FR2705660A1 - Process for the recovery and purification of a metal alloy based on enriched uranium - Google Patents

Abstract

Process for the treatment of a metal alloy based on generally highly enriched uranium and on other metal elements comprising a fluorination treatment using at least one fluorinated gas, optionally with an intermediate fluorination using a fluorinated compound, a distillation treatment of the stream of gaseous fluorides obtained, in order to obtain pure uranium hexafluoride, and a mixing treatment of the said hexafluoride with another, more depleted, hexafluoride in order to obtain the final desired isotopic content.

Description

PROCESS FOR RECOVERING AND PURIFYING BASE METAL ALLOY

URANIUM VERY ENRICHED

TECHNICAL AREA

  The invention relates to a metal alloy treatment method based on

  of highly enriched uranium to recover uranium as a compound.

  pure uranifer with lower isotopic content controlled by the medium

a dry fluoride route.

PROBLEM

  There is availability of U235 highly enriched uranium based metal alloys and generally containing significant percentages of Mo, Nb, Fe, Cr, Ni, Si, Al, Zr, Ta, W additive elements. ... By very rich, we understand ratios (U235 / U total) generally greater than 8%, but can reach or even exceed 90%. It may be for example U-Si alloys from research reactors of the MTR type enriched at most 20%, U-Al alloys, alloys for military use, etc ... These alloys can be non-irradiated (eg

  scrap, downgraded alloys ...) or irradiated.

  It is interesting to be able to recover the U235 potential contained in these supplies to use it for example in less enriched form and purified as a fuel in nuclear power reactors,

  for example PWR type, BWR ... or research reactors.

  For this, the Applicant has sought a method of treating these metal alloys to obtain a pure uranium compound having a homogeneous isotope content U235 and adapted to the new use, as well as a chemical composition and quality such as it can be used in a simple way to make the desired fuel in existing installations intended for this purpose. The chemical compound

  must in particular have a high purity and perfect homogeneity.

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DESCRIPTION OF THE INVENTION

  The invention is a U235-enriched uranium alloy metal treatment method and other metal elements comprising: a) a fluorination treatment of the alloy with at least one fluorinated gas

  to recover on the one hand a gaseous mixture containing uranium and

  other elements of the alloy in the form of gaseous fluorides which may contain oxyfluorides in the presence of oxygen in the starting alloy with incondensable gases, on the other hand a unburnt or fluorinated non-volatile solid residue, b) a distillation treatment of said gaseous mixture to recover separately from pure uranium hexafluoride, fluorides of the other elements, c) a mixing operation of said highly enriched and pure uranium hexafluoride in liquid or gaseous form with a hexafluoride of less enriched uranium in such proportions that the

  isotopic desired in the resulting mixture.

  The starting metal alloy generally contains high proportions of alloying elements such as Mo, Nb, Fe, Cr, Ni, Si, Al, Zr, Ta, W, etc. Its fluorination is carried out in solid form or divided, this division can be obtained from ingots or massive pieces by any means known to those skilled in the art, for example sawing, grinding, grinding

  cryogenic, etc ... adapted materially to the management of criticality.

  The fluorination can be carried out in one or two stages: - in one step: using fluorine gas; the exothermic reaction is controlled by introducing a generally incondensable dilution inert gas (for example nitrogen, argon, etc.). On the one hand, a gaseous mixture is obtained containing incondensable gases (mainly the inert dilution gas), gaseous fluorides and optionally oxyfluorides (highly enriched uranium and alloying elements).

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  giving volatile fluorides at the temperature of the fluorination reaction), on the other hand a solid residue consisting of impurities in the form of unburnt and non-volatile fluorides, which is optionally

treated before rejection.

  in two steps: a first step in which the uranium is converted into UF4 and a part of the metal elements of the fluoride alloy with a gaseous and non-polluting compound, fluorine, and second step o the fluorination of the solid obtained during the 1st stage is completed with the aid of a fluorine-based gas in

  conditions similar to those of one-step fluorination.

  The two-stage fluorination can be carried out according to two modes: in a first embodiment, the first step is carried out by contacting the starting alloy with gaseous anhydrous hydrofluoric acid to obtain volatile fluorides, which are to be removed after separation (for example by filtration), condensation and / or neutralization treatment and a solid residue composed mainly of solid fluorides (including all UF4 V'U), and a second step o said solid residue is treated with gaseous fluorine under conditions similar to those of one-step fluorination, as

has already been said.

  in a second embodiment, the first step is carried out by bringing the starting uranium alloy into contact with a fluorinated gas based on UF6, generally more depleted of U235, to transform all or part of U into UF4. and at least partially fluorinating the other metallic constituents of the starting alloy. In the second step, the solid obtained above is brought into contact with fluorine, possibly diluted with an inert gas, to obtain, as already seen in the one-step fluorination, firstly the mixture of gaseous fluoride and of incondensable gases, on the other hand the residue of

volatile and unburned.

  The U is transformed according to the reactions:

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  U (solid) + 2 UF6 (gaseous) ----) 3 UF4 (solid) 3 UF4 (solid) + F2 (gaseous) ----> 3 UF6 (gaseous) In this second mode, one can operate in a only reactor operating against the current. Thus, at one end of the reactor, the metal alloy is preferably introduced in pieces, at an intermediate section of the leaner gas reactor UF6 and at the other end the fluorine-based gas, the mixture of gaseous fluorides and incondensable gas being evacuated at the introduction end of

the metal alloy.

  This second mode has several advantages: the fluorination being done in two stages with the intermediate UF4 obtained by means of combined fluorine (UF6), the control of the reaction temperature is greatly facilitated and improved; a similar advantage

  is also obtained with the first mode where HF is used.

  the fact of performing, at this stage of the process, an isotopic mixture intended to lower the isotopic content of the U makes it possible to limit the criticality problems in the rest of the process and thus to considerably simplify the problems relating to the design and

regulation of installations.

  The gaseous stream resulting from the treatment with fluorine is generally filtered and condensed to be brought generally in the liquid state and stored in an intermediate capacity where the liquid state is generally maintained. the

  During this condensation, the incondensable gases are separated.

  The condensation can be carried out by the cold in one or more sucessive crystallizers kept under pressures of about 1 bar and at very low temperatures usually of about -25 C, in order to eliminate any trace of U fluoride in the incondensable gases. These gases consist mainly of a mixture of excess fluorine, dilution gas (nitrogen, argon, etc.) or incondensable fluorides (for example SiF4); they can then be rejected after a possible passage in a felling column and / or a safety filter; but they can also be recycled to the fluorination stage, as long as they do not

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  contain no gaseous fluoride gas, which allows in this

  case of less pushing the crystallization and the treatment of said gases.

  The purification treatment by distillation of the fluoride mixture stored in the intermediate capacity is carried out in one or more successive distillation columns, depending on the complexity of the mixture to be treated. These columns are generally packed, but they can be trays. They are usually equipped with a reflux condenser I0 located at the column head, and a reboiler at the bottom of the column, the fluoride mixture to be treated being introduced, generally in the state

  liquid, at an intermediate level.

  Thus, a single column may suffice when one is in the presence of a binary mixture of UF6 and a single other impurity fluoride. In this case, by adjusting the reflux rates of the condenser and the reboil, pure UF6 can be obtained either at the top or bottom of the column depending on whether the other fluoride is respectively less volatile (heavier) or more

volatile (lighter) than UF6.

  On the other hand, when one is in the presence of a complex mixture containing, besides UF6, a complex mixture of impurity fluorides, it is advantageous to operate with several successive columns. In a first column, it is sought, for example, to recover all the UF6 at the top of the column, where it is mixed with other fluorides which are lighter than itself, this UF6 then being separated from the fluorides heavier than it recovered at the bottom. of column. In the second column, said mixture of UF6 is then introduced with the light fluorides and pure UF6 is recovered at the bottom of the column while eliminating

  lighter fluorides at the top of the column.

  The sequence of these two operations can be reversed.

  In each column, the reflux ratio of the condenser and

  reboiling to arrive at the desired purity result.

  Moreover, we see that we can use additional columns

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  to separate from one another metal fluorides that it would

interesting to value.

  After the distillation treatment, pure UF6 and the various other fluorides in liquid form were obtained separately. At the top of the condensers, incondensable gases have also been separated and recovered at the temperatures of the column heads in small quantities, which are generally discarded after treatment, but which can be recycled if their

composition lends itself to it.

  Said other fluorides (other than UF6), once solidified, can be stored as such, but are preferably treated, generally hydrolysed to obtain on the one hand HF, which can then be recovered, and on the other hand the metals of the initial uranium alloy in the form of oxides more easily storable, manipulable and recoverable, said oxides can then be reduced by hydrogen if it is desired to obtain the

Uranium alloy metals.

  The pure UF6 obtained after the purification treatment is in liquid form; it can then be mixed, in this form or in gaseous form, with a metered flow of pure UF6 (generally more depleted), in the form of

  gaseous or liquid, to obtain the desired isotopic content in U235.

  Obviously, the dosed flow is a function of the respective U235 contents of the

UF6 to mix.

  The more depleted UF6 can be advantageously based on very depleted uranium (0.2% in U235) of which there are large stocks, or based on

depleted or natural uranium.

  The mixture made from UF6 in liquid or gaseous form has the advantage of simplicity and gives a resulting UF6

  having a homogeneity, both isotopic and chemical, perfect.

  After mixing, the resulting UF6, pure and at the desired isotopic content, is condensed, poured into a container for transport, storage or

use, as usual.

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  Alternatively, the fluorination step may be preceded by a step of oxidizing the alloy carried out in an oven using a gas-based

  of oxygen, possibly diluted to moderate the reaction when necessary.

  Such an operation is advantageous when the starting alloy contains metals capable of yielding volatile oxides, which then easily separate from the non-volatile oxides during the operation and are condensed separately. Preliminary purification is thus carried out which alleviates and facilitates all the subsequent operation already described of purification carried out by fractional distillation. This oxidation step is particularly advantageous when the starting alloy contains Mo or

a metal of the same kind.

  The solid oxides thus recovered, including U308, are then treated in the fluorination step, giving, as has already been said, volatile fluorides, optionally oxyfluorides, incondensable gases (of which

  oxygen from the oxides) and unburned solid residues.

DRAWINGS

  FIG. 1 represents the various steps of the process with the main optional variants: (1) represents the alloy in massive form; (11) represents the divided reduction of the alloy; (12) represents the prior oxidation variant of the alloy in massive or divided form, using oxygen (13) more or less diluted, with possible departure of volatile oxides (14); (2) represents the fluorination of the alloy, in solid or divided form, or oxide, with a fluorine-containing gas (20) and generally inert incondensable gases; as has been seen above, this fluorination can be carried out in two stages with the obtention of an intermediate UF4 with the aid of combined fluorine (HF or UF6); (21) represents the condensation of the volatile fluorides obtained in (2), starting from the incondensable gases (22) possibly at least partially recycled in (20) and / or treated in (24), for example by

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  filtration and / or washing, before discharge (25); (23) represents the heating of condensed fluorides in (21); (3) represents the fractional distillation, for purifying UF6, carried out in one or more columns; (31) represents the departure of any incondensable gases, if they have not been separated before, which can be as previously recycled to (20) and / or treated in (24) before rejection (25); (32) represents the multiple exits separated from the various fluorides of the elements of the alloy other than uranium; they can be treated in (33), generally by hydrolysis to obtain the elements in the form of oxide (34) and hydrofluoric acid in (35), then reduced by hydrogen (in 341) to obtain the metals of the alloy

(342);

  (36) represents the output of pure UF6 in general in liquid form; (37) represents the heating of UF6 to put it in gaseous form; (4) represents the mixing operation of pure UF6 in liquid or gaseous form with a more depleted UF6 (40) in liquid or gaseous form in a metered amount; (41) represents condensation and container casting of the final UF6 at

the desired isotopic content.

  Figure 2 shows the fractional distillation (reference 3 in Fig. 1)

  composed of 2 successive columns and which is used in Example 1.

EXAMPLES

  To illustrate the invention, the following examples were made from natural uranium alloys with regard to Examples 1 and 2.

  and from very enriched uranium alloy in the case of Example 3.

Example 1

  A batch of average analysis alloy fragments was used as the starting material: 5% Nb, 5% Mo, 1% Ti, 1% Fe, 0.1% Ni, the balance being natural uranium at 0.7% U235. It was ground to

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  obtain a particle size of between 1.5 and 25 mm.

  It was continuously fluorinated in a Monod reactor of outer diameter 88.9 mm, thickness 3.05 mm equipped with a thermostatic fluid cooling system. The fluorine flow rate was 455 g / h under an absolute pressure of 1.2 bar added temporarily N2 to

  maintain the reactor wall temperature at 320 + 30 C.

  Under these conditions, the nitrogen flow rate varied between 0 and 100%,

  mainly between 40 and 70% of the total flow.

  The resulting gaseous fluorides were completely collected in a crystallizer at an absolute pressure of 0.5 ± 0.1 bar, cooled to 25 ° C. with freon 113, to obtain a fluoride mixture in crystalline form. Separate incondensable gases were washed and neutralized

by an alkaline solution of KOH.

  The crystalline fluoride mixture contains all the uranium, molybdenum and niobium respectively in the form of UF6, MoF6, NbF5,

as well as traces of TiF4.

  Fluoride was collected at the base of the fluorination reactor

  all iron and nickel and 95% titanium.

  The material balance shows the following yields: Yield: U recovered in the form of crystallized UF6 / U of the starting alloy almost equal to 100% Yield: fluorine content in solid or crystalline fluorides /

  fluorine engaged in reaction equal to 95%.

  After intermediate storage in a buffer capacity, the crystallized fluorides were melted to introduce them continuously into the cycle

  of distillation composed of two successive columns.

  They were introduced at 3.3 bars absolute at the bottom 1/3 of a column packed with Monel (diameter: 55 mm, height: 1200 mm), at a continuous flow rate of 4.977 kg / h, ie 1.3 l. / h, for an average composition of (51):

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  4,300 kg / h of UF6 content 0,352 kg / h of MoF6 content 0,325 kg / h of NBF5 content

Trace of TiF4.

  The temperature equilibrium was maintained at 92 ° C. in the middle of the column (52), for a reflux flow rate (53) of 81 / h and a drawdown flow rate of

foot (54) of 0.19 l / h.

  The following mixture (55) was collected at the top of the column at 3.2 bar: UF6: 4.077 kg / h MoF: 0.352 kg / h and at the bottom of the column a mixture containing: NbF5: 0.325 kg / h UF6: 0.223 kg / h

TiF4: traces.

  The head mixture (56) which can be cyclically reprocessed by distillation adapted to separate Nb and U was introduced at the lower 1/3 of the 2nd column of the same type (diameter: 40 mm, height 2400 mm) under

3.2 bars absolute at 87 C.

  The flow rates of reflux and withdrawal were respectively: - reflux (57): 34 1 / h - withdrawal at the bottom (58): 1.14 1 / h or UF6: 4.069 kg / h withdrawal at the overhead condenser (59) : 0.12 l / h or MoF6: 0.3532 kg / h UF6: 0.002 kg / h; UF6 at the bottom of the column was collected in a 25 kg container. Its average analysis was as follows in percentage by weight brought back to uranium:

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  Mo <1 ppm Nb <1 ppm Fe <0.5 ppm Ti <0.5 ppm Ni <0.5 ppm. To these 25 kg in the liquid state at 75 ° C., 6.25 kg of pure UF6 containing 0.25% of U235, preheated in a container also at C, were added to the mixture obtained. The resulting mixture of 31.25 kg has an isotopic content of 0.61% in

U235 and is perfectly homogeneous.

Example 2

  This example illustrates the two-step fluorination using a compound

  fluorinated (UF6) of a U3Si2 type starting alloy.

  The starting alloy U3Si2 is in the form of a powder having a particle size of less than 150 μm and has the following weight composition:

U 92.6% If 7.3% to 10.1%.

  This powder represents what can be obtained from a U3Si2-based research fuel originally contained in an Al matrix, which has been previously removed by basic etching or any

another equivalent process.

  As in the previous example, we operated for practical reasons with

natural PU (0.7% in U235).

  It was carried out in a fluorination reactor identical to that of Example 1; however, considering the state of division of the powder, it was used a feed system of said powder protected by a sweep

argon.

  770 g / h of powdery alloy were introduced at the reactor head. 2112 g / h of highly depleted UF6 (0.3% in U235) at reactor bottom 516 g / h of fluorine with HF content <100 ppm more or

  less diluted by Ar to control the temperature.

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  Pressure and temperature conditions maintained in the reactor

  were identical to those of Example 1.

  A gaseous mixture containing 22 g / h of fluorine and diluting argon at 3168 g / h of UF6 of isotopic content of 0.43% in U235 was recovered at the top of the reactor.

208 g / h of SiF4.

  Solid unburnt made of AlF3 was also recovered.

  The gas stream was cooled to -25 C; the entire UF6 was condensed and vent gases containing excess fluorine and

all SiF4 (208 g / h).

  A simple degassing of the crystallized UF6, under reduced pressure while maintaining a temperature plateau, then made it possible to obtain a UF6

  containing less than 1 ppm Si and less than 20 ppm HF.

Example 3

  A batch of U3Si type uranium enriched fuel alloy fragments of medium enriched nature (m

  % U235) contained in an aluminum matrix.

  The starting material had been shredded and ground to obtain a

  particle size between 5 and 25 mm.

  The composition of the product was substantially as follows (by weight): Uranium: 80% Silicon 3% Plutonium: 6% Aluminum: 10% Other elements: 1%, the most important being

Mo, Nb, Ti, Ru and Mg.

  The product was first continuously treated in a BNSDOCiD monel reactor: internal diameter 80 mm by a current of HCI gas of 800 g / h diluted by a continuous flow rate of 500 g / h of N2. The temperature was maintained at 400 C

  and the reactor pressure at 102 KPa.

  The resulting volatile chlorides were filtered and pyrohydrolysed in an Al 2 O 3 fluidized bed reactor maintained at 400 ° C under a steam atmosphere. The oxides obtained were recovered in the foot and the gases

  Released acids were neutralized on a lime absorber.

  The nonvolatile chlorides remaining in the reactor were then fluorinated continuously at 350 ° C. under a pressure of 102 KPa. Fluoride flow

  was 150 g / h diluted by a flow rate of 500 g / h of N2.

  The volatile fluorides were filtered and then successively subjected to the following treatments: - reduction of PuF6 to PuF4 in a 350 C reactor with SF4 at a rate of

15 g / h.

  - passage of gases in a column packed with LiF pellets

maintained at 300 C.

  - Then passing over a packed column of MgF2 pellets maintained at C. The gases were then filtered, cooled in a condenser maintained at

  -20 C in which UF6 has been crystallized. The pressure was 0102 KPa.

  Incondensable gases, mainly F2.02, SF6, 02 'N2 were filtered

  and treated on a lime absorber.

  After vacuum degassing for a few hours, the condenser was warmed

  at 90 ° C and the liquid UF6 was subjected to two successive distillations.

  In the first column, we have recovered at the bottom of column heavy as NbF5. In the second column, at least a few tens of grams of MoF6 and at the bottom of the UF6 were recovered at the top.

pure.

  The activity of UF6 thus obtained was less than 2x10 -7% Ci / g U, and

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  the content of ordinary metallic impurities <50 ppm, of which Mo <1 ppm.

  Pu was localized at 99% in the SF4 reduction reactor and the remainder in the LiF column. The fission products were located in the LiF column and in the MgF2 column. The radiochemical activity checks were negative on the

  gas at the outlet of the lime absorbers.

  The safety filters placed upstream of the absorbers revealed little

  dust entrainment and very low activity.

  The process according to the invention thus leads to a very homogeneous pure final UF6, in particular as regards the U235 content; it makes it possible to recover the impurities contained in the initial alloy, in condensed form and thus generates no liquid effluent containing in particular said impurities in diluted form, effluent that would be treated before rejection. It also helps to limit the problems of

  criticality given the absence of aqueous phase.

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Claims (12)

  1. Process for treating a uranium-based metal alloy that is preferably very enriched in U235 and other metallic elements comprising: a) a treatment for fluorinating the alloy with at least one fluorinated gas to recover, on the one hand, a gaseous mixture containing uranium and other elements of the alloy in the form of gaseous fluorides which may contain oxyfluorides in the case of the presence of oxygen in the starting alloy with incondensable gases, and on the other hand a solid residue unburnt or fluorinated non-volatile, b) a distillation treatment of said gaseous mixture to recover separately from pure uranium hexafluoride, fluorides of the other elements, c) a mixing operation of said highly enriched and pure uranium hexafluoride in the form of liquid or gaseous with a less enriched uranium hexafluoride in such proportions that the   isotopic desired in the resulting mixture.   2. Method according to claim 1 characterized in that the alloy   metal is put in divided form before fluorination.   3. Method according to any one of claims 1 or 2 characterized in   that the metal alloy is treated with an oxygen-based gas before fluorination and after the possible split forming, to obtain solid oxides, including U308, and gaseous oxides which are separated.   4. Method according to any one of claims 1 to 3 characterized in   that the distillation is preceded by a step comprising the condensation of the gaseous fluorides, the subsequent separation of the incondensable gases and   the return in liquid or gaseous form of the fluorides.   5. Method according to any one of claims I to 4 characterized in   that the fluorination is carried out in one step using fluorine gas diluted by an inert gas. BNSDOCID GB 2705660A1 1   6. Method according to any one of claims 1 to 4 characterized in   that the fluorination is carried out in two stages, a first step where the alloy is treated with combined fluorine to obtain a solid containing uranium in UF4 form and a second stage where said solid is treated with a gas at fluorine base. 7. Method according to claim 6 characterized in that during the   first step the alloy is treated with gaseous anhydrous HF.   8. Method according to claim 6 characterized in that during the first step, the alloy is treated with UF6 gas, preferably more   depleted in U235 as the U of the starting alloy.   9. Process according to any one of Claims 1 to 8, characterized in   the distillation treatment is carried out using at least one distillation column, and preferably two, comprising a condenser and a reflux boiler respectively connected at the foot and at the top of said (or said) columns.   10. Process according to any one of claims 1 to 9 characterized in   that the incondensable gases separated from condensed fluorides are   recycled to the fluorination stage.   11. Method according to any one of claims 1 to 10 characterized   in that the fluorides of the other metallic elements obtained in step   (b) are separated from each other by distillation.   12. Method according to any one of claims 1 to 11 characterized   in that the fluorides of the other metal elements obtained in step (b), separated or not from each other, are processed for processing in oxide then in metal. BNSDOCID: <FR 2705660A1 I>