JPS60129698A - Method of melting and decontaminating radioactivity contaminated metal - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a melt decontamination method for obtaining a high decontamination rate in the decontamination of uranium-contaminated metals generated at nuclear facilities that handle uranium.

Conventionally, in melting and decontaminating uranium-contaminated metals, the sample is placed in a heating furnace together with a slagging agent such as an oxide of the metal, and during the melting process, the uranium compounds are transferred to slag, which has a lower specific gravity than the metal, and are separated. Ta. After the molten metal was cooled and solidified, only the slag layer was cut out and the uranium was removed. However, in this method, slag and molten metal are not completely separated, and slag components may be left behind in the metal. Therefore, a method was adopted in which the molten metal was filtered at the time of melting treatment to separate and remove the slag components. It was found that by this method, most of the uranium compounds that had migrated to the slag layer could be separated, but the uranium compounds that had combined with the molten metal to form metal compounds could not be separated. In other words, it was found that it is difficult to obtain a high decontamination coefficient even if metals contaminated with uranium compounds are decontaminated by simple melting treatment.

For example, in the case of aluminum contaminated with uranium compounds,
The experimentally obtained decontamination factor (DF) was approximately 30.

Even if slag can be completely separated by a suitable filtration method, uranium that has formed a metal compound remains in the molten metal, making it difficult to achieve a decontamination coefficient of 'e100 or higher. That is, the theoretically obtained decontamination coefficient fDF) is expressed by the following equation.

DF’=1+α where: equilibrium constant ΔG = free energy of formation R: gas constant T: temperature In the above equation, when the temperature is 1200°C, DF'ζ100 is obtained.

However, in reality, the amount of uranium attached to uranium-contaminated metal discharged from nuclear facilities is about ioppm when converted to degrees, but in order to make it possible to reuse decontamination metals, it is necessary to
It is necessary to keep it below pm. For this purpose, the decontamination factor should be set to 1.
You need to raise it to 04 or above.

Therefore, in the melting decontamination method according to the present invention, the purpose is to further increase the decontamination coefficient by removing the uranium component that remains in the metal even after melting, which could not be separated and removed by conventional methods. The present invention will be explained in detail below.

The melting decontamination apparatus of the present invention is shown in FIG. First, heating furnace 1
Place the entire sample crucible 2 in the crucible, and add uranium-contaminated metal and a slagging agent such as alumina or silica into the crucible.

A filter 4 is placed on the bottom of the crucible 2. The atmosphere inside the furnace is vacuum (approximately 10 mmHg) or inert gas.
Allows connection of air supply and exhaust equipment to control atmospheric conditions. After placing the sample in the furnace and setting the above atmospheric conditions, the temperature is raised to a predetermined temperature using the furnace heater 2 and held for a certain period of time while the uranium-contaminated metal is melted. During this time, the uranium compound in the molten metal is transferred to the slag layer, and the molten metal is transferred to the receiving crucible 5 through the filter 4. Uke is crucible 5
The molten metal that has entered the crucible 5 has already been decontaminated, but still contains residual uranium compounds.
solidifies inside.

The receiving crucible 5 has a cylindrical configuration to facilitate the transfer of the solidified metal after melting to the next process.

Next, the receiver partially melts the molded and solidified metal in the crucible 5 with a local heater 6, and the heater 6 is operated with a heater driver 7 so that the transition from the molten phase to the solid phase progresses in a quasi-equilibrium manner. Move slowly. During this melting and solidification process, if the melting point becomes low due to the influence of impurities dissolved in the liquid phase, the impurities will be concentrated in the liquid phase, and conversely, if the melting point becomes high, the impurities will be concentrated in the solid phase. Repeating this process increases the purity of the metal. This method is called the band purification method,
□It has become popular as a method for producing high-purity materials.

According to the principle of the Otono purification method, the migration of impurities in a metal rod is expressed as shown in FIG. In Figure 2,
When the melting zone advances to X, the impurity condition f at the position of
C is expressed by the following formula.

Here, C8: Initial impurity concentration Ko in the metal = Impurity segregation coefficient t: Length of the melting zone As can be seen from this equation, in order to increase the purity of the metal, the segregation coefficient Ko must be K. <1 or K11>1, and the higher the ratio to 1, the better the impurities can be separated. By the way, K was criticized for impurities in germanium and silicon. is on the order of 10-4 to 10-6, 99.99%
The above purification is possible. Even in the case of aluminum, 99.
It is said that purification of 9% or more is possible. Of course, if we take a single impurity element as an example, there will be only a similar amount of impurity, and if we compare each single element, the segregation coefficient of the element with a larger atomic number tends to be smaller than 0. For example, K of uranium in aluminum. is less than 10-4. Therefore, by band-purifying the melt-filtered aluminum and removing the end portion where uranium is segregated, the decontamination coefficient Dr of the melt-filtered aluminum becomes 100 to 1000.

As mentioned above, if the melt decontamination method according to the present invention is used, the decontamination coefficient can be reduced to 100 at maximum compared to the conventional melt method.
It is possible to further increase the level by 100 times to a level of 104. As a result, the metal after decontamination can be made to have a uranium concentration on the order of o, ooippm, which can be reused as a general metal material.

Next, we will discuss the entire industrialization of the melt decontamination method according to the present invention.

When uranium-contaminated aluminum is generated as waste material at a rate of 100 to 77 days, this aluminum is subjected to the first stage of melt filtration treatment, formed into a 10 IMφ aluminum rod and solidified, and then zone purification is proceeded at a rate of 1 cm/hour. If this is done, it will be possible to perform parallel processing using 20 band purifiers. Since the purpose of this method is not to create semiconductor materials or pure substances, it is not necessary to repeat band purification many times, and repeating it two to three times is sufficient. Therefore, industrialization is possible.

As described above, the present invention leads to the decontamination and volume reduction of radioactive waste, as well as to the utilization of waste materials.

[Brief explanation of drawings]

Figure 1 shows the melting removal of uranium contaminated metal t' according to the present invention.
(7) 1... Heating furnace, 2... Furnace heater, 3... Sample crucible, 4... Crucible filter, 5... Molten metal receiving crucible, 6... Zone melting heater, 7... Heater drive machine. Agent Patent Attorney Akio Takahashi (8) Ward 1, No. 20 Distance x->

Claims (1)

[Claims] 1. After completely melting the metal waste contaminated with radioactive substances and separating the compound system containing a large amount of radioactive substances from the molten metal system containing few radioactive substances by the difference in specific gravity, the molten metal waste is Based on the Obiha purification method, the metal is locally heated and melted, and the heater is moved slowly and in a fixed direction so that the transition from the molten phase to the solid phase occurs in a quasi-equilibrium manner. A method of melting and decontaminating radioactively contaminated metals, which is characterized in that, in particular, radioactive substances remaining in the metals are segregated to the other end. 2. Melting of radioactively contaminated metals as set forth in claim 1, characterized in that in the melting decontamination of metals, actinide elements or their compounds, which are nuclear fuel materials, are targeted as the decontaminated substances. Decontamination method.