JPH0465142B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a method for producing a corrosion-resistant zirconium alloy suitable for reactor core structural materials such as nuclear fuel cladding tubes.

(Prior art) For example, zirconium alloys such as Zircaloy-2 and Zircaloy-4 have a small thermal neutron absorption cross section and good corrosion resistance against the environment inside a nuclear reactor.
It is used as a structural material inside nuclear reactors because it has sufficient mechanical properties required as a structural material. However, in recent years, the following problems have been recognized in response to the desire to extend the service life of core structures. That is, white corrosion products called so-called nodular corrosions become scattered on the surface of the zirconium alloy structure. These corrosion products are generated and accumulated over time, eventually leading to flaking and deterioration of mechanical properties, and the accumulation of flaking leads to a decrease in heat transfer efficiency, which can lead to a reduction in heat transfer efficiency, for example. This has disadvantages such as causing local heating of the assembly and becoming a source of increased radioactivity during periodic inspections.

As a solution to the above-mentioned disadvantages, it is possible to differentiate the composition of zirconium alloys from potassium or yttrium-calcium systems (such as U.S. Pat. No. 3,261,682), or to use chemically inert metals such as gold, silver, platinum, nickel, chromium, or niobium. Coating a layer on the surface of a zirconium alloy structure (Japanese Patent Application Laid-Open No. 52-5692
(No. 1) have been proposed, but they cannot be said to be technically or economically satisfactory.

(Problems to be Solved by the Invention) The present invention seeks to provide a method for producing a zirconium alloy that has excellent corrosion resistance against nodular corrosion even when exposed to high temperatures, long periods of time, and in a highly radioactive environment. It is something.

[Structure of the Invention] (Means and Effects for Solving the Problems) The present invention will be explained in detail below.
0.07-0.13% by weight, however, the total amount of iron and chromium
For a zirconium alloy containing 0.28 to 0.37% by weight, with the remainder consisting essentially of zirconium, after heating at 870 to 1000°C and then quenching,
This is a method for producing a corrosion-resistant zirconium alloy, which is characterized in that it undergoes fine rolling and has a martensitic structure with fine precipitates at the grain boundaries, but the grain shape of the crystal grains is spherical.

The method of the present invention is based on the following knowledge. That is, according to the inventor's experiments, nodular corrosion of zirconium alloys occurs in zirconium alloys having an α-phase (h, c, p) crystal structure. By the way, zirconium adopts a b, c, c crystal structure in the β region, and by rapid cooling from the β region, it adopts a martensitic structure even though it is in the α phase (h, c, p). Moreover, the crystal structure of the zirconium alloy, which is rapidly cooled from the β region and has a martensitic structure with fine precipitates at the grain boundaries of nearly spherical grains, is further rapidly cooled and then subjected to a finishing rolling process. This was developed based on the fact that it has excellent corrosion resistance against nodular corrosion even in accelerated corrosion tests that simulate the high radiation environment inside a nuclear reactor because the degree of crystal aggregation on the surface is uniform without any change. .

The composition of the corrosion-resistant zirconium alloy of the present invention is 1.2 to 1.7% by weight of tin, 0.18 to 0.24% by weight of iron, and 0.07 to 0.13% by weight of chromium, however, the total amount of iron and chromium is 0.28 to 0.37% by weight, A zirconium alloy (Zircaloy-4) in which the balance essentially consists of zirconium can be used.

Next, the reason for limiting the composition ratio as described above will be described.

Tin is an additive that can be solid-dissolved in zirconium and increases mechanical strength, but if it is less than 1.2% by weight, no effect can be obtained from its addition, and if it exceeds 1.7% by weight, the processability is reduced. This range was set because it would be worse. In addition, iron and chromium mainly improve corrosion resistance, but if it is less than the above range, a sufficient effect cannot be obtained, and if it exceeds the above range, the precipitates become coarser and local corrosion is more likely to occur. did.

However, the above-mentioned zirconium alloy can be easily obtained by subjecting it to heat treatment at 870 to 1000°C and then subjecting it to rapid cooling treatment such as water cooling. Further, this heating-quenching process is actually inserted between, for example, the final cold rolling process and the finishing rolling process in the product manufacturing process.

In addition, in the present invention, since the (h, c, p) structure is formed by rapid cooling from the β region, if a precision rolling process is performed to further refine the shape by a few percent, the surface part will have good corrosion resistance. The degree of aggregation of (0001) planes increases and the generation of nodular corrosions is suppressed.

(Example) Next, the present invention will be explained using specific examples.

First, 1.5% by weight of tin and iron as a zirconium alloy.
An ingot of zirconium alloy (Zircaloy-4) consisting of 0.2% by weight of chromium, 0.1% by weight of chromium, and the balance zirconium is prepared, and after going through processes such as rolling, β-quenching, and α-forging sintering, it is machined into a hollow bit. Then, hot extrusion, deoxidation treatment, and cold rolling were further performed to produce a fuel cladding tube body. After that, length 50
~100cm The sample was passed through an electric furnace whose internal temperature was controlled to, for example, 900°C, and when it came out of the oven, it was cooled with water using a shower and then passed through a water tank for rapid cooling. In the above heat treatment, it is sufficient if the residence time of the cladding body in the furnace is selected to be about 5 to 20 minutes.

The cladding tube body subjected to the above quenching treatment is sequentially subjected to fine rolling and roll straightening, and then surface polishing, deoxidation,
A zirconium alloy fuel cladding tube according to the present invention was obtained by cutting to a specified length. When we cut out a part of the fuel cladding tube obtained in this way and examined the alloy structure, we found that it still had a martensitic structure (structure) with a fine array of alloying elements precipitated at the grain boundaries of nearly spherical grains. Furthermore, the degree of aggregation of (0001) planes at the surface area was increased (f value = 0.75), and it had good mechanical properties.

Note that it is desirable to be careful not to reach a high temperature of 500°C or higher after the above-mentioned heat treatment-quenching process. Therefore, even when performing normal strain relief annealing, the
It is better to choose a temperature lower than ℃. Although 500°C is a sufficient temperature to remove thermal strain, it is not high enough to change the crystal structure to α phase.

For comparison, conventional manufacturing method, i.e. 870~1000
When we examined the structure of the fuel cladding tube obtained without heat treatment at ℃ and subsequent rapid cooling treatment, we found that the grain shape was α-phase, which was close to spherical, and the precipitates were scattered within the crystal grains. Moreover, the size of the precipitates was 5 to 10 times larger than that in the case of the present invention (precipitates precipitated at grain boundaries).

The fuel cladding tube piece obtained above was heated to a temperature of 500°C.
Corrosion tests were conducted by placing each specimen in a flow type autoclave set at a pressure of 107 kg/cm ² . The cladding tube of the comparative example developed nodular corrosion after a test time of about 5 hours, and the weight increase due to corrosion was as shown by curve A in the attached diagram.
However, in the case of the cladding tube according to the present invention, no nodular corrosion was observed, and the weight increase due to corrosion was as shown by curves B and C in the attached diagram, indicating that the cladding tube had excellent corrosivity. was. Curve B is the case where processing strain relief annealing (577℃ x 2.5 hours) is performed prior to the corrosion test, and curve C is the case where thermal strain relief annealing (400℃) is performed prior to the corrosion test.
This is the case if you do the following.

As is clear from this example, the book has an equiaxed structure (consisting of nearly spherical crystal grains) with fine precipitates at the grain boundaries, a martensitic structure, and the crystal direction is aligned on the surface. The zirconium alloy according to the invention, in other words, the zirconium alloy that has been rapidly cooled from the β phase and then subjected to fine rolling, can be applied to core materials of nuclear reactors in high temperature, high pressure steam, and high radioactivity environments. Since there is no decrease in mechanical strength due to the occurrence of yular corrosion or hydrogen embrittlement due to corrosion, it can perform the required functions for a long time.

In addition, in the manufacturing method according to the present invention, the heat treatment temperature for martensite structure by β-quenching is set to 870°C.
The reason why the temperature is set at ~1000°C is because the zirconium alloy does not turn into the β phase at temperatures below 870°C, and when it exceeds 1000°C, deterioration of mechanical properties may occur. However, for the rapid cooling treatment, it is better to use water cooling or the like, especially if the cooling rate is fast.

[Effects of the Invention] As explained above, according to the present invention, corrosion resistance,
In particular, a corrosion-resistant zirconium alloy having excellent corrosion-resistant nodular corrosion properties can be obtained.

[Brief explanation of drawings]

The attached diagram is an example of a curve showing a comparison of the corrosion resistance of the corrosion-resistant zirconium alloy according to the present invention and a zirconium alloy other than the present invention.

Claims (1)

[Claims] 1. Zirconium alloy containing 1.2 to 1.7% by weight of tin, 0.18 to 0.24% by weight of iron, and 0.07 to 0.13% by weight of chromium, with the total amount of iron and chromium being 0.28 to 0.37% by weight, and the balance being essentially zirconium. The material is heated to 870 to 1000℃, then rapidly cooled, and then subjected to fine rolling, resulting in a martensitic structure with fine precipitates at the grain boundaries, but with a spherical grain shape. A method for producing a corrosion-resistant zirconium alloy.