Electromagnetic induction furnace and use of the furnace for melting a mixture of metal(s) and oxide(s), said mixture representing a corium

Abstract

An electromagnetic induction furnace which is intended to melt at least one electrically conductive material, such as an oxide and/or a metal, and which includes at least one inductor having at least one turn and at least one cooling circuit suitable for cooling at least the inductor. Said furnace is characterized in that the heat-transfer fluid of at least one cooling circuit is supercritical CO2. The invention also relates to a method for operating the furnace and to the use thereof for melting a mixture of metals (steel, zirconium, etc.) with oxides (uranium UO2, zirconium, etc.), as well as concrete components, the mixture representing a corium.

Claims

1. An electromagnetic induction furnace intended to melt at least one electrically conductive material, comprising: at least one inductor having at least one turn, a cooling circuit configured to circulate a heat transfer fluid inside the at least one turn of the at least one inductor in order to cool at least the at least one inductor, said cooling circuit for cooling the at least one inductor comprising an inlet and an outlet, a crucible for containing the at least one electrically conductive material to be melted comprising walls, and a cooling circuit configured to circulate a heat transfer fluid in order to cool the walls of the crucible, said cooling circuit for cooling the walls of the crucible comprising an inlet and an outlet, wherein the heat transfer fluid of at least the cooling circuit for cooling the walls of the crucible is supercritical CO.sub.2.

2. The furnace as claimed in claim 1, wherein the at least one inductor has at least two consecutive turns, the winding directions of said consecutive turns being reversed by forming a loop in order to reverse a direction of an electric current that passes through them, so as to form a levitation furnace.

3. The furnace as claimed in claim 1, wherein the walls of the crucible are made of an electrically conductive material so as to form a cold-crucible furnace.

4. The furnace as claimed in claim 1, the walls of the crucible or self-crucible comprising a bottom, referred to as the floor.

5. The furnace as claimed in claim 4, wherein the floor is removable.

6. The furnace as claimed in claim 4, wherein the floor comprises one or more through-orifices for discharging molten material.

7. The furnace as claimed in claim 5, wherein the floor is made of electrically conductive material.

8. The furnace as claimed in claim 1, comprising a single inductor that operates simultaneously at at least two different frequencies.

9. The furnace as claimed in claim 1, comprising two or more separate inductors, wherein at least two separate inductors operate simultaneously at different frequencies.

10. The furnace as claimed in claim 8, one of the operating frequencies being suitable for melting one or more metal(s) and another operating frequency being suitable for melting one or more oxide(s).

11. The furnace as claimed in claim 1, wherein the operating frequency or frequencies of at least one inductor is between 10 and 500 kHz.

12. A process of operating a furnace as claimed in claim 1, comprising circulating supercritical CO.sub.2 at a pressure between its critical pressure Pc, of the order of 73 bar, and 100 bar and temperatures between its critical temperature Tc, of the order of 31? C., at the inlet of at least one of said cooling circuit for cooling the at least one inductor and said cooling circuit for cooling the walls of the crucible, and 50? C. at the outlet of at least one of said cooling circuit for cooling the at least one inductor and said cooling circuit for cooling the walls of the crucible so as to have a specific heat capacity Cp of the supercritical CO.sub.2 at least equal to 4 kJ.kg.sup.?1.

13. A process of operating a furnace as claimed in claim 1, comprising circulating an alternating current in the at least one inductor, simultaneously at at least two different frequencies.

14. A process of operating a furnace as claimed in claim 1, comprising melting a mixture of at least one or more metals with one or more oxides.

15. A process of operating a furnace as claimed in claim 14, wherein the mixture is a mixture of metals comprising steel and zirconium with oxides comprising uranium UO.sub.2 and zirconium and also components of a concrete, the mixture being representative of a corium.

16. The furnace as claimed in claim 7, wherein the electrically conductive material is copper.

Description

DETAILED DESCRIPTION

(1) Other advantages and features will emerge more clearly on reading the detailed description, given by way of illustration and non-limitingly, with reference to the following figures, among which:

(2) FIG. 1 is a partially cutaway perspective view of a crucible furnace with electromagnetic induction heating capable of using the cooling circuit according to the invention,

(3) FIG. 2 is a schematic top view of a crucible furnace also with electromagnetic induction heating that forms a cold-crucible furnace also capable of using the cooling circuit according to the invention,

(4) FIG. 3 is a perspective view of a crucible-free induction furnace that forms a levitation furnace also capable of using the cooling circuit according to the invention,

(5) FIG. 4 is a perspective view of a cold-crucible furnace comprising a cooling circuit according to the invention,

(6) FIG. 5 is a perspective view showing two identical inductors having helical turns capable of being arranged concentrically one inside the other around an induction furnace according to the invention,

(7) FIG. 6 illustrates a curve of the change in the heat capacity Cp of the supercritical CO.sub.2 used in accordance with the invention as coolant for an induction furnace,

(8) FIG. 7 illustrates a curve of the variation in the temperature of the supercritical CO.sub.2 in a channel of given dimensions and at a given flowrate and, by way of comparison, the curve of the variation of water under equivalent channel dimension and flow rate conditions.

(9) Throughout the present application, the terms vertical, lower, upper, bottom, top, below and above should be understood with reference relative to an induction furnace arranged in a vertical operating configuration. Thus, in an operating configuration, the furnace is arranged vertically with its bottom through which the molten material is discharged, downward.

(10) FIGS. 1 to 3 have already been commented upon in the preamble. They are not therefore described in detail hereinbelow.

(11) Represented in FIG. 4 is a cold-crucible furnace 1 comprising at least one cooling circuit 5 in accordance with the invention, i.e in which the heat transfer fluid is supercritical CO.sub.2. Such a furnace 1 is preferably intended to carry out the melting of a charge consisting of a mixture of metal(s) and oxide(s), such as the uranium oxide UO.sub.2, representative of a corium.

(12) Such a furnace 1 comprises a copper crucible 2 surrounded by an inductor, i.e. an electromagnetic induction coil 4 having at least one turn. In the example represented, the inductor 4 comprises seven consecutive turns 41-47 that are identical and equidistant from one another.

(13) Although not represented, the sidewall of the crucible 2 is divided into a certain number of identical sections 20. This number is equal to 8 in the example of FIG. 4.

(14) The crucible 2 also comprises a bottom, referred to as the floor, which is not represented. The bottom may be removable or may be pierced with one or more through-openings in order to enable the discharge of the material or mixture of materials once this or these material(s) is (are) in the liquid state via melting.

(15) By thus dividing the sidewall of the crucible 2 into sections 20, when the alternating current passes through the turn(s) of the inductor 4, the induced currents do not remain localised at the periphery of the crucible, but go around each section 20, as already explained in the preamble in connection with FIG. 2. Thus, the current I passing through the inductor 4 (coil) induces on the crucible 2 a current which in turn induces a current inside the charge containing at least one electrically conductive material, which is housed in the crucible. The charge then melts via the Joule effect. When the molten charge has become liquid, it comes into contact with the walls of the crucible 2 that are cooled by the cooling circuit 5, which solidifies it, thus creating a self-crucible, that is to say a solid layer made from the material(s) of the charge introduced initially into the crucible 2.

(16) The use of such a cold-crucible furnace 1 is advantageous for melting a charge consisting of a mixture of uranium oxide and metal representative of a corium. Indeed, the melting point of uranium oxide is of the order of 2865? C., much higher than the melting point of the metals, in particular stainless steel. The metal at these temperatures is characterized by a virtually zero viscosity, that is to say that it may infiltrate into the smallest crack of the crucible. With the formation of the self-crucible as explained above, it is ensured, on the one hand, that the metal present in the charge to be melted cannot in any case attack the constituent metal of the walls of the crucible and, on the other hand, that the mixture of materials retains its initial purity.

(17) Preferably, an element, not represented, made of electrically insulating material is arranged between two consecutive (adjacent) sections 20. Such an insulating element serves not only to prevent leaks and decrease heat losses but also to minimize the formation of an electric arc between the copper sections 20 during the operation of the furnace.

(18) As illustrated in FIG. 4, the cooling circuit 5 according to the invention with supercritical CO.sub.2 is also divided into a number of cooling sections arranged at the periphery of the sections 20 of the crucible 2. In the example illustrated, the number of cooling sections, 5.1 to 5.8 is equal to that of the copper sections 20, i.e. eight sections. More specifically, as illustrated in FIG. 4, each section 5.1 to 5.8 comprises three tubes joined outside of the section 20 of sidewall of the crucible 2 and which open into a common collector. The tubes are advantageously made of an electrically conductive material, preferably made of copper. As regards the method of attaching the tubes to the walls of the crucible 2, any known means may be envisaged. By way of example, it is possible to envisage attachments with the aid of plates made of thermally insulating material that withstands the high temperatures.

(19) Thus, according to the invention, during the operation of the furnace, supercritical CO.sub.2 is circulated inside each tube of the cooling sections 5.1 to 5.8.

(20) Moreover, in accordance with the invention, it may be envisaged to cool the turns 41 to 48 of the inductor 4 via an additional cooling circuit inside the inductor. In other words, it is possible to envisage a circulation inside the turns 41 to 48 of the inductor 4 with supercritical CO.sub.2.

(21) According to one advantageous embodiment, when the charge to be melted consists of a mixture of oxides and at least one metal, such as a mixture representative of a corium, an alternating current that operates simultaneously at at least two different frequencies is made to flow in the inductor 4. Indeed, the temperature of the metal, such as steel typically in the vicinity of 1300? C., is substantially lower than those of the oxides, such as uranium oxide UO.sub.2 in the vicinity of 2865? C.

(22) Thus, by supplying with current at two different frequencies, one of which is suitable for induction melting of the metal(s) and the other for induction melting of the oxides, a simultaneous melting of the constituents of the mixture is ensured while ensuring mixing and therefore a homogeneous mixture, and in addition it is ensured that, throughout the melting process, the metal or metals do not come directly into contact with the walls of the crucible. Indeed, on the one hand, for a same material, the lower the induction frequency, the more the electromagnetic wave will penetrate said material and therefore generate Joule effect heating in the bulk. Moreover, as stated previously, due to the difference in melting point, oxides require higher induction frequencies and the metal(s) lower frequencies. Finally, once the melting process in the furnace is started, the metal(s) has (have) a virtually zero viscosity when the oxides begin to melt. Thus, by using a single induction frequency for the operation of a furnace according to the invention, there remains a risk of the molten metal(s) infiltrating into the smallest crack present in the walls of the crucible. There is also a risk of the metal(s) agglomerating on said walls, which would have the deleterious effect of creating a screen to the electromagnetic waves and optionally of deteriorating the inductor. Consequently, the operation of a furnace according to the invention at two different frequencies makes it possible to avoid, at the very least reduce, these risks: throughout the melting process, the metal(s) is (are) pushed back towards the inside of the crucible. A homogeneous mixture is thus obtained in an equilibrium system of the molten constituents.

(23) Thus, preferably, the operating frequencies of the inductor 4 are between 10 and 500 kHz. An effective operating frequency is in the vicinity of 100 kHz.

(24) As a variant, it is possible to envisage the use of two separate inductors, one operating with a frequency suitable for the induction melting of the metal(s) and the other operating with a frequency suitable for induction melting of the oxides. This variant is facilitated by the use of supercritical CO.sub.2, which, owing to its greater effectiveness, makes it possible to envisage a reduction by a factor of two in the diameter of the turns. Represented in FIG. 5 are two identical inductors 4A, 4B each with six helical turns 41-46 which may be arranged concentrically inside one another around the crucible 2 described above, instead of the single inductor 4. Owing to the use of supercritical CO.sub.2 as coolant according to the invention, it is possible to envisage a diameter ? of the turns of the two inductors 4A, 4B which is two times smaller than that of the single inductor 4.

(25) FIG. 6 clearly shows the substantial advantage of using supercritical CO.sub.2 instead of water as coolant for an induction furnace owing to the specific heat capacity Cp (heat capacity). Thus, it emerges from this curve from FIG. 6 that the heat capacity Cp increases very greatly between 30? C. and 40? C. and achieves a value close to 35 kJ.kg.sup.?1, i.e. a value up to 10 times greater than that of water. Therefore, according to the invention, it is possible to envisage a very substantial reduction in the diameter of the sections 20 of crucible walls and/or in the diameter of the turns of the inductor 4, up to a factor of two relative to the dimensions of existing cold-crucible furnaces.

(26) FIG. 7 additionally indicates a comparison in the variation of the temperature of the fluid in a channel of given dimensions at a given flow rate, respectively with supercritical CO.sub.2 according to the invention and an equivalence (diameter and flow rate) with water according to the prior art. It is observed that the variation in the temperature of the water is linear where as that of the supercritical CO.sub.2 increases more rapidly due to the increase in the heat capacity of the latter with the temperature. It may be deduced therefrom that with supercritical CO.sub.2 it is possible to adapt the flow rate in order to optimize the cooling capacity at the center of the crucible 2 of an induction furnace, that is to say at the location where the temperature is maximal. Thus, an advantage subsequent to the use of supercritical CO.sub.2 as coolant for a crucible induction furnace is to render the temperature in said crucible more uniform.

(27) The invention is not limited to the examples which have just been described; in particular, features of the examples illustrated may be combined with one another in variants that are not illustrated.

REFERENCE CITED

(28) [1]: Utilisation du CO2 supercritique comme solvant de substitution [Use of supercritical CO2 as substitution solvent], Guy LUMIA, Techniques de l'Ing?nieur In5