A METHOD FOR THE DIGESTION OF A URANIUM BASED MATERIAL

Abstract

The present invention relates to a method for at least partially digesting a uranium (U)-based target material which comprises at least one uranium-metal (U-Me) alloy containing Mn, Fe, Co or Ni and comprising at least a U6Me phase. By means of an accelerant in a basic solution, the uranium in U-Me alloy oxidizes to U(VI). The accelerant comprises in particular KMnO4 whilst the U-Me alloy comprises a U—Mn alloy. The alloy preferably comprises two phases of an eutectic system, in particular the U6Mn/UMn2 system. The use of the accelerant enables an enhanced digestion of the U-Me alloy.

Claims

1-19. (canceled)

20. A method for at least partially digesting a uranium-based material (U-based material, herein after) which comprises at least one uranium-metal alloy [U-Me alloy, herein after], wherein Me is selected from the group consisting of Mn, Fe, Co, Ni and combinations thereof and wherein the U-Me alloy comprises at least a U.sub.6Me phase, which method comprises the following steps: Step 1: providing the U-based material; Step 2: contacting the U-based material with a basic solution thereby forming a mixture (1); and Step 3: oxidizing at least part of said uranium in said mixture (1) to uranium (VI) by means of at least one accelerant selected from the group consisting of permanganate (MnO.sub.4.sup.2−), chromate (CrO.sub.4.sup.2−), dichromate (Cr.sub.2O.sub.7.sup.2−), perchlorate (ClO.sub.4.sup.−), chlorate (CO.sub.3.sup.−), chlorite (ClO.sub.2.sup.−), ozone (O.sub.3) and hypohalite (XO.sup.−).

21. The method according to claim 20, wherein said basic solution is a solution comprising one or more alkali or alkaline earth hydroxides.

22. The method according to claim 20, wherein said basic solution is a solution comprising one or more alkali or alkaline earth hydroxides being present in the basic solution in a concentration of at least 1.0 mol/L.

23. The method according to claim 20, wherein said basic solution comprises at least one mineral salt which is capable of reacting with hydrogen in said basic solution.

24. The method according to claim 20, wherein said basic solution comprises at least one nitrate salt which is capable of reacting with hydrogen in said basic solution, wherein said nitrate salt is present in the basic solution in a concentration of at least 1.0 mol/L.

25. The method according to claim 20, wherein said at least one accelerant is added to said mixture (1) in a maximum amount of accelerant which reacts with said U-based material in said mixture (1) or in an amount which is larger than said maximum amount or which is at least 80% of said maximum amount.

26. The method according to claim 20, wherein the uranium in said mixture (1) is oxidized by means of said at least one accelerant until at least 70.0 wt. % of the uranium contained in said mixture (1) is oxidized to uranium (VI).

27. The method according to claim 20, wherein said at least one accelerant is added in a predetermined amount to said mixture, at least 50.0% of said predetermined amount being added to said mixture (1) after said basic solution has reacted for at least 30 minutes with said U-based material.

28. The method according to claim 20, wherein said accelerant comprises permanganate.

29. The method according to claim 20, wherein said U-based material comprises said U-Me alloy in a particulate form.

30. The method according to claim 29, wherein said U-Me alloy is dispersed in an aluminum based matrix.

31. The method according to claim 29, wherein said particulate U-Me alloy has a D.sub.90 value, measured in accordance with ASTM B214-16, which is smaller than 120 μm.

32. The method according to claim 29, wherein said U-Me alloy is dispersed in said basic solution, the mixture (1) being subjected to ultrasonic waves during at least part of said oxidation Step 3.

33. The method according to claim 20, wherein the at least one U-Me alloy comprises two phases of an eutectic system, a first of said phases being said U.sub.6Me phase, and a second of said phases being a phase with a lower uranium density [lower U density phase, herein after], the at least one U-Me alloy comprising at least 5.0 wt. % of said lower U density phase.

34. The method according to claim 33, wherein the at least one U-Me alloy comprises at least 40.0 wt. % of said U.sub.6Me phase.

35. The method according to claim 33 wherein said eutectic system is selected from the group consisting of U.sub.6Mn/UMn.sub.2, U.sub.6Fe/UFe.sub.2, U.sub.6Co/UCo and U.sub.6Ni/U.sub.7Ni.sub.9.

36. The method according to claim 20, wherein the at least one U-Me alloy comprising at least 80.0 wt. % of said U.sub.6Me phase.

37. The method according to claim 20, wherein said U-Me alloy is a U—Mn alloy and said U.sub.6Me phase is a U.sub.6Mn phase.

38. The method according to claim 20, wherein said U-Me alloy comprises .sup.235U, and the U-based material is irradiated by means of neutrons before being partially digested.

39. The method according to claim 20, wherein said U-Me alloy comprises less than 20 wt. % of .sup.235U of the total amount of U, and the U-based material is irradiated by means of neutrons before being partially digested.

Description

EXAMPLES

[0076] The invention will now be described in more details with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention. In the examples reference is made to the drawings wherein:

[0077] FIG. 1 is an SEM image of the U.sub.6Mn alloy (U—Mn alloy 1);

[0078] FIG. 2A is an SEM image of a partially digested U.sub.6Mn—UMn.sub.2 76/24 alloy (U—Mn alloy 2);

[0079] FIG. 2B is a part of the SEM image of FIG. 2A with a larger magnification;

[0080] FIG. 3 is an SEM image of the U.sub.6Mn—UMn.sub.2 88/12 alloy (U—Mn alloy 3);

[0081] FIG. 4 is an SEM image of the U.sub.6Mn—UMn.sub.2 41/59 alloy (U—Mn alloy 4);

[0082] FIG. 5A is an SEM image of a partially digested U.sub.6Mn alloy particle;

[0083] FIG. 5B is a schematic drawing of the SEM image of FIG. 5A onto which the different phases/digestion products are indicated;

[0084] FIG. 6A is an SEM image of another partially digested U.sub.6Mn alloy particle showing the presence of an intermediate UO.sub.x layer in between the U.sub.6Mn core and the UO.sub.2 layer;

[0085] FIG. 6B is a schematic drawing of the SEM image of FIG. 6A onto which the different phases/digestion products are indicated;

[0086] FIG. 6C is a part of the SEM image of FIG. 6A with a larger magnification; and

[0087] FIG. 7 is an XRD diffractogram from 15° to 70° of the particles of the target digested as described in Example 5.

GENERAL PROCEDURE—PREPARATION OF U—Mn ALLOYS

[0088] All U—Mn alloys were made by arc-melting in an Arc 200 cold crucible arc-melting furnace under pure argon gas. Before melting, both metals (natural uranium and 99% Mn) were stripped of their surface oxide layer. The surface oxidation layer on the uranium was removed with 60% by volume nitric acid and wiped with acetone while manganese oxide on the manganese chips were sanded with silicon carbide paper and rinsed in acetone. For safety reasons, use was made of natural uranium but it is clear that in practice use will be made of uranium enriched in .sup.235U and the material will be irradiated to fission part of the .sup.235U to produce .sup.99Mo. A determined amount of metal uranium and metal manganese was loaded into the copper hearth, the furnace chamber was put under a vacuum of 4×10.sup.−3 mbar, backfilled with argon, then vacuumed a second time to 2.3×10.sup.−4 mbar for 1 h to ensure that as little oxygen is present in the chamber while arc-melting. To ensure homogeneity, each alloy is flipped and melted three times.

U—Mn Alloy 1: Preparation of U.SUB.6.Mn

[0089] U.sub.6Mn was prepared according to the general procedure described above and was subsequently annealed to achieve its equilibrium structure. The production of U.sub.6Mn is almost always accompanied with UMn.sub.2 along its grain boundaries (see FIG. 1) in the form of an eutectic decomposition. The U—Mn alloy contained about 3.7 wt. % of Mn and 96.3 wt. % of U.

U—Mn Alloy 2: Preparation of U.sub.6Mn—UMn.sub.2 76/24

[0090] U.sub.6Mn—UMn.sub.2 76/24 was prepared according to the general procedure described above and was subsequently annealed to achieve its equilibrium structure. An SEM image of a partly digested particle of the produced alloy showed that it contained 76 wt. % of U.sub.6Mn and 24 wt. % of UMn.sub.2. Calculated with the Lever rule, such a mixture of phases contains about 10.4 wt. % of Mn. FIG. 2A shows an SEM image of the obtained alloy after partial digestion thereof as described here after. The outer darker part is digested the lighter core part is not yet digested. The undigested alloy comprises light U.sub.6Mn grains in a darker eutectic U.sub.6Mn—UMn.sub.2 composition. FIG. 2B shows with a larger magnification the transition between the digested and the undigested part of the alloy. It can be seen that the matrix between the lighter U.sub.6Mn grains is formed by an eutectic structure consisting of a succession of lighter U.sub.6Mn and darker UMn.sub.2 lamellae.

U—Mn Alloy 3: Preparation of U.sub.6Mn—UMn.sub.2 88/12

[0091] U.sub.6Mn—UMn.sub.2 88/12, containing 7 wt. % of Mn, was prepared in the same way as U—Mn alloy 2. FIG. 3 shows an SEM image of the obtained alloy. The alloy comprises light U.sub.6Mn grains and an eutectic microstructure consisting of a succession of light U.sub.6Mn and dark UMn.sub.2 lamellae.

U—Mn Alloy 4: Preparation of U.sub.6Mn—UMn.sub.2 41/59

[0092] U.sub.6Mn—UMn.sub.2 41/59, containing 20 wt. % of Mn, was prepared in the same way as U—Mn alloys 2 and 3. FIG. 4 shows an SEM image of the obtained alloy. The alloy comprises dark UMn.sub.2 grains in a light U.sub.6Mn matrix.

Comparative Example 1: Digestion of U.SUB.6.Mn Alloy in NaOH/NaNO.SUB.3

[0093] In double-neck borosilicate reaction vessel equipped with a condenser, a 100 mL solution of 4M NaOH and 3M NaNO.sub.3 was prepared and heated to 95° C. Powered 2.3 g of U—Mn alloy 1 was immersed into the solution and magnetically stirred. At designated time points, a 2 mL aliquot was immersed into a −4° C. bath and diluted to 1.2 M NaOH. The aliquot was then put under dialysis to eliminate salts and to isolate the digested particles.

[0094] The resulting digested particles were analyzed by SEM and EDX following the general procedure.

[0095] After 5 minutes of digestion and also after 30 minutes of digestion, SEM images and EDX mapping reveals that the UMn.sub.2 located on the particle surface is oxidized to UO.sub.2. The oxidation layer can penetrate deep into the particle but primarily along the grain boundary network of UMn.sub.2. It is further observed that the U.sub.6Mn is not oxidized, even in areas where U.sub.6Mn is at the surface of the particles.

[0096] After 120 minutes of digestion, digestion was still substantially the same as after 30 minutes. The dispersion was filtered and a small elongated particle having a width of about 15 μm and a larger elongated particle having a width of about 45 μm were analyzed by SEM. The small particle predominantly consisted of UO.sub.2 whilst the larger particle contained a mixture of UO.sub.2 and U.sub.6Mn. X-ray diffraction indicated that UO.sub.2 was the only digestion product and that no sodium diuranate has formed in levels that are detectable by XRD. Additionally, U.sub.6Mn still remains even after 120 minutes of digestion.

Example 1: Digestion of U.SUB.6.Mn Alloy in NaOH/NaNO.SUB.3 .with Accelerant

[0097] In a double-neck borosilicate reaction vessel equipped with a condenser, a 200 mL solution of 4.3 M NaOH and 3.5 M NaNO.sub.3 was prepared and heated externally to 60° C.

[0098] Powered 0.57 g of annealed U—Mn alloy 1 was immersed into the solution and magnetically stirred. 0.71 g of KMnO.sub.4 was then immediately added with continued stirring. The experimental time was two hours. A 2 mL aliquot was then immersed into a −4° C. bath and diluted to 1.2 M NaOH. The aliquot was then put under dialysis to eliminate salts and to isolate the digested particles.

[0099] Cross section were analyzed (FIG. 5A). As the contrast in a BSE image depends on the density of the material, the particle can be seen with three different densities. At the core of the particle, the initial fuel can be seen that has partially digested. EDX analysis of this area shows that the average atomic percentage of this area is: Uranium 59±3%, Manganese 21±1%, and Oxygen 18±1%, which corresponds to undigested U.sub.6Mn.

[0100] The second lightest region, located immediately outside the core, is found to have the atomic percentages: Oxygen 61.5±2%, Uranium 36±3%, with less than 1% manganese and sodium, and is ascribed to UO.sub.2 resulting from the digestion of U—Mn alloy.

[0101] Finally the darkest part of the particle, at the particle edge has the atomic composition: Oxygen 66±2%, Uranium 14±3%, sodium 13±2%. This region is ascribed to a sodium diuranate region (Na.sub.2U.sub.2O.sub.7 or NADU), which is the preferred form of yellow cake and ensures a maximum recovery of .sup.99Mo and other medical radioisotopes.

[0102] FIG. 5B is a schematic drawing of the SEM image of FIG. 5A onto which the different phases and reactions are indicated. The use of the accelerant enables to produce not only UO.sub.2 as digestion product but also the required diuranate. Notwithstanding the digestion period of two hours, the sodium diuranate was however only produced at the surface of the particle, whilst the core of the particle even still contained the undigested U.sub.6Mn. The development of the sodium diuranate layer apparently slows down and/or stops the further digestion of the particles once it reaches a few micrometer in thickness.

[0103] In some particles, the development of the UO.sub.2 layer has been found. BSE images of some partially digested particles have shown that apart from the U.sub.6Mn core and the UO.sub.2 and Na.sub.2U.sub.2O.sub.7 regions there is an UO.sub.x intermediate region in between the U.sub.6Mn core and the UO.sub.2 region. FIG. 6A is an SEM image of a particle which comprises, as indicated in FIG. 6B, a succession of a U.sub.6Mn core, an intermediate UO.sub.x region, a UO.sub.2 region and an outer Na.sub.2U.sub.2O.sub.7 region. EDX analysis showed the intermediate UO.sub.x region had the atomic composition: Oxygen 55.7±2%, Uranium 27.4±3, Manganese 16.9±1%. FIG. 6C is a portion of FIG. 6A showing, with a larger magnification, the succession of the UO.sub.x region (lowermost speckled layer), the UO.sub.2 layer (more uniform lighter layer) and the diuranate layer (outer more uniform darker layer). The presence of this intermediate UO.sub.x region might explain why the further digestion of the U.sub.6Mn particles is halted after some time. The structure in the UO.sub.x region could possibly be a porous network of UO.sub.2 containing leftover manganese from the U.sub.6Mn phase and closing possibly in on itself to form a solid layer. The porous network would allow ions to pass through this area, facilitating digestion. Once the network closes ions are prevent from reaching the fresh fuel, and the digestion of the core is arrested. The NADU layer had a thickness of between about 3 μm and 5 μm. This example shows therefore that when the U.sub.6Mn particles are sufficiently small, they can be digested completely.

Example 2: Core Shell Separation of Digested U.SUB.6.Mn Alloy Via Ultrasonication

[0104] Digested particles obtained in example 1 were placed in an ultrasonic bath, and were exposed to 40 kHz of ultrasonication for 15 minutes.

[0105] BSE images revealed that the core particles of U.sub.6Mn were free of a shell layer, and were in other words no longer covered by a NADU and/or a UO.sub.2 layer. EDX analysis of one of these particles showed that it had the following atomic composition: Uranium 82±3%, Mn 13±2%, with minor amounts of oxygen, corresponding to U.sub.6Mn. Hence, when performing the ultrasonication during the digestion in the basis solution the digestion can proceed until all of the U.sub.6Mn is digested. Some shell particles could also be detected. These shell particles consisted of NADU. Most of the UO.sub.2 removed by sonication from the surface of the particles could thus be converted to the desired NADU.

[0106] Ultrasonication therefore effectively removes the shell composed of sodium diuranate and UO.sub.2, hence favoring the digestion of the undigested U—Mn alloy core.

Example 3: Digestion of a U.SUB.6.Mn—UMn.SUB.2 .76/24 Alloy in a NaOH/NaNO.SUB.3 .with Accelerant

[0107] The digestion of a U.sub.6Mn, —UMn.sub.2 76/24 alloy was carried out in a same way as in Example 1.

[0108] The resulting digested particles were analyzed by SEM and EDX.

[0109] The BSE image of a digested U.sub.6Mn—UMn.sub.2 74/24 alloy after digestion for only 15 minutes is shown in FIGS. 2A and 2B. As explained here above, the structure of the alloy consisted of U.sub.6Mn grains embedded in an eutectic matrix formed by a microstructure formed by thin parallel lamellae of U.sub.6Mn and UMn.sub.2. Notwithstanding the very thin lamellae, the UMn.sub.2 lamellae were quickly and easily digested by the basic solution containing the accelerant. The image in FIGS. 2A and 2B reveals indeed that the alloy has been digested or dealloyed after 15 minutes more than 10 μm past the particle surface. The UMn.sub.2 phase dissolves more favorably than the U.sub.6Mn phase. Islands of U.sub.6Mn can be found in the zone between the corrosion front and within the alloy surface. The corrosion front advances along UMn.sub.2 pathways, hence resulting in a percolating pathway for the digestion reagents to digest the remaining undigested U—Mn alloy.

Example 4: Digestion of a U.SUB.6.Mn—UMn.SUB.2 .Alloy in a NaOH/NaNO.SUB.3 .with Accelerant

[0110] In this example the U.sub.6Mn—UMn.sub.2 alloy is first digested in the basic solution without accelerant. It has indeed been shown in Comparative Example 1 that the UMn.sub.2 phase can be oxidized by means of the basic solution to UO.sub.2. After this initial digestion step, accelerant is added to oxidize the UO.sub.2 phase further to the diuranate phase and to digest also the U.sub.6Mn phase, and any remaining UMn.sub.2 phase. In this example, less KMnO.sub.4 is needed and consequently less MnO.sub.2 and thus less waste is produced.

Example 5: Digestion of U.SUB.6.Mn Alloy Dispersed in an Aluminum Matrix/Fuel Target

[0111] A U.sub.6Mn alloy was ground into a powder and was dispersed in an aluminum powder. The U—Mn alloy embedded in aluminum was further cladded in aluminum and was rolled to produce a target plate was cut into 4 cm by 1 cm pieces for the digestion experiments.

[0112] A solution of 4M NaOH and 3M NaNO.sub.3 was prepared in a digestion vessel. The hot water bath containing the digestion vessel was heated to 40° C. before the start of the experiment. The target piece was inserted into the digestion vessel stirred with a magnetic stir bar. The aluminum cladding slowly reacted with the digestion solution releasing H.sub.2 gas which reacted with the NaNO.sub.3 to produce NH.sub.3. After one hour, no gas was visibly produced any more from the aluminum cladding. The temperature of the hot water bath containing the digestion vessel increased during this first hour to about 50° C. and stirring continued for another 40 minutes, to help ensure no gas remained in solution and that the aluminum that was dispersed between the U—Mn alloy fuel had time to dissolve. After 110 minutes, when the reaction mixture reached a temperature of nearly 80° C., potassium permanganate was added to the reaction vessel as accelerant. The accelerant was immediately reduced to MnO.sub.2. KMnO.sub.4 was continually added until the accelerant was no longer reduced and kept its Mn.sup.VII state.

[0113] After 150 minutes of digestion, the mixture had a temperature of about 90° C. and the obtained digested U-based material was filtered from the solution.

[0114] XRD diffractogram from 15° to 70° in 2θ, shown in FIG. 7, reveals mainly sodium diuranate but also a remaining amount of U.sub.6Mn. In the same way as in Example 1, the cores of the alloy particles were still undigested and was composed of U.sub.6Mn. The cores were surrounded by a layer of NADU. Based on Examples 1 and 2, it is clear that any remaining U.sub.6Mn can thus be avoided by reducing the particle size of the U—Mn alloy in the target and/or by replacing the U.sub.6Mn alloy by a U.sub.6Mn—UMn.sub.2 alloy containing an easily digestible UMn.sub.2 phase which can expose the U.sub.6Mn grains (optionally in the form of lamellae) so as to be also completely digestible.