Plutonium aqueous chemistry

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

The electrolyte salt must be processed to recover the ionic plutonium orginally added to the cell. This can be done by aqueous chemistry, typically by dissolution in a dilute sodium hydroxide solution with recovery of the contained plutonium as Pu(OH)3, or by pyrochemical techniques. The usual pyrochemical method is to contact the molten electrolyte salt with molten calcium, thereby reducing any PUCI3 to plutonium metal which is immiscible in the salt phase. The extraction crucible is maintained above the melting point of the contained salts to permit any fine droplets of plutonium in the salt to coalesce with the pool of metal formed beneath the salt phase. If the original ER electrolyte salt was eutectic NaCl-KCl a third "black salt" phase will be formed between the stripped electrolyte salt and the solidified metal button. This dark-blue phase can contain 10 wt. % of the plutonium originally present in the electrolyte salt plutonium in this phase can be recovered by an additional calcium extraction stepO ). 

The basic electrorefining process is now being used on a production scale for the purification of non-specification plutonium metal. The technology is sufficiently well developed to permit 24-hour unattended operation of the electrorefining cells, and the quality of the product metal is highly consistent. This technology is rapidly replacing aqueous chemistry for plutonium metal purification. 

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... 

The aqueous chemistry of plutonium may well be unique in that four oxidation states co-exist in appreciable quantities. As with the trend set at NpOj, PuOJ is quite stable and its stability increases with decreasing acidity since the couples are strongly hydrogen ion dependent. 

The aqueous chemistry of plutonium is dictated by two factors its tendency to hydrolyse and its tendency to form complexes. As the plutonium ions have a high charge and relatively small ionic radius they all exhibit a strong tendency to hydrolyse quite rapidly. Kraus (9) has shown that the tendency to hydrolyse follows the order ... 

As trivalent americium has a smaller ionic potential than the ions of plutonium it hydrolyses to a much lesser extent than the various plutonium ions. However, like Pu3+, hydrolytic reactions and complex formation are still an important feature of the aqueous chemistry of Am3+. Starik and Ginzberg (25) have shown that Am(III) exists in its ionic form from pH 1.0 to pH 4.5 but above pH 4.5 hydrolysis commences and at pH 7.0 colloidal species are formed. The hydrolytic behaviour of Cm(III) resembles that of Am(III). 

The chemical properties of the actinides are much less similar to each other than those of the lanthanides, because the additional electrons added to the 5/ and 6d are bound less t tly than those of the 4/and 5d shells of the lanthanides. As shown in Table 9.4, the lanthanides in aqueous solutions exist principally in a single, tiivalent oxidation state, whereas four or more oxidation states are observed in the aqueous chemistry of uranium, neptunium, and plutonium. The actinide ions normally formed in solution by the oxidation states II through VI are M, M, M, MO2, MOj , respectively. 

Experiences with aqueous chemistry and behavior of the transuranium elements obtained in nuclear fuel reprocessing and plutonium processing are only of limited relevance for PWR primary coolants with the extremely low concentrations of these elements in a boric acid—LiOH solution of varying composition. The plutonium polymers which are formed in less acid and neutral solutions and which have been reported to show the highest plate-out potential (e. g. Wilkins and Wisbey,... 

The chemistry of plutonium ions in solution has been thoroughly studied and reviewed (30,94—97). Thermodynamic properties of aqueous ions of Pu are given in Table 8 and in the Uterature (64—66). The formal reduction potentials in aqueous solutions of 1 Af HCIO or KOH at 25°C maybe summarized as follows (66,86,98—100) ... 

Much was also learned at the Metallurgical Laboratory about the solution chemistry of plutonium during these first few years of investigation. This included elucidation of the ionic species present in aqueous solutions of different acids and determination... 

Investigations of the chemical properties of plutonium have continued in many laboratories throughout the world as it has become available. This has led to the situation where the chemistry of this relative newcomer is as well understood as is that of most of the well-studied elements. The four oxidation states of plutonium—III, IV, V, and VI—lead to a chemistry which is as complex as that of any other element. It is unique among the elements in that these four oxidation states can all exist simultaneously in aqueous solution at appreciable concentration. As a metal, also, its properties are unique. Metallic plutonium has six allotropic forms, in the temperature range from room temperature to its melting point (640 C), and some of these have properties not found in any other known metal. 

The investigation of plutonium chemistry in aqueous solutions provides unique challenges due in large part to the fact that plutonium exhibits an unusually broad range of oxidation states -from 3 to 7-and in many systems several of these oxidation states can coexist in equilibrium. Following the normal pattern for polyvalent cations, lower oxidation states of plutonium are stabilized by more acidic conditions while higher oxidation states become more stable as the basicity increases. 

Prediction of the chemistry of plutonium in near-neutral aqueous media is highly dependent on understanding reactions that may be occurring in such media. One of the most important parameters is the stability and nature of complexes formed by plutonium in its four common oxidation states. Because Pu(III), Pu(IV), and Pu(VI) are readily hydrolysed, complexation reactions generally are studied in mildly to strongly acidic media. Data determined in acid media (and frequently at high concentrations of plutonium) then are used to predict the chemical speciation of plutonium at near-neutral pH and low concentrations of the metal ion. 

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

Silver, G. L. Minor Problems in Aqueous Plutonium Chemistry, U.S. AEC Report MLM-2075, Mound Laboratory, Miamisburg, OH, 1973. 

Molten salt extraction residues are processed to recover plutonium by an aqueous precipitation process. The residues are dissolved in dilute HC1, the actinides are precipitated with potassium carbonate, and the precipitate redissolved in nitric acid (7M) to convert from a chloride to a nitrate system. The plutonium is then recovered from the 7M HNO3 by anion exchange and the effluent sent to waste or americium recovery. We are studying actinide (III) carbonate chemistry and looking at new... 

Forms of Plutonium in Seawater and Other Aqueous Solutions," to be published in Marine Chemistry (1983). 

Since transport by water is virtually the only available mechanism for escape, we will be predominantly concerned with the chemistry of aqueous solutions at the interface with inorganic solids - mainly oxides. These will be at ordinary to somewhat elevated temperatures, 20-200 C, because of the heating effects of radioactive decay during the first millennium. The elements primarily of interest (Table I) are the more persistent fission products which occur in various parts of the periodic table, and the actinides, particularly uranium and thorium and, most important of all, plutonium. 

In the present paper the chemistry of plutonium is reviewed, with particular reference to the ambient conditions likely to be encountered in natural waters. In addition, experimental work is presented concerning the effects of such variables as pH, plutonium concentration, ionic strength, and the presence of complexing agents on the particle size distributions of aqueous plutonium. In subsequent papers it will be shown that these variables, as they influence the particle size distribution of the aqueous plutonium, greatly affect its interaction with mineral surfaces. The orientation of these studies is the understanding of the likely behavior and fate of plutonium in environmental waters, particularly as related to its interaction with suspended and bottom sediments. 

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