JPH1048395A - Method for collecting fine particle in cutting time of in-pile structural material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a simple ventilation system which requires no replacement work by passing bubbles made in the cutting time of an in-vessel structural material through a metal mesh to subdivide them and then letting them ascent in the water. SOLUTION: An in-vessel structural material 2 is submerged in a pool 3 and is cut by a plasma cutter 1. On this occasion, an Ar gas emitted into the water becomes bubbles containing metal fine particles generated in the cutting work. Accordingly, a metal mesh 4 is installed in a diffusion preventing cover 5, and the diameter of a bubble is shortened by lessening the size of the mesh. The smaller bubbles ascent in the water, whereby it take only a shorter time for the fine particles to move to the surface of the bubbles to increase the quantity of moving fine particles per time, and improve the efficiency in the collection of them in the water. Consequently, a required collecting efficiency can be obtained by setting the diameter of a bubble and the depth of the water. Moreover, the other metal fine particles emitted into the air are sucked through the cover 5, and then the air is fed to an exhaust stack 8 by an air pump 6. Therefore, a simple ventilation system where it is unnecessary to replace a filter can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear reactor, and more particularly, to a method for minimizing the amount of fine metal particles released into the air during a cutting operation in water to reduce the volume of structural material inside the reactor.

[0002]

2. Description of the Related Art The conventional technology is "Reactor dismantling and aerosol" (Yoshio Ikezawa: Aerosol Research, 5 (3), 212-2).
16 (1990)), it is performed in water to reduce scattering of radioactive metal fine particles, and the atmosphere containing the fine particles released from the water is sucked using a local exhaust device to collect the radioactive fine particles. After being released to the environment.

[0003]

In the above-mentioned prior art, the generated highly radioactive metal fine particles are released from water into the air in a working room, and then processed in a ventilation system. Since the metal fine particles generated at this time have a particle size of submicron or less, it is necessary to use a filter such as HEPA or ULPA which has a high collection efficiency, and also requires replacement work at regular intervals due to an increase in pressure loss. .

An object of the present invention is to provide a simple ventilation system that does not require replacement work or the like.

[0005]

The above object is achieved by allowing bubbles formed at the time of cutting to pass through a grid-like metal mesh to fragment the bubbles, and then rising in the pool water.

[0006] During the plasma cutting operation, the carrier gas rises in the state of bubbles containing metal fine particles in the carrier gas.

The fine particles in the bubbles move to the surface of the bubbles by the action of diffusion, inertia, gravitational sedimentation, etc., and then move into water and are removed.

[0008] By reducing the bubble diameter, the time required for the particles to move to the bubble surface is shortened, the amount of particles transferred per hour is increased, and the trapping efficiency is increased.

[0009]

Embodiments of the present invention will be described below. FIG. 1 shows a ventilation system for cutting a structural material, which comprises a plasma cutter 1, a structural material in a furnace 2, a metal mesh 3, a cover for preventing diffusion 4, an air pump 5, a working room 6, a building 7, and an exhaust pipe 8. You. 20mm thick SUS304 carried out of the reactor,
The in-furnace structural material 2 having a width of 1 m is submerged in a pool having a depth of 4 m in the work room 7. Plasma cutting machine 1 for underwater furnace structural materials
Cut horizontally by remote control of. After flowing Ar gas at a flow rate of 100 l / min from a nozzle having a diameter of 1 cm, plasma is supplied at a supply voltage of 30 kW to generate a plasma arc. The nozzle of the cutter is set at a distance of 5 cm from the surface of the structural material 1, and the cutter is moved at 500 mm / min to cut. At this time, the Ar gas released into the water becomes bubbles containing metal fine particles generated at the time of cutting, passes through the metal mesh 3 of 2 mm square attached to the diffusion prevention cover 4 and has a diameter of about 2 times the mesh size. It breaks into bubbles of .4cm and rises in the water. In order to suppress the diffusion of the metal fine particles released from the water into the air into the work chamber 6, the metal fine particles are sucked through the diffusion prevention cover 4, then sucked by the air pump 5, sent to the exhaust pipe 8, and discharged to the environment. . The mass concentration of the fine particles having a particle diameter of 20 nm generated by the cutting decreased from 19.5 g / cm ³ to 3 × 10 to ⁴ g / cm ³ while rising 3 m in the pool water. Particle removal rate DF in pool water at this time
The calculated value of (= 1 / collection efficiency) is 64600, which is about 67 times larger than the case where bubbles are not subdivided.
The target collection efficiency of radioactive fine particles in the work of cutting the structural material inside the furnace is set at 99.99% or more, and it has been found that the required collection efficiency can be obtained.

The mesh size is 1.5 mm square,
Assuming that the bubble diameter is 0.3 cm, the calculated value of DF is 2.1 × 10 ⁷
And increases by 21649 times. It was found that the DF was increased by 323 times by reducing the bubble diameter by 25%, and the DF was greatly increased by reducing the bubble diameter. Even when the cutting position was reduced to 2 m and the depth was reduced to 2 m, the calculated value of DF was 62100, indicating that the target collection efficiency could be obtained.

From the above, it was confirmed from the present example that metal fine particles could be easily collected.

[0012]

According to the present invention, radioactive metal fine particles generated at the time of cutting can be collected by dividing air bubbles, so that the ventilation system in the work room can be simplified.

Further, the cutting operation at a shallow water depth becomes possible, and the degree of freedom of the operation is increased.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of one embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Plasma cutting device, 2 ... Furnace structural material, 3 ... Metal mesh, 4 ... Diffusion prevention cover, 5 ... Air pump, 6 ... Work room, 7 ... Building, 8 ... Exhaust cylinder.

Claims (2)

[Claims] 1. A method for collecting fine particles at the time of cutting a structural material in a furnace, characterized in that bubbles in the water are cut during the cutting operation of the structural material in the furnace. 2. The method according to claim 1, wherein fine particles are cut at the time of cutting the in-furnace structural material using a grid-like metal mesh for subdividing the air bubbles.