GB2298080A - Decontamination of zircaloy with the aid of a slag by a cold crucible melting operation with continuous drawing of the ingot - Google Patents

Abstract

A process for the decontamination of a zircaloy-based alloy by the transfer of radioactivity into a slag during the melting of the alloy, involves the use of a slag containing at least one cryolite (3MF-AlF 3 , in which M is an alkali metal), the process making it possible to obtain a metallic matrix having a better resistance to leaching by water.

Description

DECONTAMINATION OF ZIRCALOY WITH THE AID OF A SLAG BY A COLD CRUCIBLE MELTING OPERATION WITH CONTINUOUS DRAWING OF THE INGOT DESCRIPTION TECHNICAL FIELD The present invention relates to the field of the decontamination of reactor can or hull waste, which are highly ss-γ contaminated α waste. The waste from PWR reactors are zircaloy.

contamination comprises actinides among which plutonium is the most important element. - contamination is constituted by fission products and activation products. The fission products are 3H, 90Sr, Zr, 99Tc, Ru, 135 137 144 147 151 154 Cs, Cs, Cs, Cs, Ce, Pm, Sm, Eu.

54 55 60 The activation products consist of the following elements: fln, Fe, Co, 59Ni, 63Ni, 93Zr, 94Nb, 119Sn, 125Sb. They result from constituent elements present in hulls and must therefore be eliminated at source, i.e. by the choice of ores used in the production of the alloys of the hulls. Following irradiation in reactors, said alloys must make it possible to at least minimize, if not eliminate, said activation products.

Thus, the object of the present invention is a process for the decontamin ation of zircaloy in α radionuclides and ss-γ fission products.

PRIOR ART A process is known for the melting of stainless steel by direct cold crucible induction with continuous drawing of the ingot and in the presence of an oxide or fluoride-type slag. The principle of this process will be summarized in conjunction with fig. 1.

The essence of the process is a copper, cylindrical crucible 2 cooled by a circulation of water. Reference numeral 4 designated a cooling water inlet and outlet collector. The crucible is also surrounded by a solenoid inductor 6, which creates a magnetic field, the crucible being by design transparent to the electromagnetic field.

By means of a distribution opening 8, the metal to be melted is regularly introduced into the crucible 2, where it is heated by direct induction at a frequency of 10 kHz. The eddy currents generated by the alternating electromagnetic field created by the induction flow in the periphery of the metal mass and dissipate there the energy by the Joule effect. The interaction between the electromagnetic field and the currents induced in the metal produces electromagnetic forces directed towards the interior of the molten metal bath, which have the effects of necking, which gives the molten bath a dome shape and stirring from the centre towards the outside of the bath, which homogenizes the composition and temperature of the metal.

The metal is melted in the presence of a flux or slag introduced by means of a distribution opening 10. Reference numeral 12 designates an opening for introducing a gas such as argon and numeral 14 an outlet making it possible to collect gases given off during melting. The latter takes place in zone 16, located above zone 18, in which there is a progressive solidification due to the presence of a cooled hearth 20, located at the base of the solidification zone 18. The cooled hearth 20 is downwardly mobile and it is thus possible to draw the molten metal1 which sets into solid ingots as soon as it passes out of the installation. This process makes it possible to transfer most radioactive elements to the slag. The same process makes it possible to melt zircaloy canning waste.

The slags which are usable at present and in fact used are fluorides such as CaF2, BaF2 and Lay3, either singly or in mixtures. They are stable with regards to the reducing properties of the zircaloy, which can also solubilize the radioactive elements. The combination of the two latter properties of zircaloy makes it possible to assist the maintenance of the radionuclides therein and consequently limits their transfer to the slag.

The solubilizing power of said slags can be evaluated by uranium oxide dissolving tests making it possible to evaluate the capacity of the slag for the collection of hull contaminating oxides.

A measuring cell has been used for this purpose and is formed by an induction furnace for heating the melting flux contained in a graphite crucible.

A solid uranium oxide bar is rotated in the molten flux. The uranium concentration in the flux is then determined throughout the dissolving test.

Two factors can influence the oxide dissolving kinetics in a given flux, namely the temperature and rotation speed of the bar.

It can be considered that the kinetics of the medium is governed by a first order kinetic equation of type: dC V = dt = kA(CS-C) in which A represents the solid-liquid contact surface, V the liquid flux volume, k the transfer coefficient, C the oxide concentration in the flux at time t, and C the saturation concentration.

S The integration of this equation leads to the following expression:

For each of the tests table I gives the limit solubility values (C ) and the S transfer coefficient (k).

TABLE I Slag Composition # Rotation speed k C (wt.%) ( C) (r.p.m.) (10-3cm/s) (wt.%) CaF2-BaF2 30-70 1250 350 9.4 0.029 CAF2-BaF2 30-70 1250 250 6.5 0.033 CaF2-BaF2 30-70 1250 150 4.3 0.027 CaF2-BaF2 30-70 1300 250 8.4 0.040 CaF2-BaF2 30-70 1200 250 4.0 0.038 CaF2-BaF2 10-90 1250 250 8.3 0.028 CaF2-BaF2 50-50 1250 250 5.7 0.040 It is possible to deduce from these results that the solubility of uranium oxide is very low in CaF2-BaF2 and has a maximum of 0.04t.

In the same way, a zirconia (ZrO2) dissolving test in a CaF2-BaF2 flux (50-50 wt.X) indicates that the maximum weight solubility is 0.04%, with a transfer coefficient of 5.7 x 10 3 cm/s.

DESCRIPTION OF THE INVENTION Thus, the slags used in the prior art have a low maximum solubility, so that the slag quantity must be adequate to ensure that it is not saturated during a melting operation.

The present invention seeks to solve this problem of the low maximum solubility.

It therefore relates to a process for the decontamination of an alloy containing zircaloy by transferring radioactivity into a slag during the melting of the alloy and is characterized in that the slag contains at least one cryolite of formula 3MF-AlF3, in which M is an alkali metal. M can e.g.

be chose from among sodium, potassium or lithium.

The saturation oxide concentrations are 10 to 200 times higher than those obtained, e.g. with CaF2-BaF2. With cryolites, the saturation effect by oxides is also significant, but it occurs after dissolving a much larger oxide quantity (a few %).

A secondary problem which may arise under certain process performance conditions is that of the variation between the melting temperature of the cryolite used and the use temperature. This problem can be solved by increasing the melting temperature of the slag and/or by decreasing the melting temperature of the alloy.

According to an embodiment, for increasing the melting temperature of the slag, the cryolite is mixed with a refractory fluoride.

This adjuvant also makes it possible to bring about a spontaneous crust removal and obviates a supplementary chemical engineering operation consisting of separating the slag from the ingot. This fluoride can e.g. be calcium fluoride (CaF2).

According to another embodiment, for decreasing the melting temperature of the alloy, to the same is added metals such as iron or nickel.

Another secondary problem solved by a special embodiment of the invention is that of the formation of volatile ZrF4.

For this purpose, the invention proposes adding to the alloy-slag mixture, an element crystallizing with part of the Zr of the matrix in the same structure as the alloy crystallized with Zr.

This addition makes it possible to ensure that there is no transfer of aluminium from the slag to the zirconium, so that the slag is stabilized.

This addition also favours the obtaining of an ingot for which the deterioration of the elements forming it is low and whose radioactivity confinement properties are high. The addition also has a beneficial effect on the constituent alloy of the ingot and increases the resistance of the latter to leaching by water. The added element can be aluminium.

The invention also relates to a metal ingot obtained by a process like that described hereinbefore.

In particular, an element crystallizing with part of the zirconium of the alloy may have been added to the slag-alloy mixture, crystallization taking place in the same structure as the alloy crystallized with Zr. This element can be aluminium, the ingot containing lava phases penetrated by the aluminium. In particular, the lava phases may contain Zr2 (AlNi).

Other aspects of the invention can be gathered from the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the invention can be better gathered from the following description of embodiments given in an explanatory and nonlimitative manner with reference to fig. 1 showing an apparatus for performing a continuous drawing, cold crucible melting process.

DETAILED DESCRIPTION OF EMBODIMENTS A process according to the invention e.g. uses an apparatus like that already described in conjunction with fig. 1.

The slag proposed by the invention is chosen from among cryolites of formula 3MF-AlF3, in which M is an alkali metal, e.g. chosen from among potassium, sodium or lithium. The slag can also be a mixture of said cryolites.

Finally, the slag can contain in majority form either a cryolite or a cryolite mixture.

During the melting of the alloy, the radioactive elements are transferred to the slag and trapped in the latter. Compared with the prior art slags, the choice of cryolites makes it possible to more easily get round the reducing power and solubilizing power of zircaloy with respect to radionuclides. In addition, these radionuclides are maintained in cryolite-type slags throughout the drawing operation, which represents another aspect of solving the problem caused by the prior art slags.

Uranium oxide (U02) dissolving tests carried out in cryolites were performed for evaluating the capacity of the latter to collect hull-contaminating oxides.

The tests were carried out under conditions identical to those described hereinbefore for evaluating the solubilizing power of CaF2-BaF2. The measurement cell is identical and a solid uranium oxide bath is rotated in the molten slag.

The maximum uranium oxide concentration and the transfer speed into the slag were measured as a function of the temperature and stirring, i.e. the rotation speed of the uranium oxide bar for different cryolites and several mixtures. The results are collected in the following table II.

TABLE II No. Slag Composition # # k C (wt.%) ( C) (r.p.m.) (10-3 cm/s) (wt.%) 1 Na3AlF6 100 1050 250 5.6 3.0 2 Na3AlF6 100 1200 250 8.5 6.0 3 Na3AlF6-CaF2 65-35 1200 250 7.0 2.9 4 Na3AlF6-CaF2 82.5-17.5 1200 250 8.1 3.6 5 K3AlF6 100 1100 - - 5.6 6 l 6 100 1000 250 6.8 2.6 7 Li3AlF6 100 1100 250 9.5 5.6 8 Li3AlF6 100 1200 250 14.0 7.3 9 Li3AlF6-CaF2 60-40 1200 250 9.5 5.2 The essential result of these tests if that the saturation uranium oxide concentrations are 10 to 200 times higher than those obtained with CaF2-BaF2.

With cryolites, the saturation effect by the oxides is also significant, but it occurs after the dissolving of a larger oxide quantity (a few %).

The melting temperatures of the cryolites are between 8000C and just above 1000 C (8000C for Li3AlF6, 1010 C for Na3AlF6 and 10200C for K3AlF ). In 6 certain cases, the difference between the melting point of the cryolite (particularly in the case of lithium cryolite) and the melting point of the alloy in the crucible is significant. To reduce this difference it is possible to either increase the melting point of the slag or decrease the melting point of the alloy, both being simultaneously performable.

In the case of lithium cryolite, preference is given to the addition of calcium fluoride. The addition of a refractory fluoride such as CaF2 also permits an easier separation between the ingot and the slag than when the latter is constituted by pure cryolite.

In the case of lithium cryolite and for stabilizing the binary Li3AlF6-CaF2 mixture, it is possible to form a ternary mixture by adding lithium fluoride (e.g. a weight mixture of 40X Li3AlF6, 20X CaF2, 40% LiF or 30% Li3AlF6, 30% LiF and 40X CaF2). The solubilizing power of LiF for oxides is 10 times better than that of the equal-weight mixture CaF2-BaF2, but is 10 times inferior to that of Li3AlF6. In general terms, for stabilizing a binary mixture constituted by cryolite (3MF-AlF3) and fluoride, it is possible to form a ternary mixture by adding MF.

In order to decrease the melting point of the alloy, it is possible to add in a small proportion metals such as iron or nickel or an iron-nickel mixture. The weight addition of iron or nickel varies between 1 and 10%.

The melting slag must also be chemically stable during the melting of the metallic alloy, which is very rich in zirconium. In order to avoid the formation of a volatile ZrF4 compound, it is possible to directly add aluminium to the alloy-slag mixture. This avoids any transfer of aluminium from the slag to the zirconium, so that the slag is stabilized.

Moreover, aluminium addition has a beneficial effect on the final metal alloy matrix or ingot, because it contributes to increasing the resistance of said matrix to a leaching by water. Thus, the alloy contains lava phases in which it is in particular possible to identify Zr2Ni. The leaching of such an alloy leads to a dissolving of the nickel. Aluminium has the property of penetrating the lava phases forming compounds of type Zr2(AlNi), which are also integral with the metal matrix of the alloy and stable during leaching by water, the nickel not being dissolved. It is preferable to add an aluminium mass equal to 1 to 10% of the total mass.

More generally, it is possible to obtain a stable matrix with good radioactivity confinement properties by adding in a small proportion (approximately 1 to 10% of the total mass) elements crystallizing in the same structure with part of the zirconium of the matrix.

The other advantage of this substitution of Zr by Al and/or another element is a decrease of the alloy reducing power and in particular zirconium, which is favourable for the decontamination of the latter.

Claims (13)

1. Process for the decontamination of a zircaloy-based alloy by the transfer of radioactivity into a slag during the melting of the alloy, characterized in that the slag contains at least one cryolite of formula 3MF-AlF3, in which M is an alkali metal.

2. Process according to claim 1, wherein M is lithium, sodium or potassium.

3. Process according to claim 1 or 2, wherein a refractory fluoride is added to the cryolite.

4. Process according to claim 3, the added fluoride being calcium fluoride (CaF2).

5. Process according to claim 3, the cryolite-fluoride mixture being stabilized by MF addition.

6. Process according to one of the preceding claims, the melting point of the alloy being decreased by the addition to the latter of a metal.

7. Process according to claim 6, the metal added to the alloy being chosen from among iron, nickel or mixtures thereof.

8. Process according to one of the preceding claims, wherein addition takes place to the slag-alloy mixture of an element crystallizing with part of the zirconium of the alloy matrix in the same structure as the alloy crystallized with Zr.

9. Process according to claim 8, the added element being aluminium.

10. Metal ingot obtained by a process according to one of the claims 1 to 9.

11. Metal ingot according to claim 10, an element crystallizing with part of the zirconium of the alloy having been added to the slag-alloy mixture, crystallization taking place in the same structure as the alloy crystallized with Zr.

12. Ingot according to claim 11, the added element being aluminium, the ingot containing lava phases penetrated by the aluminium.

13. Ingot according to claim 12, the lava phases containing Zr2 (AlNi).