FR2667533A1 - Method for cutting up a metal tube by induction, and application to the separation of irradiated fuels from their metal sheath - Google Patents

Abstract

The invention makes it possible to cut a metal tube up by pointwise melting brought about by an inductor. It consists mainly in heating the tube (10) by means of an inductor (4) in order to give rise to the melting of a weak point of the tube (2), thus giving rise to a slit (12) at the ends of which the accumulation of current density causes a permanent rise in the temperature in order to widen the slit (12) thus created. The start point may be brought about by a hole (2) made beforehand in the tube (10). Application to the separation of sheaths from bars of irradiated nuclear fuels.

Description

METHOD FOR CUTTING A METAL TUBE BY INDUCTION
AND APPLICATION TO THE SEPARATION OF IRRADIATED FUELS
OF THEIR METALLIC SHEATH
DESCRIPTION
FIELD OF THE INVENTION
The invention generally relates to
the opening of metal tubes, and in particular
separation of bars or stacks of pellets
nuclear fuel irradiated from the cladding
metal surrounding them.

PRIOR ART
In nuclear reactors, and in
in particular, fast neutron reactors, the
fissile material is in the form of pellets
most often consisting of a mixed oxide
uranium (U02) and plutonium (Pu02). These pastilles
are stacked inside a metal sheath
to form a nuclear fuel needle.

A large number of these needles are then assembled
in bundles in generally section sleeves
hexagonal, to form fuel assemblies
nuclear. Once used, the beam must be
dismantled and the fuel must be reprocessed.

The first step in reprocessing
irradiated fuels from reactors
nuclear consists of separating the fuel from its
metallic sheath. Indeed, it is always better
to separate nuclear fuel material from its
protective sheath before treating the fuel
nuclear irradiated, so as to separate the products
of nuclear fuel material fission.

Several processes are currently
used. It is common practice to shear
mechanically assemblies of fuel in short sections and then cause the preferential dissolution of the combustible material.

This type of process requires complex and slow mechanical shears. In addition, its maintenance is problematic because this shear is exposed to radioactive conditions and the shearing causes a release of dust which contaminates the region surrounding the shears and which must be eliminated.

 It is also possible to put the sheath in contact with an embrittlement agent reacting with the metal of the sheath to cause its embrittlement, then to disintegrate the sheath.

 One can also consider removing the sheath by melting the metal sheath, but the separation between the liquid metal and the pellets of solid fuels is very delicate.

 On the other hand, it is known to cut metal sheets and tubes using a thermal device, such as a blowtorch. This kind of device makes it possible to cause a very strong and very localized rise in temperature so as to melt the metal part. The melting point then being moved, this causes the part to be cut. The flame of a torch does not allow the cladding of nuclear fuel cladding, since the latter are almost in contact behind the cladding.

 The object of the invention is to remedy these drawbacks by proposing a method for cutting a metal tube, applicable to the cutting of a metal sheath surrounding a stack of pellets of irradiated nuclear fuels.

SUMMARY OF THE INVENTION
A first main object of the invention is therefore a method for cutting a metal tube, of the type consisting in cutting the tube by fusion very quickly and very localized on a cutting line.

 The main embodiment provides that the fusion of the tube is carried out by heating the tube by induction with a sufficient current so that the current density at the weak point or around the weak point causes, by a very localized rise in temperature, the fusion of the metal tube at the weak point or around the weak point, the rest of the tube remaining at a temperature much lower than the melting temperature. Cutting continues by moving the melting point along the cutting line to create a slit.

 According to one aspect of the invention, a mechanically weak point in the tube is created beforehand.

 The induction is produced by an inductor which can be either mobile to move the melting point on the fixed tube, or fixed. The tube can then be movable to move the melting point along the cutting line on the tube. This melting point can also be displaced by a natural progression of the two ends of the slit, thanks to the accumulation of induced currents.

 The weak point in the tube may consist of a hole.

 It can also consist of a small piece of metal of a different nature from that of the tube, to form an alloy whose melting point is lower than that of the metal of the tube.

 The treatment of spent fuel is the subject of the following main applications.

 A first application consists in using the process previously summarized to separate a nuclear fuel rod from its metallic cladding which surrounds it.

 The second application of the method according to the invention consists in separating all the bars of a bundle of nuclear fuels from the claddings which each surround a bar. In this case, the process continues by moving the entire beam of the bar in the electromagnetic field created by a large diameter inductor, so as to cut all the bars simultaneously.

LIST OF FIGURES
The invention and its various characteristics will be better understood on reading the description which follows, appended to several figures representing, respectively - FIG. 1, an explanatory diagram relating to the birth
of a melting point created by induction
in the method according to the invention; - Figure 2, an explanatory diagram relating to displacement
from the melting point of Figure 1; - Figure 3, an explanatory diagram relating to a phenomenon
loopback inductive that may occur
in a metal tube; - Figure 4, a diagram showing a tube to be cut,
surrounded along its entire length by
inductor; - Figure 5, a diagram representing a tube to be cut
moving in the middle of an inductor
short and fixed; and - Figure 6, a fuel assembly placed at
inside a large inductor
diameter.

DETAILED DESCRIPTION OF AN IMPLEMENTATION
OF THE INVENTION
Like cutting sheets or tubes using a blowtorch, the method according to the invention consists in creating a very rapid and very localized fusion on the tube and in moving this melting point according to the chosen cutting line.

 Referring to Figure 1, the fusion is created according to the invention by induction.

 The first phase of the process consists in previously creating a weak point in the tube to be cut. This weak point can be a hole, but can also be a crack, or a thinning of the thickness of the tube. In general, this point must cause a very localized increase in the current density in order to be able to melt well before the rest of the tube, during a rise in temperature. Indeed, the main thing is to be able to exploit for a determined period of time the fact that the tube can have a place in the molten state without the rest of the tube melting, so as to initiate the cutting by fusion.

 The rise in temperature of the tube is caused by induction. The inductor 4 is placed around the tube 10 to be cut, preferably coaxially. The inductor is generally made in the form of a solenoid 4, several turns of which surround the tube 10. When it is traversed by a sufficiently strong current, the inductor 4 causes the birth of electric current inside of the tube 10. The horizontal arrows drawn on this tube 10 indicate the direction of the current, the direction of the latter depending on the direction of the current inside the inductor 4.

 In the case shown in Figure 1, the tube 10 and the inductor 4 have been shown in a vertical position. The induced currents flowing through the tube 10 are therefore horizontal.

 The presence of the hole 2 or of a crack causes an increase in the current density around the hole 2. When this current density is sufficient, the melting temperature of the metal of the tube 10 can be reached in two very localized zones around the hole 2. In fact, the induced current lines being horizontal, the two opposite points 6 and 8 of the hole 2, disposed respectively at the top and at the bottom thereof, undergo the increase in current density because the induced current flowing horizontally at the height of hole 2 is deflected by the latter and is forced to pass either below or above it. The two upper points upper 6 and lower 8 are therefore subject to an accumulation of induced currents. If these are sufficiently intense, there is a fusion of these two points upper 6 and lower 8, while the rest of the tube is keeps at a lower temperature.

 The intensity of the current flowing in the inductor 4 is therefore determined so that these two upper 6 and lower 8 points are in fusion, while the rest of the tube is maintained at a temperature below the melting temperature of the metal constituting the tube. 10.

 Furthermore, as shown in FIG. 2, if an induced current circulation is caused at different heights of the tube 10, and close to the starting point of the fusion, in this case the initial hole 2, the upper points 6 and lower 8 tend to move away from the hole going up and down respectively, always because of the accumulation of induced currents above and below the hole 2. A vertical slit 12 then appears on the surface of the tube 10.

 The presence of induced current at different heights of the tube 10 can be caused by a simple vertical displacement of the inductor 4.

 It is also possible to create the start of the cutting of the tube 10 by the simple fact of heating the latter until a first point of the tube begins to melt. To facilitate the fusion of this first point, it is possible to place in contact with the tube 10 a small piece of metal, of a different nature, forming with the metaL of the tube an alloy having a melting point lower than the melting point of the metal constituting the tube. This small piece of metal can be integrated into the tube, during its manufacture. It is also possible to cause a local thinning of the tube 10 to cause the weak point.

 The frequency of the induced alternating current must be chosen so that the penetration depth of the induced currents in the tube 10 is just greater than or equal to the thickness of the tube 10. In fact, a too low frequency leads to a depth of too high penetration and poor electrical and thermal efficiency.

 As shown in Figure 3, too high a frequency leads to too low a penetration depth. There is then a loopback of the currents induced in the very thickness of the tube 10.

In other words, the induced currents form a closed circuit with two paths in opposite directions 14 and 15 in the same thickness of the tube 10. The induced currents then do not go around the circumference of the tube 10 and a hole 16 or a breach in the tube 10 will not cause an accumulation of current density at the edge of this hole 16.

 Several ways to move the initial melting point are possible to obtain a slot for cutting a tube.

 In fact, with reference to FIG. 4, it is possible to place the tube 10 inside an inductor 20 continuously surrounding the tube 10 to be cut. The progression of the melting point is then done by the simple fact that the upper and lower points, marked respectively 6 and 8 in Figure 1, cause by their fusion the displacement of the lines of induced currents, this along the generator on which is the initial point of fusion. By therefore placing the weak point, in this case a hole 2, at the bottom of the tube 10, a slot 12 will therefore appear both downward and upward relative to the hole 2, along the generatrix of the tube. Whatever the location of the starting point of the fusion, the tube 10 can always be cut. The evacuation of the molten metal can be done by the fact that the tube can be arranged horizontally, the drops of molten metal being collected by a container or an object, placed inside the tube in the case where the slit is on the upper part of the tube, or placed outside the latter in the case where the cutting slit is placed in the lower part of the tube.

 With reference to FIG. 5, it is possible to use a short inductor 22. In this case, it is necessary to create a relative displacement of the tube 10 relative to the inductor 22 in order to be able to advance the melting point and to create a slot 12 along a generatrix of the tube 10. The latter can therefore be fixed and the inductor 22 mobile.

It is also possible to have a fixed inductor 22 and to move the tube 10 inside of it, as shown in FIG. 5.

 Used fuels from nuclear reactors generally come in two forms.

 The first consists of insulated tubes containing the fuel pellets. The insulated tubes, like those of FIG. 5, can be cut by a simple scrolling in the electromagnetic field created by the solenoidal inductor 22, traversed by a current of selected frequency.

One can for example cut in this way a zircaloy tube 10 mm in diameter by scrolling at the speed of 10 cm / s in an inductor supplied at a frequency of 100 KHz, with a power of 7 kW.

 As shown in Figure 6, the second consists of an assembly containing several tubes separated by spacing grids 34 and held by end caps 36 and foot 38. It is possible to simultaneously open all the tubes 30 of the 'assembly by scrolling this entire assembly in the field created by the long inductor 32 of very large diameter. Indeed, the tubes 30 placed inside the assembly are also bathed by the electromagnetic field created by the inductor 32. The fact that the field is a little weaker inside simply entails an opening time a little longer for these tubes placed inside the assembly. The tubes placed on the periphery are opened earlier, but do not heat more after their opening, since the induced currents can no longer loop back due to the presence of the cutting slot.

 In the case of the opening of tubes of oxidizable alloy, in this case zircaloy, the opening must obviously be made under a neutral atmosphere to avoid oxidation of the tubes.

 In the context of an application to the cutting of zircaloy cladding of nuclear fuel rods, the resistivity of this cladding is equal to approximately 1.10-6 ohms x m, and the thickness of the cladding is equal to 0.5 mm. The heating frequency is then between 60 and 120 KHz, the intensity of the current in the inductor being of the order of 1000 A.

 The thermal cutting process which has just been described therefore allows the opening of the tube by means of a very simple mechanism, at a high speed.

In the case of the opening of fuel tubes, the separation between the cladding and the fuel is then carried out by a conventional dissolution of the fuel in nitric acid.

 The cutting method according to the invention can be applied whenever it is desired to cut tubes longitudinally without using traditional mechanical cutting tools.

Claims (10)

 1. Method for cutting a metal tube, of the tube cutting type by very rapid and very localized fusion along a cutting line, characterized in that it consists in - heating the tube (10, 30) by induction with a  sufficient current for the current density  at the weak point (2) or around the weak point (2)  causes, by a very localized elevation of the  temperature, the tube melting in or around the  weak point (2), the rest of the tube (10, 30) remaining  at a temperature much lower than the temperature  for melting the metal constituting the tube (10, 30)  and - move the melting point along the line  of cutting constituted by a generator of  tube (10, 30) to create a slot (12).  2. Method according to claim 1, characterized in that it consists in previously creating a mechanically weak point (2) in the tube (10, 30) before its use.  3. Method according to claim 1 or 2, characterized in that the induction is created by a mobile inductor (4, 22) movable along the tube to be cut (10).  4. Method according to claim 1 or 2, characterized in that the induction is obtained by a fixed inductor (4, 20, 32), the tube being moved inside the inductor (20).  5. Method according to claim 2, characterized in that the weak point is a hole (2).  6. Method according to claim 2, characterized in that the weak point is a small piece of metal of a different nature from that of the tube (10) to form an alloy whose melting point is lower than that of the metal of the tube ( 10).  7. Method according to any one of the preceding claims, characterized in that the inductor (4, 20, 22, 32) is a solenoid.  8. Method according to claim 4, characterized in that the inductor (20) fixed surrounds the tube to be cut (10) over its entire length, the melting point (2) moving along the generator on which it is placed, by increasing the temperature on the two ends of the slot (12), placed on said generator.  9. Application of the method according to any one of the preceding claims for separating a nuclear fuel rod from the metal cladding which surrounds it.  10. Application of the method according to any one of claims 1 to 8, for separating the bars (30) of an assembly of nuclear fuels from the claddings which each surround a bar.