JPH0938831A - Remote-control underwater cutting method and remote-control underwater electric discharge machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a remote cutting for replacing a structure in a nuclear reactor. SOLUTION: A discharge current is supplied to an electrode 1 of an underwater electric discharge machine actuator 4 via a twist-type electricity supply cable 5a and a capacitor and an object is cut under electric discharge machining conditions of discharge time 700-900μs, out-of-discharge time 100-300μs, peak current 150-200A, thereby making it possible to apply an electric discharge machining to an underwater remote cutting operation in a preventive maintenance technique relative to replacing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a remote control underwater cutting method and an apparatus for carrying out the method, and more particularly to underwater cutting of a relevant part in a preventive maintenance technique such as replacement of reactor internal structure in a nuclear power plant. It relates to electrical discharge machining in pure water to achieve it.

[0002]

2. Description of the Related Art There are various preventive maintenance techniques for nuclear reactor internal structures in a nuclear power plant, but it is difficult to secure a sufficient working space in the nuclear reactor. Also, since the atmosphere dose is high, it is necessary to carry out the replacement work with the reactor filled with water. Moreover, since the water depth of the lower structure of the nuclear reactor is about 25 m, it is necessary to adopt the remote control cutting work which requires high technology.

The surrounding environment in the nuclear reactor will be described with reference to FIGS. The inside of the reactor pressure vessel is composed of a reactor pressure vessel 40 forming an outer vessel, a shroud 41 forming an inner vessel, and a baffle plate 42, and various structures are arranged therein. . Further, the baffle plate 42 is provided with an access hole cover (AHC) 43 for an operator to access during the construction of the plant.

For this reason, the preventive maintenance techniques up to now are difficult to cope with, and there are the following problems in order to realize the replacement work of the reactor internal structure.

(1) It is necessary to cut / remove the object in water having a depth of about 25 m.

(2) It is necessary to develop a remote control underwater cutting device that can be moved to a narrow space in the reactor pressure vessel, and can be installed at a work site in water at a depth of 25 m and easily operated remotely. is there.

(3) In order to cope with the above (1), it is conceivable to adopt electric discharge machining technology, but the application conditions (in oil / power supply cable, etc.) are significantly different from general electric discharge machining technology, Furthermore, since the equipment required for electrical discharge machining must be able to move in a narrow space inside the reactor and to sit with high accuracy in the lower part of the reactor, it is necessary to use the electrical discharge machining technology for underwater Cutting is practically impossible to adopt.

(4) Regarding (3) above, there is no record of electric discharge machining in pure water, and the environment is inconvenient for conventional electric discharge machining. It is extremely difficult to establish.

(5) From the above (3) and (4), even if the conventional electric discharge machining is adopted for the cutting work of the reactor internal structure, the cutting work takes a long time, and the replacement work related to this cutting It has a big impact on the whole.

This is the same in the preventive maintenance technology of the general structure in the nuclear reactor, and the situation is extremely difficult to deal with.

[0011]

As described above, conventionally, the cutting technique in the preventive maintenance technique related to the replacement of the internal structure of the reactor has been preferred from the viewpoint of the special environmental conditions in the reactor and the underwater operation by remote control. Was not established.

An object of the present invention is to provide a remote control underwater cutting method and apparatus effective for working in a narrow space in a nuclear reactor and in pure water having a water depth of 25 m.

Another object of the present invention is to provide an electric discharge machining method and apparatus as a machining technique for realizing underwater cutting by remote control, and particularly to underwater electric discharge machining under a special environmental condition of a water depth of 25 m / pure water. The object is to establish a suitable electric discharge machining method and apparatus, support preventive maintenance related to replacement of structures in a nuclear reactor, and improve reliability of a nuclear power plant.

[0014]

The present invention provides a discharge time of 7
00-900μs, discharge pause time 100-300μ
s, a cutting method and device for electric discharge machining an object to cut at high speed by repeatedly generating short-cycle electric discharge in water between an electric discharge machining actuator and an object to be cut under an electric discharge condition of 150 to 200 A Is.

In order to cut the object by electric discharge machining in pure water with a water depth of 25 m, a power source for electric discharge machining is installed on the floor of the building, and the electric discharge machining actuator is made to penetrate to the position of the object to be cut in water. A discharge condition is optimized by supplying power to the underwater electric discharge machining actuator from the electric power source for electric discharge machining through a twist type power supply cable and making the rise of the discharge current steep by a capacitor.

[0016]

Function: Discharge time: 700-900 μs, discharge rest time: 100-300 μs, peak current: 150-200 A
The electric discharge that is repeatedly generated in water in a short cycle between the electric discharge machining actuator and the object to be cut under the electric discharge condition of No. 6 cuts the object at high speed.

The twist type power supply cable reduces the impedance from the electric power source for electric discharge machining to the underwater electric discharge machining actuator, and the capacitor supplements the rise of the discharge current to improve the current waveform.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A remote control underwater cutting method and apparatus according to the present invention improves general electric discharge machining technology so that it can be applied to work in a facility environment of a nuclear power plant, and further In reactor water (pure water) / water depth 25 m / narrow space, etc.]
In consideration of the above, by establishing a new electrical discharge machining condition, an electrical circuit configuration, and an actuator for underwater electrical discharge machining that is accessible to the object to be cut, a remote control to support preventive maintenance of the structure in the reactor An underwater cutting method and device.

1 to 7, a remote control underwater cutting method and apparatus suitable for an access hole cover (AHC) preventive maintenance technique will be described as an embodiment of the present invention.

FIG. 1 shows the principle of electric discharge machining in liquid. The remote control cutting method in this embodiment is a method for cutting and removing the AHC installed in the lower part of the reactor pressure vessel, and it is a remote work in a narrow space in water with a depth of 25 m. Therefore, the underwater electric discharge machining technology is adopted.

In submerged electric discharge machining, an intermittent spark discharge 3 is continuously generated by energizing a workpiece 2 through a power supply cable and an electrode 1 from a discharge power source, and the spark discharge is eroded. This is an electrical processing method for scraping the workpiece 2 into a desired shape by the action. In this electric discharge machining, basically, five kinds of phenomena as shown in FIG. 1 are repeated and the machining progresses.

In general electric discharge machining, mineral oil, which is most suitable for electric discharge machining, is used as the machining liquid, and the power supply cable for supplying the discharge current is within 3 m so as not to deteriorate the electrical characteristics. The short one is adopted and machining is performed under the most suitable conditions for electric discharge machining.

However, since the underwater electric discharge machining method and apparatus applied to the remote control underwater cutting according to the present invention is an electric discharge machining under special environmental conditions applied to the preventive maintenance technology of the structure in the nuclear reactor, it is generally used. Unlike the electric discharge machining technology adopted in the mold machining and machining of precision machine parts, which is currently undertaken, the new conditions described below must be overcome.

(1) In order to apply it to the repair of the internal structure of a nuclear reactor, the working fluid is electric discharge machining in reactor water (pure water) instead of mineral oil.

(2) Due to environmental restrictions of the nuclear power plant, from the EDM power source temporarily placed on the 6th floor of the reactor building to the underwater EDM actuator installed near the AHC in the lower part of the reactor pressure vessel. The length of the power supply cable is about 40 m or more, which is 10 times or more the length of the power supply cable used for general electric discharge machining.

(3) The electric resistance increases due to the length of the power supply cable, and the electric discharge machining performance is significantly reduced.

(4) The electric discharge machining performance is remarkably lowered due to the head pressure corresponding to a water depth of 25 m.

The remote-controlled underwater cutting method and apparatus according to the present invention overcomes such inconvenient conditions for electrical discharge machining by improving the apparatus configuration and electrical discharge machining conditions (parameters) as described below, and the electrical discharge machining is performed. Making processing applicable to preventive maintenance technology.

FIG. 2 shows a system configuration of an underwater electric discharge machining apparatus for performing electric discharge machining applied to remote control underwater cutting according to the present invention. This electric discharge machine is configured such that an electric discharge machining actuator 4 is seated on a workpiece 2 to be cut in a reactor pressure vessel, and an electric discharge machine power supply device 6 temporarily placed on the sixth floor of a reactor building is used. The actuator 4 is supplied with power via the power supply cable 5a and the control cable 5b to generate an electric discharge between the electrode 1 and the workpiece 2. Further, a pump 7 and a filter 8 for sucking and collecting sludge generated in water during electric discharge machining through a suction port 1a formed in the electrode 1 are installed on the 6th floor of the reactor building.

3 to 5 specifically show the underwater electric discharge machining actuator 4, FIG. 3 is a front view thereof, FIG. 4 is a plan view thereof, and FIG. 5 is a side view thereof.

This underwater electric discharge machining actuator 4 hangs down the narrow part in the reactor to obtain the baffle plate 42.
It is configured to be seated on the upper portion of the AHC 43 which is attached by welding at two locations (a and a) on the circumference of the. The actuator 4 is provided with a cylindrical electrode 1 for cutting the AHC welded portion (c) shown in FIG. 10, and the overall shape is a substantially cylindrical shape. Then, a frame portion 11 constituting a skeleton of the underwater electric discharge machining actuator 4, a stepping motor 12 for vertically moving the electrode 1, a ball screw 13 for converting rotation of the stepping motor 12 into vertical movement of the electrode, Support plate 1 for supporting the electrode 1
4. The leg portion 15 supporting the entire actuator is provided.

Further, this underwater electric discharge machining actuator 4
Is equipped with a hanging metal fitting 16 for moving the narrow portion to the relevant portion at a water depth of 25 m while suspending and hanging the narrow portion, and further, in order to cope with the case where the relevant portion is not flat, A screw slide mechanism 17 is provided at the lower end of the leg 15 so that the height can be adjusted. The underwater electric discharge machining actuator 4 is configured to have a height of 350 mm or less and a diameter of 760 mm or less so that it can pass through the narrow space.

Next, in order to realize the electric discharge machining suitable for the remote control underwater cutting according to the present invention, the following improvements were carried out with respect to the problems in the conventional electric discharge machining technique, and the optimum machining conditions were established. The cutting speed has been improved.

There are the following three main factors for improving the cutting speed by electric discharge machining.

(1) Increase the peak current value during discharge.

(2) The rounding of the discharge current waveform (pulse waveform) is improved.

(3) Establish optimum electrical discharge machining conditions.

When a single wire power supply cable used in general electric discharge machining is adopted in this underwater electric discharge machine,
The peak current value at the time of discharge is about 130 A, and the current interruption period becomes long. Therefore, in order to reduce an increase in electrical resistance such as impedance due to an increase in resistance and inductance of the power supply cable due to the extremely long length of the power supply cable, one independent power supply cable is provided for each electrode, Furthermore, by adopting a twist type power supply cable in which a plurality of power supply cables are twisted at a pitch of 300 mm or less, it is possible to suppress the increase in impedance and increase the peak current value to 200A.

Further, the power feeding cable is extremely long (40
Therefore, in order to prevent the rising of the rectangular wave of the pulse current supplied at the time of discharge from reaching the peak current and the prolongation of the intermittent period of the discharge, the proximity of the electrodes should be avoided. By installing a capacitor with an optimum capacity (22.4 μF) and compensating for the delay in the rise of the pulse current with the current that instantaneously flows from the capacitor, it was possible to shorten the time until disconnection.

FIG. 6 shows a single-wire power supply cable used for general electric discharge machining and shows a current waveform before adding a capacitor. FIG. 7 shows a twist type power supply cable according to the present invention. And the current waveform after the addition of the capacitor is shown.

Next, the optimum electric discharge machining conditions in pure water having a water depth of about 25 m in the underwater electric discharge machining method of this embodiment will be described.

The optimum electric discharge machining conditions for the electric discharge machining according to the present invention are, as shown in FIG. 7, that the discharge time is the minimum time until the current value reaches the peak value, and the next discharge is stabilized even in the discharge pause time. The number of repeated discharges per unit time can be increased by shortening the number of discharges per unit time.

Table 1 below shows the electric discharge machining conditions in the prior art and the electric discharge machining conditions according to the present invention.

[0044]

[Table 1]

The discharge time is 1500 to 1800 μ
s is shortened to 700 to 900 μs, 200 to 400 μs is set to 100 to 300 μs for the discharge rest time, and 100 to 130 A is increased to 150 to 200 A for the peak value of the discharge current. It was confirmed that the cutting speed can be improved from 1.5 to 1.7 mm / Hr to 4.1 to 4.5 mm / Hr.

As described above, by establishing the underwater electric discharge machining technique suitable for the remote control underwater cutting method in water at a water depth of 25 m, it is possible to apply the electric discharge machining cutting to preventive maintenance of the reactor internal structure. By establishing the optimum electric discharge machining conditions, the reduction of the cutting machining speed in pure water can be remarkably improved, and the safety and reliability of the nuclear power plant can be greatly contributed.

[0047]

According to the present invention, the following effects can be obtained.

(1) By adopting the electric discharge machining technology, the machining actuator that penetrates into the cutting work position can be made compact, and remote control (operation) for cutting machining in a narrow space such as in a nuclear reactor. Became possible.

(2) By adopting a twist type power supply cable and a capacitor, the discharge characteristics in a special environment such as a long power supply cable and a water depth of 25 m / in pure water are improved, and it becomes possible to apply the electric discharge machining technology. .

(3) In combination with the above item (2), the cutting speed can be improved by establishing the optimum electric discharge machining conditions in the conditions that are a very special environment in electric discharge machining.

(4) By improving the cutting speed, the cost of preventive maintenance measures for replacement can be significantly reduced.

(5) From the above items (1) to (4), it becomes possible to apply electric discharge machining to remote underwater cutting of many structures in a nuclear reactor.

(6) When all of the above are combined, the reliability and economical efficiency of the preventive maintenance technology for the entire reactor internal structure are improved, and the safety of the nuclear power plant can be greatly contributed.

[Brief description of drawings]

FIG. 1 is a phenomenon explanatory diagram showing the principle of electric discharge machining in liquid.

FIG. 2 is a system configuration diagram of an underwater electric discharge machining apparatus for performing electric discharge machining applied to remote control underwater cutting according to the present invention.

FIG. 3 is a front view of an underwater electric discharge machining actuator that can be placed in the underwater electric discharge machining apparatus according to the present invention.

FIG. 4 is a plan view of an underwater electric discharge machining actuator that can be placed in the underwater electric discharge machining apparatus according to the present invention.

FIG. 5 is a side view of an underwater electric discharge machining actuator that can be placed in the underwater electric discharge machining apparatus according to the present invention.

FIG. 6 is an electric discharge waveform diagram in an electric discharge machine constituted by a conventional technique.

FIG. 7 is a discharge waveform diagram in the remote control underwater electric discharge machining apparatus according to the present invention.

FIG. 8 is a perspective view showing an internal structure of a reactor pressure vessel.

FIG. 9 is a cross-sectional plan view of the lower portion of the reactor pressure vessel.

FIG. 10 is a vertical sectional side view showing a mounting state of an access hole cover.

[Explanation of symbols]

 1 Electrode 2 Workpiece 4 Underwater EDM Actuator 5a Power Supply Cable 5b Control Cable 6 Power Supply Device for EDM

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takashi Sakurai 3-1-1, Saiwaicho, Hitachi, Ibaraki Hitachi Ltd. Hitachi factory (72) Inventor Noriaki Goto 3-chome, Saiwaicho, Hitachi, Ibaraki No. 1 Hitachi Ltd. Hitachi factory

Claims (10)

[Claims] 1. A remote control underwater cutting method for cutting a structure in a plant in water, wherein a discharge time is 700 to 900 μs, and a discharge pause time is 100.
A remote control underwater cutting method characterized by cutting an object by electric discharge machining with a peak current of 150 to 200 A for 300 μs. 2. The remote controlled underwater cutting method according to claim 1, wherein the structure inside the nuclear reactor is used as a cutting target, and the target is cut by electrical discharge machining in pure water having a water depth of 25 m. 3. The object to be cut according to claim 2,
A remote control underwater cutting method characterized in that the manhole cover is fixed to the lower part of the reactor pressure vessel by welding. 4. The electric discharge machining according to claim 2 or 3, wherein an electric power source for electric discharge machining is installed on a floor of a reactor building,
A remote control underwater cutting method characterized in that the underwater electric discharge machining actuator is made to penetrate to the position of a cutting target and power is supplied to the underwater electric discharge machining actuator from a power supply for electric discharge machining via a power supply cable. 5. A remote-controlled underwater cutting apparatus for cutting a structure in water in a plant, wherein a discharge time is 700 to 900 μs, and a discharge pause time is 100.
A remote control underwater cutting device characterized in that power is supplied to an underwater electric discharge machining actuator under a discharge condition of up to 300 μs and a peak current of 150 to 200 A. 6. The underwater electric discharge machining actuator according to claim 5, wherein the structure inside the nuclear reactor is an object to be cut.
A remote control underwater cutting device, which cuts the object by electrical discharge machining in pure water having a water depth of 25 m. 7. The object to be cut according to claim 5,
A remote controlled underwater cutting device, which is a manhole cover fixed to the lower part of the reactor pressure vessel by welding. 8. The electric power source for electric discharge machining according to claim 5 or 6, wherein an electric power source for electric discharge machining is installed on a floor of a reactor building, an underwater electric discharge machining actuator is made to penetrate to a position of an object to be cut, and a power supply cable is used. The remote control underwater cutting device is characterized in that power is supplied to the underwater electric discharge machining actuator from the device. 9. The remote control underwater cutting device according to claim 8, wherein the power feeding cable is a twist type cable. 10. A remote control underwater electric discharge machining apparatus according to claim 5, wherein the underwater electric discharge machining actuator is provided with a capacitor for assisting a discharge current.