JPS5815080B2 - Solidification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention reduces the amount of surfactant added when solidifying radioactive concentrated waste liquid containing sodium sulfate with asphalt, and achieves solidification of asphalt that is homogeneous and stable in water and can be scaled. Regarding the method.

In nuclear power plants, especially boiling water nuclear power plants, the steam generated in the reactor is sent to a turbine through a dryer to generate electricity. At this time, the steam drain water discharged from the turbine is sent to a condenser where it is condensed. It becomes water and is recycled again as reactor water.

In addition to radioactive substances, this condensate contains what is called a type of rust-like crud (mainly composed of iron compounds) generated from piping or equipment, and in order to remove these,
A condensate desalination equipment using ion exchange resin is installed, and the ion exchange resin after desalination is regenerated with sulfuric acid and caustic soda.

The waste liquid after regeneration contains 2 to 5% by weight of sulfuric acid, which is produced by mixing the two, and about 1.
Contains about 2% by weight of metal compounds such as iron, cobalt, and nickel, and salt.

Since this regenerated waste liquid is frequently generated and contains a lot of waste, it is necessary to treat it effectively, and it is generally treated by solidification or the like.

During solidification, in order to minimize the amount of final solidified material, the recycled waste liquid is first reduced to a solid content of approximately 1 in an ordinary evaporator.
It is concentrated to about 5 to 20% by weight.

The main component of this radioactive concentrated waste liquid is sodium sulfate, and its pH value is within the range of about 9-12.

Solidification was conventionally done with cement, but in order to remove excess water from the solidified material and produce a more homogeneous solidified material, asphalt was developed in which concentrated radioactive waste liquid was dispersed in heated and melted asphalt to evaporate the water. A solidification method has been developed.

To solidify asphalt, concentrated waste liquid at about 60°C and asphalt at about 140°C are supplied to an evaporator heated to about 230 to 245°C by a heating medium, and uniform dispersion, mixing, and heating are performed at the same time. Re.

As a result, excess moisture is removed by evaporation, sodium sulfate and other solids are dispersed in the asphalt, and the asphalt containing solids at about 170°C is extracted from the bottom of the evaporator and stored in a container such as a drum. It is cooled and solidified.

The solidified asphalt obtained in this way is disposed of by ocean dumping, storage on land, or burial, but since the solidified material contains radioactive materials, its uniformity and safety are especially important. required.

In this case, asphalt solidification is used to disperse the solid content, which is mainly composed of sodium sulfate in the concentrated waste liquid, into the solidified material in an anhydrous state, but in order to achieve more uniform dispersion, a considerable amount of surfactant is added. If the amount of this surfactant is insufficient, crystals of sodium sulfate will adhere to the walls of the evaporator and the stirrer, making long-term stable operation difficult.

Further, when the pH value of the concentrated waste liquid is high, the amount of surfactant may not necessarily exhibit a surfactant effect corresponding to the amount.

Therefore, in consideration of such cases, the amount is generally more than about 10% by weight based on the solid content in the concentrated waste liquid.

However, since surfactants are added dissolved or dispersed in water, as the surfactant increases, the volume of concentrated waste liquid increases.
Therefore, the amount of evaporated liquid in the evaporator also increases, which is undesirable.

In addition, after solidification, sodium sulfate, which is dispersed in an anhydrous state in the solidified asphalt, absorbs water when immersed in water, obtains crystallization water, and gradually expands in volume, and the solidified material It gradually becomes porous and loses stability.

The purpose of the present invention is to reduce the amount of surfactant added when solidifying radioactive concentrated waste liquid containing sulfuric acid with asphalt, thereby achieving stable operation and obtaining a homogeneous solidified asphalt that is stable in water. An object of the present invention is to provide a solidification method.

To summarize the present invention, the solidification method of the present invention involves adding calcium nitrate to the waste liquid and adjusting its pH value to approximately 4.5-8.5
The method is characterized in that the amount of surfactant is adjusted within the range of 3 to 10% by weight based on the solid content of the waste liquid.

As mentioned above, radioactive concentrated waste liquid containing sodium sulfate as a main component (hereinafter referred to as concentrated waste liquid) has a pH value of about 9 to 12.
When solidifying this with asphalt, it is not uniformly dispersed in the asphalt as it is, so it is necessary to add a considerable amount of surfactant.

As a result of our studies focusing on this point, the present inventors have found that by lowering the pH value of the concentrated waste liquid, we can significantly reduce the amount of surfactant added and perform stable operation. The present invention was achieved by discovering that a stable asphalt solidified body can be formed by using the same method.

That is, according to the present invention, calcium nitrate is added to the concentrated waste liquid, and its pH value is lowered to about 4.5 to 8.5, and about 3 to 10% by weight is added to the solid waste in the waste liquid. By adding the relevant surfactant, stable asphalt solidification operation can be performed.

If the pH value of the concentrated waste liquid falls below this range, there will be problems of corrosion of the evaporator and other equipment piping, and water quality will deteriorate.

Furthermore, if the pH value of the concentrated waste liquid is above this range, the effect of the surfactant of about 10% by weight or less will not be sufficiently exhibited.

Although it is desirable for operation to reduce the amount of surfactant added as much as possible, the amount can be reduced to about 3 weight coefficients by lowering the pH value.

The desirable pH value range is 5 to 7, and correspondingly, the appropriate amount of surfactant to be added is about 3.5 to 5.5% by weight.

Examples of the pH adjuster in the present invention include inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, and phosphoric acid.

Organic acids such as acetic acid are undesirable from a corrosion standpoint.

By the way, the present inventors have discovered that calcium nitrate is effective as a pH adjuster, and that by adding it to the concentrated waste liquid, some of the sulfate ions of sodium sulfate in the concentrated waste liquid can be converted into calcium salts, that is, calcium sulfate. It was confirmed that the solidified material could be stabilized by changing its shape.

From the viewpoint of simply adjusting the pH of concentrated waste liquid, the use of water-soluble calcium compounds other than calcium nitrate is also considered, examples of which include calcium chloride, hydroxide, and acetate; Salts, acetates, etc. are undesirable from the viewpoint of corrosion of equipment, and chlorides in particular do not have the effect of reducing the pH value.

In addition, in the case of hydroxide, the liquid becomes alkaline and the pH of the waste liquid decreases.
Since the H value is further increased, the solidified material tends to swell, which is undesirable, and the same applies to alkali metal hydroxides.

In this respect, calcium nitrate has high solubility in water and is most suitable from the viewpoint of corrosion and stability of solidified material.

The amount of calcium nitrate compound added is about 2 to 5 sulfate ions in the concentrated waste liquid, based on the weight of calcium ions.
% is sufficient, thereby reducing the pH value of the concentrated waste liquid to approximately 8
．． If it can be kept below 5.

By simply adding a calcium nitrate compound, the addition rate of surfactant can be reduced, and if necessary, the pH
Further, an inorganic acid or the like can be added for adjustment.

Even if all of the sulfate ions in the waste liquid are combined with calcium ions, there will be no problem with solidification itself, but if the calcium ions are increased more than necessary, the amount of solidified substances will increase and the solids content rate will be sacrificed. Undesirable.

The addition of the inorganic acid and/or calcium nitrate compound to the waste liquid is preferably carried out in a feed tank that supplies the concentrated waste liquid to the solidification evaporator, thereby achieving a uniform dispersion in the asphalt. I can do it.

Solidification according to the present invention uses a known evaporator, and supplies concentrated waste liquid with pH adjusted with an inorganic acid and/or calcium nitrate compound and a surfactant added thereto together with asphalt. This can be easily achieved by solidifying the mixture while evaporating it, storing it in a suitable container (usually a drum), and cooling it.

At this time, the solid content concentration in the concentrated waste liquid is 15 to 20% by weight, whereas the density of the solidified material containing radioactive materials using asphalt of the same amount or more than the solid content contained is Approximately 1.2 to 1.4, penetration is 30 to 6
It is desirable that the solid content is about 0 and the solid content mixing rate is about 30 to 40% by weight.

Next, the present invention will be explained using comparative examples and actual cases, but the present invention is not limited by these in any way.

Comparative Example An aqueous solution was prepared by adding sulfuric acid and caustic soda to a test aqueous solution containing 18% by weight of sodium sulfate to change the pH value of the aqueous solution.

A surfactant was further added to this aqueous solution in varying amounts.

Each aqueous solution obtained in this manner and at 140°C
Straight asphalt heated to (penetration 40/60
) into a model plant evaporator (
The mixture was heated to about 235° C.) and the water was evaporated while mixing.

Note that the straight asphalt was 1.5 times (by weight) the solid content in the concentrated waste liquid.

FIG. 1 shows the results of operating conditions depending on changes in the pH value of the aqueous solution and the surfactant addition rate (solid content (weight percentage relative to solid content)).

In addition, in the figure, ○ marks are good, Δ marks are slightly defective, and × marks are defective (
Deposition and adhesion of sodium sulfate to the evaporator and generation of friction noise).

The graph also shows the boundary line between the good operation area and the poor operation area.

From the graph in Figure 1, the pH value of the aqueous solution is approximately 4.5 to 8.
It can be seen that by suppressing the amount to about 5, the operating condition becomes good and the addition rate of the surfactant can be reduced to about 3 to 10% by weight.

Example: Concentrated waste liquid containing 15% by weight of sodium sulfate was added to an aqueous calcium nitrate solution (calcium ions were added to
(equivalent to 5% by weight) was added to the concentrated waste liquid, and the pH value of the concentrated waste liquid was varied to prepare aqueous solutions, and to this concentrated waste liquid, surfactants were further added in varying amounts.

Asphalt solidification operation was carried out in the same manner as in the comparative example using each of the concentrated waste liquids thus obtained.

Figure 2 shows the results of the operating conditions as a result of changes in the pH value of the concentrated waste liquid and the surfactant addition rate.

1. Note that the O, Δ, and X marks and graphs in the figure have the same meanings as in FIG. 1.

From the graph in Figure 2, the pH value of the aqueous solution is about 4.5-8.
It can be seen that by controlling the amount to about 5, the operating conditions become good and the addition rate of the surfactant can be reduced to about 3 to 10% by weight.

Furthermore, as is clear from the comparison between FIG. 1 and FIG. 2, according to the present invention, the range of good operation at pH 8.5 is expanded compared to the comparative example, so it is less susceptible to fluctuations in the amount and concentration of concentrated waste liquid. On the other hand, it is easy to manage.

When the solids of Comparative Examples and Examples at pH 8.5 were immersed in water, the solids of Comparative Examples gradually became porous, but the solids of Examples were stable.

As described above, by adjusting the pH value of the concentrated waste liquid according to the present invention, it is possible to reduce the amount of surfactant added, and to perform stable asphalt solidification operation to obtain a high-quality solidified asphalt. I can do it.

Further, by using a water-soluble calcium compound such as calcium nitrate as a conditioning agent, there is an advantage that a more homogeneous and stable asphald solidified body in water can be obtained.

[Brief explanation of the drawing]

Figures 1 and 2 are graphs showing the influence of the relationship between the pH value of the test aqueous solution and concentrated waste liquid and the addition rate of surfactant on the operating state of asphalt solidification, and Figure 1 is a comparison. EXAMPLE FIG. 2 is a graph of an embodiment of the present invention.

Claims (1)

[Claims] 1. When solidifying an alkaline radioactive concentrated waste liquid containing sodium sulfate as a main component with asphalt, calcium nitrate is added to the waste liquid and its pH value is adjusted to about 4.
5 to 8.5 and adding a surfactant in an amount of about 3 to 10% by weight based on the solid content in the waste liquid. 2. The solidification method according to claim 1, wherein the pH value of the waste liquid is adjusted with an inorganic acid. 3. The solidification method according to claim 1, wherein the pH value of the waste liquid is adjusted with calcium nitrate. 4. The solidification method according to any one of claims 1 to 3, wherein calcium nitrate is added in an amount of about 2 to 5% based on the weight of calcium ions to the sulfate ions in the waste liquid.