Plutonium nitrate solution from fuel reprocessing is to be converted either to plutonium dioxide for fuel fabrication or it may be converted to intermediate compounds suitable for reduction to plutonium metal. Direct thermal decomposition of the nitrate solution to plutonium dioxide is possible, but it requires very pure feed solutions. Industrial-scale operations usually begin with the precipitation of plutonium peroxide or plutonium oxalate, which result in further decontami­nation of impurities. Routes to the production of plutonium metal may involve hydrofluorination of plutonium dioxide, peroxide, or oxalate to form the anhydrous tetrafluoride for metaUothermic reduction. Alternatively, anhydrous plutonium halides may be formed by precipitation of PuF3 or CaF2 — PuF4 , either of which can be reduced directly to the metal, or PuF3 may be converted to PuF4 or PuF4-Pu02 for subsequent reduction. Preparation of PuCl3 from calcined Pu02 is another alternative.

Figure 9.6 Solubility of Pu(OH)4 and Pu(IV) polymer as a function of acidity. (From Rai and Seme fRIJ.)

H20’I40g * As contained Pu

The important industrial-scale conversion operations are described below, followed in Sec.

4.7 by the description of processes to produce plutonium metal.

Plutonium peroxide. Plutonium nitrate solutions containing plutonium of any oxidation state can be used as the feed solution for peroxide precipitation, because plutonium in aqueous solu­tions is converted to the tetravalent state by hydrogen peroxide. A flow sheet of the peroxide precipitation process is shown in Fig. 9.7 [Ml, C2]. A solution of 30 to 50 percent H2 02 is added to the plutonium nitrate-nitric acid solution slowly to promote precipitation of large and easily filtered hexagonal crystals. About three peroxide oxygen atoms are required per plutonium atom. A low temperature, about 30°C or less, is desirable to reduce peroxide decomposition. Peroxide precipitation equipment used at the U. S. Savannah River plant is refrigerated so that precipitation can occur at 15°C, followed by digestion at 6°C, to minimize decomposition of H202 by impuri­ties in the feed [Ml]. The filtered plutonium peroxide cake can be calcined at 150°C to form plutonium oxide, although a final temperature of 900°C is required to produce stoichiometric Pu02. Alternatively, the peroxide cake can be dried at 25 to 55°C and fluorinated with HF to form PuF4 , for subsequent reduction to the metal.

Peroxide precipitation achieves excellent decontamination from cationic impurities, because there are so few metals that form peroxide precipitates. However, special care must be taken in

the preparation and handling of solid and dissolved peroxides, which can decompose explosively, especially in the presence of iron and other impurities that catalyze decomposition.

Phitonium(IV) oxalate. The tetravalent plutonium oxalate can be precipitated from a nitric acid solution of Pu(IV) nitrate according to the flow sheet of Fig. 9.8 [H3]. Hydrogen peroxide is added for valence adjustment to Pu(IV), either before or during the addition of oxalic acid. Best precipitations occur at a temperature of 50 to 60°C and for time periods of oxalic acid addition in the region of 10 to 60 min. Higher temperatures and more rapid addition of oxalic acid result in finely divided and gummy precipitates. The nitric acid concentration must be adjusted so that the final oxalate slurry is in a solution between 1.5 and 4.5 M HN03. At lower acid concentrations coprecipitation of impurities is favored and the precipitate is finely divided, whereas at higher acid concentrations the solubility of Pu(IV) oxalate is unsuitably high and the precipitate is thixotropic. The oxalate cake is washed and then calcined at 300°C, followed by restructuring at 900°C to form stoichiometric Pu02. If the final product is to be plutonium metal, the oxalate can be fluorinated directly with HF and oxygen to form PuF4 for subsequent reduction.

The Pu(IV) oxalate process achieves decontamination factors of about 3 to 6 for zirconium — niobium, 12 for ruthenium, 60 for uranium, and 100 for aluminum-chromium-nickel. As com­pared with peroxide precipitation, the oxalate process achieves less decontamination from im­purities, but the solutions and solids are more stable and safer to handle. It is more suitable for processing solutions containing high concentrations of impurities that would catalyze peroxide decomposition.

Plutonium(III) oxalate. If the plutonium solution from fuel reprocessing is concentrated by sorption on a cation-exchange resin, the plutonium eluent will be a nitrate solution of stabilized trivalent plutonium. This solution may be a logical candidate for the relatively simple precipitation of Pu(III) oxalate. The Pu(III) oxalate Pu2(C204)3 -9H20 can be easily precipitated by adding oxalic acid, either as a solution or a solid. Precipitation conditions are not critical, and the Pu(III)

Figure 9.8 Flow sheet for the precipitation of Pu(IV) oxalate. (From Cleveland [C2], by per­mission.)

oxalate precipitate settles rapidly and is easy to filter. The oxalic acid can be added rapidly, with a digestion period of about a half hour.

If the Ри(Ш) nitrate is not stabilized against oxidation, hydriodic acid, hydroxylamine or ascorbic and sulfamic acid may be used as a reducing agent. Problems of handling and corrosion result with HI. If appreciable Pu(IV) were present, more stringent care would have to be taken to avoid unmanageable precipitates of Pu(IV) oxalate, as discussed above.

The Pu(III) oxalate is calcined to form the dioxide or hydrofluorinated with HF and oxygen to form PuF4 , as has been described above for the tetravalent oxalate.

When process solutions of Pu(III) are available, Pu(III) oxalate precipitation may be desirable if impurity levels are too high for the PuF3-precipitation process described later.

Pu02 from direct calcination of Pu(N03 )4. The precipitation steps of the above processes can be avoided by the direct calcination of the plutonium nitrate solution to Pu02. Calcination has been carried out at 350°C in a liquid-phase screw calciner. Half a mole of ammonium sulfate per mole of plutonium is added to the feed solution to increase the production of reactive Pu02. The calcination time and temperature must be low enough to minimize sintering, which would other­wise reduce the chemical reactivity of the oxide particles for subsequent conversion to a halide.

Direct calcination of Pu(N03)4 involves no chemical separations that could remove impuri­ties, so a highly pure plutonium nitrate feed solution is required. The plutonium dioxide product can be hydrofluorinated to PuF4, or it can be used as a feed for the formation of PuCl3. Direct calcination has received less industrial-scale application than the precipitation processes described above [C2].

Plutonium trifluoride. Plutonium trifluoride can be converted directly to plutonium metal, or it is an intermediate in the formation of PuF4 or PuF4 — Pu02 mixtures for thermochemical reduc­tion, as described in Sec. 4.8. The stabilized Pu(III) solution, produced by cation exchange in one of the Purex process options for fuel reprocessing, is a natural feed for the formation of plu­tonium trifluoride, as is shown in the flow sheet of Fig. 9.9 [03]. A typical eluent solution from cation exchange consists of 30 to 70 g plutonium/liter, 4 to 5 M nitric acid, 0.2 M sulfamic acid, and 03 M hydroxylamine nitrate. The sulfamic acid reacts rapidly with nitrous acid to reduce the rate of oxidation of Pu(III) to about 4 to 6 percent per day. Addition of ascorbic acid to the plutonium solution just before fluoride precipitation reduces Pu(IV) rapidly and completely to Pu(III).

Addition of 2.7 to 4 Mhydrofluoric acid results in the precipitation reaction

Pu(N03 )i(aq) + 3HF(o<?) — PuF3 (s) + 3HN03 (aq) (9.47)

with a solubility product for PuF3 of 2.4 ± 0.4 X 10"16. Easily filterable precipitates result from controlled rate of addition of the reagents and by maintaining a HN03/HF ratio of at least 4. In contrast to PuF4, the PuF3 precipitate is crystalline and contains no water of crystallization, so that it is easily dried to the anhydrous salt desirable for metallothermic reduction to plutonium metal. The filtered trifluoride cake is washed with 0.8 M HF and dried by ambient air. Anhydrous PuF3 is produced by further drying with warm air, followed by heating to 600°C in argon to remove remaining volatile impurities [B6].

The PuF3 process does not attain the degree of decontamination from cationic impurities that can be achieved in peroxide or oxalate precipitation, but it is acceptable when the plutonium nitrate feed contains no more than a few hundred parts of uranium and aluminum per million of plutonium. This process has been in routine use at the U. S. plant at Savannah River [B6,03].

Plutonium tetrafluoride. Precipitation of PuF4 by adding hydrofluoric acid to a Pu(TV) nitrate solution is impractical because the hydrated precipitate PuF4 -23H20 is amorphous and difficult to Alter, and it is difficult to dehydrate to the anhydrous material necessary for subsequent reduc-

Pu concentrate

0. 6 5//min

Balch size’ I to 2 kg Pu

tion to the metal. Vacuum dehydration of the precipitate at 200°C yields PuF4 — H2 0, and further heating results in the trifluoride [Cl]. When heated in a moist atmosphere above 300°C, PuF4 hydrolyzes to Pu02 [K2]. Therefore, the more difficult process of hydrofluorination of a solid is necessary to obtain anhydrous PuF4.

If the plutonium to be fluorinated is the plutonium peroxide cake, as in one of the processes used at the U. S. Savannah River plant, the air-dried cake is reacted with HF gas at 600°C. The reaction time is quite sensitive to sulfate containment in the oxalate cake, which interferes with fluorination and requires a longer time for reaction of the oxalate with HF. The interfering sulfate is that present due to a sulfuric acid wash of the cation-exchange resin prior to peroxide precipita­tion.

Alternatively, PuF4 can be formed from Pu(III) or Pu(IV) oxalate cake by drying the cake in air at 100 to 120°C and fluorinating in HF at 400 to 600°C, or the dried oxalate can be calcined at 130 to 300°C in air to form Pu02, which is then hydrofluorinated in HF to form PuF4. The hydrofluorination temperature must equal or exceed the calcination temperature, and the latter must be kept below 480°C to prevent formation of refractory oxide [Ml]. Similarly, PuF4 can be prepared by hydrofluorinating Pu02 or PuF3.

With the availability of anhydrous plutonium trifluoride, as discussed in the previous section, the equipment problems associated with the direct conversion of PuF3 to PuF4 with HF and oxygen can be avoided by roasting the trifluoride in oxygen to form a mixture ofPuF4 and Pu02, according to the reaction

4PuF3 + 02 -*• 3PuF4 + Pu02 (9.48)

The PuF4-Pu02 mixture is suitable for metallothermic reduction, as discussed in Sec. 4.8.

Air-dried PuF3 cake is roasted in an inert atmosphere at 150 to 200°C for 1 і to 3 h, and then in an oxygen atmosphere at 400 to 600° C. This is one of the processes that has been em­ployed at the U. S. plant at Savannah River [C2,03].

The CaF2‘PuF4 process. A process for forming plutonium tetrafluoride, without the attendant corrosion problems of dry hydrofluorination, involves the precipitation of the double salt CaF2’PuF4 from a solution of Pu(N03)4. By contrast to the PuF4*2.5H20 precipitate, the CaF2 -PuF4 precipitate is less soluble, is readily dried, and may be directly reduced to the metal.

A flow sheet for the precipitation of CaF2 — PuF4 is shown in Fig. 9.10 [C2]. A solution of plutonium and calcium nitrates in 4 to 5 M HN03 is added to HF solution of 6 M or less. The precipitate is washed with dilute HN03-HF solution and dried at 300°C in argon or nitrogen to form the anhydrous CaF2 — PuF4, which must be crushed to particles suitable for reduction to the metal.

The process is attractive in its simplicity. However, in the subsequent metallothermic reduc­tion the CaF2 diluent absorbs a portion of the heat of reaction otherwise needed for slag melting. Also, there is less decontamination from impurities than in the case of the other precipitation processes described earlier.

Plutonium trichloride. Although PuCl3 is more hygroscopic than the plutonium fluorides, and although it generates less heat of reaction in subsequent metallothermic reduction to the metal,

Waste Figure 9.10 Flow sheet for the precipitation of CaF2*PuF4. (From Cleveland [C2], by permis­sion.)

the production of PuCl3 is motivated by the reduced shielding requirements. Because of the relatively weak reaction of plutonium alphas with chlorine to produce neutrons, the neutron emission of PuCl3 is one-sixty-fourth that of PuF4 [C2]. Also, the slag from PuCl3 reduction melts at a much lower temperature than the fluoride slags (see Sec. 4.8).

Plutonium dioxide, prepared by direct calcination of the nitrate or calcination of the peroxide or oxalate precipitates, can be chlorinated to PuCl3 by HC1-H2, gaseous ССЦ, or phosgene (COClj), the latter resulting in the most rapid reaction.

Chlorination of nitrate-calcined oxide has been carried out in a fluidized bed at 500° C. Oxide from oxalate calcination has been chlorinated in a continuous screw calciner at 250 to 350°C. Because many impurities form volatile chlorides under these conditions, relatively good decon­tamination from impurities results. Consequently, this is a logical conversion step to follow the direct calcination of Pu(N03 )4.

It is essential that PuCl3 be handled only in a very dry atmosphere, otherwise hygroscopic moisture accumulation can result in excessive pressures during subsequent reduction to the metal.