Uranium nitrate complexes

Principle of Separation. Uranium forms a nitrate complex that is extractable into ethyl acetate (as well as other organic extractants). Thorium does not readily form an extractable nitrate complex. When ethyl acetate is contacted with an aqueous solution, the uranium-nitrate complex is partitioned favorably into the ethyl acetate whereas thorium nitrate is not. The distribution of the metal ion between the two phases is expressed as D = Corganic/Caqueous where C is the concentration in moles or dps per unit volume in the respective phases. The thorium remains in the aqueous phase and is precipitated as the hydroxide for counting. 

Hexavalent. Nitrate complexation with actinide ions is very weak, and the determination of the formation constants for aqueous nitrate solution species is extremely difficult. Under aqueous conditions with high nitric acid concentrations, complexes of the form An02(N03)(H20)x+, An02(N03)2(H20)2, and An02(N03)3 (An = U, Np, Pu) are likely to be present. Solids of the anionic trisnitrato complex have been isolated for U and Np however, minimal structural data have been obtained. Solid uranyl nitrate, U02(N03)2-xH20, is obtained as the orthorhombic hexahydrate from dilute nitric acid solutions and as the trihydrate from concentrated acid. The Np analog can be precipitated from a mixed aqneons HNO3 and MeCN solntion by the addition of 18-crown-6. Multiple structural determinations have been made for the hexavalent uranium nitrate complexes, and all show the common formula unit of... 

Tributyl phosphate (TBP) in paraffinic diluent can be used to isolate uranium and plutonium from a nitric acid ( 6moll ) medium, with uranium extracted as a neutral uranium nitrate complex ... 

The rate (kinetics) and the completeness (fraction dissolved) of oxide fuel dissolution is an inverse function of fuel bum-up (16—18). This phenomenon becomes a significant concern in the dissolution of high bum-up MO fuels (19). The insoluble soHds are removed from the dissolver solution by either filtration or centrifugation prior to solvent extraction. Both financial considerations and the need for safeguards make accounting for the fissile content of the insoluble soHds an important challenge for the commercial reprocessor. If hydrofluoric acid is required to assist in the dissolution, the excess fluoride ion must be complexed with aluminum nitrate to minimize corrosion to the stainless steel used throughout the facility. Also, uranium fluoride complexes are inextractable and formation of them needs to be prevented. 

Uranium Purification. Subsequent uranium cycles provide additional separation from residual plutonium and fission products, particularly zirconium— niobium and mthenium (30). This is accompHshed by repeating the extraction/stripping cycle. Decontamination factors greater than 10 at losses of less than 0.1 wt % are routinely attainable. However, mthenium can exist in several valence states simultaneously and can form several nitrosyl—nitrate complexes, some for which are extracted readily by TBP. Under certain conditions, the nitrates of zirconium and niobium form soluble compounds or hydrous coUoids that compHcate the Hquid—Hquid extraction. SiUca-gel adsorption or one of the similar Hquid—soHd techniques may also be used to further purify the product streams. 

It can be shown that the virial type of activity coefficient equations and the ionic pairing model are equivalent, provided that the ionic pairing is weak. In these cases, it is in general difficult to distinguish between complex formation and activity coefficient variations unless independent experimental evidence for complex formation is available, e.g., from spectroscopic data, as is the case for the weak uranium(VI) chloride complexes. It should be noted that the ion interaction coefficients evaluated and tabulated by Cia-vatta [10] were obtained from experimental mean activity coefficient data without taking into account complex formation. However, it is known that many of the metal ions listed by Ciavatta form weak complexes with chloride and nitrate ions. This fact is reflected by ion interaction coefficients that are smaller than those for the noncomplexing perchlorate ion (see Table 6.3). This review takes chloride and nitrate complex formation into account when these ions are part of the ionic medium and uses the value of the ion interaction coefficient (m +,cio4) for (M +,ci ) (m +,noj)- Io... 

Since publication of this work, Japanese researchers have undertaken an effort to demonstrate the feasibility of direct dissolution of U02 from spent nuclear fuels by the TBP-HN03 complex in SC-C02.49 Ultimately, the project is directed at the extraction of both uranium and plutonium from mixed oxide fuels and from irradiated nuclear fuel. Ideally, soluble uranyl and plutonium nitrate complexes will form and dissolve in the C02 phase, leaving the FPs as unwanted solids. As in the conventional... 

In the second separation, uranium is extracted from thorium. Ethyl acetate is the extractant for uranium, which is bound in a nitrate complex. Thorium remains in the aqueous phase. Thorium is then co-precipitated with Nd(OH)3 to avoid absorption of the beta particles emitted by 234Th and 234mPa by the large amount of NH4N03 if the solution were simply evaporated and counted. The filter with Nd(OH)3 is mounted on a planchet for counting beta and alpha particles. 

Uranium(vi) nitrate complexes have been discussed in Section 11.5.4, but uranium forms complexes in the 4-4 state that are generally similar to those of thorium. 

A similar scheme has been used for example, for the recovery on anion exchanger of uranium as a nitrate complex from its mixture with thorium [17, p. 317], or for the purification of nickel from calcium on cation exchanger[18]. 

The dibenzoylmethane method is often used after uranium has been extractively separated from other metals as the nitrate complex [78,79], or by the carbonate method (see procedure below). 

X These coefficients were not used in the NEA-TDB uranium review [92GRE/FUG] because they were evaluated by Ciavatta [80C1A] without taking chloride and nitrate complexation into account. Instead, Grenthe er a/, used 8(U02, X) = (0.46 + 0.03) kg-moP, for X = CP, CIO4 and NO3. ... 

The kerosene fraction is now subjected to a second solvent extraction. Addition of iron(II) sulfamate, Fe(NH2S03)2, and shaking of the kerosene fraction with water, results in the formation of plutonium(III) nitrate which is partitioned into the aqueous layer. [U02][N03]2 resists reduction, is com-plexed by TBP and remains in the organic layer. Separation of the two solvent fractions thus separates the uranium and plutonium salts repeated extractions result in a highly efficient separation. The extraction of [U02][N03]2 from kerosene back into an aqueous phase can be achieved by adding nitric acid under these conditions, the uranium-TBP complex dissociates and [U02][N03]2 returns to the aqueous layer. 

As can be seen, the nitrate complexes are, in the main, the dominant thorium species in solution. As there are no experimental values for the enthalpy of formation of these complexes (for either thorium or uranium), only the difference between the enthalpies of solution of the pentahydrate and the tetrahydrate has been used (see Section X. 1.3.2) in conjunction with other results, to evaluate (Th(N03)4-4H20, cr, 298.15 K). 

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