JP2657810B2 - Stainless steel tubular elements with improved wear resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION INDUSTRIAL APPLICATION The present invention relates to a high pressure,
A chromium-nickel stainless steel tubular element designed for use in high temperature water.

[Conventional technology]

In this type of reactor, the cooling water often contains a boron product, which comes into contact with the tubular elements, some of which are often slid along in guides, I'm going to rub it.

This occurs especially in the stainless steel sleeve of the reactivity control rod cluster. In order to control the reactivity of the reactor, the control rods are moved along guide tubes provided on the fuel system and along guides provided on the inner top of the reactor. In particular, the reactor is “load following”
The cycle and amplitude of the movement of some control rods when used in a load follower mode are determined by the core rel.
It is often necessary to systematically exchange some clusters during oading.

It has already been proposed to apply a coating to the outer surface of the tubular element subject to friction to reduce its wear. For example, electrolytic plating of hard chromium and chemical plating of nickel have been performed.

[Problems to be solved by the invention]

However, electrolytic chromium plating is brittle, while chemically deposited nickel can contaminate the main circuit of the reactor. Also, coatings containing chromium carbide to which a nickel-chromium bonding alloy has been added have poor behavior under radiation exposure.

Accordingly, it is an object of the present invention to improve the wear resistance of a stainless steel tubular element. It is another object of the present invention to increase the durability of the tubular element to thermal cycling while maintaining its properties under radiation exposure in a water cooled reactor.

[Means for solving the problem]

According to the present invention, there is provided a method of manufacturing a tubular element, typically a control rod sheath for a nuclear reactor, whose outer surface is made of chromium-nickel stainless steel nitrided to a depth of 15 to 40 microns.

Advantageously, the nitriding is performed in the ionized state.
Tubular elements are rarifed containing nitrogen and hydrogen
The plasma created by the discharge in the atmosphere undergoes and active nitrogen ions are implanted at a high enough temperature to cause deep diffusion of the ions.

In order to avoid sensitization of the material due to dechroming of the matrix of the tubular element, which is detrimental to corrosion resistance in oxidizing media, it is advisable to use processing times and temperatures low enough that the processing layer does not exceed 40 μm. desirable. In practice, about
It is possible to process for several hours at a temperature of 500 ° C.

According to the present invention, there is also provided a reactivity control rod for a pressurized water reactor in which the surface of a tubular element manufactured by the above-described manufacturing method is passivated. Passivation is carried out by maintaining the nitrided tubular element in an oxidizing plasma at a passivation thickness not exceeding the nitriding depth and at a temperature below 500 ° C. for a selected time longer than 1 hour. .

〔Example〕

Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings, but it is needless to say that the present invention is not limited to the following embodiments.

FIG. 1 shows one rod 10 of a control cluster of a pressurized water reactor. The rod 10 and other rods are fixed to the arm of a spider 12 for simultaneously moving a plurality of rods vertically.

The rods slide within a guide tube 14, which belongs to the fuel system and is connected to an end nozzle 16.

Each rod is provided with a sheath or sleeve 18 made of chromium-nickel stainless steel, often AISI 304 steel, the main components of which are: Ni:
8.50-11% by weight, Cr: 17-19% by weight, carbon: maximum 8% by weight, Mn: maximum 2% by weight. The rest is iron, except for inevitable impurities. Pellets 20 (or slag) of neutron absorber are deposited in the sleeve. Sleeve 18
Is hermetically sealed by an upper plug 22 and a lower plug 24 also made of stainless steel. As an example, sleeve 18 can have an outer diameter of 9-25 mm and a wall thickness of about 0.4-1.5 mm. The sleeve may be made thicker as the diameter increases.

Guide tube 14 also generally made of stainless steel
In order to reduce wear and tear of the sheath 18 during friction sliding on the surface, a surface nitriding treatment with a depth of 15 to 40 μm, typically 20 to 40 μm is performed.

The nitriding is performed in the apparatus shown schematically in FIG. The apparatus of FIG. 2 comprises a sealed enclosure 26, which includes a vacuum pump 28 for reducing the internal pressure, usually to a value of 30 to 200 Pascals, and a mixture of nitrogen and hydrogen. An inflow pipe for introduction is provided. The enclosure 26 comprises a plurality of sleeves to be treated.
A suspension for receiving the load is included. The operation sequence is performed as follows.

The sleeve to be treated is cleaned and its lower plug 24
Are fitted. After a thermal buffer, which may be made of stainless steel slag, is placed in each sleeve 18, a temporary upper plug 32 is secured to each sleeve. The plug is used to hold the sleeve during coating. The sleeve 18 is placed in an enclosure 26, the part of which is close to the upper plug 32 is advantageously masked in order to avoid nitriding of the stainless steel in this area.

Each part is ionically etched in the enclosure 26 under a pressure of about 30 Pa. The enclosure is then scavenged to generate a low pressure (total pressure not exceeding 200 Pa and a partial nitrogen pressure of, for example, 40-60 Pa) nitrogen and hydrogen atmosphere. A voltage between 300V and 1000V is applied by a generator 34 between the metal enclosure 26 and the sleeve 18 to be treated. Activated nitrogen ions formed by dissociation of the gaseous mixture by the discharge are bombarded on the surface of the sleeve 18 by ion bombardment.

Over the entire length of the sleeve but above ambient temperature
Means (not shown) are provided for maintaining the temperature below 510 ° C. (typically about 500 ° C.) to effect the diffusion of nitrogen ions, which can be conventional. This operation is continued until the implantation depth reaches the desired value. However, in practice, the processing time does not exceed 6 hours.

The enclosure 26 is again scavenged and filled with an oxygen-containing oxidizing atmosphere at approximately the same pressure as during nitriding. In this case, plasma containing active oxygen ions is generated, and the nitride layer is oxidized to a limited extent. This process is 400-4
It is carried out at a temperature of 80 ° C. until a suitable passivation is achieved. Generally, the duration is greater than one hour and less than the time to passivate the metal to a depth greater than that achieved by nitrogen ions.

The sleeve 18 is then removed from the enclosure 26. The temporary upper plug 32 is removed, the sleeve 18 is loaded with an absorbent material, and the final upper plug is placed.

Alternatively, the sleeve is closed with a neutron absorber before the ion nitriding process.

The invention is applicable to tubular elements used in nuclear reactors other than control rods.

The invention is particularly applicable to rod clusters that have other functions and whose surface must withstand friction or vibration wear.

It is also possible to perform the same treatment on the inner surface of the tubing that receives the friction of the member sliding inside.

[Functions and effects of the invention]

As described above, according to the method for producing a water-cooled and decelerated sheath for a nuclear reactor from the chromium-nickel stainless steel tubular element of the present invention, the nitriding treatment is durable while being as shallow as 15 to 40 μm. Provided is a method for manufacturing a water-cooled and decelerated reactor sheath having improved water resistance, and this method provides an excellent sheath which does not contaminate the main circuit of the reactor and which does not deteriorate under radiation exposure. Is what you do. Further, in this method, since the nitriding treatment is relatively shallow, the chromizing property originally expected as chromium-nickel stainless steel is not destroyed.

Further, according to the reactivity control rod for a pressurized water reactor of the present invention, chromium-nickel stainless steel exhibits excellent wear resistance even in relatively shallow treatment due to the synergistic action of nitriding and passivation. can do.

[Brief description of the drawings]

FIG. 1 is a schematic partial cross-sectional view showing one rod of a control cluster having a sheath to be processed according to the present invention, and FIG. 2 is a schematic view of a sheath processing apparatus according to an embodiment of the present invention. 18 ... sleeve (or sheath), 20 ... neutron absorber, 22 ... upper plug, 24 ... lower plug, 26 ... enclosure.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Jean-Michel, Coutourie France, 69100 Villeurbanne, Alley-Marcel-Achard 6 (56) References JP-A-62-35289 (JP, A) JP-A-60-1985 262955 (JP, A) JP-A-60-117187 (JP, A) JP-A-62-5541 (JP, A) JP-A-60-35989 (JP, A) JP-A-61-295377 (JP, A) US Patent 4704168 (US, A)

Claims (8)

(57) [Claims] 1. A reactivity control rod for a pressurized water reactor having an outer sleeve made of chromium-nickel stainless steel and having a neutron absorbing material in the sleeve, wherein the sleeve has a thickness of 15 to 40 μm. A surface nitriding treatment to a depth of: and a passivation control rod for a pressurized water reactor, wherein the sleeve is passivated at a maximum depth of the nitriding treatment. 2. A sleeve made of chromium-nickel stainless steel is prepared, the sleeve is closed with a plug, and the sleeve is maintained in a nitrogen-hydrogen atmosphere under a reduced pressure of 30 to 200 Pascal, and is discharged into the atmosphere. To generate nitrogen ions, so that the outer surface of the sleeve in the radial direction is 15 to 40 μm.
A method for producing a water-cooled and decelerated reactor sheath, comprising nitriding by a depth of m. 3. The plug closing the upper part of the sleeve is a temporary plug, and a plurality of sleeves are simultaneously arranged in the enclosure, and these sleeves are provided by the temporary upper plug. 3. The method of claim 2, wherein the suspension is performed and the electrical discharge is performed by connecting a generator between the enclosure and the sleeves. 4. The method of claim 2, wherein said nitriding is performed at a temperature not exceeding 510 ° C. for a connection time of up to 6 hours. 5. The method of claim 2 including the step of performing a pre-treatment of ion etching in said enclosure prior to nitriding in said enclosure. 6. The method of claim 2 wherein the sleeve is filled with a thermal buffer prior to nitriding and closed by a plug. 7. The method according to claim 2, wherein the sleeve is filled with a neutron absorbing material before the nitriding treatment and the sleeve is closed. 8. The method of claim 2 further comprising the step of superficially passivating over a depth at most equal to the nitriding depth by generating a plasma in an oxidizing atmosphere immediately after the nitriding treatment.