JPS59179791A - Surface treatment of coated pipe for nuclear fuel rod - Google Patents

Abstract

PURPOSE:To improve durability by subjecting the inside circumferential surface of a coated pipe for a nuclear fuel rod to a final annealing treatment at a specific temp. after pickling and washing and subjecting the inside circumferential surface thereof to polishing by a sandblasting method and the outside circumferential surface thereof to mechanical polishing. CONSTITUTION:A coated pipe 1 for a nuclear fuel rod obtd. after final rolling by the conventional method is first subjected to a pickling treatment on the inside circumferential surface 6 thereof by a soln. mixture composed of a nitric acid and a hydrofluoric acid by which the ruggedness and an oxide layer or scale formed on the surface 6 is removed. An aq. aluminum nitrate soln. is run in the pipe 1 to remove practically all the hydrofluoric acid stuck thereon. The pipe is then subjected to a final annealing treatment at about 580 deg.C and the surface 6 is subjected to polishing by a sandblasting method and an outside circumferential surface 7 to desired mechanical polishing, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for surface treatment of cladding tubes for nuclear fuel rods used in boiling water reactors (BWRs), and particularly to prevent cladding tubes from being damaged by hydrides generated inside the cladding tubes. The present invention relates to a surface treatment method that can effectively and reliably prevent this.

Generally, a nuclear fuel rod obtained from the above-mentioned cladding tube IK made of this type of Zircaloy has an intermediate tube 1, as illustrated in FIG.
The internal KUO, nuclear chamber; accommodates the required number of separate pellets 2, and the pellets 2 are located in the upper part of the cladding tube 1, in a space called the blennium 3, which is formed to store the gas generated due to nuclear fission. Plenum spring 4&'C of the ammunition is pressed in the longitudinal direction of the tube, and both ends of W1 [
Now, the end plug 6.5 is plugged, and the inside of the tube is sealed.

By the way, since these 11 cladding tubes play an important role of retaining nuclear fission products, it is necessary that the inner and outer circumferential surfaces 6 of the intermediate tube 1 are uniformly formed and in a clean state B. It is said that

For this reason, the cladding tube 1 is subjected to surface treatment, but conventionally, after the intermediate tube 1 is subjected to a final annealing treatment,
The durability of the pipe 1 is improved by pickling its inner and outer peripheries (Mj 6.7) with a mixture of nitric acid and fluorine, cleaning it, and then oxidizing it to form a 6.7Vc oxide film on both sides. I was trying to make him do it.

However, it has been reported that when nuclear fuel rods obtained from the cladding tube 1 with such conventional surface treatment are used in a large-sized power reactor, some of the rods are damaged soon after the start of use, and the damage situation has been observed. As a result, it became clear that a lump of hydride was generated inside the cladding tube 1, and a crack from this hydride penetrated through the cladding tube 1, leading to the breakage.

Therefore, as a result of examining the formation of hydrides that are the cause of such damage, the following causes were considered.

First, the inner and outer circumferential surfaces 6 and 7 of the cladding tube 1 are cleaned after being pickled, but even if the neutralization cleaning is carefully carried out, water-insoluble zirconium fluoride (ZrF, )
is attached to the inner circumferential surface 6.7, and especially zirconium (ZrF) is attached to the inner peripheral surface 6.7.
4) is attached to the inner circumferential surface 5, moisture in the air sealed in the cladding tube 1 reacts with the zirconium (ZrF4) in the high temperature atmosphere during reactor operation. , ZrF4 +2H20=Zr02 +j, HF,
This increases the possibility that the inside of the cladding tube 1 will be damaged by the generated hydrofluoric acid.

In order to solve this problem, sufficient care should be taken to prevent the moisture in the air sealed inside the cladding tube 1 from being adsorbed to the nuclear fuel pellet 2.
Since it is a porous member made of unglazed ceramic, it is impossible to completely absorb moisture.

Next, the second factor is that if even a small amount of hydrofluoric acid remains, the above-mentioned hydride generation phenomenon will definitely occur.

In order to solve this problem, the inner and outer peripheral surfaces 6.
7. Care must be taken to ensure that no hydrofluoric acid remains after the pickling treatment.

The last factor that can be considered is that the oxide film applied to the inner and outer circumferential surfaces 6.7 of the cladding tube 1 is extremely thin, having a thickness of several microns, and the nuclear fuel pellet 2.
Due to the sliding, the pellet 2 and the inner circumferential surface 6 of the cladding tube 1 rub against each other, causing the membrane to peel off, and the peeled part reacts with the moisture in the air and becomes hydrogenated, causing the cladding tube 1 to become brittle. This means that the

In order to solve this problem, the above-mentioned oxide film thickness may be reduced to a certain value, but in practice it is difficult to eliminate the peeling caused by the nuclear fuel pellet 2.

This invention was devised in view of the current situation, and its purpose is to apply residual hydrofluoric acid used in the treatment to the inner and outer peripheral surfaces of the cladding tube used for nuclear fuel rods. A cladding tube for nuclear fuel rods that does not adhere to hydrofluoric acid and can completely prevent the subsequent generation of hydrofluoric acid in conventional examples, and as a result, improves the durability of the tube without applying an oxide film to the tube. The present invention aims to provide a method for surface treatment.

In order to achieve this objective, in this invention, the cladding tube for nuclear fuel rods obtained through final rolling by a conventional method,
After sequentially pickling and cleaning the inner circumferential surface, the inner tube was
A final annealing treatment is carried out at a temperature of about 0.degree. C., and the inner circumferential surface of the interpipe is polished by sandblasting, and the outer circumferential surface of the interpipe is subjected to desired mechanical polishing.

Hereinafter, this invention will be explained in detail with reference to the process diagrams of FIGS. 1 and 2 used to explain the conventional example. Using Zircaloy with added elements, arc melting is repeated several times in a vacuum atmosphere using an electrode obtained by solidifying this to obtain an ingot, which is then forged and then hot extruded after heat treatment. to form a blank tube.

The hollow hollow tube thus obtained is then processed into a small-diameter, thin-walled hollow tube by repeated cold rolling and vacuum annealing, resulting in the final product as shown in Figure 2. In the present invention, the material obtained by the step of rolling treatment (5) is first subjected to pickling treatment (B).

This pickling treatment (B) is performed on the inner circumferential surface 6 VC of the cladding tube 1 with a mixed solution of nitric acid and hydrofluoric acid, as in the conventional method, and removes the unevenness and oxide layer formed on the circumferential surface 6. Scale is thereby removed.

After this, the interpipe 1 is subjected to a cleaning process (C);
C) l"l: This is carried out by flowing an aqueous aluminum nitrate solution into the pipe 1, and most of the hydrofluoric acid attached during the pickling treatment (B) is thereby removed.

Next, the cladding tube 1 is subjected to a final annealing treatment (2), in which O is 570°C to 590°C in a vacuum heat treatment furnace with a high vacuum atmosphere better than 10' Torr.
It is conveniently carried out at temperatures between 3130°C and 585°C.

At this stage, the cladding tube 1 is finally annealed at a temperature of about 580°C.
Zirconium fluoride (ZrF), which was not removed by
This is because 4) sublimates at a temperature of about 580°C.

Thereafter, the cladding tube 1 is shaped to be straight, and then the inner circumferential surface 6 of the intermediate tube 1 is polished by sandblasting.

The same treatment (g) is performed by blowing out fine powder of silicon carbide (5 inches) from a thin nozzle, and thereby the annealing treatment CD
) K, the thin scale generated on the inner circumferential surface 6 of the stepped pipe 1 is completely removed by polishing.

Cladding tube IV whose inner peripheral surface 6 has been finished in this way
For c, mechanical polishing (01) is applied to the outer circumferential surface.

The polishing may be performed by grinding with a centerless grinder, straddling belt grinding, or sand blasting method as described in M, and the outer peripheral surface 7 is cleaned by mechanical polishing by a desired means.

Thereafter, the cladding tube 1 is sent to an inspection step C), and in the same step (G), the polished state of its inner and outer circumferential surfaces 6.6 is inspected.

The cladding tube 1 that has passed the inspection is then sent to the nuclear fuel rod manufacturing process (2). In the same process (2), the required number of nuclear fuel pellets are placed inside the cladding tube 1 as described above with reference to FIG. The plenum 31C formed on the upper part of the pellets 2 is loaded with a plenum spring 3, and end plugs 4 are installed at both ends of the cladding tube 1. The inside of the tube 1 is sealed.0 Here, to show a specific example of the present invention, a large diameter, thick-walled Zircaloy-2 tube is rolled in several steps, and after the final annealing process, the outer diameter is 12.6 mm and the wall thickness is 5. It was made into a small-diameter, thin-walled tube made of crab 091.

After this, when the open tube was pickled, the thickness of the Crab 002 body was reduced, and after that, cleaning, final annealing, and internal surface polishing (
C) After applying (D) (g), external polishing treatment (g)
The wall thickness of the pipe was 0.04 with the same treatment (G).
Finally, the outer diameter was 12.5 mm and the flesh was 1 mm.
A cladding tube of 8E length was obtained.

Inspection of the inner and outer circumferential surfaces of the cladding tube thus obtained revealed that no hydrofluoric acid or insoluble zirconium fluoride was present on the inner circumferential surface, and that the inner circumferential surface was smooth. This was confirmed.

As explained above, according to the present invention, the pickling treatment is performed before the final annealing treatment, and the pickling treatment is performed at around 580°C, which is the sublimation temperature of zirconium fluoride, and is Since the inner and outer circumferential surfaces of the tube are polished by sandblasting, etc., moisture in the air adsorbed by VC, residual hydrofluoric acid, and nuclear fuel pellet VC during reactor operation is removed by Zr.
The hydrofluoric acid produced by reacting with F4 no longer damages the cladding tube, and the inner and outer peripheral surfaces of the cladding tube are
Since it is polished smooth by sandblasting or other means, it is possible to prevent the nuclear fuel pellet from rubbing against the inner circumferential surface of the intermediate tube and causing damage to the inner circumferential surface, and as a result, the coating can be coated without forming an oxide film layer. The durability of the pipe can be improved, and the reliability of the coating and pipe as a whole can be improved by an inexpensive means that does not require unpleasant oxide coating treatment.

[Brief explanation of drawings]

Figure 1 is a partially cutaway 101 diagram of a nuclear fuel rod, and Figure 2 is:
It is a process block diagram showing a surface treatment method for a cladding tube for nuclear fuel according to an embodiment of the present invention. Surface A...Final rolling treatment B...Pickling treatment C...Cleaning treatment D...Final annealing treatment E...Inner peripheral surface polishing treatment F... ...Makoto Ito, Patent Attorney and Patent Attorney for the Patent Applicant for External Surface Polishing Process Figure J

Claims (1)

[Claims] Nuclear fuel rod cladding tube (
After sequentially pickling and cleaning the inner circumferential surface of the inner tube, the inner tube is subjected to a final annealing treatment at a temperature of approximately 580°C, and the inner circumferential surface of the inner tube is polished by sandblasting. A method for surface treatment of cladding tubes for nuclear fuel rods in which desired mechanical polishing is applied to each.