JPH0320719B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a method for melting and decontaminating radioactively contaminated aluminum, particularly for decontaminating aluminum contaminated with radioactive materials such as nuclear fuel materials generated in nuclear facilities, especially facilities that handle nuclear fuel materials. Regarding melt decontamination method. [Background of the Invention] The melting decontamination method for radioactively contaminated metals involves adding a slag agent mainly composed of inorganic oxides to radioactively contaminated metals, and heating and melting the mixture to selectively release radioactive materials into the slag. This is a method of separating and recovering metals by extraction and inclusion. A specific example of this method will be explained with reference to FIG. In addition, the metal contaminated with uranium as a radioactive substance is placed in the crucible 1, and is further made of a mixed powder or powder of inorganic oxides (silicate SiO ₂ , calcia CaO, etc.) and fluorides (calcium fluoride CaF ₂ etc.). Approximately 10 wt% of the slag agent is added to the amount of metal to be treated. This crucible is placed in a closed electric furnace 2 that is isolated from the outside air, and the inside of the furnace is evacuated to approximately 0.1 Torr.Then, the pressure is adjusted to a constant level by vacuum or by introducing an inert gas such as argon from an inert gas cylinder 5. Heat to raise temperature. Incidentally, whenever heat treatment is carried out under vacuum conditions, and at appropriate times when heat treatment is carried out under constant pressure, the gas is evacuated through a chemical trap 3 or filter provided for exhaust gas treatment. For heating and melting, the temperature is raised at a constant rate by the heater 6 and the current control device 7 with a program mechanism, and the melting is performed for a certain period of time. During the melting process, the crucible separates into molten slag 8 in the upper layer and molten metal 9 in the lower layer due to the difference in specific gravity, and in this process, uranium compounds are incorporated into the slag. Melting decontamination of radioactively contaminated metals is performed, and the factors most involved in decontamination are thought to be the type of slag agent and heating conditions. Conventionally, it has been confirmed that relatively high decontamination efficiency can be obtained by using a slag agent for steel refining in the melting treatment of metals, mainly _iron and iron alloys. −
By using an Al ₂ O3-based slag agent with an optimized composition ratio, it is possible to decontaminate metal materials to a level close to the uranium concentration before contamination. However, when the contaminated material is aluminum or its alloy (hereinafter abbreviated as aluminum), it is difficult to apply such a slag agent for decontaminating iron materials. Possible reasons for this are as follows. The melting point of the slag agent for decontaminating iron materials is 1200℃ or higher, which is much higher than the melting point of aluminum (approximately 660℃), so the melting process temperature for molten slag and aluminum must be performed at 1200℃ or higher. Under such high temperatures, the redissolution reaction of the uranium compounds in the slag into aluminum tends to proceed. That is, in the concept of metallurgical physical chemistry, it is generally evaluated that the following reaction equilibrium is established between molten slag and metal (M). UX+MU+MX...(1) Here, U: Uranium U: Oxygen or Fluorine If the equilibrium constant K of equation (1) is 1 or more, the reaction proceeds to the right and elemental uranium is produced. In this case, uranium forms intermetallic compounds with the molten metal and dissolves, making decontamination impossible. Now, if we calculate the equilibrium constant in the reaction equation (1) for iron and aluminum, it will be as shown in Table 1. As is clear from the table, in the case of iron, the equilibrium constant is sufficiently small, indicating that the uranium oxide form is stable. On the other hand, in the case of aluminum, the equilibrium constant is around 1, which indicates that if reaction (1) is the dominant constant, uranium oxide is easily reduced by aluminum and decontamination is extremely difficult.

[Purpose of the invention]

The present invention targets aluminum materials, which are difficult to decontaminate due to their high reducing effect on compounds of radioactive contaminants. Our goal is to provide the optimal decontamination method. [Summary of the invention] For melting and decontaminating aluminum and its alloys, which are highly active among metals contaminated with radioactive substances such as uranium.

The types of slag agents suitable for [Table] and the necessary points for melting conditions are considered to be as follows. As already mentioned, aluminum and uranium oxide tend to react to generate reduced uranium element, and the uranium element thus generated forms an intermetallic compound with aluminum and dissolves into the metal. If this happens, decontamination will be impossible.
On the other hand, in the early stage of the melting process when the temperature is not high, it is thought that the migration reaction of uranium compounds into the slag is progressing. Therefore, it is considered that the uranium compound once incorporated into the slag can be included in the slag without undergoing a reduction reaction with aluminum if it exists as a stable complex with a slag agent. Therefore, we will summarize the conditions for aluminum melting decontamination. (1) After the slag agent takes in the uranium compound and undergoes a reaction, it forms a complex with the uranium compound in a stable chemical form, thereby suppressing the reduction reaction of the uranium compound by aluminum. (2) After the reaction of incorporating uranium compounds into the slag agent is completed, a re-dissolution reaction of uranium into the metal can be considered, so the melting temperature and time should be optimized. (3) When selecting the type of slag agent, choose the optimal conditions that will maximize the amount of collection slag ions needed to promote the collection reaction of uranium compounds. (4) The melting point, viscosity, and chemical stability of the slag must be appropriate. Up to now, no knowledge has been obtained regarding the slag agent for aluminum melting and decontamination and the melting conditions, so the basis for selecting the optimal slag agent that led to the invention will be explained based on experimental results. Table 2 shows the experimental results. From Table 2, oxides generally have a high melting point, so they are not suitable as slag agents for aluminum, and their decontamination effects are also low. In the case of fluoride-based slag, the decontamination effect is higher than that of oxide-based slag, but it is still insufficient. In other words, the target level for decontamination is the uranium concentration in raw aluminum.
It shows a much higher level compared to 1ppm. However, in the case of fluorides, considering the practical types of fluorides and their combinations,
There are over 100 types of slag agents,
If fluorides and their composition ratios are optimized, there remains the possibility that they will be effective in melt decontamination. Next, we searched for chloride-based products and found that they were even more effective in decontamination. However, most chlorides are deliquescent, as represented by salt (NaCl), and even if melting and decontamination is completed, the slag that adhered to the ingot will deliquesce when the ingot is removed. Wet the surface. If such recontamination occurs, the benefits of melt decontamination are lost. In addition, liquid contaminants are handled, which poses a storage problem. Among chlorides, potassium chloride (KCl) and barium chloride (BaCl ₂ ) are stable compounds with relatively little deliquescent property. Although the deliquescent problem can be avoided by using these chlorides, the decontamination effect is not sufficient. Therefore, a mixed slag agent containing chloride and fluoride was invented. By using this,
It is possible to take the issue of chemical stability and decontamination efficiency a step further. however,
Moreover, it is one order of magnitude higher than the target level for decontamination. Examples of improvements to this problem will be described later, but the reason why halides such as chlorides and fluorides were effective in melting and decontaminating aluminum in the previous examples will be explained based on the principle of decontamination. The reasons why halide-based slag agents are effective are as follows. First, since a low melting point slag close to the melting point of aluminum is obtained, the decontamination reaction between the molten slag and the metal can proceed almost simultaneously with the melting of the aluminum. Further, since halides have a strong ionization tendency, produce a large amount of uranium-trapping ions, and have a viscosity one to two orders of magnitude lower than that of inorganic oxides, it is thought that the reaction between slag and metal is completed in a short time. Next, we will discuss the optimal slag composition for decontamination among chloride and fluoride-based slag agents.

[Embodiments of the invention]

In the apparatus for melting radioactively contaminated aluminum shown in FIG. 1, uranium contaminated aluminum was melted according to the procedure described above. In this example, a certain amount of uranyl nitrate solution was applied to the surface of the metal rod so that the uranium contamination concentration was 500 ppm, and then placed in a crucible together with a slag agent equivalent to 10 wt% of the metal and heated and melted at a maximum temperature of 1000°C. The melting time was kept constant for 30 minutes, and after cooling, the ingot was removed from the crucible and the uranium concentration in the ingot was measured. The results of the examples are shown in Table 2 and FIGS. 3 and 4. Table 2 summarizes the results of various slag agent selection tests for aluminum material treatment. Figure 3 shows the basicity of the fluoride slag agent, that is, the ratio of the number of moles of the slag component with weak interion attraction to the slag component with strong interion attraction, and the uranium concentration in the ingot obtained for that molar ratio. This is an example showing that a range of basicity of 0.5 to 2 is suitable. Figure 4 shows representative slag 14LiF- from the fluoride and chloride mixed slag agents that had the greatest decontamination effect as a result of the slag agent selection test shown in Table 3.
An example is shown in which the temperature effect using a 76KCl-10BaCl ₂ slag agent was confirmed. FIG. 4 shows the relationship between the uranium concentration in the aluminum ingot and the melting temperature. The melting point of this slag agent is approximately 700℃,
In the example, the minimum temperature is higher than the melting point of the slag agent.
It is carried out at 780℃. In addition, the degree of uranium contamination is
It is carried out as 2000ppm. As a result, the lower the processing temperature, the lower the uranium concentration in the ingot.
This shows that the decontamination effect is increasing. Using this method, we were able to achieve the target decontamination level, which is almost the same level as the uranium concentration in aluminum materials, which is 0.9 ppm. [Effects of the Invention] As described above, according to the method of the present invention, oxidized substances of radioactive contaminants can be reduced and easily incorporated into molten metal, and aluminum materials, which are generally difficult to decontaminate, can be reduced. The radioactively contaminated aluminum material is melted by melting it at the lowest possible temperature with a slag agent that contains chloride and whose composition is adjusted so that the melting point is not more than 200℃ higher than the melting point of the metal material. Decontamination efficiency can be improved.
By performing such optimization, it becomes possible to reduce the uranium concentration in the decontaminated aluminum material to the concentration level in the raw material.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the configuration of the melt processing equipment, Figure 2 is the relationship between slag basicity and decontamination coefficient during melt decontamination of iron materials, and Figure 3 is the relationship between fluoride slag basicity and decontamination coefficient during aluminum melt decontamination. Figure 4 shows the relationship between aluminum melting temperature and decontamination effect. 1... Crucible, 2... Closed electric furnace, 3... Chemical (NaF) trap, 4... Exhaust pump,
5... Inert gas cylinder, 6... Heater, 7...
...Electric furnace control device, 8... Molten slag, 9... Molten metal.

Claims (1)

[Claims] 1. A method for melting radioactively contaminated metal in which a slag agent made of inorganic salts is added to aluminum or an aluminum alloy contaminated with a radioactive substance, the mixture is heated and melted, and the radioactive substance is included in the slag. A method for melting and decontaminating radioactively contaminated aluminum, characterized in that the slag agent is a mixture of an inorganic chloride and an inorganic fluoride. 2. The components of the slag agent are adjusted so that the melting point of the slag agent is at most 200°C or lower than the melting point of the treated aluminum material, and the melting treatment is carried out at a low temperature at which the slag agent and the aluminum material melt. Method for melting and decontaminating radioactively contaminated aluminum as described in Section 1. 3. The slag agent contains at least two types of inorganic chlorides in its composition, one of which has a strong ionic attraction of the compound, the other has a weak attraction, and the molar ratio of the latter to the former is 0.5 or more. The method for melting and removing radioactively contaminated aluminum according to item 1 or 2. 4. The method for melting and decontaminating radiation-contaminated aluminum according to claim 1 or 2, which uses potassium chloride and barium chloride, which are compounds characterized by low deliquescent properties, as the inorganic chlorides.