JPS6360818B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a method for producing metallic zirconium that is lined on the inner surface of a cladding tube made of a zirconium alloy that coats nuclear fuel pellets. [Background of the Invention] At present, fuel cladding tubes that house nuclear fuel in nuclear reactors are used in nuclear reactors, so they must (1) have excellent corrosion resistance, (2) be non-reactive and have good thermal conductivity. (3) high toughness and ductility; and (4) small neutron absorption cross section. Zirconium alloys are widely used as fuel cladding tubes because they satisfy the above characteristics. However, although fuel cladding made of zirconium alloy is an excellent fuel cladding under steady conditions,
When load fluctuations in a nuclear reactor are large, stress corrosion cracking may occur due to corrosion caused by iodine gas released from nuclear fuel and stress caused by expansion of fuel pellets, and there is a risk of damage. As a method of preventing stress corrosion cracking of fuel cladding, various metal barriers are provided between nuclear fuel pellets and cladding. In the case of cladding tubes using zirconium alloys, composite cladding tubes lined with pure zirconium as a metal barrier are used (Japanese Patent Laid-Open No.
54-59600). The reason for this is that pure zirconium maintains its softness during neutron irradiation compared to zirconium alloy, reduces local strain generated in the zirconium alloy cladding, and has the effect of preventing stress corrosion cracking. However, according to the inventors' experiments, the pure zirconium layer (hereinafter referred to as zirconium liner)
It was found that extremely high purity is required to maintain softness during irradiation. Especially in high combustion conditions, zirconium liners require purity levels of crystal bar zirconium to be effective. When the purity is on the sponge zirconium level, the degree of radiation hardening is large and it cannot be expected to be sufficiently effective as a liner. The conventional manufacturing method for crystal bar zirconium is to produce a zirconium crystal bar by iodizing sponge Zr and chemical vapor deposition, as shown in FIG.
However, this method has an extremely slow reaction rate and cannot be mass-produced, so the Zr obtained is extremely expensive. Conventionally, as a method for refining zirconium, a method is known in which chloride of zirconium is reduced with an active metal such as Mg to produce an ingot that can be processed by vacuum arc melting. In the case of this method, deoxidation is not possible even in vacuum arc melting, and the oxygen present during reduction continues in the product as it is, making it impossible to produce low-oxygen zirconium. [Object of the Invention] An object of the present invention is to provide a method for mass-producing zirconium metal for composite fuel cladding tubes, which has an oxygen concentration on the order of crystal verzirconium and has a low degree of irradiation hardening. [Summary of the Invention] The present inventors have discovered that it is oxygen among the impurity elements in metallic zirconium that induces radiation hardening in the zirconium liner material, and that other impurity elements such as Al, C, Cr, Hf, We focused on the fact that Fe and other elements contained in ordinary sponge zirconium have almost no effect on radiation hardening.
Therefore, it is not necessary for the zirconium liner material to intentionally reduce the impurity elements other than oxygen to the crystal bar zirconium level, and a sponge zirconium level is sufficient. The present invention was achieved based on this knowledge and as a result of extensive research into a method for producing zirconium that can bring only the oxygen concentration to the same level as crystal verzirconium. That is, in the present invention, a zirconium raw material or a molten raw material thereof is placed in a hearth mold, and a heat source with an energy density of 50 W/mm ² or more is irradiated onto the surface of the raw material in a vacuum atmosphere to remove the raw material below the irradiation point. is melted from the surface to the bottom,
The process of relatively scanning the irradiation point over the entire surface of the raw material in the hearth mold is repeated one or more times to reduce the amount of oxygen in the raw material.
This is a method for producing metal zirconium for composite fuel cladding tubes, which is characterized in that the obtained ingot is remelted in a vacuum atmosphere or in an inert gas to obtain a processable ingot. In particular, 250 W/mm ² or more is preferable. [Embodiment of the Invention] Fig. 2 is a flow diagram showing an embodiment of the present invention, in which Zr sponge is used as a raw material, and the raw material is inserted into a hearth mold using a melting furnace using, for example, an electron beam as a heat source. Repeat the operation of melting the Zr sponge one part at a time while moving the hearth at least once. The rod-shaped ingot after melting is then remelted in a vacuum atmosphere or an inert gas atmosphere to produce an ingot with a low oxygen concentration that can be processed. Next, as in the conventional manufacturing method, a composite cladding billet is formed using Zr alloy (Zircaloy) and the low oxygen concentration Zr ingot manufactured by the present invention, and a predetermined composite fuel cladding is formed by hot extrusion and tube reduction processing. A tube is created. As a raw material, Zr sponge or its dissolved material is used, which has an oxygen concentration of 400 ppm or more and a total amount of impurities other than oxygen of 1000 to 5000 ppm. In vacuum melting using an electron beam, approximately 400 to 1500 ppm of oxygen dissolved in Zr sponge is dissolved.
It can be lowered to 300ppm or less. Generally, it is known that the dissociation pressure of zirconium oxide (ZrO ₂ ) is very low, and that it will not dissociate into zirconium and oxygen unless the vacuum level is about 10 ^-18 torr (1800°C). However, it has become clear that if the oxygen concentration in zirconium is in the range of 400 to 1500 ppm or less, it can be removed by evaporation in the form of lower oxides (ZrO). That is, in the case of zirconium, when comparing the vapor pressure of zirconium itself and the vapor pressure of a lower oxide of zirconium, the latter is larger (vapor pressure ratio ZrO/Zr=10 ² ). Therefore, if vacuum melting is efficiently performed using an electron beam, it is possible to reduce the oxygen content. For this purpose, it was found that the energy density of the electron beam is most important. The reason for this is that when the energy density is increased, the surface of the melt pool becomes extremely hot, and oxygen is evaporated and removed in the form of ZrO. FIG. 3 shows the relationship between the amount of oxygen in zirconium and the energy density during dissolution. The oxygen reduction effect can be obtained at an energy density of 50 W/mm ² or more. The higher the degree of vacuum, the better, but the vapor pressure of zirconium is 4×10 ^-5 torr (at a melting temperature of 2200K)
Therefore, if the degree of vacuum is too high, the evaporation loss of zirconium will increase, which is not preferable. The degree of vacuum is
10 ⁻⁴ to 10 ⁻⁶ torr is preferred. The higher the number of melts, the lower the oxygen content, which corresponds to the time the melt pool is exposed to vacuum.
Therefore, the number of times of melting can be reduced as the hearth movement speed is made slower. The Haas melting method using an electron beam can be performed by the method shown in FIG. In Figure 4, 1
is the filament, 2 is the cathode, 3 is the anode,
4 and 5 are focusing coils, and dotted lines indicate electron beams. The zirconium raw material 6 is gradually moved in the direction indicated by the arrow A in the figure, and the surface of the raw material is irradiated with an electron beam to melt it one part at a time. In the figure, 7
8 is a melting part and 8 is a solidification part. The melted portion 7 reaches the bottom of the hearth mold 10 of the hearth 9. That is, the raw material 6 below the irradiation point of the electron beam is melted from its surface to its bottom.
By scanning the hearth 9 in the direction of the arrow A as described above, the irradiation point is moved over the entire surface of the raw material 6. In the present invention, the same effect can be obtained by appropriately arranging the melting raw materials in a melting container (for example, a water-cooled copper hearth) whether the melting raw materials are in the form of a powder or a rod. Another important process of the present invention is to melt the hearth-shaped ingot into a rod and then process it into a composite cladding tube. In the present invention, the process of melting sponge zirconium or its melting material in a vacuum atmosphere, the melting method of low-oxygen ingots, and the method of melting low-oxygen ingots include vacuum arc melting, as long as it is a method that can re-melt the low-oxygen ingots. , plasma arc melting is also possible. As zirconium obtained by such a method, for example, Al100ppm by weight, hereinafter,
C500ppm or less, Cr300ppm or less, Hf200ppm or less, Fe1000ppm or less, Pb200ppm or less,
Nb 200ppm or less, Ni 100ppm or less, Si 200ppm or less, Ta 200ppm or less, Sn 100ppm or less,
W100ppm or less, N80ppm or less, O400ppm or less,
The total amount including other impurities is 1000 to 5000 ppm, and the remainder is Zr. This metallic zirconium is lined on the inner surface of the fuel cladding, and the zirconium alloy that coats nuclear fuel pellets has a Sn content of 1.20 to 1.70% by weight.
Fe0.07~0.20%, Cr0.05~0.15%, Ni0.03~0.08
%, Fe, Cr, Ni total 0.18~0.38%, balance Zr and impurities Zirconium alloy, or by weight Sn1.20~1.70%, Fe0.18~0.24%, Cr0.07~
0.13%, Fe, Cr, Ni total 0.28-0.37%, balance Zr
A zirconium alloy consisting of impurities and impurities is desirable. Table 1 shows the hearth melting conditions using an electron beam, which is one step of the present invention. Table 2 shows the analysis results of the amount of impurity elements in the zirconium raw material used in the test. The raw materials for Examples 1 to 3 are ASTM B-351-
Made of 79 grade R60001 sponge zirconium,
It is a rod shape with a diameter of 8 mm. In Example 4, the reactor
It is made of grade zirconium and has a spongy shape.

【table】

[Table] Table 3 compares the amounts of oxygen, nitrogen, and hydrogen between hearth melting using an electron beam and rod melting.

[Table] As is clear from Table 3, electron beam hearth melting is much more effective in reducing the amount of oxygen in sponge zirconium than electron beam rod melting. In electron beam hearth melting, a zirconium ingot with an oxygen concentration of 300 ppm or less can be obtained in one melting cycle. FIG. 5 shows the relationship between the number of times of dissolution and the amount of oxygen for Examples 2 and 4. 1 is the curve of Example 4, and 2 is the curve of Example 2. In both cases, the amount of oxygen decreases with the number of times of dissolution, but the degree of this decrease is several orders of magnitude higher in Example 4. When the number of melting times is 5 or more, the oxygen amount in Example 4 falls below 100 ppm. The higher the energy density, the lower the amount of oxygen. FIG. 6 shows the relationship between the number of times of melting and hardness for Example 4. As the amount of oxygen decreases, the hardness also decreases,
Bitkers hardness is 100 (Hv) after melting 3 times or more
The hardness is approximately the same as that of Crystal Bar Zr. FIG. 7 shows the relationship between the energy density of the electron beam and the number of times of melting for Examples 1 to 4. In the diagram, white circles indicate that the amount of oxygen in zirconium is less than 300 ppm.
Black circles have an oxygen content of 300 ppm or more. The shaded area in the figure is the optimal combination of power density and melting frequency according to the present invention. Next, a large number of hearth ingots as shown in Example 4 were manufactured and subsequently melted into a large ingot having a diameter of 56 mm and a length of about 300 mm in an electron beam melting furnace. The amount of oxygen in the ingot after melting is the same as that in hearth ingots.
It was around 200ppm. The following composite fuel cladding manufacturing method is the same as the conventional method. First, the outer billet is made of a zirconium alloy (Sn1.52%, Cr0.11%, Fe0.13%, Ni0.05%, balance Zr by weight) with an outer diameter of 79.30 mm, an inner diameter of 34.55 mm, and a length of 250 mm. I made a hollow tube. The inner billet was fabricated into a hollow tube with an outer diameter of 32.55 mm, an inner diameter of 21.25 mm, and a length of 253 mm by processing the above-mentioned zirconium ingot. Then, the inner billet was inserted into the outer billet to produce a double tube, and thereafter hot extrusion, cold rolling, and annealing were performed in the same manner as the usual cladding tube processing method. Table 4 shows the dimensions of the final finished pipe and the measurement results of the zirconium liner thickness.

〔Effect of the invention〕

As described above, according to the present invention, a zirconium material with a low oxygen concentration can be obtained in mass production at a low cost, making it easy to manufacture highly reliable and high-performance fuel cladding tubes with extremely little irradiation hardening.

[Brief explanation of drawings]

Figure 1 is a manufacturing process diagram of a composite fuel cladding tube using conventional crystal verzirconium material;
The figure is a manufacturing process diagram showing one embodiment of the present invention, Figure 3 is a diagram showing the relationship between energy density and oxygen content, Figure 4 is an explanatory diagram showing the Haas melting method using an electron beam, and Figure 5 is a diagram showing the relationship between energy density and oxygen content. FIG. 6 is a diagram showing the relationship between the number of hearth melts and the amount of oxygen in zirconium according to the present invention, FIG. 6 is a diagram showing the relationship between the number of hearth melts and the hardness of zirconium, and FIG. 7 is a diagram showing the relationship between the energy density of the electron beam and the number of melts. DESCRIPTION OF SYMBOLS 1... Filament, 2... Cathode, 3... Anode, 4, 5... Focusing coil, 6... Zirconium raw material.

Claims (1)

[Scope of Claims] 1. A zirconium raw material or its molten raw material is placed in a hearth mold, and a heat source with an energy density of 50 W/mm ² or more is irradiated onto the surface of the raw material in a vacuum atmosphere to reduce the temperature below the irradiation point. The raw material is melted from the surface to the bottom, and the process of relatively scanning the irradiation point over the entire surface of the raw material in the hearth mold is repeated one or more times to reduce the oxygen content in the raw material. A method for producing metallic zirconium for composite fuel cladding. 2 In claim 1, the zirconium raw material or its melting material has an oxygen concentration of 400 ppm.
The total amount of impurities other than oxygen is 1000~
A method for manufacturing zirconium metal for composite fuel cladding, which is 5000 ppm sponge zirconium. 3. The method for producing metal zirconium for a composite fuel cladding tube, as set forth in claim 1, wherein the step of melting the zirconium raw material or its melting material is a step of melting with an electron beam. 4. The method for producing metal zirconium for a composite fuel cladding tube, as set forth in claim 1, in which the oxygen content in the raw material is reduced to 400 ppm or less by the melting.