JPS6145999A - Method for melting and decontaminating radioactively contaminated aluminum materials - 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 relates to a method for melting and decontaminating radioactively contaminated aluminum materials, and is particularly applicable to nuclear fuel materials generated at fuel manufacturing facilities, reprocessing facilities, etc. of nuclear facilities. This melt decontamination method is suitable for efficiently extracting uranium compounds present in the molten material of aluminum and its alloys contaminated with radioactive substances into the slag and decontaminating them by heating and melting them with the addition of a slag agent. It is about the method.

[Background of the invention]

The principle of the melting and decontamination method for radioactively contaminated metals is to selectively remove radioactive materials by heating and melting metals whose surfaces are contaminated with radioactive materials by adding a slag agent mainly composed of inorganic oxides or inorganic halides. It is a type of refining purification method that involves extracting and incorporating metals into molten slag to radioactively clean them.

A specific example of this method will be explained using FIG.

First, a metal piece whose surface was contaminated with a radioactive uranium compound is placed in a porcelain crucible 1, and an inorganic compound (
Add slag powder or particles consisting of an inorganic oxide or an inorganic halide. After this crucible 1 is placed in an electric furnace 2 having a closed structure, the inside of the furnace is evacuated and filled with inert gas, the pressure is adjusted to a constant pressure under negative pressure, and the temperature is increased by heating. The reason for this sealed structure is not only to prevent metal oxidation, but also to prevent radioactive materials from scattering into the surrounding area due to bumping or other causes during heating.

When the pressure inside the furnace increases during the heating process, sodium fluoride (Na
F) Exhaust with an exhaust pump 4 through a chemical trap filled with pellets and a normal absolute filter 3. 5 is an inert gas cylinder. Heating and melting is carried out by using a heating element 6 and a programmable load current control device 7 to measure the temperature inside the furnace with a platinum-rhodium thermocouple, raising the temperature at a constant rate, and holding it at a predetermined temperature for a certain period of time. 9. Normally, a slag agent with a low melting point and light specific gravity is first melted to form molten slag 8, and then the metal is melted to become molten metal 9.

Radioactive substances attached to the metal surface during the melting process are taken into the molten slag 8, and separated in the crucible 1 into molten slag 8 in the upper layer and molten metal 9 in the lower layer due to the difference in specific gravity. .

If it is cooled and solidified after melting for a certain period of time, it can be relatively easily separated and recovered into vitrified slag and clean metal lumps. The greatest effect of the method of melting radioactively contaminated metals described above lies in the significant volume reduction of waste, but also in the fact that contaminated metals can be decontaminated at the same time.

Here, factors governing decontamination include the type and component ratio of the slag agent, and melting conditions such as melting temperature and time.

Conventionally, melting treatment of iron, copper, and aluminum materials has been carried out in consideration of the factors governing the melting and decontamination described above. Table 1 shows specific examples of the results of melting and decontamination of uranium-contaminated metals. As shown in Table 1, when iron and copper are melted at 1650 to 1500 C for about 30 minutes using a metal oxide (silicic acid-calcia HCaO-8ing) slag agent as the slag material, decontamination on the order of 103 is achieved. It can be seen that the ratio is obtained.

As a result, it can be seen that decontamination can be achieved to a concentration level of uranium that is close to or lower than the naturally occurring uranium concentration level originally contained in the raw material before decontamination. However, aluminum materials are difficult to decontaminate, and unless the decontamination efficiency is increased by two orders of magnitude, the uranium concentration cannot be reduced to the level of the shell material. The reason why it is difficult to decontaminate aluminum materials is that the activity differs depending on the metal type.In other words, according to the theory considered in the metallurgy and refining field, the reaction between molten slag and metal follows the reaction formula shown below. Therefore, decontamination becomes difficult under conditions where a reaction equilibrium is established and uranium compounds are easily reduced by metals.

UX + MjiU ten stables (1) also castle Here, U; uranium X; oxygen or fluorine M; metal The reaction equilibrium constant is expressed by the following equation: be done.

Here, a; activity N of each component; abundance γ of each component; activity coefficient of each component. Also, K can be a constant determined thermodynamically and can also be expressed by the following equation.

K=e x p (-ΔG/7RT') −
...-(3) R: Gas constant (cat/motK) T: Melting temperature ('K) Qualitatively, when the equilibrium constant K in equation (1) is 1 or more,
If the reaction tends to proceed to the right, elemental uranium is produced.

In this case, elemental uranium and metals do not form intermetallic compounds and remain in the metal phase, making decontamination impossible. K
The magnitude of can be calculated from the relationship between temperature and free energy of formation of various metal oxides shown in FIG.

Compared to copper and iron, which have small values, the free energies of formation of aluminum and uranium oxides have large values, producing very active and stable oxides. The reaction equilibrium constant between copper and iron and uranium oxide is the second
As shown in the table, very small values are expected to produce almost no reduced uranium. On the other hand, in the case of aluminum, the value is around 1, and reduced uranium is easily generated. This is considered to be the main reason why it is difficult to decontaminate uranium-contaminated aluminum.

Another factor that changes the equilibrium constant K is temperature. Regarding the relationship between the reaction equilibrium constant of uranium oxide and aluminum and temperature, as shown in Table 2, which shows the values at representative temperatures, the value tends to become larger as the temperature decreases.

Although the K value is one of the factors governing decontamination efficiency, it is thought that there are other factors that govern decontamination efficiency, and in the case of aluminum decontamination, the melting temperature is important. Figure 3 is a diagram showing the temperature effect of melting and decontaminating pure aluminum.
As the melting temperature is lowered from 1000C to 5ooC, the uranium concentration in the At (aluminum) ingot decreases. This is thought to be because increasing the melting temperature causes the uranium compound once incorporated into the slag to dissolve into the metal again, but the detailed reason is unknown. In any case, lowering the melting temperature improves the decontamination effect, but it is still higher than the uranium concentration level in the raw material, and in order to achieve the target decontamination level, the decontamination efficiency must be increased by an order of magnitude.

This has been difficult to achieve using conventional melt processing methods.

[Purpose of the invention]

The present invention has been made in view of the above, and its purpose is to improve the decontamination efficiency of radioactively contaminated aluminum materials by reducing the activity of aluminum. The purpose of the present invention is to provide a melt decontamination method.

[Summary of the invention]

A feature of the present invention is that in the process of adding a slag agent made of an inorganic compound to radioactively contaminated aluminum material and heating and melting the material, at least one type of dissimilar metal that is effective in reducing the activity of aluminum is contained. The point is that it is added so that the amount is at least a predetermined value.

[Embodiments of the invention]

An embodiment of the method of the present invention will be described in detail below with reference to FIGS. 1 to 8.

Before explaining the embodiments, first, the basic concept of the present invention will be explained.

Among metals contaminated with radioactive substances such as uranium, aluminum is more active than other metals such as iron and is difficult to decontaminate. The reason for this is, as already mentioned, that aluminum and uranium oxide tend to undergo a reaction as shown by the following formula.

When formula (2) is applied to the equilibrium constant, it becomes as follows.

a tloz ' aAtNuo 2 ' N A 1
γ[102@γAt (5) The results of calculating equation (5) are shown in Table 2. Here, in the case of pure aluminum, the activity coefficient γAt is 1. Still, U (uranium), UO2 (uranium dioxide), At
Since 203 (aluminum oxide) is a trace amount, γυ, γo
The true values of o2 and γ are unknown and are 40g, but they can be considered constant regardless of the reaction amount, so C:
As rU・γ /γUO2, equation (5) is as follows h
It can be expressed as t203.

Therefore, depending on some factors, the activity aA of aluminum
Assuming that t=NAt·γAt decreases, since K is constant, Nuo2 increases by an amount commensurate with the decrease, but Ntr should decrease. In other words, since the amount of reduced uranium produced is relatively reduced, decontamination efficiency increases. In the conventional example, one of the reasons why the decontamination efficiency improved as the temperature was lowered is that the activity of aluminum, that is, the activity coefficient, decreased. However, it was found that the desired decontamination efficiency could not be achieved by temperature effects alone. Therefore, as a result of further examining the causes of lowering the activity aAZ of aluminum, we found that
It has been found that when aluminum contains different metals to form an alloy, aAt may decrease.

This will be specifically explained below, taking as an example the case where magnesium (Mg) is used as an additive metal.

Originally, magnesium is an active metal like aluminum as shown in FIG. 2, but when aluminum and magnesium are mixed and melted, an intermetallic compound of A4-Mg is formed. At this time, the activity coefficient γAt of aluminum decreases as the magnesium component increases, as shown in FIG.

On the other hand, as the magnesium component increases, the activity coefficient γ of magnesium increases, but when the abundance of magnesium is low, it shows a value of around 0.5.

This shows that the activities of both metals are decreasing. Furthermore, as shown in FIG. 5, A/, -Mg
When an intermetallic compound of Further decrease.

Next, the reaction equilibrium constant K between the alloyed aluminum material and uranium oxide will be calculated. Assuming that an intermetallic compound A/, -Mg-, corresponding to the mixing ratio of aluminum and magnesium is produced, the following reaction will be considered.

・・・・・・(7) Here, K=ex, p(-ΔG/RT)
・−・−・(9)ΔG−,(iγAt・ΔG,,+m
γ is Δ~, 0)−(ΔGAJI−Mg+ΔGuoz)
--(In the LdlQO1 formula, At203 and Mg
Change in free energy of formation of O ΔG a1. The reason why ΔGM and O are multiplied by the activity coefficient is that At which apparently contributes to the reaction
This is because the amount of Mg component is decreased. kt in Figure 6
- The relationship between the equilibrium constant of the reaction represented by equation (7) and the Mg abundance ratio in the Mg alloy is shown. From Figure 6, Mg abundance ratio O~
It can be seen that the value decreases exponentially as the Mg amount increases between 0.5 and 0.5. However, when the Mg abundance ratio exceeds 0.5 t-, the value increases again because the influence of magnesium activity becomes dominant. Therefore, if the amount of magnesium added is 50 or more, it will have the opposite effect. However, since the present invention is about decontaminating aluminum materials,
Adding more than 50'1 of magnesium is impractical and goes against the concept of reducing the amount of waste.

Now, when 5T of magnesium is added to an aluminum material contaminated with uranium and the equilibrium constant K in equation (7) decreases, it is thought that theoretically non-reduced uranium compounds will be incorporated into the slag. Therefore, the decontamination coefficient is inversely proportional to K. However, as a factor governing decontamination,
In addition to the thermodynamically determined equilibrium constant described above, the ionic properties of the slag are determined by the type and composition ratio of the slag agent.

There is an optimum composition ratio that maximizes the amount of slag ions effective for collecting uranium in the molten slag, and basicity is used as a representative indicator of ionic properties. Generally, slag agents are a mixture of several types of inorganic compounds, and slag agents with a relatively high ionization rate are classified as basic, and slag agents with a low ionization rate and strong covalent bonding are classified as acidic. = Expressed as the molar ratio of ash base component/acidic component. Unlike the slag agent for treating iron materials, the slag agent for Chilminiram melting treatment requires a low melting point slag agent because the melting point of aluminum is as low as 6.60C. For this reason, fluoride or chloride-based slag agents are used. The results of melt decontamination of aluminum materials shown in the conventional example were obtained by using a slag agent in the basicity range where the decontamination effect is highest, taking ionic properties into consideration. Therefore, up to now, what is considered to be the most suitable slag agent has been used. According to the results of attempts made so far by selecting various slag agents, the target decontamination efficiency cannot be achieved by simply changing the slag agents, so changing the properties of the metal side will improve the decontamination effect. The melting decontamination method according to the present invention was devised as a method for increasing this.

Hereinafter, a description will be given of the state of melting decontamination when magnesium is added as a different metal to reduce the activity of aluminum.

FIG. 7 is a diagram showing the influence of the magnesium addition rate on the melting decontamination effect, with the horizontal axis representing the Mg abundance rate and the vertical axis representing the U concentration of the At ingot.

It can be seen that as the amount of magnesium contained in the pure aluminum material increases, the amount of uranium in the aluminum ingot after decontamination decreases, and the decontamination efficiency improves exponentially. When the magnesium abundance rate exceeds 5%, the uranium concentration (i
It can be seen that it is possible to lower the value down to (below p). This makes it possible to achieve the target decontamination level, but the question of whether to increase the amount of magnesium added to further improve the decontamination effect is based on technical considerations such as an increase in the amount of waste and economic losses caused by adding additives. It is necessary to take this into consideration when making a decision.

The effects of adding magnesium have been explained above using specific examples.
Copper (Cu) is a metal that has similar effects. The At-C11 alloy similarly produces stable intermetallic compounds, and as the amount of copper added increases,
The activity of aluminum decreases, showing the same tendency as shown in the figure. Furthermore, the decontamination efficiency can be similarly improved for aluminum materials containing copper.

Similar effects can also be expected by adding other metals such as iron or chromium, but in this case, the amount added cannot be increased. The reason is.

As mentioned in the conventional example of melting treatment of aluminum materials, the melting temperature is the second factor that affects the decontamination effect, and the lower the treatment temperature, the higher the decontamination effect; As shown in Figure 3, decontamination is difficult with treatment at temperatures above 900-1000°C.Incidentally, the melting points of At-Fe and At-Cr alloys when added at 5% are 5% and 5%, respectively.
ooc, ss.

In such a case, it is necessary to set the treatment temperature to 9 ooc or higher, because the effect of adding iron and chromium is offset by the negative decontamination effect that accompanies the temperature rise. From this point of view, it is desirable that the melting point of the alloy be below the melting point of aluminum, 660C, or at most around 700C, when at least 1 (1%) of the different metal to be added is added.

Next, specific examples will be described. Using the apparatus for melting radioactively contaminated aluminum shown in FIG. 1, a uranium-contaminated aluminum material was melted according to the processing procedure described above. In this example, a metal rod with uranium equivalent to 500 ppm adhered to the surface of an aluminum material and a slag agent (inorganic halide Li
F-KO2-Back) powder was placed in a porcelain crucible 1, and further heated to O to 5 according to the test conditions. '2% magnesium was added.

Next, after putting the crucible 1 into the electric furnace 2, heating by the heating element 6 is started, and when the temperature inside the furnace reaches 800C, it is maintained for 30 minutes. However, the inside of the furnace is first evacuated by the exhaust pump 4, and then argon gas is supplied from the inert gas cylinder 5, and the pressure inside the furnace is increased from 200 Torr to 500'l'or.
Keep r. After melting at the above temperature for a certain period of time, the ingot was allowed to cool, and the ingot in the crucible 1 was taken out, and the uranium concentration in the ingot was measured using a radioactivity measuring device. The results are shown in FIG. It can be seen from FIG. 7 that when the magnesium abundance rate is 5 degrees, the ura/corrosion degree in the aluminum ingot is equal to 9279 degrees in the raw material before contamination. Therefore, the amount of magnesium to be added to the aluminum material is preferably 4 or more. In addition, in the case of an aluminum alloy material that already contains magnesium, it is sufficient to remove more than the insufficient amount. If the contaminated aluminum alloy material already contains 5% or more of magnesium, it is possible to decontaminate the uranium concentration to the pre-contamination level without adding magnesium. . In other words, in facilities that handle nuclear fuel materials such as uranium, alloyed aluminum materials containing 5 or more magnesium may be used for equipment or piping made of aluminum materials; can be sufficiently melted and decontaminated without further adding magnesium vinegar.

The same can be said of aluminum materials containing copper. According to the results of melting and decontaminating aluminum materials containing 5-inch steel, it was possible to decontaminate the uranium concentration to below IIP, which is almost the same as the uranium concentration in the raw material.

FIG. 8 is a block diagram of an apparatus for melting radioactively contaminated metal using an electroslag remelting furnace (EAR furnace). E.S.
The R furnace melts and decontaminates fixed-shaped pipes contaminated with radioactivity by directly using it as a melting electrode.The R furnace uses radioactively contaminated metal as a consumable electrode 10, and inserts molten slag 8 into the thick section flow supply transformer. Power is input from the power supply device 11 to the molten slag 8 mk
Flow. At this time, Joule heat is generated in the molten slag 8, and the molten slag 8 serves as a heat source to melt the electrode 10.

When the electrode 10 is made of aluminum, a necessary amount of magnesium is added by the metal supply device 12 at the same time. Also,
At the same time, the required amount of slag agent is added to the slag agent supply tank 13. The molten aluminum is solidified and laminated in a water-cooled copper crucible 14 to form an ingot 15. The atmosphere surrounding the furnace is set to negative pressure, argon gas is supplied from an inert gas cylinder 5, and exhaust gas that may be contaminated with radioactivity is exhausted by an exhaust pump 4 via a filter 3. Note that as the electrode 10 melts and wears out, the electrode drive motor 17
The electrode drive mechanism 18 is driven by the electrode holder 1.
An electrode 10 is lowered via 9.

The melting decontamination method according to the present invention can also be carried out using a melting treatment apparatus having the configuration shown in FIG. In addition, 20 is a guarantee
It is noku-.

According to the above-described melt decontamination method of the present invention, magnesium (
By adding uranium (or copper) to a content of 4 or more, it is possible to reduce the activity of aluminum, and as a result, the uranium concentration in the raw material before contamination can be reduced to the level of the raw material. This makes it possible to handle aluminum waste generated at nuclear facilities without distinguishing it from general materials.

Note that the metal added to the aluminum material in the melt decontamination method according to the present invention includes silicon in addition to magnesium and copper. On the other hand, although the metal to be decontaminated is aluminum, it can also be applied to melting and decontaminating other active metals such as titanium and silicon. Furthermore, it is also applicable to aluminum materials contaminated with radioactive contaminants other than uranium, for example, compounds of transuranium elements such as plutonium.

In addition, this melting decontamination method can also be applied to metal materials contaminated with radioactive substances such as cobalt, manganese, and chromium generated at nuclear power plants. In particular, radioactive substances produced at nuclear power plants are close to iron, and the free energy of formation of its oxides is small, so compounds of radioactive substances are easily reduced. Therefore, it is necessary to reduce the activity of the contaminant metal, and the method according to the present invention can be applied thereto.

Regarding the melting processing equipment, Fig. 1 uses an external heating method using a heating element 6, and Fig. 8 uses an electroslag remelting furnace, but induction heating and arc melting are also available. good. The ESR furnace shown in FIG. 8 is suitable for continuous melting treatment.

Here, we will discuss the relationship with magnesium and copper contained in aluminum materials, which are generally used as industrial materials. Generally, the purpose of including magnesium, copper, and other metals in industrial standard aluminum materials is to improve corrosion resistance and
This is to increase the strength.

The amount of magnesium added is 1 to 5% depending on the application, but generally, except for special aluminum alloys, there is no example in which the amount of magnesium added exceeds 5%. Moreover, there is no example of adding magnesium for the purpose of increasing the decontamination effect, and generally when melting radioactively contaminated aluminum materials, the desired decontamination effect cannot be obtained without the addition of magnesium or the like.

〔Effect of the invention〕

As explained above, according to the present invention, there is an effect that the decontamination efficiency of aluminum materials contaminated with radioactivity can be improved by lowering the activity of aluminum.

[Brief explanation of drawings]

Figure 1 is a diagram showing the configuration of the melting treatment equipment, Figure 2 is a diagram showing the relationship between temperature and free energy of formation of various metal oxides, Figure 3 is a diagram showing the temperature effect of melting and decontamination of pure aluminum, 4
The figure shows M in a 100 (lx) melt of At-Mg alloy.
Diagram showing the relationship between the g abundance rate and the activity coefficient of each metal, 5th
The figure shows the relationship between the Mg abundance ratio of At-Mg intermetallic compounds and the change in the free energy of formation. Figure 6 shows the relationship between the Mg abundance ratio at 1073'K and the reaction equilibrium constant between At-Mg alloy and uranium oxide. Relationship diagram, Figure 7 shows uranium contamination level 500
Figure 8 is a diagram showing the relationship between the Mg content in the aluminum material of p and the uranium concentration in the aluminum ingot after decontamination. . 1... Porcelain crucible, 2... Electric furnace, 3... Chemical trap and filter, 4... Exhaust pump, 5...
... Inert gas cylinder, 6 ... Heating element, 7 ... Load current control device, 8 ... Molten slag, 9 ... Molten metal, 10 ... Radioactively contaminated metal consumable electrode, 11 ... - Large current supply transformer power supply device, 12... Metal supply device, 1
3... Slag agent supply device, 14... Water-cooled copper crucible, 15... Ingo soot, 17... Electrode drive motor,
18... Electrode drive mechanism, 19... Electrode holder, 2o
...Protective cover.

Claims (1)

[Claims] 1. Adding a slag agent made of an inorganic compound to an aluminum material contaminated with radioactivity and heating and melting the same to incorporate radioactive substances into the slag to radioactively clean the aluminum material. A radioactive melting decontamination method characterized by adding at least one type of dissimilar metal effective in reducing the activity of aluminum in the heating and melting process so that the content is at least a predetermined value. Method for melting and decontaminating contaminated aluminum materials. 2. The dissimilar metal is one in which an intermetallic compound produced by containing and melting the dissimilar metal exhibits a standard free energy of formation change as large as possible on the negative side. Method for melting and decontaminating radioactively contaminated aluminum materials as described in . 3. The dissimilar metal has a melting point of the intermetallic compound of 900.
2. A method for melting and decontaminating radioactively contaminated aluminum materials according to claim 2, wherein the temperature does not exceed .degree. 4. The dissimilar metal is magnesium, copper, or a mixture thereof, and is added such that the content thereof is 4% or more, including those already contained in the aluminum material. The described method for melting and decontaminating radioactively contaminated aluminum materials.