Plutonium impurities

Nuclear wastes are classified according to the level of radioactivity. Low level wastes (LLW) from reactors arise primarily from the cooling water, either because of leakage from fuel or activation of impurities by neutron absorption. Most LLW will be disposed of in near-surface faciHties at various locations around the United States. Mixed wastes are those having both a ha2ardous and a radioactive component. Transuranic (TRU) waste containing plutonium comes from chemical processes related to nuclear weapons production. These are to be placed in underground salt deposits in New Mexico (see... 

M. N. Myers, Mbsorption Spectra of Plutonium and Impurity Ions in Nitric Mcid Solution, HW-44744, General Electric Co., 1956. 

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. 

Other Pyrochemical Processes. The chemistry of pyrochemi-cal separation processes is another fertile area of research e.g., new molten salt systems, scrub alloys, etc. and the behavior of plutonium in these systems. Studies of liquid plutonium metal processes should also be explored, such as filtration methods to remove impurities. Since Rocky Flats uses plutonium in the metal form, methods to convert plutonium compounds to metal and purify the metal directly are high-priority research projects. 

Plutonium Oxide Dissolution. All four sites dissolve impure PuO, residues in concentrated HND3 (10 to 14M) containing HF (<0.3M). Whereas material calcined at temperatures of... 

Salt Processing of Impure Plutonium Dioxide to High-Purity Metal, U.S. DOE Report LA-9154-MS, Los Alamos National... 

Electrorefining. Impure plutonium metal from MSE and Direct Oxide Reduction (DOR) is sent to an electrorefining operation (J3). In plutonium electrorefining, impure plutonium... 

Spent anode residues from electrorefining (which contain approximately 20-30 percent of the plutonium fed to the process) are either recycled back to electrorefining, or, if high enough in impurities, are oxidized and sent to oxide dissolution. The spent salt is sent to aqueous dissolution (see Figure 1). 

Pyroredox. This is a three-step plutonium purification process ( 3). Impure plutonium metal is reacted with ZnCl2 in a solvent salt of KC1-CaCl 2 > as follows ... 

Impurities less chemically reactive than plutonium follow the zinc. The salt and zinc are allowed to solidify and are separated. The PuCl3, contained in the salt, is reacted with calcium, according to the reaction ... 

The principle of the electrorefining process is basically simple plutonium is oxidized at a liquid metal anode containing impure metal feed and the resulting Pu+3 ions are transported through molten salt to a cathode where pure metal is produced. 

This PUCI3 also acts as a salt-phase buffer to prevent dissolution of trace impurities in the metal feed by forcing the anode equilibrium to favor production (retention) of trace impurities as metals, instead of permitting oxidation of the impurities to ions. Metallic impurities in the feed fall into two classes, those more electropositive and those less electropositive than plutonium. Since the cell is operated at temperatures above the melting point of all the feed components, and both the liquid anode and salt are well mixed by a mechanical stirrer, chemical equlibrium is established between all impurities and the plutonium in the salt even before current is applied to the cell. Thus, impurities more electropositive than the liquid plutonium anode will be oxidized by Pu+3 and be taken up by the salt phase, while impurities in the electrolyte salt less electropositive than plutonium will be reduced by plutonium metal and be collected in the anode. 

Those impurities less electropositive than plutonium that are already present in the molten anode will remain in the anode. 

The anode residues must be chemically processed to recover the plutonium remaining in the residues. This may amount to about 10% of the feed mass if delta alloy is the feed metal. Either aqueous or pyrochemical processes may be used for anode recovery. One pyrochemical process used for recovery utilizes oxidation of the plutonium with zinc chloride to form plutonium chloride salt, followed by calcium reduction of the PUCI3 contained in the salt phase to produce pure plutonium metal (the impurities follow the zinc metal obtained from the oxidation reaction and are discarded to waste). Impurities more stable than calcium chloride remain in the salt phase and are also... 

Mullins, L.J. Christensen, D.C. Babcock, B.R. "Fused Salt Processing of Impure Plutonium Dioxide to High Purity Metal", Los Alamos Nat. Lab. Report LA-9154-MS also Symposium on Actinide Recovery from Waste and Low Grade Sources, ACS, New York City August 23-28, 1981 (in press). 

Americium Extraction (more commonly referred to as Molten Salt Ex-or MSE). This process is specifically designed to reduce the americium content of the plutonium metal. (Am241 spontaneously grows into plutonium as a result of Pu241 decay.) When the impure metal contains more than 1000 ppm of americium, it is run through the MSE process. Otherwise, it bypasses the MSE step and proceeds directly to electrorefining. 

Plutonium Electrorefining. Plutonium electrorefining principles are summarized in Refs. 1,3,9. Briefly, the process consists of oxidizing plutonium from an impure metal feed at the anode and reducing it to pure metal at the cathode. 

NaCl-KCl-PuCl3-MgCl2, under near-equilibrium conditions. Virtually all of the impurities concentrate in the anode. Of the impurities usually present in plutonium, only americium concentrates in the salt. 

Anode Residue Recovery. Approximately 10% of the plutonium present in a Pu- lwt% Ga electrorefining feed, ends up in the anode residue. This residue also contains most of the impurities which were present in the metal feed. The following pyrochemical processes have been considered for recovering plutonium from this residue. 

After a study of the three alternatives we concluded that pyroredox offered the most promise for anode residue recovery. Pyroredox is a molten-salt process in which plutonium metal is oxidized chemically into the salt phase and then reduced chemically into the metal phase. Most of the impurities are not oxidized and remain in the metal residue. Thus, for a Pu-Ga anode residual, the reactions would be ... 

Plutonium pyrochemical processes are now the principal tools at Los Alamos for producing large amounts of high purity plutonium metal from impure metal and oxide scrap. Pyrochemical processing was selected because of its cost effectiveness. The processes are highly compact and require little floor space and manpower to operate. The processes are also operationally efficient in that one or two steps can be used to supplant multi-step operations found in the classical aqueous chemistry flow streams. The... 

Lisa Townsend, a technician in the Radiochemistry section of the Actinide Analytical Chemistry Group, analyzes bulk components and impurities in plutonium-238 materials used to fabricate heat sources used in space exploration. She utilizes a combination of ion exchange and solvent extraction techniques and determines component concentrations using alpha and gamma radio-counting instrumentation. 

It has been established that plutonium hydrolysis products exhibit colloidal behaviour (147-151) and may adsorb onto minerals and other surfaces to form radiocolloids. However, it is difficult to determine whether a radiocolloid is a true colloid or a pseudocolloid formed by adsorption of the plutonium species onto other colloidal impurities in the solution (152). In some cases both forms may be present... 

The yield and rate of the tantalothermic reduction of plutonium carbide at 1975 K are given in Fig. 3. Producing actinide metals by metallothermic reduction of their carbides has some interesting advantages. The process is applicable in principle to all of the actinide metals, without exception, and at an acceptable purity level, even if quite impure starting material (waste) is used. High decontamination factors result from the selectivities achieved at the different steps of the process. Volatile oxides and metals are eliminated hy vaporization during the carboreduction. Lanthanides, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, and W form stable carbides, whereas Rh, Os, Ir, Pt, and Pd remain as nonvolatile metals in the actinide carbides. Thus, these latter elements... 

This process is particularly useful for the preparation of pure plutonium metal from impure oxide starting material (111). It should also be applicable to the preparation of Cm metal. Common impurities such as Fe, Ni, Co, and Si have vapor pressures similar to those of Pu and Cm metals and are difficult to eliminate during the metallothermic reduction of the oxides and vaporization of the metals. They are eliminated, however, as volatile metals during preparation of the actinide carbides. 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

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