JPH066761B2 - Hearth for producing zirconium and method for producing the same - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a hearth for producing zirconium material and a method for producing the same, and more particularly to a half used for refining and melting zirconium under vacuum using an electron beam. And a manufacturing method thereof.

[Conventional technology]

Currently, the fuel cladding tube that contains the nuclear fuel of the nuclear reactor is used in the nuclear reactor, so (1) it has excellent corrosion resistance, (2)
It must 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 requirements. Fuel cladding made of zirconium alloy is an excellent fuel cladding under steady-state conditions, but when the load fluctuation of the reactor is large, it is caused by corrosion by iodine gas released from nuclear fuel and expansion of fuel pellets. Due to the action of stress, stress corrosion cracking occurs and there is a risk of damage.

As a method of preventing stress corrosion cracking of a fuel cladding tube, it is known to provide various metal barrier cushions between the nuclear fuel pellet and the cladding tube. In the case of a cladding tube using a zirconium alloy, there is a composite cladding tube lined with pure zirconium as a metal barrier (JP-A-54-59600).
issue). The reason for lining pure zirconium is that pure zirconium retains its softness during neutron irradiation as compared with zirconium alloys, so it buffers the interaction between the fuel cladding tube and the nuclear fuel pellets, and stress caused by the expansion of the fuel pellets. It functions to prevent the cracks from advancing into the zirconium alloy of the fuel cladding tube. As a result, stress corrosion cracking is prevented.

According to experiments conducted by the inventors, it has been found that the pure zirconium lining layer (hereinafter referred to as “zirconium liner”) needs to have an extremely high purity in order to maintain softness during irradiation. did. In particular, zirconium liners under high combustion conditions require a crystal bar zirconium grade purity. In the case of sponge zirconium grade, the degree of radiation curing is large and the effect as a liner cannot be expected sufficiently.

On the other hand, in the zirconium liner, oxygen and iron are considered to be the main elements as impurities that affect the main properties such as irradiation effect and chemical reactivity. In particular, oxygen affects the hardness of zirconium, and the lower the concentration of oxygen, the softer the zirconium is retained. In addition, since iron has a low solubility in zirconium, it tends to exist as a precipitate. The area around the precipitate is chemically active and easily corroded. Further, particularly when precipitates on the grain boundaries are applied to the zirconium liner, they act as stress concentration areas and become the origin of cracking. Therefore, lowering the oxygen concentration of the zirconium liner and softening the liner means that the thermal expansion of the nuclear fuel pellet causes the nuclear fuel pellet to come into contact with the fuel cladding tube and spread the inner surface of the fuel cladding tube when the fuel cladding tube is expanded. It has the function of relieving the stress received. As a result, the tolerance for damage to the fuel cladding tube increases. Further, reducing the iron concentration to reduce the number of precipitates reduces the number of sites that are the starting points of cracks, and has the effect of making it less susceptible to chemical attacks such as iodine, which is a corrosive fission product.

The conventional method for producing crystalline perzirconium is to iodize sponge zirconium as described in Chapter 5 of the document “Metalliergy of Zcrconuim” (Lustman and Keize),
This is due to the chemical vapor deposition of zirconium liquid crystal rods. However, in this method, the reaction rate is extremely slow and mass production is not performed, so that the zirconium obtained is extremely expensive.

On the other hand, vacuum melting using an electron beam is used as a refining and melting method for refractory metals and alloys. As a feature of this, it is possible to hold the substance to be dissolved in the groove of the hearth at a high temperature in a high vacuum, whereby the impurity element having a high vapor pressure contained in the substance to be dissolved is evaporated and removed. As a mechanism for removing gas elements such as oxygen, hydrogen and nitrogen, hydrogen and nitrogen are desorbed from the surface of the molten metal into a vacuum in the form of atoms or H ₂ and N ₂ molecules, while oxygen alone is in a gas phase. The probability of migration is low and its removal is based on the phenomenon of volatilization and deoxidation, which evaporates in the form of volatile oxides. In the case of zirconium, oxygen can be removed by evaporation in the form of a lower oxide (ZrO). The efficiency of removing the impurity element can be understood by comparing the vapor pressure of zirconium itself with the vapor pressure of the impurity element. For example, regarding the removal of iron and oxygen, comparing the vapor pressures with zirconium
/ Zr = 10 ⁵ and ZrO / Zr = 10 ² , which shows that high-purity zirconium can be produced by efficiently performing electron beam melting.

In order to carry out highly efficient purification dissolution of zirconium using an electron beam, it is necessary to study the dissolution method.
In the rod melting method of manufacturing a rod-shaped ingot by irradiating the melted material with an electron beam and melting it down, the time for which the electron beam is irradiated per unit volume is short, and the molten part is kept at a high temperature. Is difficult and is not suitable as a method for purification and dissolution. Further, in the hearth melting method in which the material to be melted is inserted into the hearth mold and the hearth or electron gun is moved, the scanning speed of the beam is adjusted to increase the time for which the electron beam is irradiated per unit volume. It is also possible to maintain the melting part at a high temperature, which is effective as a highly efficient method for purifying zirconium.

In order to keep the melted portion at a sufficiently high temperature, it is necessary that the hearth mold does not react with the melted portion to cause a eutectic.
Since the conventional hearth mold is made of copper, the hearth mold must be water-cooled before use. That is, as shown in the horizontal sectional view (A), the plan view (B), and the vertical sectional view (C) of FIG. 2, the hearth groove portion 2 of the hearth body (basic hearth) 1 is shown.
A water cooling pipe 3 is provided below. Therefore, zirconium in the melt was not uniformly maintained at a high temperature, and variations in the refining effect were observed. Therefore, by using a high melting point material for the hearth mold, water cooling of the hearth mold can be removed, and the melted portion can be uniformly maintained at a high temperature, and a uniform refining effect can be brought about.

[Problems to be solved by the invention]

As the high melting point hearth, it is necessary to consider (1) reactivity with zirconium, (2) wettability, (3) heat resistance, (4) workability, etc. in the applied material. Refractory metals such as tungsten, molybdenum, and niobium are 1500 ~ 16 with zirconium.
Since a eutectic crystal easily occurs at around 00 ° C, there is a problem that zirconium is contaminated with the eluted metal. In addition, SiC, MoSi ₂ and T, which are electrically conductive and have a high melting point,
High melting point ceramics composed of i, Zr, or Hf carbides or borides are extremely difficult to sinter, and are usually formed by high pressure sintering called hot pressing, making it difficult to manufacture hearths of the required size and shape. There was a problem of being.

In order to solve the above problems, the present invention provides a hearth capable of efficiently producing high-purity zirconium and a method for easily producing such a hearth.

[Technical means for solving problems]

In order to achieve the above-mentioned object, the first invention of the present application is an electrically conductive hearth having a groove portion filled with molten zirconium, in which at least the groove portion surface is non-oxide,
A hearth for producing zirconium, which is characterized by being formed of a high melting point electrically conductive ceramics layer which is not reactive with zirconium.

Further, the second invention of the present application molds a hearth body having a groove portion filled with molten zirconium by using at least one type of oxide or metal having a good moldability as a hearth base material, and at least the surface of the groove portion is formed by plasma spraying. A method for producing a hearth for producing zirconium, which comprises coating a high melting point ceramics powder which is a non-oxide and has no reactivity with zirconium.

In the above-mentioned structure of the present invention, the hearth base material must be electrically conductive because it melts zirconium using an electron beam. The groove surface filled with molten zirconium must be formed of a high melting point electrically conductive ceramic layer which is non-oxide and has no reactivity with zirconium. This is because the oxide is mixed in zirconium and the vapor pressure of the oxide is low. The ceramic layer must not be reactive with or eutectic with zirconium. This is to prevent contamination of zirconium. Further, the ceramic layer must have a melting point higher than that of zirconium, and must be electrically conductive because it melts zirconium using an electron beam.

As such ceramic materials, SiC, MoS
At least one selected from the group consisting of borides or oxides of i ₂ , Ti, Zr, and Hf can be used.

[Action]

When such a hearth according to the first invention of the present application is used for melting and producing zirconium by an electron beam, it is possible to prevent oxygen as an impurity from being mixed into zirconium. Also,
Since it is not necessary to provide a cooling means such as a cooling pipe in the hearth, a high-purity zirconium body can be produced with high efficiency by heating the entire zirconium to a high temperature.

Further, according to the second invention of the present application, since the predetermined basic hearth can be molded from the hearth base material using an oxide or metal having good moldability, the difficulty in molding the base material is alleviated. Moreover, since the ceramic material can be coated on the groove by plasma spraying, the coating can be performed easily and quickly. The characteristics of plasma spraying are that high-temperature plasma salt can be used, high melting point metal and ceramics can be sprayed, there is a wide choice of spraying materials, and the flight speed of the spraying material is as high as the speed of sound. Therefore, the adhesion strength between the spray coating and the base material is high, and since plasma salt is a neutral salt, it has various characteristics such as a small degree of deterioration of the spray material due to oxidation and reduction. A hearth having good properties can be manufactured quickly and efficiently.

〔Example〕

 Next, examples of the present invention will be described.

FIG. 1 is an external view showing an embodiment of the hearth for producing zirconium according to the first invention of the present application. FIG. 1 (A) shows a groove section, (B) shows a plan view, and (C) shows a vertical section.
The hearth body 1 is provided with a groove 2 filled with molten zirconium. A high melting point electrically conductive ceramic layer 4 which is non-oxide and has no reactivity with zirconium is provided between the groove 2 and the hearth body 1.

In this embodiment, ZrO ₂ having electrical conductivity and excellent heat resistance was used as the base material of the hearth body 1. This ZrO ₂
Since is an oxide, it has good moldability. ZrB ₂ was surface-coated on the groove 2 of the hearth body 1 by plasma spraying. Zr coated on the surface of the groove 2 of the hearth body 1 using plasma spraying
The conditions for forming the B ₂ sprayed coating are shown in Table 1 below.

Since ZrO ₂ and ZrB ₂ have very high adhesion strength,
The ceramic layer of the hearth after melting zirconium can prevent peeling due to thermal stress. Further, by examining the adhesion strength between the hearth base material and the thermal spray material, the hearth base material can be made of a refractory metal such as tungsten, molybdenum, or niobium. Furthermore, when producing a high melting point hearth, it was difficult to produce a high melting point hearth by itself, but since plasma spraying was applied to the method for producing a high melting point hearth,
It is also possible to form a sprayed coating by using various kinds of high melting point ceramics as two or more layers.

Next, FIG. 3 shows an external configuration diagram of an electron beam melting apparatus for producing a high-purity zirconium material by using the hearth for producing zirconium according to the first invention of the present application.

The main melting apparatus is composed of a melting furnace 20 and a main drawing exhaust device 21 and a roughing exhaust device 22, and an electron beam gun 6 which is a heat source generating part is provided in the upper part of the melting furnace 1 in the lateral direction. The exhaust device 21 for main evacuation and the exhaust device 22 for rough evacuation are connected through the vacuum exhaust pipe 8. Exhaust device 21 for main draw
, An oil diffusion pump 7, and a roughing exhaust device are mechanical booster pumps 9. Further, a liquid nitrogen trap 10 is provided above the intake of the oil diffusion pump 7 to provide a mechanism for increasing the cooling rate of the injected steam. The switching between the roughing evacuation device 22 and the main evacuation device 21 is performed by a mechanism in which the roughing evacuation is performed until the inside of the melting furnace 20 is evacuated to 0.1 Torr, and thereafter, the main evacuation is performed. Also,
A table for driving the hearth mold is installed in the melting furnace 20, and the electron beam gun 6 is fixed, and the zirconium sponge in the hearth is melted by driving the table. In the actual purifying and melting process, the composite high melting point hearth shown in FIG. 1 and the conventional water-cooled barrel hearth having the tube shown in FIG. Then, the zirconium material was manufactured.

FIGS. 4 and 5 show a comparison of the oxygen content and the iron content contained in the zirconium sponge refined and dissolved by the composite high melting point hearth and the water-cooled barrel hearth, respectively, and the efficiency of the purifying property in each hearth is shown. Is considered. 4 and 5
As is clear from the figure, the composite high melting point hearth was able to remove impurities more efficiently and dissolve zirconium. The efficiency was improved by about 50% for oxygen and about 30% for iron. It is also considered that the purification efficiency of impurities such as oxygen and iron or less is also improved. The reason why the efficiency of removing impurities is improved is that the high melting point hearth can heat the zirconium as a whole.

〔effect〕

As described above, according to the present invention, the molten zirconium filled in the groove portion in the hearth for producing zirconium can be entirely heated, so that impurities in zirconium can be completely removed. In addition, since there is no dissolution of impurities in zirconium by the ceramic layer coated in the groove, high-purity zirconium can be manufactured. Since the groove portion filled with zirconium is made of a ceramic layer having a high melting point, it is not necessary to cool the lease base material. Therefore, high-purity zirconium can be efficiently produced.

Further, when manufacturing a hearth for producing zirconium, the base material is formed of a material having good moldability, and ceramic coating is applied thereto by plasma spraying, and ceramic coating is applied to the groove surface filled with zirconium material. You can Therefore, the hearth for producing such zirconium can be processed quickly and easily. Further, since plasma spraying is used, the adhesion between the ceramic layer and the hearth base material is improved, and peeling of the ceramic layer due to the thermal stress of the hearth can be prevented. Therefore, the heat resistance of the hearth is improved.

[Brief description of drawings]

FIG. 1 is a block diagram of a hearth for producing zirconium according to the present invention, FIG. 2 is a block diagram of the same hearth having a conventional water cooling tube, FIG. 3 is a block diagram of an electron beam melting apparatus, and FIG. 4 is zirconium melting. FIG. 5 is a graph showing the relationship of the amount of oxygen after that, and FIG. 5 is a graph showing the relationship of the amount of iron after dissolving zirconium. 1 ... Hearth base material, 2 ... Groove part, 3 ... Water cooling pipe, 4 ... Ceramic layer, 6 ... Electron beam gun, 7 ... Oil diffusion pump, 8 ... Vacuum exhaust pipe, 9 ... Mechanical booster pump, 10 ... Liquid nitrogen trap, 11 ... Valve, 20 ... Melting furnace, 21 ... Exhaust device for main draw, 22 ... Exhaust device for rough draw.

Claims (4)

[Claims] 1. A high-melting point electrically conductive ceramics layer having a groove portion filled with molten zirconium, wherein the surface of the groove portion is at least a non-oxide and has no reactivity with zirconium. A hearth for manufacturing zirconium whose patent is formed by. 2. The hearth for producing zirconium according to claim 1, wherein the ceramic layer is at least a group consisting of SiC, MoSi ₂ , Ti, Zr, and Hf carbides, and Ti, Zr, and Hf borides. A hearth for producing zirconium, which is formed of one kind. 3. A hearth main body having a groove portion filled with molten zirconium is formed by using at least one kind of oxide or metal having good formability as a hearth base material, and at least the surface of the groove portion is subjected to plasma spraying so as not to Is an oxide,
A method for producing a hearth for producing zirconium, which comprises coating a high melting point ceramic powder having no reactivity with zirconium. 4. The method for producing a hearth for producing zirconium according to claim 3, wherein the high melting point ceramic powder is SiC, MoSi ₂ , a carbide of Ti, Zr, Hf, or an oxidation of Ti, Zr, Hf. A method for producing a hearth for producing zirconium, which is at least one powder of the group consisting of