JPS6126898A - 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] [Field of Application of the Invention] The present invention involves adding a slag agent to metal contaminated with radioactive materials generated in nuclear facilities, etc. that handle nuclear fuel materials, heating and melting the metal, and adding the radioactive material to the slag. The present invention relates to melt decontamination methods for radioactively contaminated metals that are suitable for providing a slag agent composition that optimizes the efficiency with which radioactive materials are transferred to slag.

[Background of the invention]

The metal melting decontamination method is a method in which a slag agent mainly composed of inorganic oxides is added to radioactively contaminated metals, and the slag agent is heated and melted to incorporate the radioactive materials into the slag agent, which is then separated and recovered from the metal. A specific example of this method will be explained with reference to FIG. First, metals contaminated with radioactive uranium are placed in a crucible, and then mainly silicic acid (Sin2) and calcia (Cab) are placed in a crucible.
A suitable amount of a slag agent consisting of 1Qwt% or less is added to the metal. This crucible 1 is used in a closed electric furnace 2 which is equipped with a filter to prevent contaminants from scattering and takes measures to treat exhaust gas.
After the inside of the furnace is evacuated through a chemical trap 3 and an exhaust pump 4, an inert gas (in an inert gas cylinder 5) is filled and the pressure is controlled to be constant. Next, when the metal and slag agent are melted by heating with heater 6, crucible 1
Inside, the molten slag 7 and the molten metal 8 are separated into two layers due to the difference in specific gravity, and in this process, uranium compounds are incorporated into the slag.

In the process of melting and decontaminating radioactive pollutants, the slag agent is the one most responsible for the decontamination effect. Conventionally, no particular optimization has been taken in selecting this slag agent.For example, for iron materials, SiO□-CaO=A7203-based slag agent or 5in2-CaO, which is generally used in metal refining and refining, has not been particularly optimized. -3rd for Fe20° type slag agent
Components were added and their decontamination effects were evaluated. However, even if the targeted decontamination effect could be obtained, it was unclear whether this was the optimum composition ratio among the slag compositions.

[Purpose of the invention]

An object of the present invention is to provide a method for melting and decontaminating radioactive contaminants that is most suitable for maximizing the decontamination effect of the added slag agent on radioactive contaminants.

[Summary of the invention]

When metals contaminated with radioactive substances such as CRAN are melted, ion reactions are considered as one of the mechanisms by which uranium compounds migrate from the molten metal to the slag layer. In other words, uranyl ions (UO') generated in the molten metal
2+) and slag ions generated in the molten slag are considered to associate at the molten metal-slag interface, form a slag-uranyl complex through an ionic reaction, and transfer to the slag layer.

Slag ion + uranyl ion - slag/cranyl complex ・・・・・・・・・(
1) Taking the 5iO1-CaO system as a typical slag agent as an example, CaO has a high ionization tendency, and Ca0-
It easily dissociates into Ca''+02-, while SiO2 has a strong covalent bond and takes in the free 02- ion to form 5io2+2.
It is said that it is easy to exist as a stable ionic molecule as o2--Sin. The above chemical reaction can be expressed as follows.

SiO□-1-2CaO: SiO+2Ca" ・-
... (2) Here, if the slag ion that acts to collect the usinyl ion is S r O nee, the complex obtained by the reaction of formula (1) becomes the following product.

slO knee + 2UO; j + = S i 04 (0
02) 2 -...4') Now, if equation (3) is dominant in decontamination, the greater the amount of Sin knee ions produced, the higher the decontamination effect. When the mixing ratio of 5io2-CaO-based slag is varied and the total amount of Sin knee ions produced in a certain amount of slag is calculated for the Cab/5i02 molar ratio, the relationship shown in FIG. 2 is obtained. It can be seen from the figure that the ratio is maximum when the molar ratio is 2.

Generally, oxides with high ionic bonding properties such as C and aO are distinguished from basic oxides, and oxides with high covalent bonding properties such as 5in2 are distinguished from acidic oxides. As a more strict regulation, the valence of each metal cation and oxygen anion is z
", z-, and when the distance between both ions is a (A), the attractive force Ie between both ions is expressed by the following equation, and basicity and acidity are distinguished by the value of .

In formula (4), inorganic oxides with an I value of less than 1 are considered basic, and inorganic oxides with a large I value of 1 or more are considered acidic. To give a representative example,
Ca0=0.7, 5iQ2=2-8. At20s=1
．． Ruru at 9. In multi-component slag agents, basicity is used as a definition to indicate the ionic properties of the slag.

The Cab/5in2 molar ratio shown in FIG. 2 corresponds to the basicity shown by equation (5) (1), indicating that the amount of Sin knee ions produced is maximum when the basicity is 2.

When the basicity is less than 1, the absolute amount of Si07ios is insufficient and Ca"+ and 02- are in excess. On the other hand, when the basicity is 3 or more, SiO2 is in excess and j>SiO□ polymerizes. It behaves as a cyclic anion.For example, 5in2 becomes 2
When molecular polymerization occurs, the reaction for producing 5i2Q, tonatori, and silicate anions is as follows.

51204+2CaO=SizOney+2Ca”,
...+...46) In the case of formula (6), focusing only on the amount of Si207 produced, the maximum value is shown when the basicity is 1.

Therefore, if silicate anions containing polymers in the slag contribute to the uranium scavenging reaction, the decontamination effect will be effective in the basicity range of 2 or less and 1 or more, where the amount of silicate anions produced is maximum. It is more reasonable to think that it will be the maximum. Based on this theoretical basis, using a typical acidic-m-based mixed oxide 5i02 A403 CaO slag agent,
Melting tests were conducted on iron materials contaminated with uranium compounds. The results are shown in FIG.

The experimental conditions shown in FIG. 3 are as follows: the concentration of uranium attached to the surface is equivalent to 500 ppm, the amount of slag added is 1 gwt% of the weight of the iron material, and the heating temperature is about 6500 x. The figure shows that the decontamination coefficient reaches its maximum when the basicity is around 1.5. Here, the decontamination coefficient was determined by (amount of uranium contaminated with metal)/(amount of uranium in metal after decontamination).

The decontamination coefficient can be derived theoretically, and if it is assumed that 8i04(UCh)2 generated by the reaction in equation (3) is involved in the decontamination reaction, the decontamination coefficient DF can be expressed by the following equation. can.

Here, the weight of Ws' slag agent WM　Weight of metal (%　) Weight percentage in varnish slag, [Ran] Weight percentage in two metals, L: indicates the distribution ratio of the composite oxide.

From equation (7), it can be seen that DF' depends on the distribution ratio of the slag/uranyl ion complex. However, generally L
It is said that L depends on the concentration of the substance to be distributed, and L does not necessarily take a constant value with respect to the concentration of uranium, which is a pollutant. On the other hand, it is not possible to uniformly define a sufficient F value that is optimal for complete decontamination; rather, a higher DF value is required. When actually targeting low-level contaminated waste, the surface contamination level is io-"μCi/r;
``Hereafter, it is considered that the concentration is about 100 ppm, so for example, if the concentration of uranium attached to iron is 1001) pm, it is necessary to contaminate the uranium concentration in general waste to 0.02 I) pm, and the F value is 5 x 10 ” becomes. Looking at the graph in Figure 3 from this point of view, S i 02-At
20s=Basicity is 1 to 1 when using CaO-based slag
It can be seen that a satisfactory decontamination effect can be obtained by using a slag agent between 20 and 20%. Taking Panski data into account, it is considered that basicity between 0.5 and 30 can be used for decontamination purposes.

On the other hand, even if the basicity is within the same range, the decontamination coefficient will vary greatly if the slag agent composition changes. For example, as shown in FIG. 3, when SiO2F e205-CaO-based slag is used, the decontamination coefficient deteriorates by about one order of magnitude. However, the optimum basicity value exists in the range of 1 to 2, which is the same tendency as in the case of using the 5in2-Az2o, -CaO-based slag. This is S i 02
- When Cao-based slag is used as a base, there is an optimum basicity value in the range of 1 to 2 that maximizes the decontamination effect, and as expected from the uranium collection mechanism, the basicity of SiO ions is The amount produced and the decontamination effect tend to be almost the same.

8102 At203-CaQ based slack agent niote,
Although the decontamination coefficient is generally not high when At20s is replaced with Fe20g, the decontamination effect may be improved if a third component of 5i02 A4Os CaO-based slack agent is added.

Table 1 shows an example of improvement in decontamination coefficient by adding a third component. In the table, there are two possible reasons for the improvement in the decontamination coefficient due to the addition of the third component. One reason is that the basicity shifted to the more optimal basicity range of 1 to 2, and the other reason is that the third component has a unique decontamination reaction promoting effect.

Therefore, the conditions for selecting the optimal slag composition are to use slag components of a type that has essentially high decontamination effects and to adjust the component ratio so that the basicity range is optimal. This allows the uranium concentration in the treated metal to be equal to or lower than the uranium concentration in the raw material.

The present inventors investigated a method for selecting a slag agent that increases the decontamination coefficient of materials contaminated with radioactive substances and enables melting and decontamination under optimal conditions. As a result, the basicity of the slag agent was 0.
Decontamination conditions can be optimized by using a slag agent with a composition in the range of 5 to 3, that is, the molar percentage of basic inorganic oxides in the slag is in the range of 33 to 75, with the remainder being acidic inorganic oxides. This was discovered based on the estimation of the decontamination mechanism and the results of experiments, leading to the completion of the present invention.

That is, the S 1o2-CaO-Aj20s-based slag agent must be within the range shown in the ternary component diagram shown in FIG. If the basicity is within this range, C
aO to other basic inorganic oxides, SiO□ and A40s
It is also possible to obtain a high decontamination effect by changing to other acidic inorganic oxides. Furthermore, inorganic halogen compounds, of which inorganic fluoride is a typical example, have a high tendency to ionize, cut bonds in polymerized silicic acid, and have the effect of increasing silicate ions, thereby enhancing the decontamination effect. Since inorganic halogenides exhibit a high tendency to ionic binding, they can be substituted for basic oxides. Therefore, even if equimolar halide and basic oxide are substituted, the basicity can be calculated by considering the halide as a basic oxide, and the basicity can be adjusted to a range of 0.5 to 3 for decontamination. It is effective as a slag agent. Among inorganic halides, fluoride is particularly effective.

The above has described the composition of a slag agent aimed only at the melting decontamination effect, but in reality, the melting point of the slag agent is also a condition for selecting a slag agent. It is preferable that the melting point of the slag agent be close to the melting point of the target metal and as low as possible. Therefore, even if we take the S i 02 kl 20s-CaO-based slag agent shown in Fig. 4 as an example, the basicity is in the range of 0.5 to 3, and the melting point is lower than the melting point of iron, 1550t:'. Even more limited.

Adding a halide is effective in lowering the melting point of slag, and it is necessary to add a portion of the halide in order to enhance the decontamination effect.

In addition to iron, applicable metals in the present invention include iron alloys including stainless steel and copper.

′! Contaminated nuclides include transuranium elements such as plutonium and other radioactive elements in the form of stable oxides, and the present invention can also be applied to the decontamination of metals contaminated with these radioactive substances. be.

[Embodiments of the invention]

In the apparatus for melting radioactively contaminated metal shown in FIG. 1, uranium contaminated metal was melted according to the operating procedure described above. In this example, a uranium contamination concentration equivalent to 500 ppm was applied to the surface of 99% pure iron, and then placed in a crucible together with 10 wt% FJlQ various slag agents, and the uranium concentration in the obtained ingot was measured after heating and melting at approximately 1650"C. The decontamination coefficient was determined. The results of the examples are shown in Figure 3 and Table 1. In Figure 3, an example of the composition of the slag agent in which the highest decontamination coefficient was obtained for -C3 in the optimum basicity range is Morti. 12810
2-61CaO27At20 g (basicity = 156)
Met. A1 to 3 in Table 1 are S i 02-Az2o
The basicity was changed by changing the composition ratio of the 3-CaO-based slag, and examples were given for A1 with a basicity of 1 or less, examples of I62 with a basicity of 1 to 20, and examples of A3 with a basicity of 2 or more.

All of them show a decontamination coefficient of 10' or more, but when the basicity is around 1.5, a decontamination coefficient of 104 or more is obtained. This suggests that if used, it is possible to decontaminate contaminated metals of the order of sooppm down to the level of raw materials. Next, in 54-6 of Table 1, ya 1
An example in which a third slag component is added to the slag agent having the composition shown in FIG. By adding M-go to A4, the basicity shifted from 0.82 to 1.03, which resulted in a slight increase in the decontamination coefficient.

A5 shows an example in which MgO was replaced with CaF. CaF2 has a higher decontamination effect than MgO, and this example shows that the decontamination effect can be enhanced by replacing a portion of the basic oxide with fluoride. A6 shows an example in which NjO was added instead of MgO or CaF2, and in this case, the decontamination coefficient increased significantly. Due to this reaction, NiO+Fe=FeO+Ni, it is easy to undergo reduction, increasing the concentration of oxygen ions near the molten slag-metal interface, and these ions are considered to be actively transferred, and as a result, they are considered to be ions that collect uranium compounds. This is thought to be because it promotes the production of slag ions such as SiO7.

〔Effect of the invention〕

As described above, according to the present invention, after determining the slag component type suitable for the molten metal and the slag component type to be added to adjust the melting point of the slag and the chemical properties of the metal after treatment, the slag An optimal decontamination slag agent can be provided by adjusting the composition ratio so that the basicity of the slag agent becomes around 1.5.

[Brief explanation of the drawing]

Figure 1 is a flow diagram of the melt processing equipment, Figure 2 is 5iQ2-
A diagram showing the relationship between the basicity of CaO-based molten slag and the amount of Sin knee ions produced (calculated value). Figure 3 is a diagram showing the relationship between basicity and decontamination coefficient. Figure 4 shows the relationship between basicity and decontamination coefficient of CaO-based molten slag. The figure which limits the composition ratio in the range of degree 0.5-3 is shown. l... Crucible, 2... Closed electric furnace, 3... Chemical trap, 4... Exhaust pump, 5... Inert gas cylinder, 6... Heater, 7... Molten slag, 8・
...molten metal.

Claims (1)

[Scope of Claims] 1. A method for melting radioactively contaminated metal in which a slag agent mainly consisting of an inorganic oxide is added to a metal contaminated with a radioactive substance and then heated and melted to incorporate the radioactive substance into the slag. A slag having a composition in which the content of inorganic oxides is in the range of 33 to 75 mol percent, the remainder is inorganic oxides that are more acidic than the oxides, and the basicity is between 0.5 and 3. 1. A method for melting and decontaminating radioactively contaminated metals, characterized by using a chemical agent. 2. In claim 1, part or all of the basic inorganic oxide is replaced with an inorganic halide, the sum of both being in the range of 33 to 75 mol percent, and the remainder being less than the basic oxide. A method for melting and decontaminating radioactively contaminated metals, characterized by using a slag agent having a composition consisting of an inorganic oxide biased towards the acidic side. 3. A method for melting and decontaminating radioactively contaminated metal according to claims 1 and 2, characterized in that the radioactively contaminated metal is iron or an alloy containing iron. 4. Radioactivity according to claim 3, characterized in that the slag agent added contains at least calcium oxide as a basic oxide and silicic acid or silicic acid and aluminum oxide as an acidic oxide. Method for melting and decontaminating contaminated metals.