GB2281809A - Process and apparatus for cutting up by induction elements of a nuclear installation - Google Patents

Abstract

The induction is generated by the passage of an electric current in an inductor 2 having a flat induction coil positioned facing the part of the element 1 to be cut up. The cutting operation is under remote control and the time for the cutting-up of a melting pot is reduced to 76 hours compared to cutting by reciprocating saw. <IMAGE>

Description

PROCESS AND APPARATUS FOR CUTTING UP BY INDUCTION ELEMENTS OF A NUCLEAR INSTALLATION DESCRIPTION The present invention relates to the cutting up by induction of elements used in the construction of a nuclear installation.

It more particularly applies to the cutting up of melting pots.

The dismantling of nuclear installations is a problem which more particularly preoccupies the authorities responsible for nuclear safety and protection, together with operators of nuclear power plants. For several decades various dismantling means have been used and experience has shown that it is necessary to improve the existing dismantling equipments or create new equipments.

Therefore numerous countries have started up research and development programmes in this connection.

As nuclear installations are of different types and sizes, it is necessary to have a large variety of means for performing dismantling operations. The latter are generally performed by cutting up by means of one of the numerous existing thermal, electrothermal, pyrotechnic, mechanical and similar processes. A particular cutting up process is chosen as a function of numerous criteria, namely the effectiveness of cutting up, its costs, performance constraints, the level of noxious substances emitted and the material to be cut up.

It is known to carry out cutting by means of an abrasive wheel, cable and disk, reciprocating saw, circular saw, high pressure water jet with or without an abrasive, as well as drilling cutting tools. The size and weight of these tools makes it difficult to remotely manipulate them.

Certain components of nuclear installations are particularly difficult to cut up, which can be due to the nature of the material from which they are formed, their thickness and their state. This is particularly the case with iron, chromium and nickel alloy melting pots such as INCONEL 601, which have a thickness of about 10 mm. In addition, spent melting pots have a residual glass deposit on their inner wall which increases the cutting up problems. This makes it impossible at present to cut up such pots. Tests carried out with a reciprocating saw have revealed that approximately two months are needed for cutting up such a pot.

Thermal cutting processes are generally very efficient and make it possible to cut up considerable steel thicknesses. However, they generate considerable quantities of clinker, smoke and aerosols, so that they must be combined with ventilating, filtering and confinement means.

FR-A-2 667 533 also discloses a process making it possible to separate irradiated fuels from their metal sheath. It consists of the induction heating of the metal sheath in order to bring about the melting of a small portion of the sheath and propagate the melting action so as to create a slit. The induction is generated by the passage of an electric current in a solenoid surrounding the metal sheath. This process makes it possible to correctly separate irradiated fuels from their sheath without contaminating the said fuels by the molten metal.

FR-A-2 080 952 discloses a process for the treatment of irradiated ceramic nuclear fuel cartridges. The metal sheath surrounding the fuel undergoes local heating by the induction generated by a coil placed around the cartridge.

When a part of the cartridge located in the induction coil is exposed to an appropriate high frequency magnetic field, the metal sheath breaks.

These induction cutting processes are particularly suitable for the cutting up of relatively small diameter tubes, because they require the installation of tubes in the solenoids. They cannot be used for the cutting up of voluminous, cumbersome parts having a random shape.

In another field, namely that of welding, induction heating devices are known permitting the preheating of the parts to be welded. For this purpose use is made of an inductor incorporating a flat coil into which is passed an electric current. Heating is obtained by placing the coil in the vicinity of the parts to be welded.

In order to bring about a rapid cutting up of parts which are particularly resistant to mechanical cutting, such as spent melting pots, the inventor of the present invention had the idea of cutting up said parts by induction, which is not conventionally generated from a solenoid surrounding the part, but from an inductor of the type simply used for heating parts to be welded.

Therefore the invention relates to a process for cutting up by induction an element used in the construction of a nuclear installation with a view to its dismantling, the induction being generated by the passage of an electric current in an induction coil, characterized in that the induction is produced by a flat coil positioned facing the part of the element to be cut up.

The invention also relates to an apparatus for cutting up by induction an element used in the construction of a nuclear installation with a view to its dismantling, incorporating an induction coil and means for the electrical connection of each end of said coil to an electric power supply, characterized in that the coil is a flat coil.

To bring about greater efficiency, the coil preferably has two turns.

Also with a view to obtaining greater efficiency, the coil is preferably elongated.

The apparatus advantageously comprises means for cooling the coil by the circulation of a cooling fluid.

The coil can be hollow so as to permit the passage of the cooling fluid between its two ends.

The electrical connection means can be formed by two parts per coil end, one part used for the electrical connection to said coil end and one part used for the circulation of the cooling fluid.

They can comprise two assemblies, each assembly being constituted by a first, tubular electrical conductor connected by one of its ends to one end of the coil so as to ensure the continuity of the cooling circuit and by its other end to a cooling fluid circulation duct and a second electrical conductor connected to the electric power supply and in electric and thermal contact with the first conductor.

The two assemblies can be fixed to one another by fastening means between the second conductors with the interposing of an insulator between these second conductors.

The invention is described in greater detail hereinafter with reference to a non-limitative embodiment and the attached drawings, wherein show: Fig. 1 A cutting equipment provided with an inductor according to the invention positioned at the end of a telemanipulator arm.

Fig. 2 An inductor according to the invention insulated from the equipment with which it is associated.

Fig. 3 The circulation of current in the inductor coil at a given instant.

Fig. 4 The currents induced in the part to be cut up when the current in the coil flows in the direction shown in Fig. 3.

Fig. 5 The thermal effect generated in the part to be cut up by a two-turn inductor according to the invention.

Fig. 6 The thermal effect generated in the part to be cut up by a single-turn inductor according to the invention.

Fig. 1 shows a cutting equipment according to the invention ready for cutting up an element 1 used in the construction of a nuclear installation, said element 1 e.g.

being a melting pot. The inductor 2, shown in isolation in Fig. 2, is supported by a transformer fixed to the end of a telemanipulator arm 3. Apart from the said transformer, said end of the arm 3 supports the electrical connections and the cooling water circuit of the inductor, the assembly carrying the overall reference 4.

As is shown in Fig. 1, cutting up takes place within a dismantling cell 5 provided with a table 6 used as a support for the element to be cut up. One of the walls 7 defining the cell 5 maintains the telemanipulator 8 and is provided with an inspection window 9 permitting the observation of the part to be cut up. A plurality of cables 10 passing through the wall 7 carries the supply current and cooling water from the generator 11 located in the control zone of the apparatus up to the inductor 2.

For example, the equipment can comprise an ELVA generator of the Maximinac type having a maximum output power of 50 kW. This apparatus is conventionally used during hard and soft soldering, forging of small parts, surface hardening and preheating. It is connected to the three-phase mains, 380 V, 50/60 Hz. The mains voltage is firstly rectified and then converted in an inverter in order to obtain an alternating current with a frequency between 10 and 30 kHz. This alternating current is appropriately matched to the inductor coil. The generator output is connected to the inductor coil by means of a transformer mechanically fixed to the telemanipulator arm end.

Fig. 2 is a detail of the inductor 2, which is obtained by assembling and welding several elements. It comprises a flat coil 20 which, in the embodiment shown here, has two turns. The coil is hollow to permit the circulation of a cooling fluid. The ends of the coil 20 are connected by welding to hollow, conducting branches 21, 22. The connection between-the coil 20 and the branches 21, 22 takes place in such a way as to ensure not only the electrical connection, but also the continuity of the cooling circuit between the end 23 of the branch 21 and the end 24 of the branch 22. Fast connections ensure the hydraulic connection between the ends 23, 24 and the connections of the cooling circuit carried by the telemanipulator arm end.

The electrical connection with the secondary of the transformer carried by the end of the telemanipulator arm takes place via electric terminals 25, 26, which are two plates bent at right angles. The first portions 27, 28 of these plates are fixed by screwing to the output terminals of the transformer so as to ensure an electrical and mechanical connection of the inductor. The second portions 29, 30 are firmly fixed to one another by an insulating screw system, e.g. made from Teflon, accompanied by the interposing of an insulating plate 31. The branches 21, 22 are respectively welded to the terminals 25, 26 so as to ensure the electrical continuity up to the terminal 20.

Figs. 3 and 4 should be considered together. Fig. 3 shows the flow of alternating current through the two-turn coil of the inductor at a given instant, the coil being positioned facing and at a limited distance from the element 1 to be cut up. In regions facing the coil turns in the element 1 are produced induced currents circulating in the manner shown in Fig. 4.

In order to illustrate the thermal effect generated in the element 1 to be cut up, Fig. 5 is a diagram or graph of the temperature T to which is exposed that part of the element facing the operating coil 20 for a cross-section like that indicated by the arrows in Fig. 3. It can be seen that the turns of the coil used have sections of different shapes. The outer turn has a rectangular section, whose sides parallel to the element 1 are longer than the sides perpendicular to said element 1. The inner turn has a rectangular section, whose sides parallel to the element 1 are shorter than the sides perpendicular to said element 1. Around the turns of the coil 20 is represented the magnetic induction produced by the current flowing through the coil.

The diagram of Fig. 5 shows the evolution of the temperature in the element 1 as a result of the induced currents. The latter passes through a maximum in the longitudinal axis of the coil and then drops rapidly on either side of said maximum and with small rises level with the outer turn. It is clear that the elongated shape given to the coil favours an accumulation of thermal energy along the longitudinal axis of said coil.

In the heated zones of the element 1, the induction produces forces which are exerted in directions indicated in Fig. 5. In the zone facing the longitudinal axis of the coil is created a force F, which presses back the molten metal in order to initiate the formation of a slit which is to be extended. The forces f1 and f2 due to the external turn avoid any rise of the molten metal on the sides. This leads to a relatively precise cutting up of the element 1.

A single-turn coil does not bring about such a result.

This is illustrated by the coil 40, which only has one turn and for which the temperature curve is wider.

All the elements constituting the inductor are preferably made from copper.

Tests performed have shown that the invention has a definite interest. They were carried out for a frequency of the voltage applied to the coil of approximately 12 kHz.

The continuous output power was approximately 46.2 kW. The generator supplied to the primary of the transformer located at the end of the telemanipulator arm an a.c. voltage of 420 V peak-to-peak for an intensity of the current of 110 A peak-to-peak. The voltage at the terminals of the inductor was max 40 V for an intensity between 1500 and 6000 A. The coil was formed by a 7 x 10 mm copper tube with a thickness of 1 mm. The cooling consisted of a water flow of 12 1/mien for a pressure of 5 bars.

Under these conditions a melting pot was cut up in 76 hours instead of 2 months with a reciprocating saw and this included the necessary manipulations. No problem is caused by the glass stuck on the interior of the pot.

Claims (9)

1. Process for cutting by induction an element used in the construction of a nuclear installation with a view to the dismantling thereof, the induction being generated by the passage of an electric current in an induction coil, wherein the induction is produced by a flat coil positioned facing that part of the element which is to be cut up.

2. Apparatus for cutting by induction an element used in the construction of a nuclear installation with a view to its dismantling, incorporating an induction coil and means for the electrical connection of each end of said coil to an electric power supply, wherein the coil is a flat coil.

3. Apparatus according to claim 2, wherein the coil has two turns.

4. Apparatus according to claim 2, wherein the coil is elongated.

5. Apparatus according to claim 2, wherein means for cooling the coil by the circulation of a cooling fluid are provided.

6. Apparatus according to claim 5, wherein the coil is hollow in order to permit the passage of the cooling fluid between its two ends.

7. Apparatus according to claim 6, wherein the electrical connection means are constituted by two portions per coil end, one portion used for the electrical connection to said coil end and one portion used for the circulation of the cooling fluid.

8. Apparatus according to claim 7, wherein the electrical connection means incorporate two assemblies, each assembly being constituted by a first, tubular electrical conductor and connected by one of its ends to one end of the coil ensuring the continuity of the cooling circuit and by its other end to a cooling fluid circulation duct and a second electrical conductor connected to the electric power supply and in electrical and thermal contact with the first conductor.

9. Apparatus according to claim 8, wherein the two assemblies are fixed to one another by fastening means between the second conductors with the interposing of an insulator between said second conductors.