JPH0972998A - Method for solidifying radioactive waste - Google Patents

Abstract

(57) [Summary] [Structure] In a vitrification process in which radioactive solid waste is mixed with molten glass and the waste is contained in glass,
After the solid waste is vitrified, the vitrification is performed again using glass materials having different compositions. [Effect] In vitrification of radioactive solid waste, crack generation due to thermal strain is suppressed and the water resistance of the glass is secured to improve the radionuclide containment performance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vitrification treatment method for radioactive waste generated from nuclear facilities.

[0002]

2. Description of the Related Art Generally, a hydraulic inorganic solidifying material such as cement is used for solidifying waste having a low radioactivity level such as chemical waste liquid generated from a nuclear power plant. On the other hand, for wastes with high levels of radioactivity generated from reprocessing plants, etc., solidification materials that are denser than cements such as glass have been studied from the viewpoint of reducing radionuclide leakage.

For glass solidification, New Glass Vol. 9 N
o.2 pp. As described in 26-31, borosilicate glass or the like having high durability is melted at a temperature of a thousand and several hundreds of degrees Celsius, and mixed with waste to form a solidified body. In this case, the target waste is only liquid waste (high-level concentrated waste liquid), and a method has been developed in which the glass raw material is made into a fibrous form, and the waste liquid is impregnated and supplied to the melting furnace.

[0004]

When the prior art is applied to the solidification of solid waste (metal, ceramics, etc.), the coefficient of thermal expansion of glass, which is the solidifying material, differs from that of the waste by about one digit. It is expected that strain will occur during cooling and cracks will occur. Further, when solidified using a low-temperature molten glass with a small amount of strain generation, the water resistance of the glass is reduced compared to conventional glass, although the solidified body can be suppressed,
It is not preferable from the viewpoint of containment performance of radionuclides.

The object of the present invention is to suppress the generation of cracks due to thermal strain in vitrification of radioactive solid waste and to secure the water resistance of the glass, thereby improving the containment performance of the radioactive nuclide contained in the waste. To improve.

[0006]

To achieve the above object, the present invention provides a vitrification process in which radioactive solid waste is mixed with molten glass to confine the waste in glass, after vitrifying the solid waste. , Glass materials having different compositions are used for vitrification again.

[0007]

The coefficient of thermal expansion of glass is on the order of 10 ^-6 / ° C on average, but 10 ^-5 / ° C for metals among solid waste,
Ceramics are on the order of 10 ^-7 / ° C, and there is an order of magnitude difference from glass, which is a solidifying material.

When encapsulating such solid waste in the molten glass, a difference in strain occurs between the glass and the waste during cooling, and stress acts on the glass. The stress generated at this time is represented by the amount of thermal strain of the glass, that is, the product of the coefficient of thermal expansion and the temperature at the time of glass solidification, and when this stress exceeds the tensile strength of the glass, cracks may occur in the glass. Is expected.

That is, since the solidification temperature of borosilicate glass, which has been conventionally vitrified for high-level waste liquid, is as high as about 1100 (° C.), the absolute value of the strain difference at the contact surface between waste and glass is high. There is a problem that it becomes large and cracks easily occur.

On the other hand, the inventors have confirmed by experiments that the difference in strain can be reduced and the occurrence of cracks can be prevented by lowering the glass solidification temperature. In the experiment, as a low temperature molten glass having a low vitrification temperature, a lead-based glass containing lead (Pb) oxide as a main component, arsenic (As) -tellurium (Te).
A glass solidified body was prepared by using three types of chalcogen-based glass containing a sulfur (S) oxide as a main component and vanadium-based glass containing a vanadium (V) -phosphorus (P) oxide as a main component. Table 1 shows the solidification temperature of each glass and the results of examining the presence or absence of cracks on the glass surface under a microscope.

[0011]

[Table 1]

From these results, it was confirmed that the generation of cracks can be prevented by using the low temperature molten glass.

However, Table 2 shows the results of the water resistance test of the glass measured at the same time as this experiment.

[0014]

[Table 2]

As a result, it was found that the water resistance was lower than that of borosilicate glass. That is, it was found that the low-temperature molten glass has a new problem that the water resistance required for the radionuclide containment performance decreases.

In the present invention, solid waste is first solidified with a glass material for preventing cracks, and then glass is again solidified with a glass material for ensuring water resistance, thereby preventing cracks from occurring and water resistance. Can be secured.

[0017]

EXAMPLES Example 1 An example of the present invention will be described below. In this example, silica gel spheres having an average particle diameter of 1 mm were used as a solid waste simulating body, and a vitrification test was conducted. The filling amount of the silica gel spheres was made so as to occupy 10 wt% of the final vitrified body. In addition, the glass material was borosilicate glass alone, and was solidified with lead-based glass, chalcogen-based glass, vanadium-based glass, and then solidified again with borosilicate glass. It was The test results are shown in Table 3.

[0018]

[Table 3]

The prepared vitrified body was cut and the cross section was observed under a microscope. As a result, it was confirmed that cracks did not occur in any case. Further, as a result of measuring the dissolved amount of the glass when the vitrified body was immersed in pure water for one month, a value almost equal to the dissolved amount of the borosilicate glass alone was obtained in all cases, and water resistance could be secured. . That is, the solid waste according to the present example is first solidified with a glass material that prevents the occurrence of cracks, and then by vitrification again using a glass material that ensures water resistance, the occurrence of cracks is prevented, and the water resistance is improved. It was shown that it is possible to secure

Further, as an unexpected effect, when solidified alone with borosilicate glass, the contact surface between the solid waste and the glass reacts with each other to change the quality of the glass material and reduce the glass physical properties such as the strength of the vitrified body. In some cases, however, it was found that the solidification with the low temperature molten glass in advance suppresses the reaction with the solid waste and maintains the physical properties of the vitrified body.

It should be noted that, as for the vitrification for preventing the occurrence of cracks at this time, it was obtained that the same effect can be obtained by solidifying the solid waste into a lump or by solidifying it so as to coat the surface layer. .

(Embodiment 2) Another embodiment of the present invention will be described below. In this example, as in Example 1, a vitrification test was conducted using silica gel spheres having an average particle size of 1 mm as a solid waste simulating body. The filling amount of the silica gel spheres was made so as to occupy 10 wt% of the final vitrified body.
As for the glass material, two cases were carried out: vanadium-based glass alone and solidified with lead-based glass, and then solidified again with vanadium-based glass, and crack generation and water resistance were examined respectively. Table 4 shows the test results.

[0023]

[Table 4]

As a result, the prepared vitrified body was cut and the cross section was observed under a microscope. As a result, it was confirmed that cracks did not occur in any case. In addition, as a result of measuring the amount of dissolved glass when the vitrified body was immersed in pure water for one month, it was 10 when vanadium glass was used alone.
^{It was -4} (g / cm ² ), whereas it was 10 ^-6 (g / cm ² ) when solidified beforehand with lead-based glass, and a new finding was obtained that the dissolution amount was low. It is considered that this is because the contact surface layer of the vanadium glass and the lead glass covering the solid waste reacted to generate a compound having excellent water resistance. At this time, no deterioration of the glass physical properties such as the strength of the vitrified body was observed.

Example 3 An example of vitrifying the solid waste of the present invention. Figure 1 shows the flow of the solidification system.

Solid waste is put in the storage tank 1, and glass as a solidifying material is put in the storage tank 2. Powder (frit) or lump (cullet) is preferable. A predetermined amount of glass is supplied to the melting furnace 3 through a hopper for constant amount supply or a feeder to be melted. The solid waste is supplied to the net container 4 in a predetermined amount via a hopper or a feeder for fixed amount supply. Here net container 4
Consists of a mesh with an opening smaller than the particle size of the solid waste. The solid waste placed in the net container 4 is immersed in the melting furnace 3 to coat the surface of the solid waste with glass,
Be lifted. The glass-coated solid waste is discharged from the net container 4 to the waste hopper 5, and is further supplied to the solidification container 6 in a predetermined amount via a hopper or a feeder for quantitative supply.

On the other hand, a glass having a composition different from that of the above-mentioned glass is put in the storage tank 7, and the glass is supplied to the solidification container 6 in a predetermined amount through a hopper or a feeder for constant quantity supply. The solidification container 6 may be made of metal or ceramics, but alumina is preferable from the viewpoint of the coefficient of thermal expansion. The solidification container to which the glass and the solid waste are supplied is transferred to the stirring furnace 8 and fixed in the furnace. An agitator 9 having an elevating function is installed in the agitation furnace. The inside of the furnace is maintained at a temperature corresponding to the working point of the glass, and the glass in the container melts and becomes ready for mixing with the solid waste. Then, the stirrer 9 is inserted into the solidification container to stir the melt. After stirring for several minutes, the stirrer 9 is pulled out and the solidification container 6 is cooled in the slow cooling chamber 10.
Then, it is slowly cooled so as not to generate sudden strain, and a solidified body is prepared.

[0028]

According to the present invention, in vitrification of radioactive solid waste, the generation of cracks due to thermal strain is suppressed, and the water resistance of the glass is ensured to contain radioactive nuclides contained in the waste. The performance can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram of solid waste solidification processing according to an embodiment of the present invention.

[Explanation of reference numerals] 1, 2, 7 ... Storage tank, 3 ... Melting furnace, 4 ... Net container, 5 ... Waste hopper, 6 ... Solidification container, 8 ... Stirring furnace, 9 ... Stirrer, 1
0 ... slow cooling room.

Claims (5)

[Claims] 1. A vitrification process in which radioactive solid waste is mixed with molten glass and contained in glass, wherein the solid waste is vitrified and then vitrified again using a glass material having a different composition. Method for solidifying radioactive waste. 2. The method for solidifying radioactive waste according to claim 1, wherein the solid waste is solidified with a low-temperature molten glass and then vitrified using glass materials having different compositions. 3. The method for solidifying radioactive waste according to claim 2, wherein the low-temperature melting glass contains at least one kind of lead-based glass, vanadium-based glass, and chalcogen-based glass. 4. The method for solidifying radioactive waste according to claim 3, wherein the solid waste is solidified with a low-temperature molten glass and then solidified again with borosilicate glass. 5. After solidifying the solid waste with lead glass,
The method for solidifying radioactive waste according to claim 3, wherein the glass is solidified again using vanadium-based glass.