JPS6252277B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for thermally decomposing waste ammonium nitrate containing radioactive residues, and more specifically, the present invention provides a method for thermally decomposing waste ammonium nitrate containing radioactive residues, and more specifically, a method for thermally decomposing waste liquid containing radioactive residues, and in particular for safely thermally decomposing it and converting it into nitrogen monoxide NO and nitrogen dioxide.
This invention relates to a thermal decomposition treatment method for ammonium nitrate waste liquid containing radioactive residue, which minimizes the generation of NO ₂ and minimizes the radioactive residue after thermal decomposition, making the treatment simple and easy.

The components of ammonium nitrate waste fluid generated from the nuclear fuel reprocessing process consist of water, ammonium nitrate, and insoluble residue.The insoluble residue contains radioactive materials, and the amount of radioactive materials has been increasing recently, so it should be reduced as much as possible. Therefore, there is a need for a processing method that allows safe storage. On the other hand, ammonium nitrate in the ammonium nitrate waste liquid is thermally decomposed,
It is known that the ammonium nitrate salt is released as a harmless gas, and if this thermal decomposition method is adopted, it is naturally thought that it is more beneficial than storing all the ammonium nitrate salts in the ammonium nitrate waste liquid containing radioactive residues as a solid.

By the way, conventional methods for thermally decomposing ammonium nitrate waste liquid containing radioactive residues include liquid film decomposition, spray pyrolysis, and fluidized bed pyrolysis. In the liquid film decomposition method, the waste liquid flows down as a film on the inner surface of a reaction tube, and the reaction tube is heated from the outside to 200℃ using a heater.
It is thermally decomposed by heating to 300℃.
Although safety can be ensured by controlling the amount of waste liquid retained in the reaction tube by controlling the flow rate, radioactive residue that does not decompose will adhere to the inner surface of the reaction tube.
It is necessary to periodically clean the inner surface of the reaction tube, which increases the amount of waste and requires treatment of the cleaning water. This method also has the disadvantage that a large amount of recombined ammonium nitrate is generated.

In the spray pyrolysis method, as shown in Japanese Patent Publication No. 52-22751, waste liquid is sprayed into a primary decomposition furnace maintained at 180°C to 210°C, and then nitrous oxide in the pyrolysis gas is removed.
N ₂ O is thermally decomposed and rendered harmless in a secondary decomposition furnace kept at 600℃ to 1000℃, and because the primary decomposition temperature is kept low, the amount of NO, NO _2, etc. generated is small; Similar to the liquid film decomposition method, it is necessary to dispose of radioactive residues attached to the inner surface of the reaction tube, and there is a problem that waste disposal is difficult.

As shown in Japanese Unexamined Patent Publication No. 52-7369, the fluidized pyrolysis method involves thermally decomposing waste liquid by spraying it into a fluidized bed maintained at 180℃ to 290℃, which reduces the amount of NO, NO ₂ , etc. generated. However, since the fluidized bed also flows into the gas treatment system, it not only increases the size of the device, but also has the disadvantage that the fluidized particles also become radioactive waste, resulting in a large amount of waste. That is, the conventional thermal decomposition method of ammonium nitrate waste liquid is not a preferable thermal decomposition method from the viewpoint of radioactivity management and waste disposal when radioactive residue is present.

The present invention improves the conventional technology as described above, and makes it possible to easily manage radioactive residue after thermal decomposition and treat waste, while minimizing the generation of nitrogen monoxide NO, nitrogen dioxide NO ₂ , etc. However, it is an object of the present invention to provide a thermal decomposition method for ammonium nitrate waste liquid containing radioactive residues that can be carried out safely.

In order to achieve the above objects, the present invention has the following features:
The ammonium nitrate waste liquid containing radioactive residue is put into a processing container, and it is thermally decomposed in a primary decomposition furnace while controlling the decomposition temperature between 250℃ and 310℃, and the recombined ammonium nitrate generated by the primary thermal decomposition is heated to 350℃. Re-decomposition is carried out in a secondary decomposition furnace heated to 600 °C to 600 °C, and the two-stage decomposition allows the decomposition to occur safely and while suppressing the generation of harmful gases, and the radioactive residue after thermal decomposition is removed only into the processing container. It is characterized by making it easier to dispose of radioactive waste by leaving a residue.

The present invention will be explained in more detail below.

Typical thermal decomposition reactions of ammonium nitrate are shown in reaction formulas (1) to (4) below.

(1) NH ₄ NO ₃ NH ₃ +HNO ₃ (2) NH ₄ NO ₃ →N ₂ O+2H ₂ O (3) NH ₄ NO ₃ →N ₂ +2H ₂ O+1/2O ₂ (4) 2NH ₄ NO ₃ →N ₂ +2NO+4H ₂ O The decomposition model differs greatly at about 310℃,
Below 310°C, reactions (1) and (2) are the main reactions, but above that, reactions (3) and (4) are the main reactions. reaction (1),
Compared to (2), reactions (3) and (4) are both exothermic reactions,
Because the reaction rate is fast and difficult to control, there is a risk of explosion, and the decomposition produces harmful nitrogen monoxide NO and nitrogen dioxide NO ₂ as gases.

At temperatures below about 310℃, the endothermic reaction of reaction (1) and reaction (2) occur.
Since the exothermic reaction and the exothermic reaction occur simultaneously, an equilibrium temperature exists, and the temperature can be easily controlled without runaway. However, in thermal decomposition at 310° C. or lower, ammonium nitrate decomposed (dissociated) in reaction (1) recombines in the atmosphere, so decomposition can be completed completely.

Here, the present invention safely performs the primary thermal decomposition reaction by suppressing the decomposition temperature within the primary decomposition furnace to 250°C to 310°C, and completely removes the recombined ammonium nitrate generated in the primary decomposition furnace at 350°C to 600°C. be decomposed into

The decomposition in the primary decomposition furnace is performed while the ammonium nitrate waste liquid containing radioactive residue is kept in the processing container, so that the radioactive residue after pyrolysis exists only in the processing container,
This reduces the volume of radioactive residue (waste) after thermal decomposition to a minimum, and facilitates its recovery and treatment.

In the present invention, the primary decomposition temperature is set at 250°C to 310°C.
The reason why the secondary decomposition temperature was set at 350°C to 600°C is as follows.

In other words, in order to decompose safely and quickly through primary decomposition, the decomposition temperature must be equal to or higher than the decomposition temperature of equations (1) and (2) above (approximately 210°C or higher), and the decomposition temperature of equations (3) and (4) above should be higher than (about
310℃ or below), but low temperature (approximately 210℃~
(250℃), the decomposition rate is slow and the amount of undecomposed ammonium nitrate increases, so the primary decomposition temperature is set at 250℃ to 310℃.
And so.

In secondary decomposition, the decomposition rate is slow at low temperatures (below 350°C), and the metal materials used in this type of equipment (equipment and piping, etc.) (e.g., hot-rolled stainless steel sheets, seamless nickel iron for heat exchangers) The strength of alloy pipes) decreases at high temperatures compared to room temperature, especially at 600°C.
℃ or higher, so the secondary decomposition temperature is
The temperature was 350°C to 600°C.

When the treatment method of the present invention is used, primary decomposition can be carried out safely because the decomposition temperature in the primary decomposition furnace is relatively low, and only the recombined residual ammonium nitrate generated in the primary decomposition furnace is completely removed in the secondary decomposition furnace. Since the generation of nitrogen monoxide NO and nitrogen dioxide NO ₂ is suppressed to a small amount only by the secondary decomposition furnace, the volume of radioactive residue (waste) after thermal decomposition is reduced to a minimum as mentioned above. Since it exists only in the processing container in a processed state, its collection and other processing can be performed very simply and easily.

Next, an example of the implementation of the present invention will be specifically described with reference to the accompanying drawings.

FIG. 1 is a processing system diagram according to the present invention. Reference numeral 1 denotes a primary decomposition furnace, in which a processing container housing chamber surrounded by a heat insulating material is formed, and a processing container 7 containing waste liquid is placed therein. Further, a heater 11 is arranged around the processing container storage chamber. Further, in this primary cracking furnace, a space 12 is formed above the storage chamber. 2 is a secondary cracking furnace, 3 is a cooling air intake, 4 is a harmful gas collection device (for example, an alkaline solution spray cleaning device), 5 is a dehumidifier, and 6 is an exhaust fan.

A sample (components: water 80% by weight, ammonium nitrate 12% by weight, virtual ash content Al(OH) ₃ and Fe(OH) ₃ each 4% by weight) was placed in the waste liquid treatment container 7 (φ100mm x H150mm), and the sample was poured into the upper part. The dimensions of space 12 are W300mm x L300
It is installed in the primary decomposition furnace 1 with a size of mm
The temperature was controlled at ℃. The temperature inside the waste liquid treatment container during heating changed as shown in FIG. In Figure 2, A shows the water evaporation process and B shows the ammonium nitrate decomposition process, which took about 15 minutes.

The decomposition gas generated from inside the waste liquid treatment container is
After passing through the upper space 12 of the primary cracking furnace, it is led to the secondary cracking furnace 2.

The secondary cracking furnace (dimensions: φ50 mm x L 300 mm) has a set temperature of 400°C to 420°C, and decomposes the recombined ammonium nitrate generated in the primary cracking furnace 1. The amount of recombined ammonium nitrate at the outlet of the primary cracking furnace is 30 to 40% by weight (processing amount: 60 g).
The amount of undecomposed ammonium nitrate at the outlet of the secondary cracking furnace was reduced to 1 to 2% by weight. In addition, the cracked gas composition at the outlet of the secondary cracking furnace during the ammonium nitrate decomposition process is H ₂ O: 42.7%,
NO ₁₊₂ : 1.9% Other cracked gases (N ₂ , O ₂ ,
_N2O ): 55.4%. Also, no explosion occurred during the decomposition.

The decomposed gas leaving the secondary decomposition furnace 2 was cooled with outside air as shown in Fig. 1, and then led to a harmful gas collection device 4 where it was washed with alkaline water, and further water was removed with a dehumidifier 5. It was then vented to atmosphere.

After the experiment, processing container 7 installed in the primary decomposition furnace
When the sample was taken out and the residue was analyzed, it was found that the residue was only virtual ash and no ammonium nitrate was observed, and it was easy to remove the residue from the waste liquid treatment container.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing a thermal decomposition treatment method for ammonium nitrate waste liquid containing radioactive residues according to the present invention;
FIG. 2 is a diagram showing temperature changes inside the processing container. 1...Primary cracking furnace, 2...Secondary cracking furnace, 3...
Cooling air intake, 4... Harmful gas collection device, 5...
... Dehumidifier, 6 ... Exhaust fan, 7 ... Waste liquid processing container, 11 ... Heater, A ... Moisture evaporation process, B ...
...Ammonium nitrate decomposition process.

Claims (1)

[Claims] 1. Put ammonium nitrate waste liquid containing radioactive residue into a processing container, and thermally decompose it in a primary decomposition furnace while controlling the decomposition temperature between 250°C and 310°C.
The recombined ammonium nitrate generated in the primary decomposition furnace is re-decomposed in a secondary decomposition furnace heated to 350°C to 600°C, and the two-stage decomposition is used to decompose safely and while suppressing the generation of harmful gases, A method for thermal decomposition treatment of ammonium nitrate waste liquid containing radioactive residues, characterized in that the radioactive residues after thermal decomposition are left only in the treatment container.