Plutonium nitrate complexes

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... 

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. 

Plutonium(III) in aqueous solution, Pu " ( 4)> is pale blue. Aqueous plutonium(IV) is tan or brown the nitrate complex is green. Pu(V) is pale red-violet or pink in aqueous solution and is beUeved to be the ion PuO Pu(VI) is tan or orange in acid solution, and exists as the ion PuO. In neutral or basic solution Pu(VI) is yellow cationic and anionic hydrolysis complexes form. Pu(VII) has been described as blue-black. Its stmcture is unknown but may be the same as the six-coordinate NpO (OH) (91). Aqueous solutions of each oxidation state can be prepared by chemical oxidants or reductants... 

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). 

Plutonium(V) exhibits little tendency to complex with inorganic anions. By contrast Plutonium(VI) complexes with chloride and nitrate anions but these complexes are much less stable than the corresponding Pu(IV) complexes. 

The ability of 2,3-dihydroxybenzoyl-N-glycine to form a stable complex with plutonium is demonstrated in Fig. 2. In the absence of 2,3-dihydroxybenzoyl-N-glycine plutonium nitrate hydrolyzed and could not be eluted from the column. In the presence of 2,3-dihydroxybenzoyl-N-glycine the plutonium recovery averaged 80%. 

Step 9. Add 1 mL of 50% NaNOz solution to the dissolved sample in the flask [or to the small sample from Step 2] to adjust the Pu oxidation state to +4 and form the anionic nitrate complex of plutonium. Swirl to mix and let sample stand overnight, or at least two hours if the sample must be rushed. The sample is now approximately 50 mL of 8 M HN03, and ready for the plutonium column. 

Nitrate Complexes. Although there is spectrophotometric evidence for the existence of nitrate complexes of plutonium(III),... 

A similar scheme has been used to remove Zr and Nb while concentrating plutonium [15]. In this case use is made of the strong dependence of the stability of nitrate complexes of plutonium on the concentration of HNO3. Nitrate complexes are sorbed from concentrated (6 mol/L) solution of HNO3 on anion exchanger introduced in the NO3 form. At lower concentrations of HNO3 the complexes dissociate and the sorbed ions are displaced by nitrate ion in the diluted solution (1 mol/1 of HNO3) employed for flow reversal in column II. 

Plutonium Purification. The same purification approach is used for plutonium separated from sediments or seawater. In case reduction may have occurred, the plutonium is oxidized to the quadrivalent state with either hydrogen peroxide or sodium nitrite and adsorbed on an anion exchange resin from 8M nitric acid as the nitrate complex. Americium, curium, transcurium elements, and lanthanides pass through this column unadsorbed and are collected for subsequent radiochemical purification. Thorium is also adsorbed on this column and is eluted with 12M hydrochloric acid. Plutonium is then eluted from the column with 12M hydrochloric acid containing ammonium iodide to reduce plutonium to the non-adsorbed tervalent state. For seawater samples, adequate cleanup from natural-series isotopes is obtained with this single column step so the plutonium fraction is electroplated on a stainless steel plate and stored for a-spectrometry measurement. Further purification, especially from thorium, is usually needed for sediment samples. Two additional column cycles of this type using fresh resin are usually required to reduce the thorium content of the separated plutonium fraction to insignificant levels. 

The principle of the Purex process, now commonly used for processing irradiated uranium by solvent extraction, is illustrated in Fig. 1.18. The solvent used in this process is a solution of tributyl phosphate (TBP) in a high-boiling hydrocarbon, frequently n-dodecane or a mixture of similar hydrocarbons. TBP forms complexes with uranyl nitrate [U0i(N03)2] and tetravalent plutonium nitrate [Pu(N03)4] whose concentration in the hydrocarbon phase is higher than in an aqueous solution of nitric acid in equilibrium with the hydrocarbon phase. On the other hand, TBP complexes of most fission products and trivalent plutonium nitrate have lower concentrations in the hydrocarbon phase than in the aqueous phase in equilibrium. 

From experimentation with supercritical fluid extraction it is known that neither plutonium nitrate or oxides nor americium nitrate or oxides are soluble in carbon dioxide without the aid of soluble complexing agents. Even with the aid of selected complexing agents plutonium extraction efficiency from spiked... 

Plutonium is a very reactive metal and oxidizes readily in moist air. In finely divided form, plutonium metal is pyrophoric (Taylor 1973). Plutonium exhibits five oxidation states from plutonium(lll) to plutonium(VII). The four lower oxidation states are stable in solution and may co- exist in the same solution. Complex (coordination) compounds are formed with many of the common inorganic anions, such as plutonium nitrate (Pu(NO 3) 4). 

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 ... 

Ammonium ions, tetradecyldimethylbenzyl-liquid—Liquid extraction, 1, 548 Ammonium molybdate, 3,1257 Ammonium nitrate, hydroxyl-as plutonium(IV) reductant Purex process, 6, 949 Amphotericin B metal complexes, 2, 973 a-Amylase zinc, 6, 607 Anabaena spp. 

In the case of the monofluorocomplexes of quadrivalent plutonium, it is obvious that the lower values obtained in chloride and nitrate media are due to complexing by these ions these results will not be discussed further. In HCIO4 media the data for the first two fluoride complexes are quite self-consistent and well within the same order of magnitude as these reported for the other quadrivalent actinides (12, 89). An extensive comparison would extend beyond the scope oT tKTs paper. In the case of PuF3+, extrapolation of bi to zero ionic strength is not warranted as such in view of the limited number of data. However, in the case of ThF3+ where the data extend over a very wide range of ionic... 

Dissolution. Plutonium is solubilized in nitric acid solutions at Rocky Flats. The feed material consists of oxide, metal and glass, dissolution heels, incinerator ash and sand, slag, and crucible from reduction operations. The residues are contacted with 12M HNO3 containing CaF2 or HF to hasten dissolution. Following dissolution, aluminum nitrate is added to these solutions to complex the excess fluoride ion. 

The spent fuel element is still mainly UO2 and is dissolved in aqueous nitric acid, which is oxidizing enough to take the uranium to the VI oxidation state as UC>22+(aq) and Pu to Pu4+(aq) (the uranyl ion U022+ can be regarded as hydrolyzed U6+ see Section 13.6). Treatment of the solution of uranyl and plutonium(IV) nitrates with either an iron(II) salt or SO2 will reduce all the Pu to Pu3+(aq), which is not extractable with TBP, but will leave the uranium(VI) untouched (see Exercise 15.5). The solution is then equilibrated with TBP (which is immiscible with water) or TBP in an alkane solvent. The U022+ forms a neutral complex containing both TBP and the nitrate ions, which axe present in large excess ... 

Berthon, C. Chachaty, C. NMR and IR spectrometric studies of monoamide complexes with plutonium(IV) and lanthanide(III) nitrates, Solvent Extr. Ion Exch. 13 (1995) 781-812. 

Horwitz et al. showed that the trivalent americium is coordinated to three CMPO molecules and three nitrate anions in an overall neutral complex, another molecule of nitric acid being hydrogen bound to each of the carbamoyl oxygen atoms.157 158 Several molecules of CMPO are included in complexes formed with other cations such as plutonium. 

The plutonium solubility increased in the presence of increased NaN03, NaOH, and NaA102 concentrations. According to the literature, Pu(V) should be the stable oxidation state in alkaline NOjT-NO solutions (8). It has been observed that the solubility of Pu(V) increases as tne NaOH concentration increases (8.11) probably this occured due to formation of the more soluble anionic hydroxide complexes of Pu(V) such as PuO,(OH)2 (11). Sodium nitrate and NaAlO, may have increased Pu(V) solubility through complexation. Sodium nitrate also may have increased plutonium solubility by oxidizing the less soluble Pu(IV), initially present in the tracer solids, to Pu(V). 

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