JPH024878B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for solidifying radioactive waste, and particularly to a method for solidifying radioactive waste containing soluble salts using a hydraulic solidifying material.

[Prior art]

Reducing the volume and solidifying radioactive waste generated from nuclear power plants is important in securing storage space within the facility and stabilizing the waste.
In addition, these wastes need to be solidified in some form before final disposal. One way to reduce the volume of radioactive waste is to dry and powder concentrated waste liquid (main component Na ₂ SO ₄ ) and granular ion exchange resin slurry to remove water, which accounts for most of the volume of radioactive waste, and solidify it. Methods are being considered.

It has been confirmed that this method can reduce the volume to a fraction of that of the conventional method of directly turning waste liquid or slurry into cement. This method, which is highly effective in volume reduction, has the drawback that a stable solidified body cannot necessarily be created using a hydraulic solidifying agent such as cement or alkali silicate. This is because the hydraulic solidifying material is used by kneading the solidifying material with water, and this water is reabsorbed by the dry waste, causing the dry waste to expand in volume and causing damage to the solidified material.

For example, when pellets containing Na ₂ SO ₄ as the main component are solidified using an alkali silicate composition as a solidification agent, the solidification material temperature and curing temperature are room temperature. The problem is that the pellets absorb the reaction water generated during the process, causing the pellets to swell and causing cracks in the solidified product, resulting in a failure to form a healthy solidified product.

On the other hand, in order to prevent water absorption of pellets, known as the published patent publication, JP 53-8880,
The method of impregnating or coating pellets with plastic etc. requires a complicated treatment process and has technical problems such as uniform coating. Furthermore, there are other problems such as a reduction in the amount of filling due to the thickness of the coating material and high cost, so it cannot be said to be a practical method.

In addition, as a measure to prevent water absorption of pellets,
In order to shorten the curing time, a method of increasing the paste temperature has been considered, but this is not a practical method because the curing is accelerated and the viscosity increases, making injection difficult.

[Purpose of the invention]

An object of the present invention is to provide a solidification method that prevents the waste from swelling as described above and forms a healthy solidified body when solidifying once dried radioactive waste with a hydraulic solidification material. There is a particular thing.

[Summary of the invention]

The present invention is characterized by the discovery that the cause of waste swelling is that the waste absorbs free water and reaction product water in the solidifying material and incorporates it as crystal water to form hydrates. The point is that the curing reaction is carried out at a curing temperature equal to or higher than the crystal water formation temperature.

The present invention was made through the following considerations. As an example, a case where the main component of the waste is Na ₂ SO ₄ and the solidifying material is an alkali silicate composition will be described below.

Na ₂ SO ₄ absorbs water and forms hydrates of Na ₂ SO ₄ .10H ₂ O. The inventors believe that the cause of crack formation in the solidified material is swelling of the pellets due to the formation of hydrates. It was confirmed.

The solubility curve of Na ₂ SO ₄ is shown in Figure 1. According to this, it can be seen that hydrates are theoretically not produced at temperatures above 32.4°C.

On the other hand, the solidification time of the alkaline composition is controlled by the solidification environmental temperature. Its characteristics are shown in Figure 2. When the pellets are heated (e.g. 60°C), the solidifying material around the pellets is at a high temperature, so the curing time is quite short as shown in Figure 2, and the solidifying material at a place away from the pellets (e.g. 32.5 ℃) is 1/7 of the curing time. Therefore, by warming the temperature of the pellet in advance, it is possible to rapidly harden the solidifying material near the pellet and selectively form a layer of cured material on the surface layer of the pellet.

The water permeability test results of the cured product of the above alkaline composition are shown in FIG. As you can see from this figure, the cured material has about 10 times the water resistance of concrete. It is necessary to form a hardening agent hardening film on the pellet surface as soon as possible after solidification injection, which can sufficiently protect the pellets from the free water of the solidification material and reaction product water generated until the solidified material is completely hardened. There is. In order to form a film as quickly as possible, it is better to raise the pellet temperature as much as possible, but in this case, the temperature of the injected solidifying agent will be indirectly increased, accelerating the curing reaction and increasing the temperature of the solidifying agent. This causes an increase in viscosity and impairs solidification flow into the pellet gaps. For this reason, the temperature at which the pellets are heated is preferably about 60°C, which is the temperature at which a thickness sufficient to withstand the complete curing time is obtained. Figure 4 shows 60 pellets.
The temperature distribution when heated to ℃ is shown.

As a result of the above studies, a healthy solidified product can be obtained by forming a hardening agent coating on the pellet surface that is thick enough to withstand water during the time it takes for the solidifying material to completely harden. Further, this pellet surface coating can be formed by heating the pellet in advance, and its thickness can be adjusted by adjusting the temperature at which the pellet is heated. Furthermore, before the solidifying agent film is formed on the pellet surface, water absorbed into the pellet is incorporated as crystal water of Na ₂ SO ₄ and becomes Na ₂ SO ₄・10H ₂ O. In order to prevent volumetric expansion, the material is hardened and cured at a temperature of 32.5℃.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. In this example, radioactive waste pellets containing Na ₂ SO ₄ as a main component are solidified in 200 drums as an alkali silicate composition solidifying material. First, the pellets 1 stored in the pellet hopper 2 are heated to 60°C using the heater 6. Next, the solidified material 3 in the solidified material tank 4 is heated to 32.5° C. by the heater 6. Furthermore, approximately 310 kg of heated pellets 1 are filled into a 200 drum can 5, and then approximately 120 kg of heated solidification material 3 is injected. Container 5 with heater 6
Heat and cure at 32.5℃ to solidify. The solidified product produced in this manner is sound and has no cracks due to water absorption and swelling of the pellets.

According to this embodiment, a healthy solidified body of radioactive waste pellets can be provided only by adjusting the temperature.

In the above examples, an alkali silicate composition was used as the solidifying material, but the same solidifying method can be used with blast furnace cement, Portland cement urea resin, etc., and the same effect can be obtained, but Since it has a high solubility, it is necessary to insolubilize the waste to prevent this.

In the above embodiments, the waste was in the form of pellets, but the same effect can be obtained even if it is in the form of powder.
However, as shown in FIG. 7, it is necessary to add a mixer 10 to the system.

In the above example, Na ₂ SO ₄ was used as waste.
However, sodium borate (Na ₂ B ₄ O ₇ ), which is waste from PWR blunts, and other wastes (e.g., calcium sulfate, sodium phosphate, etc.) cannot be heated above their respective crystal water binding temperatures. Similar handling is possible by adjusting the temperature.

In the above example, the waste pellets were heated to 60°C.
Although the temperature was heated to 32.5°C, the same effect was obtained. Additionally, the temperature control system is simplified, but since no coating is formed, pellets may swell due to failure of control.

〔Effect of the invention〕

According to the present invention, since the pellets are heated, after the solidifying agent is injected, the solidifying agent rapidly hardens on the surface of the pellet, forming a dense layer of the hardened material. This layer prevents the pellets from absorbing free water before curing and reaction product water during the curing process, and is therefore effective in preventing swelling of the pellets.

Furthermore, by setting the curing temperature to be equal to or higher than the crystal water binding temperature, there is an effect of preventing deterioration of the waste due to the formation of hydrates.

Also, without coating the pellets,
Since a healthy solidified body can be obtained only under curing conditions, it is effective in simplifying the system, improving volume reduction performance, and improving economic efficiency.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the relationship between the solubility of Na ₂ SO ₄ and temperature, Figure 2 is a characteristic diagram showing the relationship between the solidification time of the alkaline composition and the solidification environment temperature, and Figure 3 is the characteristic diagram showing the relationship between the solidification time of the alkaline composition and the solidification environment temperature. Figure 4 is a characteristic diagram showing the temperature distribution when pellets are heated, Figure 5 is a schematic diagram showing the pellet solidification method according to this example, and Figure 6 is a characteristic diagram showing the concrete water permeability. It is a schematic diagram which shows the powdery waste solidification method by the modification of an example. 1...Pellet, 2...Pellet hopper, 3
...Solidifying material, 4...Solidifying material tank, 5...200
Drum, 6...Heater, 7...Waste, 8...
...Waste tank, 9...Mixer, 10...Mixing tank, 11...Solidified material.

Claims (1)

[Claims] 1. A method in which dry radioactive waste containing soluble salts is placed in a container and solidified by adding a hydraulic solidifying agent, wherein the temperature of the waste and the solidifying material is
A method for solidifying radioactive waste, characterized by solidifying the waste by maintaining the waste at a temperature equal to or higher than the water-of-crystal formation temperature. 2. The method for solidifying radioactive waste according to claim 1, wherein the radioactive waste is a dried pellet-like material. 3. The radioactive waste according to claim 1 or 2, characterized in that the solidification material and the radioactive waste are heated to a temperature higher than the crystallization water formation temperature of the radioactive waste and mixed. Solidification method. 4. Claims 1 to 3, characterized in that when the main component of the soluble salt of the radioactive waste is sodium sulfate, the solidifying material is mainly composed of an alkali silicate. A method for solidifying radioactive waste as described in any of the above.