JPH0470599A - Solidification method for radioactive waste - Google Patents

Abstract

PURPOSE:To suppress excessive heat generation from solidifying material and prevent cracking and the like due to thermal strain by adding a heat absorption material reacting with a solidifying material to a liquid hardening solidifying material during the process of solidifying radioactive waste with heat generation. CONSTITUTION:The material to be added to a solidifying material should be such one that incurs disolution, phase change or adsorption in the paste of mixture of the solidifying material waste, or reacts with the solidifying material (including mixing water) or the waste resulting in positive entropy change (heat absorption change). To prevent spattering of the mixing water due to boiling and evaporation, such materials that incurs these changes below 100 deg.C are favored. Favorable materials include urea to utilize disolution and ammoniates such as nitrates like silver nitrate etc. and urea is the most favorable considering interaction with the solidifying material. As for materials to utilize phase change, ice, acetaldehyde etc, are favorable. As for materials to utilize reaction, such materials reacting with water absorbing the reaction heat, is the most favorable.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a method for stably solidifying radioactive waste such as concentrated waste liquid and heat-generating metals generated from nuclear power plants and nuclear fuel reprocessing plants in a container. This invention relates to a solidification method suitable for.

[Conventional technology]

Reducing the volume and solidifying radioactive waste generated by nuclear power plants is not only important for securing storage space within the facility, but also necessary for land storage, which is one of the final disposal methods. It is essential.

The main waste generated from nuclear power plants (boiling water reactors) is concentrated waste liquid from the regeneration of ion exchange resins used in cooling water purification systems (waste accumulated in the mesh section of ion exchangers). The main component is Na2S○4), and the concentrated waste liquid can be used as it is or after drying and powdering.
A generally known method is to mix it with a hydraulic inorganic solidifying material such as cement and solidify it in a container. Cement-based solidifying materials are: ■ Solidifying operations are easy.

■ It is inexpensive.

■ Excellent long-term durability.

It has the following advantages and is particularly suitable for solidifying low-level radioactive waste.

For such solidified radioactive waste, it is necessary to avoid deterioration due to the reaction between the waste to be filled and the solidification material as much as possible, but this is not possible as much as possible, but it is necessary to avoid deterioration due to the reaction between the waste to be filled and the solidification material. (February 1983), sodium sulfate, which is the main component of concentrated waste liquid, reacts with one of the components of cement, which is a solidifying agent, to generate cement bacilli and generate heat. There is a risk of expansion and destruction of the solidification container. For this reason, as a solidification material for sodium sulfate, as described in the above-mentioned literature, cement with few components that react with sodium sulfate (for example, blast furnace cement) is used, or artificial lightweight aggregate, sand, jiyari, etc. are used. A method of diluting the solidifying agent by mixing it with cement is known.

On the other hand, it is known that the reaction between sodium sulfate and cement components proceeds in the initial stage of mixing, and significantly accelerates the hardening speed of Cement 1. For this reason, the viscosity of the paste made by adding water to cement powder becomes excessive, the pot life is shortened, and workability is deteriorated. On the other hand, in JP-A No. 6
There is a method of slowing down the reaction rate by adding a setting retarder, as described in Japanese Patent No. 4-12358.

According to this method, since it has the effect of lowering the heat of hydration, it is also possible to avoid the problem of abnormally rising solidification temperature due to reaction promotion.

[Problem to be solved by the invention]

In the above-mentioned conventional technology, in the method of using cement (for example, blast furnace cement) with a small amount of components that react with sodium sulfate to produce cement bacillus, excessive heat generation caused by the hardening accelerating effect of sodium sulfate is not taken into account. First, there is a problem in that thermal distortion occurs due to an abnormal rise in the temperature of the solidified body, and cracks occur in the solidified body.

In addition, in the method of diluting the solidification material with artificial lightweight aggregate, etc., the filling ratio of waste packed into the solidification container is increased,
No consideration was given to reducing the amount of waste, and there was a problem in maintaining a predetermined solidified body strength.

Furthermore, the method of adding a setting retarder to the solidifying material does not take into account the negative effects of the retarder on the normal curing reaction of the solidifying material, and slight differences in the amount added may cause poor curing. There was a problem in that the physical properties of the solidified product deteriorated due to the rising of breathing water (surface layer separated water) and waste.

The purpose of the present invention is to prevent a decrease in the amount of waste filling and a decrease in the physical properties of the solidified material due to poor curing when solidifying concentrated waste liquid containing sodium sulfate as a main component using a hydraulic solidifying agent such as cement. method to suppress excessive heat generation of the solidified material,
The objective is to prevent the occurrence of cranks due to thermal distortion and ensure the integrity of the solidified body.

[Means to solve the problem]

The structure of the radioactive waste solidification method of the present invention for solving the above problems is that when exothermic radioactive waste is solidified using a hydraulic solidification material, it reacts with the solidification material and becomes endothermic. The solution is to add a substance that repels .

The above-mentioned solidifying material is a hydraulic inorganic solidifying material, and is particularly effective for solidifying materials containing calcium silicate as a main component. Examples include Portland cement (including early strength, very early strength, and moderate heat), blast furnace cement, silica cement, and fly ash cement.

Currently, the above wastes include concentrated waste liquid containing sodium sulfate as its main component or its dry powder, and in the future, used ion exchange resins, channel boxes, burner poison, and other exothermic metal wastes. Conceivable.

The substance added to the solidification material dissolves and undergoes a phase change in the mixed paste of solidification material and waste.

Suitable materials are those that cause adsorption or a reaction between the solidifying material (including kneading water) and the waste, and the entropy change accompanying the change is positive (that is, an endothermic change).

Further, in order to prevent dissipation due to boiling and evaporation of the kneading water as much as possible, a substance that causes such a change at a temperature of 100° C. or lower, preferably 80° C. or lower is suitable.

For those that utilize dissolution, consider kneading water as a solvent,
Urea (ΔH = 15 kJ/moQ), silver nitrate (MuH = 22.6 kJ/maQ), sodium nitrate (ΔH =
Nitrates such as 20 kJ/maQ, and ammonium salts such as ammonium chloride (ΔI (=15 kJ/moQ)) are suitable, but urea is optimal in consideration of interaction with the solidifying material.

For example, ice (melting, ΔH
=6kJ/moQ), acetaldehyde (evaporation, ΔH=
27.2 kJ/mon), alcohols, low melting point polymers, etc. are suitable. Ice is suitable because it is consumed in a water utilization reaction with the solidifying agent after the phase change.

The most suitable substances that utilize reactions are those that have an endothermic reaction with water.

[Effect]

The substances added to the solidifying agent are dissolved in the paste.

It causes either a phase change or an adsorption 2 reaction, and absorbs the surrounding heat. This action alleviates the acceleration of hardening due to the interaction between the solidifying material and the waste, and the excessive heat generation of the solidifying material during the initial stage of curing of the solidified material caused by heat generation from the waste, thereby reducing the internal temperature of the solidified material.

Therefore, it is possible to suppress the occurrence of cracks in the solidified body due to thermal strain, and the soundness of the solidified body can be ensured.

Since cement-based solidifying materials have low thermal conductivity, when the solidified body generates excessive heat as described above, the temperature difference between the center temperature of the solidified body and the surrounding temperature becomes large, and thermal cracks are likely to occur. Furthermore, the low thermal conductivity means that external cooling is less effective in preventing such thermal cracks. Therefore, in order to uniformly cool a medium such as a cement-based solidifying material, the present invention, which absorbs heat by utilizing an internal reaction, is effective.

The effect of the additive of the present invention will be explained below, taking as an example the case where concentrated sodium sulfate waste liquid is solidified using cement.

The main hydration reaction of cement can be written as equation (1).

CaS +3H20-+0.5C3S2H3+ 1.5
Ca (OH)2+ 120caQ/ g...(1) Here, Cs S represents tricalcium silicate, which is the main component of cement, and Cs52Hs represents its hydrate [(Note) C: CaO, S: Si abbreviation].

By the hydration reaction of formula (1), 12 per 1 g of C3S
I have a fever of 0 caD. In this case, the problem is (
1) The rate of water utilization reaction in equation 1), which is the factor that determines the maximum solidification temperature. Compared to the heat generation rate associated with the curing of ordinary cement, when mixed with concentrated sodium sulfate waste liquid,
The inventors' experiments have shown that the curing reaction of cement is accelerated and the rate of heat generation increases by 2 to 3 times. As a result, it was found that when a solidified body of 200Q scale is formed, the internal temperature of the solidified body rises to a maximum of 120° C., and fine racks are generated inside the solidified body.

When urea is used as an additive in the present invention, urea dissolves in kneading water in a paste made by adding water to cement, and absorbs an amount of heat corresponding to the standard enthalpy of solution.

...(2) Due to the reaction in equation (2), the heat generation rate during the initial stage of curing of the solidified body can be reduced by ~50% compared to the case without additives, and the internal temperature of the solidified body can be suppressed to a level where thermal cracks do not occur. becomes possible.

〔Example〕

Embodiments of the present invention will be described using FIGS. 1 to 5.

FIG. 1 is a flow diagram of a solidification method according to a first embodiment of the present invention.

The configuration of FIG. 1 is as follows: 1 is a concentrated waste liquid tank, 2 is an additive tank, 3 is a solidification material tank, 4 is a main kneader tank,
5 is a solidification container.

Next, the function shown in Figure 1 is as follows.
A predetermined amount of concentrated waste liquid containing about 20% O+ is supplied to the main mixer 4, and then a predetermined amount of dry cement is supplied from the solidifying agent tank 3 to the main mixer tank 4. A certain amount of the additive of the present invention is supplied from the additive tank 2 several minutes before the end of the mixing of the concentrated waste liquid and the solidification material, and after a predetermined mixing time, the solidification container 5
It is injected into a solid state.

In this case, suitable cements include Portland cement, blast furnace cement, silica cement, and fly ash cement, and additives that cause changes in the cement paste such as dissolution, phase change, adsorption, and reaction with cement components. , a substance with the property that the enthalpy change during the process is positive is selected. - Examples include inorganic salts such as urea, nitrates, and ammonium salts that absorb heat when dissolved in kneading water, lower alcohols that have a low boiling point (below 100℃) and absorb heat of vaporization, and low melting point (below 100℃) ) is suitable. Alternatively, a method of replacing part of the kneading water with cut ice is also possible. It is desirable that all of these additives cause the above-mentioned changes and absorb heat at temperatures below 100°C, preferably below 80°C.

Next, a specific example of this embodiment will be shown below. Table 1 shows the composition of solidified bodies suitable for carrying out the present invention.

Table 1 Here, the waste liquid is an aqueous solution containing 20% by weight of sodium sulfate. With this composition, the waste liquid/cement ratio is approximately 0.
It is 5.

A solidified body prepared with the composition shown in Table 1, a solidified body using the same waste liquid with a waste liquid/cement ratio of 20.5 without adding urea, and a solidified body with a water/cement ratio of 0.5 without adding sodium sulfate. A thermocouple was inserted into the center of the solidified body to measure the change in temperature at the center of the solidified body over time. The solidified volume was 200Q. The measurement results are shown in FIG. In Figure 2, curve 6 is a solidified product with the composition shown in Table 1, curve 7 is a solidified product with a waste liquid/cement ratio of 0.5 and no urea added, and curve 8 is a solidified product with a water/cement ratio of 0.5. shows the temperature change of each. Table 2 summarizes the results shown in Figure 2.

It was found that in the hydration reaction of Class 0 blast furnace cement, the hardening was clearly accelerated by the action of the waste liquid containing sodium sulfate as the main component, and the exothermic rate increased approximately 2.5 times. For this reason,
The temperature at the center of the solidified body rises to a maximum of 120°C, and at this time,
A minute thermal crank occurred. This rank becomes apparent when the solidified material comes into contact with water.

On the other hand, in the case of a solidified product containing urea, the urea gradually dissolves in the paste and absorbs the heat of dissolution from the surroundings, reducing the rate of heat generation, and the maximum center temperature of the solidified product only rises to 75°C. Ta. It was found that the decrease in compressive strength after hardening was slight, and the normal hardening reaction of cement was not inhibited. In this example, the thermal
No cracks were observed, and the solidified material remained sound even after immersion in water.

According to this embodiment, excessive heat generation due to the synergistic effect of the waste and the solidifying agent can be alleviated without reducing the amount of waste (waste liquid) filled or reducing the strength due to inhibition of the hardening reaction of the solidifying agent. This makes it possible to prevent cracks caused by thermal strain during the curing period of the solidified body, and to ensure the soundness of the solidified body.

FIG. 3 is a flow line of a solidification method according to a second embodiment of the present invention.

In the configuration shown in FIG. 3, 9 is a dryer tank, 10 is
The kneading water tank 11 is a solidifying material kneading machine tank, and the other symbols are the same as in FIG. 1.

This example is a solidification method suitable for drying and pulverizing the concentrated waste liquid and then kneading it with a cement-based solidifying material to form a solidified body.

The functions of FIG. 3 will be explained below.

In FIG. 3, the concentrated waste liquid is supplied from the concentrated waste liquid tank 1 to the dryer tank 9, and the concentrated waste liquid becomes a dry powder.

On the other hand, cement, which is a solidification material, is mixed into the solidification kneader tank 1.
1, dry cement supplied from the solidifying agent tank 3 and kneading water supplied from the kneading water tank 10 are pre-kneaded. At this time, the viscosity of the cement paste must be lower than usual. For this purpose, either the water/cement ratio is set to 0.4 or more, or a high performance water reducing agent containing β-naphthalene sulfonate as a main component is added.

A fixed amount of the cement paste thus prepared is supplied to the main kneader tank 4, and the dried powder of the concentrated waste liquid is gradually added thereto. The addition of the dry powder is stopped before the viscosity of the paste in the main kneader tank 4 exceeds the natural injection limit, and then a predetermined amount of the endothermic additive of the present invention is supplied from the additive tank 2. The paste kneaded for several minutes in the main kneader tank 4 is poured into a solidification container 5, and after curing becomes a solidified product.

In this example, since the waste liquid is supplied as a powder, it has the advantage that the amount of waste filled per solidified body can be more than twice as high as when adding liquid. A part of it dissolves in the mixing water and accelerates the hardening reaction of cement. For this reason, there was a problem in which the temperature of the solidified body rose too much due to excessive heat generation in the early stage of curing the solidified body.

Here, as in the first embodiment, by adding an additive such as urea that has a property of endothermic change in the paste, the temperature of the solidified body can be maintained in a range where thermal cracks do not occur.

According to this embodiment, in addition to the effects described in the first embodiment,
There is an advantage that the waste filling rate can be further increased.

FIG. 4 is a flow diagram of the solidification method according to the third embodiment of the present invention.6 In FIG. are the same.

This embodiment is a solidification method suitable for solidifying cut pieces of channel boxes and parable poisons, which are waste products of other heating elements, using a hydraulic inorganic solidification material.

Part of the exothermic metal waste in this example is radioactive,
It continues to emit constant decay heat over a long period of time. When solidifying with a hydraulic solidifying material, it is necessary to suppress the waste filling rate so that the temperature of the solidified body does not rise above 100°C, preferably above 80°C. In this case, the calorific value reaches its maximum between 20 hours after filling the solidifying material and the hydration reaction of the hydraulic hardening material is remarkable, and the temperature of the solidified material reaches 100°C.
There may be times when you bend down. In this case, voids and thermal cracks are generated inside the solidified body due to boiling of the kneading water, and the physical properties of the solidified body are significantly deteriorated.

In this example, the endothermic agent of the present invention is mixed into a hardening material paste such as cement, and the endothermic change of the endothermic agent is used to prevent an abnormal rise in the temperature of the solidified body at the initial stage of hardening agent injection. It is.

In FIG. 4, the solidification container 5 is filled with exothermic metal cut pieces 12 in advance. As for the solidifying material, a fixed amount of hydraulic hardening material powder is supplied from the solidifying material tank 3, and a fixed amount of kneading water is supplied from the kneading water tank 10.

The mixture is supplied to the main kneader tank 4 and thoroughly kneaded.

Thereafter, a predetermined amount of the endothermic agent is added from the additive tank 2 to the main kneader tank 4, and after a few minutes of mixing, it is poured into the solidification container 5 filled with the heat-generating metal cut pieces 12. At this time, in order to sufficiently inject the solidifying material paste into the gaps between the heat generating metal pieces filled in the solidifying container 5, it is desirable that the viscosity of the solidifying material paste be 3000 cp or less. For this purpose, β-
It is advisable to add a small amount of a high performance water reducing agent whose main component is naphthalene sulfonate.

According to this embodiment, even if the heat-generating metal cut pieces 12 are filled at a volumetric filling rate of about 50%, the temperature of the solidified body does not exceed 80°C, and therefore voids may occur due to boiling and evaporation of the kneading water, and thermal distortion may occur. It is possible to prevent the occurrence of thermal crank and maintain the integrity of the solidified material.

FIG. 5 is a flow diagram of a solidification method according to a fourth embodiment of the present invention.

In the configuration shown in FIG. 5, 14 is an ice shaver tank;
Other symbols are the same as in FIG. 1.3.4.

In this example, when exothermic radioactive waste is solidified using a hydraulic hardening material, an additive that absorbs heat through phase change, especially melting, is added to the solidifying material paste to alleviate excessive heat generation in the solidified material. This is a solidification method.

As shown in Figure 5, the basic process is Example 2 (Figure 3)
Similarly, the concentrated waste liquid is dried and powdered using the dryer tank 9, mixed with the solidifying material paste created in the solidifying material kneading machine tank 11 in the hot wire machine tank 4, and poured into the solidifying container 5. , solidify.

When ice is used as the heat absorbing material, shaved ice is created in the ice shaver tank 14 and supplied to the main kneader tank 4. in this case,
It is preferable to reduce the amount of kneading water by the same weight as the amount of added ice.6 Table 3 shows an example of a specific solidified body composition.

Table 3 In this example, the initial temperature of the solidifying material paste was set at 20 to 3
It is effective in reducing the temperature by 0℃. This is particularly effective when dealing with waste that generates excessive heat during crosstalk, such as when solidifying ion exchange resin. Ice has the advantage that it absorbs ambient heat, turns into water, and is consumed in a hydration reaction with the solidifying material, so that it does not have any adverse effect on the solidified material.

In addition to ice, low-melting point polymers and organic compounds are used as additives in the solidifying material paste that melt at temperatures below 100°C and have the property of internally cooling the solidified material by absorbing the heat of fusion. can. For example, polyethylene with a low degree of polymerization, polyvinyl alcohol, higher fatty acids such as maleic anhydride, and their esters, N-vinyl derivatives, β
-Vinylnaphthalene, etc. These organic compounds have an average endotherm of nearly 100 caQ/g upon melting, and can effectively lower the temperature of the solidified material.

When the low melting point organic substance of this example is used, when the temperature of the solidifying material paste gradually decreases after the maximum heat generation period, it solidifies again. In particular, polyethylene and polyvinyl alcohol form a water-repellent film and close the voids. The action of such additives provides a secondary effect of improving the water resistance of the solidifying material.

〔Effect of the invention〕

According to the present invention, excessive heat generation of the exothermic radioactive waste solidified body can be alleviated by the action of the endothermic agent added to the solidification material paste, so that the internal temperature of the solidified body is always maintained at 10%.
It has the effect of being able to maintain the temperature below 0°C, preferably below 80°C. Therefore, it is possible to prevent cracks due to thermal strain and voids due to evaporation of kneading water from forming in the solidified body, and the soundness of the solidified body can be ensured.

In addition to the above effects, a high waste filling rate can be achieved, and the solidified material produced has radiation resistance, high strength,
It has the effect of being able to maintain various properties such as chemical resistance.

[Brief explanation of the drawing]

FIG. 1 is a flow diagram of a solidification method according to a first embodiment of the present invention;
Fig. 2 is a comparative diagram showing the effect of the present invention as a change in the temperature at the center of the solidified body over time, Fig. 3 is a flow diagram of the solidification method of the second embodiment of the present invention, and Fig. 4 is a diagram of the solidification method according to the present invention. FIG. 5 is a flow diagram of the solidification method according to the fourth embodiment of the present invention. 1... Concentrated waste liquid tank, 2... Additive tank, 3.
...Solidifying agent tank, 4...Main kneader tank, 5...
Solidification container, 6... Solidified body of (cement + waste liquid + urea), 7... Solidified body of (cement + waste liquid), 8... Solidified body of cement only, 9... Dryer tank, 10 ...
Kneading water tank, 11...Kneading machine tank with solidification, 12.
・Heat-generating metal cut piece.

Claims (1)

[Claims] 1. When pyrogenic radioactive waste is solidified using a hydraulic solidifying material, a substance that reacts with the solidifying material and repels endothermic properties is added. A method of solidifying radioactive waste. 2. The solidification method of radioactive waste according to claim 1, wherein the radioactive waste is a concentrated waste liquid containing sodium sulfate as a main component or a dried powder thereof. 3. The method of solidifying radioactive waste according to claims 1 and 2, wherein the hydraulic solidifying material is Portland cement or blast furnace cement. 4. In the solidification method according to claims 1 to 3, the solidification material includes:
1. A method for solidifying radioactive waste, which comprises adding a substance that absorbs heat by dissolution, phase change, adsorption, or reaction with solidified body components at 100° C. or lower. 5. In the solidification method according to claims 1 to 3, the solidification material includes:
A method for solidifying radioactive waste characterized by adding urea. 6. In the solidification method according to claims 1 to 3, the solidification material includes:
A method for solidifying radioactive waste characterized by adding shaved ice. 7. In the solidification method according to claims 1 to 3, the solidification material includes:
A method for solidifying radioactive waste characterized by adding an organic compound with a low melting point. 8. The solidification method of radioactive waste according to claim 7, wherein the organic compound added to the solidification material is a polymer with a low degree of polymerization. 9. The solidification method of radioactive waste according to claim 8, wherein the polymer added to the solidification material is polyethylene.