formed on the surface layer portion of the sheath 4, which is formed between the sheath component 4 and the coating component 7 a containing the metal that promotes passivation of the sheath 4. Will be.

  The manufacturing procedure is as follows. First, the portion corresponding to the welded portion 10 of the sheath 4 (only the parallel portion (length (X) portion) of the sheath inner surface 4a facing the tie rod 1 from the end position of the welded portion 10 shown in FIG. 4). The sheath component 7a is applied, and the sheath 4 to which the coating component 7a is applied is bent from the center line (see FIG. 5 (a)) and formed into a U shape (see FIG. 5 (b)). ), The U-shaped opening is inserted into the tie rod 1 extending in the four directions from the cross shape, and the two are welded together (see FIG. 5 (C)). It is fixed by the welding part 10 (refer FIG. 4).

  Since the welded portion 10 is solidified after the sheath 4 and the tie rod 1 are mixed and dissolved, the coating component 7a and the diffusion layer 9a have disappeared. However, since a geometric gap structure remains in the vicinity of the welded portion 10, crevice corrosion can be suppressed by the presence of the coating component 7a and the diffusion layer 9a in the gap portion.

  As the film component 7a for the stainless steel sheath 4, for example, a film that promotes passivation can be formed by using Cr, which is a component contained in stainless steel.

  FIG. 6 is a photograph taken by an electron microscope after a corrosion test of an untreated stainless steel test piece and a test piece provided with a Cr film and a diffusion layer on the surface by vapor deposition. (1) in FIG. 6 is a micrograph of an untreated test piece, and (B1) is a micrograph in which a part of (A1) is enlarged. On the other hand, (2) in FIG. 6 is a micrograph of a Cr-added test piece, and (B2) is a micrograph in which a part of (A2) is enlarged. In the corrosion test, a low strain rate tensile test (SSRT test) was performed in high temperature water at 288 ° C. until the strain amount became 15%.

  In the untreated test piece (1), as shown in FIG. 6 (B1), the surface was cracked and traces of corrosion were observed. On the other hand, in the Cr application test piece (2), as shown in (B2) of FIG. 5, a streaky pattern was seen in the formed Cr film, but no crack was generated.

  From this test result, it was found that corrosion of stainless steel as a test piece can be suppressed by applying a film with a component (for example, Cr) that promotes passivation.

  In particular, in the Cr application test piece (2), as described above, streaky patterns were generated in the Cr film, but no cracks were generated in the Cr film. The inventors think that this is because a Cr diffusion layer having a continuous concentration gradient is applied from the Cr film on the surface to the base material (stainless steel) of the Cr application specimen (2). It was.

  Therefore, the inventors of the present invention chemically analyzed the cross sections near the respective surfaces of the untreated test piece and the Cr film-coated test piece, and obtained the Cr concentration distribution. The obtained Cr concentration distribution is shown in FIG. (1) of FIG. 7 shows the distribution of Cr concentration near the surface of the untreated test piece, and (2) of FIG. 7 shows the distribution of Cr concentration near the surface of the Cr film-coated test piece. ing.

  As shown in the figure, in the untreated specimen (1), the Cr concentration is substantially uniformly distributed with the Cr concentration of the base material in the depth direction from the surface. On the other hand, in the Cr film-applied test piece (2), the Cr concentration is highest on the surface of the Cr film, and in the depth direction from the surface of the Cr film, the base metal at a depth of 5 μm from the surface of the Cr film. A Cr diffusion layer having a concentration gradient that continuously decreases to the Cr concentration is observed (see (2) in FIG. 7).

  The streak-like pattern observed in (2) of FIG. 6 is considered that the thickness of the Cr film is reduced by cracking or peeling of the Cr film. However, the present inventors considered that the crack reaching the base material could be suppressed without exposing the base material because the Cr diffusion layer was present on the base material side of the Cr coating.

  A schematic diagram of the applied Cr film is shown in FIG. As shown in the figure, a plurality of coating components 7a are applied in a film shape to a stainless steel plate 8 as a base material, and a diffusion layer 9a having a continuous concentration gradient is applied from the surface of the Cr coating 7 to the stainless steel plate 8. I think.

  The result of measuring the depth of cracks observed when crevice corrosion occurs is shown in FIG. From the measurement results shown in FIG. 9, it was found that cracks start from about 3 μm in the initial stage of corrosion. Therefore, it is desirable that the thickness of the Cr film shown in (2) of FIG. 7 is greater than 3 μm, and since the Cr film is a film having a thickness of 5 μm, it can be said that cracking of the base material could be suppressed.

  In FIG. 6, a physical vapor deposition method, a so-called PVD (Physical Vapor Deposition) method is used. As a method for providing the Cr film 7 and the diffusion layer 9a, a solution containing Cr, which is a kind of metal that promotes passivation, dissolved in any one of sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid and chromic acid. It is possible to use. As a solution containing Cr, a sergeant desire containing chromic acid and sulfuric acid is used, and the concentration of chromic acid is 250 g / L and the concentration of sulfuric acid is 25 g / L. The temperature of the solution is heated to a temperature in the range of 40-60 ° C. In addition, the same effect can be expected by spraying a metal that promotes passivated heat-dissolved to the sheath, and a plurality of methods described above can be applied in combination.

  According to the present embodiment as described above, the gap formed in the vicinity of the welded portion between the sheath and the tie rod includes a film containing a metal that promotes passivation and a diffusion layer of the metal. In addition to being able to suppress the occurrence of crevice corrosion, a diffusion layer is also applied to the inside of the base material together with a sufficiently thick film, so that even if the surface film cracks due to residual stress etc., it becomes passivated Since the component that promotes is present in the diffusion layer, the occurrence of corrosion can be suppressed by passivation without exposing the base material itself.

  FIG. 10 shows a second embodiment of the reactor control rod according to the present invention. The same parts as those in the first embodiment shown in FIG.

  In the present embodiment shown in FIG. 10, the tie rod 1 is deactivated on the tie rod outer surface 1 a in the gap formed by the end portion of the sheath 4 and the sheath 4 and the tie rod 1 in the vicinity of the welded portion 10 of the tie rod 1. A film component 7b containing metal and a metal diffusion layer 9b are formed. That is, a coating component 7b containing a metal that promotes passivation of the tie rod 1 is formed on the surface of the tie rod 1 facing the sheath 4 in the gap formed by the sheath 4 and the tie rod 1, and the coating component 7b A diffusion layer 9b is formed on the surface layer portion of the tie rod 1, and the diffusion layer 9b is formed between the coating component 7b containing the metal that promotes passivation of the tie rod 1 and the tie rod 1. Will be.

  As shown in FIG. 11, the manufacturing procedure is as follows. First, a portion corresponding to the welded portion 10 of the sheath 4 (see FIG. 11A) bent and formed into a U shape (from the end position of the welded portion 10 shown in FIG. 10). The coating component 7b is applied only to the parallel portion (length (Y) portion) of the outer surface 1a of the tie rod facing the sheath 4. The sheath 4 is applied to the tie rod 1 (see FIG. 11B) on which the coating component 7b is applied. The U-shaped opening portion of the tie rod 1 is inserted into a portion extending from the cross shape of the tie rod 1 in all directions, and the two are welded (see FIG. 11C). It is fixed by the part 10 (refer FIG. 10).

  Also in the present embodiment, as in the first embodiment, since the welded portion 10 is solidified after the sheath 4 and the tie rod 1 are mixed and dissolved, the coating component 7b and the diffusion layer 9b disappear. However, since a geometric gap structure remains in the vicinity of the welded portion 10, crevice corrosion can be suppressed by the presence of the coating component 7b and the diffusion layer 9b on the tie rod 1 side of the gap portion.

  As the film component 7b for the tie rod 1 made of stainless steel, for example, a film that promotes passivation can be formed by using Cr, which is a component contained in stainless steel.

  By adopting such a configuration of the present embodiment, it is possible to obtain the same effects as those of the first embodiment described above.

  FIG. 12 shows a third embodiment of the reactor control rod according to the present invention. In the present embodiment shown in the figure, one end of the sheath 4 on the handle 2 side and a position of 1/2 or less of the total length in the axial direction of the sheath 4 from the one end toward the other end of the sheath 4 on the lower support member 3 side. The configuration of the first embodiment or the configuration of the second embodiment described above is applied to the area between the two.

That is, the sheath 4 is placed in the gap formed by the end of the sheath 4 in the range of a half of the total length L ₀ from the upper end of the hafnium elliptic tube 5 and the sheath 4 and the tie rod 1 in the vicinity of the welded portion 10 of the tie rod 1. Alternatively, a nuclear reactor control rod having a sheath 4 in which a coating component 7a (or 7b) containing a metal that promotes passivation of the tie rod 1 and a metal diffusion layer (not shown) is formed.

With such a configuration of the present embodiment, not only it is possible to obtain the same effect as in Example 1 and 2 described above, the first range of 2 of the total length L ₀ from the upper end of the hafnium elliptic tube 5 It is a part where the amount of neutron irradiation received during use in the nuclear reactor is high, and it is possible to apply the surface treatment only in this range, and there is an effect that the manufacturing cost can be suppressed by limiting the range. In addition to ferrite, a surface treatment of a noble metal can be used as long as the film is not damaged or dissolved after the film component 7a (or 7b) is applied.

  Although not specifically illustrated and described, Example 1 and Example 2 may be applied to form a film component on either the sheath 4 side or the tie rod 1 side. That is, in addition to the configuration of FIG. 4, a metal diffusion layer may be formed on the surface layer portion of the tie rod 1 on which the coating component 7 a containing a metal that promotes passivation of the sheath 4 is formed. In addition to the configuration of 10, a metal diffusion layer may be formed on the surface layer portion of the sheath 1 on which the coating component 7b containing a metal that promotes passivation of the tie rod 1 is formed.

  In each of the above-described embodiments, Cr has been described as an example of a coating containing a metal that promotes passivation of the sheath or tie rod and a diffusion layer of the metal, but in addition to Cr, chromium, nickel, titanium, zinc Of course, a film containing a metal containing at least one of niobium and manganese and a metal diffusion layer may be used.

  The present invention can be applied to an in-furnace structure and in-furnace equipment, including a stainless steel control rod used in a nuclear power plant.

  DESCRIPTION OF SYMBOLS 1 ... Tie rod, 1a ... Tie rod outer surface, 2 ... Handle, 2a ... Tongue part of handle, 3 ... Lower support member (or fall speed limiter), 3a ... Tongue part of lower support member, 4 ... Sheath, 4a ... Sheath Inner surface, 5 ... Hafnium elliptic tube, 5a ... Upper hafnium, 5b ... Lower hafnium, 6 ... Pin, 7 ... Cr coating, 7a, 7b ... Coating component, 8 ... Stainless steel plate, 9a, 9b ... Diffusion layer, 10 ... Welded part .

Claims (10)

A tie rod having a substantially cross-shaped cross section, a handle attached to an upper end portion of the tie rod, a lower support member attached to a lower end portion of the tie rod, an upper end portion on the handle, and a lower end portion on the lower support member Four sheaths attached to the tie rods and having a substantially U-shaped cross-section and extending from the cruciform of the tie rods in all directions, and neutron absorption arranged inside each of the sheaths In a reactor control rod comprising a hafnium member that is a material, the end of the sheath and the tie rod being welded,
In the gap formed by the sheath and the tie rod in the vicinity of the end of the sheath and the welded portion of the tie rod, a coating containing a metal that promotes passivation of the sheath or the tie rod and a metal diffusion layer are formed. Reactor control rod, characterized by The reactor control rod according to claim 1,
A film containing a metal that promotes passivation of the sheath is formed on the surface of the sheath facing the tie rod in the gap, and the metal layer is formed on a surface layer of the sheath where the film is formed. A nuclear reactor control rod characterized in that a diffusion layer is formed. Reactor control rod according to claim 2,
The nuclear reactor control rod, wherein the metal diffusion layer is formed between the sheath containing a metal that promotes passivation of the sheath and the sheath. In the nuclear reactor control rod according to claim 2 or 3,
A nuclear reactor control rod, wherein a metal diffusion layer is formed on a surface layer portion of the tie rod on which a film containing a metal that promotes passivation of the sheath is formed. The reactor control rod according to claim 1,
A film containing a metal that promotes passivation of the tie rod is formed on the surface of the tie rod facing the sheath of the gap, and the surface layer of the tie rod on which the film is formed has a metal layer. A nuclear reactor control rod characterized in that a diffusion layer is formed. Reactor control rod according to claim 5,
A nuclear reactor control rod, wherein the metal diffusion layer is formed between a film containing a metal that promotes passivation of the tie rod and the tie rod. Reactor control rod according to claim 5 or 6,
A nuclear reactor control rod, wherein a metal diffusion layer is formed on a surface layer portion of the sheath on which a film containing a metal that promotes passivation of the tie rod is formed. Reactor control rod according to claims 1 to 7,
In the region between one end of the sheath on the handle side and a position that is less than or equal to ½ of the total length in the axial direction of the sheath from the one end toward the other end of the sheath on the lower support member side Alternatively, a nuclear reactor control rod characterized in that a coating containing a metal that promotes passivation of a tie rod and a metal diffusion layer are formed. The nuclear reactor control rod according to any one of claims 1 to 8,
The metal that promotes passivation of the sheath or tie rod is a component that promotes passivation of stainless steel, and contains at least one of chromium, nickel, titanium, zinc, niobium, and manganese. Control rod. Reactor control rod according to claims 1 to 9,
The reactor control rod according to claim 1, wherein the coating containing a metal that promotes passivation of the sheath or tie rod has a thickness of 3 μm or more.

Images