Plutonium precipitate

During plutonium reconversion in a reprocessing plant, oxalate supernatant is normally generated during plutonium precipitation by oxalic acid as plutonium oxalate. This is further converted into plutonium oxide. This waste solution will have the following composition uranium (U) 5 gm dm , plutonium (Pu) 25 mg dm, mthenium (Ru ° ) 0.0032 mCi dm, cesium (Cs ) 0.003 mCi dm , nitric acid 3 M, and H2C2O4 0.1 M. 

Melting—for obtaining ceramics on the basis of gamet/perovskite for immobilization of plutonium precipitates of complex chemical composition. 

Mullins and Leary [29] patented a method of separating americium from plutonium which involves bubbling a mixture of oxygen and argon gas into a molten salt containing both elements. Plutonium precipitates as PuOj, whereas americium stays in solution. 

A single-region converter fueled with natural or slightly enriched uranium oxide as a suspension avoids the problems of plutonium precipitation, phase separation, and corrosion mentioned above. The advantage of such a converter reactor for power production is the elimination of radiation damage and fuel bumup problems encountered with solid-fuel elements however, the problem of radiation damage to the reactor pressure vessel must be considered. 

Initially, the only means of obtaining elements higher than uranium was by a-particle bombardment of uranium in the cyclotron, and it was by this means that the first, exceedingly minute amounts of neptunium and plutonium were obtained. The separation of these elements from other products and from uranium was difficult methods were devised involving co-precipitation of the minute amounts of their salts on a larger amount of a precipitate with a similar crystal structure (the carrier ). The properties were studied, using quantities of the order of 10 g in volumes of... 

The wastes from uranium and plutonium processing of the reactor fuel usually contain the neptunium. Precipitation, solvent extraction, ion exchange, and volatihty procedures (see Diffusion separation methods) can be used to isolate and purify the neptunium. 

The plutonium usually contains isotopes of higher mass number (Fig. 1). A variety of industrial-scale processes have been devised for the recovery and purification of plutonium. These can be divided, in general, into the categories of precipitation, solvent extraction, and ion exchange. 

Kilogram quantities of americium as Am can be obtained by the processing of reactor-produced plutonium. Much of this material contains an appreciable proportion of Pu, which is the parent of Am. Separation of the americium is effected by precipitation, ion exchange, or solvent extraction. 

Uranium. The uranium product from the PUREX process is in the form of uranyl nitrate which must be converted to some other chemical depending on anticipated use. One route to MO fuel is to mix uranium and plutonium nitrates and perform a coprecipitation step. The precipitate is... 

Plutonium. The plutonium nitrate product must be converted to MO fuel if it is to be recycled to lightwater reactors. Whether from a plutonium nitrate solution or a mixed U/Pu nitrate solution, the plutonium is typically precipitated as the oxalate and subsequendy calcined to the oxide for return to the fuel cycle (33). 

Evidence foi the anionic complex PuCP is the precipitation of complex halides such as Cs2PuClg from concentrated HCl (aq). The ability of Pu(IV) to form stable nitrate complexes provides the basis for the Purex and ion-exchange (qv) process used in the chemical processing of Pu (107). Pu(VI) is similar to Pu(IV) in its abihty to form complex ions. Detailed reviews of complex ion formation by aqueous plutonium are available (23,94,105). 

The plutonium extracted by the Purex process usually has been in the form of a concentrated nitrate solution or symp, which must be converted to anhydrous PuF [13842-83-6] or PuF, which are charge materials for metal production. The nitrate solution is sufficientiy pure for the processing to be conducted in gloveboxes without P- or y-shielding (130). The Pu is first precipitated as plutonium(IV) peroxide [12412-68-9], plutonium(Ill) oxalate [56609-10-0], plutonium(IV) oxalate [13278-81-4], or plutonium(Ill) fluoride. These precipitates are converted to anhydrous PuF or PuF. The precipitation process used depends on numerous factors, eg, derived purity of product, safety considerations, ease of recovering wastes, and required process equipment. The peroxide precipitation yields the purest product and generally is the preferred route (131). The peroxide precipitate is converted to PuF by HF—O2 gas or to PuF by HF—H2 gas (31,132). 

Oxalates. Stable oxalates of Pu(III), Pu(IV), and Pu(VI) are known. However, only the Pu(III) and Pu(IV) oxalates are technologically important (30,147). Brilliant green plutonium(III) oxalate [56609-10-0] precipitates from nitric acid solutions containing Pu(III) ions upon addition of oxaUc acid or sodium oxalate. The composition of the precipitate isPu2(C20 2 10H2O. A homogeneous oxalate precipitation by hydrolysis of diethyl oxalate at... 

C minimises plutonium loss ia the filtrate and decreases filtering time (166). Heating the compound to ca 270°C ia air or to 460°C ia absence of air results ia the formation of PUO2. Yellow-green Pu(IV) oxalate [26588-74-9] Pu(C20 2 6H20, precipitates from acidic solutions of Pu(IV) upon addition... 

The reason for the ultramicrochemical test was to establish whether the bismuth phosphate would carry the plutonium at the concentrations that would exist at the Hanford extraction plant. This test was necessary because it did not seem logical that tripositive bismuth should be so efficient in carrying tetrapositive plutonium. In subsequent months there was much skepticism on this point and the ultramicrochemists were forced to make repeated tests to prove this point. Thompson soon showed that Pu(Vl) was not carried by bismuth phosphate, thus establishing that an oxidation-reduction cycle would be feasible. All the various parts of the bismuth-phosphate oxidation-reduction procedure, bulk reduction via cross-over to a rare earth fluoride oxidation-reduction step and final isolation by precipitation of plutonium (IV) peroxide were tested at the Hanford concentrations of... 

Iodine was determined by an iodometric titration adapted from White and Secor.(3) Instead of the normal Carius combustion, iodide was separated from the samples either by slurrying in 6M NaOH, or by stirring the sample with liquid sodium-potassium (NaK) alloy, followed by dissolving excess NaK in ethanol. Precipitated plutonium hydroxides were filtered. Iodine was determined in the filtrate by bromine oxidation to iodate in an acetate buffer solution, destruction of the excess bromine with formic acid, acidifying with SO, addition of excess KI solution, and titrating the liberated iodine with standard sodium thiosulfate. The precision of the iodine determination is estimated to be about 5% of the measured value, principally due to incomplete extraction of iodine from the sample. 

Plutonium, deposited on soil, moves downwards with a rate which depends on precipitation and soil properties. In dry, sandy areas, the downward rate may be 1 mm/year, while in rainy areas it may be 10 times higher (3). The rate is considerably reduced in clay soil. 

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

The existence of such a process can be indirectly evidenced by the irregular change of decrement observed in free carbonate (also bicarbonate) concentrations near pH = 10 as illustrated in Figure 3. This may be ascribed to the consumption of free carbonate ions for the formation of the plutonium hydroxocarbonate precipitate. The reactions involved in this process are interpreted as follows ... 

After the stipulated leaching period, the waters were sampled before and after filtration through 0.05 ym Nuclepore filters and the oxidation-state distribution of plutonium in the filtrates was determined as follows PrF3 carrier precipitation for Pu(III) and (IV), PrF3 precipitation following NaHS03 reduction for total plutonium, and thenoyltrifluoroacetone (TTA) extraction... 

The Los Alamos MPL can accept either plutonium nitrate solution or plutonium oxide as feed. If the feed is the nitrate solution, then the process steps are precipitation of... 

Figure 3 shows a flowsheet for plutonium processing at Rocky Flats. Impure plutonium metal is sent through a molten salt extraction (MSE) process to remove americium. The purified plutonium metal is sent to the foundry. Plutonium metal that does not meet foundry requirements is processed further, either through an aqueous or electrorefining process. The waste chloride salt from MSE is dissolved then the actinides are precipitated with carbonate and redissolved in 7f1 HN03 and finally, the plutonium is recovered by an anion exchange process. 

Impure plutonium oxide residues are dissolved in 12M HN03-0.1M HF under refluxing conditions, and then the plutonium is recovered and purified by anion exchange. Plutonium is leached from other residues, such as metal and glass, and is also purified by anion exchange. The purified plutonium eluate from the anion exchange process is precipitated with hydrogen peroxide. The plutonium peroxide is calcined to the oxide, and the plutonium oxide is fluorinated. The plutonium tetrafluoride is finally reduced to the metal with calcium. 

The complete chemistry of plutonium 1 iquid-to-solid conversion processes, especially peroxide and oxalate precipitation, should be further studied. Research and development of direct thermal denitration methods should also be pursued. 

The chemistry of waste treatment processes and the development of new processes are fertile areas of research work. The speciation of plutonium in basic and laundry wastes is needed. For example, if soluble plutonium complexes in basic wastes can be destroyed, perhaps ultrafiltration could replace the flocculent-carrier precipitation process. The chemistry of plutonium(VII) and of ferrites—a candidate waste treatment process—needs to be explored.(23)... 

We are not aware of any previous studies of the removal of plutonium or americium from (NH )2ZrF6-NHltF-NH N03 solutions. For ready plant-scale application, precipitation, sorption on inorganic materials, or batch solvent extraction processes may all be satisfactory. An inexpensive inorganic material with great selectivity and capacity for sorbing actinides, and with suitable hydraulic properties, would be especially attractive. 

High-Level Waste Hydrated iron oxide (so-called "sludge") precipitates when NaOH is added to HLW and carries down almost all the plutonium and most of the other actinides in the HLW. 

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