Separation of low levels of actinides by selective oxidation/reduction and co-precipitation with neodymium fluoride

A systematic study of separating the actinides from each other in 1 M hydrochloric acid media has been carried out using selective oxidation/reduction processes followed by coprecipitation with neodymium fluoride. We have optimized two such procedures, one with bromate and another with permanganate, for the sequential separation of Am, Pu, Np, and U isotopes. The first procedure involves oxidation of Pu, Np, and U to +6 state in 1 M HCl media at 85° C with 30% NaBrO₃ and separation from trivalent Am by collecting the latter on the first NdF₃ coprecipitated source. Plutonium is then reduced and converted to +4 oxidation state with 40% NaNO₂ at 85°C, while Np and U are kept oxidized with additional bromate in 50–70°C hot solution, thus separating Pu by collection on a second NdF₃ source. At this stage, Np present in the filtrate is reduced with hydroxylamine hydrochloride and separated from U by collecting on a third source. Subsequently, U is reduced with 30% TiCl₃ and co-precipitated on a final source. The second procedure, which employs KMnO₄ in 1 M HCl media at 60–85°C for oxidizing Pu, Np, and U, and separating from Am, produced MnO₂ which is collected along with Am on the coprecipitated NdF₃. This MnO₂ is dissolved on the filter itself with 1 mL of acidified 1.5% H₂O₂ without any degradation of the -spectra. After evaporating the filtrate to destroy H₂O₂, Pu, Np, and U are separated by following steps similar to those in the bromate procedure. The recoveries of the actinides with both procedurés are >99%. The decontamination factors are between 10³ and 10⁴. The precision and accuracy of measurements, as expressed by the relative standard deviation of replicate analyses, are within 5%. Absolute detection limits for a one-day count on a 600 mm² detector at 32% counting efficiency and 450 mm² detector at 27% counting efficiency are about 2.7×10^–4 and 3.2×10^–4 Bq, respectively. These procedures have been applied to the analysis of actinides in environmental samples.     

Simultaneous electrodeposition of actinides

A new system for simultaneous electrodeposition of U, Np, Pu, Am and Cm has been developed. The system consists of (NH₄)₂C₂O₄–H₂SO₄–HCl. The effects on recovery of pH, current density, interfering ions and the amount of added HCl have been studied. The optimum condition for simultaneous electrodeposition of actinides has been recommended. Under the recommended condition recoveries of U, Np, Pu, Am and Cm have been obtained by using²³²U,²³⁷Np,²⁴¹Am,²⁴²Pu and²⁴⁴Cm. The counting sources prepared are uniform, adherent and suitable for -spectrometry.     

Determination of stability constants of U(VI), Np(VI) and Pu(VI) with chloride ions by extraction chromatography

The complex formation of U(VI), Np(VI) and Pu(VI) with chloride ions was studied in HClO₄−HCl solutions at ionic strength of 2.0 and [H⁺]=2.0M by the method of extraction chromatography using dilute HDEHP as the stationary phase.     

Thermal decomposition of Np(IV) and Pu(III, IV) oxalates

Thermal decomposition of Pu(C₂O₄)₂·6H₂O, Pu₂(C₂O₄)₃·10H₂O and Np(C₂O₄)₂ ·6H₂O has been studied by using combination of gas chromatography, infrared spectroscopy, spectrophotometry and complex thermal analysis. We also investigated the decomposition of Pu oxalate under its -radiation. The reduction of Pu(IV) to Pu(III) has been confirmed. We found Np(V), which is formed from Np(IV), on the basis of infrared and absorption spectra of the intermediate compounds.     

Gas-phase coordination complexes of U(VI)O2(2+), Np(VI)O2(2+), and Pu(VI)O2(2+) with dimethylformamide

Electrospray ionization of actinyl perchlorate solutions in H₂O with 5% by volume of dimethylformamide (DMF) produced the isolatable gas-phase complexes, [An^VIO₂(DMF)₃(H₂O)]²⁺ and [An^VIO₂(DMF)₄]²⁺, where An = U, Np, and Pu. Collision-induced dissociation confirmed the composition of the dipositive coordination complexes, and produced doubly- and singly-charged fragment ions. The fragmentation products reveal differences in underlying chemistries of uranyl, neptunyl, and plutonyl, including the lower stability of Np(VI) and Pu(VI) compared with U(VI).     

Sorption characteristics of237Np,238Pu and241Am in sedimentary materials

Adsorption experiments were performed to measure distribution coefficients of²³⁷Np(V),²³⁸Pu(IV) and²⁴¹Am(III) for sedimentary sequential chemical extraction of the adsorbed radionuclides was carried out with water, CaCl₂, KCl, NH₂OH−HCl, K-oxalate and H₂O₂ solutions, to elucidate their dominant sorption mechanisms. The distribution coefficient of²³⁷Np was two orders of magnitude smaller than that of²³⁸Pu and²⁴¹Am. Most of²³⁷Np adsorbed was extracted with CaCl₂ solution and its sorption was controlled by a reversible ion exchange reaction. The adsorbed²³⁸Pu was mainly extracted with NH₂OH−HCl+K-oxalate solution and its sorption was possibly controlled by irreversible reactions.     

The studies on absorption behavior of calix[4]-bis-crown-6 chromatographic resin and its application to the separation of HLW

In this paper, to separation Cs(I) effectively from HLW, a kind of chromatographic rein was synthesized through immobiling calix[4]-bis-crown-6 on the macroporous polystyrene -divinylbenzene polymeric material. The synthesized chromatographic resin showed high ability to absorb Cs(I). While the radioactive elements Pu(III), Np(V), U(VI), Th(IV), as well as all of the fission and non-fission products showed almost no sorption towards chromatographic rein. The separation of Cs from HLW can be realized through adjusting the valence of Np and Pu or adding H₂C₂O₄. All the results showed that the application of the new chromatographic rein to the analysis of HLW is possible.     

Radiochemische Bestimmung von Plutonium in Urin bei Gegenwart von Komplexen Dekorporationsmitteln

The interfering effect of DTPA or EDTA on the coprecipitation of Pu from urine with Ca(Mg)-phosphate is cancelled nearly by adding an excess of Ni²⁺ (about 1.8 g/l) to the urine samples before precipitating. After subsequent extraction with Alamine-336 (10% in xylene), reextraction with 10N HCl/NH₄I, and electrodeposition on stainless steel, the recoveries of Pu are about 70% in comparison with 85% in absence of DTPA or EDTA.      

Interaction of Aluminium (III) with the f-Family Ions Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) in the Course of Simultaneous Hydrolysis

The interaction of Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) with Al(III) in solutions at pH 0–4 was studied by the spectrophotometric method. It was shown that, in the range of pH 3–4, the hydrolyzed forms of neptunyl and plutonyl react with the hydrolyzed forms of aluminium. In the case of Pu(VI), the mixed hydroxoaqua complexes (H₂O)₃PuO₂(-OH)₂Al(OH)(H₂O)₃ ²⁺ or (H₂O)₄PuO₂OAl(OH)(H₂O)₄ ²⁺ are formed at the first stage of hydrolysis. Np(VI) also forms similar hydroxoaqua complexes with Al(III). The formation of the mixed hydroxoaqua complexes was also observed when Np(IV) or Pu(IV) was simultaneously hydrolyzed with Al(III) at pH 1.5–2.5. The Np(IV) complex with Al(III) has, most likely, the formula (H₂O) _n (OH)Np(-OH)₂Al(OH)(H₂O)₃ ³⁺. At pH from 2 to 4.1 (when aluminium hydroxide precipitates), the Np(V) or Nd(III) ions exist in solutions with or without Al(III) in similar forms. When pH is increased to 5–5.5, these ions are almost not captured by the aluminium hydroxide precipitate.     

Studies on the recovery of Pu(IV) from hydrochloric acid-oxalic acid solutions using an anion exchange method

Sorption of Pu(IV) from hydrochloric acid-oxalic acid solutions has been investigated using different anion exchangers, viz., Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) for the recovery of plutonium from plutonium oxalate solutions. Distribution ratios of Pu(IV) for its sorption on these anion exchangers have been determined. The sorption of Pu(IV) from hydrochloric acid solutions decreases drastically in the presence of oxalic acid. However, addition of aluminium chloride enhances the sorption of plutonium in the presence of oxalic acid, indicating the feasibility of recovery of plutonium. Pu(IV) breakthrough capacities have been determined with a 10 ml resin bed of each of these anion exchangers at a flow rate of 60 ml per hour using a solution of Pu(IV) with the composition: 6M HCl+0.05M HNO₃+0.1M H₂C₂O₄+0.5M AlCl₃+100 mg.l^–1 Pu(IV). The 10% Pu(IV) breakthrough capacities for Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) are 15.0, 8.9 and 6.2 g of Pu(IV) l^–1 of resin respectively.     

Redox determination of uranium in presence of plutonium in low phosphoric acid medium and the recovery of plutonium

Quantitative determination of uranium in (U, Pu)O₂ fuels is usually done by the DAVIES-GRAY method. High concentrations of phosphoric acid in the analytical waste generated by this method make the revocery of plutonium rather complex. Studies on the recovery of plutonium from nitric acid medium containing different concentrations of H₃PO₄ by conventional anion-exchange procedure reveal that more than 90% of the plutonium can be easily recovered when the phosphoric acid concentration is less than 0.5 M in the solution. A method was developed for the determination of uranium in the presence of plutonium, which involves the reduction of U(VI) to U(IV) by Fe(II) in a medium of 3.5M H₃PO₄ +4.5M H₂SO₄ instead of 10–11M H₃PO₄ so as to have the H₃PO₄ concentration 0.6M in the waste. A number of determinations of uranium in UO₂(NO₃)₂ working standard solutions and (U, Pu) synthetic solutions with uranium at the 3–7 mg level were carried out by this method. The precision obtained was better than ±0.2% and the accuracy was also within the precision limits. The resulting analytical waste generated was directly subjected to anion exchange separation for the recovery of plutonium which was found to be more than 90%.     

A novel Np–Pu separation procedure based on extraction with di-(chlorophenyl)-dithiophosphinic acid in nitric solution

The extraction behavior of Pu(III), Pu(IV), Np(IV) and Np(V) with di(chlorophenyl)-dithiophosphinic acid (DCPDTPA) in toluene from nitric acid solutions was studied systematically. In aqueous solution with high nitric acid concentration, the extraction capability (represented by distribution ratio D) for Pu and Np in different valences with DCPDTPA comes as D _Np(IV) > D _Pu(IV) > D _Np(V) > D _Pu(III). A new radiochemical procedure for Np/Pu separation based on DCPDTPA extraction was proposed and tested with simulated samples. The recoveries of Np and Pu are as high as 80 % after the whole separation procedure, with the decontamination factor of trivalent lanthanide fission product element (e.g. Eu) greater than 1.5 × 10⁴. The decontamination factor of Pu–Np is 2.0 × 10³, while the decontamination factor of Np–Pu is greater than 4.8 × 10³ after additional purification.     

Reduction kinetics of Pu(IV) and Np(VI) by N,N-dimethylhydrazine,and its potential application in nuclear fuel reprocessing

Reduction kinetics of Pu(IV) by N,N-dimethylhydrazine (NNDMH) were studied by spectrophotometry, and the reduction rate equation in 3M (mol/dm³) nitric acid was obtained. The reduction properties of NNDMH for U(VI), Np(VI), and Pu(IV) was studied in the mixture solution of trin-butylphosphate diluted to 30 vol.% by n-dodecane (30% TBP) and 3M nitric acid. It was confirmed that NNDMH selectively reduce Np(VI) to Np(V) without affecting the valences of U(VI) and Pu(IV) in a few minutes. Numerical simulation indicated that 99.9% of Np was separated from U and Pu applying NNDMH for a mixer-settler.     

One step U-Pu-Cs-Ln-steel separation using TRU preconditioned extraction resins from Eichrom for application on transmutation targets

A one step Cs, lanthanides (Ln), Pu and U separation using TRU resin (Eichrom), for subsequent isotopic analyses, is presented for samples of transmutation targets containing a predominant matrix of steel elements. Cs, Ln, Pu and U are successively eluted using minor volumes (<12 mL) of 2M HNO₃, 4M HCl, 0.075M oxalic acid in 1M HCl media and 0.1M ammonium hydrogen oxalate in 0.02M HNO₃ media, respectively. Accurate and precise isotopic compositions obtained for these elements by thermal ionization mass spectrometry or multiple collector inductively coupled mass spectrometry show neither significant fractionation nor contamination during the complete analytical process.     

Titrimetric determination of uranium in the presence of plutonium in sulphuric acid medium

Present work summairzes a method for the estimation of uranium in the presence of plutonium involving the reduction of uranium to U/IV/ and plutonium to Pu/III/ by Zn/Hg/ followed by the selective oxidation of Pu/III/to Pu/IV/with HNO₃ catalyzed by molybdate in the presence of large sulphate concenration [5M H₂SO₄+1.5M /NH₄/₂SO₄]. The oxidation of U/IV/ by K₂Cr₂O₇ is then carried out in the presence of excess of Fe/III/ and Al/NO₃/₃ to a sharp potentiometric end point. R.S.D. obtained for 20 determinations of uranium /3–6 mg/ was 0.3% in the presence of 0.35 mg of plutonium. Larger quantity for plutonium was found to interfere.     

Voltamperometric determination of uranium and plutonium in alkaline solutions

The simultaneous determination of U(VI), Pu(VI), Pu(V) in 0.5–4.0 M NaOH has been elaborated by means of classical and differential pulse voltamperometry. U(VI) is determined with a dropping mercury electrode (DME) at the half-wave potential of E_1/2=–0.89 V vs. Ag/AgCl reference electrode due to reduction to U(V). The limiting current or peak heights are proportional to uranium(VI) concentration in the range of 1.3.10^–7–3·10^–4 M U(VI). Deviation from proportionality is observed for higher concentrations due to polymerization of uranates. Pu(VI) and Pu(V) are determined with a platinum rotating electrode at E_1/2=–0.02 V due to the reaction Pu(VI)+e^–»Pu(V) and with DME at E_1/2=–1.1 V due to the reduction to Pu(III). The limiting currents of both Pu(VI) and Pu(V) are proportional to their concentrations in the range of 4·10^–6–1.2·10^–3 M Pu. The determination of U(VI), Pu(VI), Pu(V) is not interfered by the presence of the following salts: 2M NaNO₃, 2M NaNO₂, 1.5M NaAlO₂, 0.5M NaF and ions of Mo(VI), W(VI), V(V), Cu(II). The presence of CrO ₄ ^2– and FeO ₂ ^– ions disturbs the determination of U(VI) in 1–4M NaOH, however, contribution of the reaction Fe(III)+e^–»Fe(II) to uranium reduction peak can be calculated from the height of the second peak Fe(II)+2 e^–»Fe(0).     

242Pu as tracer for simultaneous determination of 237Np and 239,240Pu in environmental samples

A procedure has been developed using ²⁴²Pu as tracer for simultaneous determination of ²³⁷Np and ^239,240Pu in environmental samples. The validity of the method has been demonstrated by ICPMS and a-spectroscopy for up to 10 gram soil and sediment, seawater up to 200 litres. The paper describes a suitable chemical procedure for Np and Pu including a quantitative pre-concentration of neptunium and plutonium, preparation of Np⁴⁺ and Pu⁴⁺, Np(NO₃)₆ ^2- and Pu(NO₃)₆ ^2-. The ratio of ²³⁷Np/²⁴²Pu (or ²³⁷Np/²³⁹Pu) before and after the procedure has been determined using 10 g soil (free from Np and Pu) R _before/R _after = 1.004±3.3% (S.D n = 20) and 1 litre seawater R _before/R _after = 1.019±1.9% (S.D., n = 12). Results from the intercomparison samples IAEA-135, IAEA-381 and from environmental samples are presented.     

Crystal Structure of the Mixed-Valence Neptunium Compound Na6[(NpVO2)2(NpVIO2)(MoO4)5] · 13H2O

The crystal structure of the mixed-valence Np(V) and Np(VI) compound Na₆[(Np^VO₂)₂(Np^VIO₂)(MoO₄)₅] · 13H₂O was determined. The structure is built of the anionic layers [(Np^VO₂)₂(Np^VIO₂)(MoO₄)₅] ^6n- _n with the Na⁺ cations and crystal water molecules between them. The Np(V) and Np(VI) atoms in the anionic layers are ordered. The motif of the anionic layer is close to that found in Mg₂[(UO₂)₃(SeO₄)₅] · 16H₂O. The isostructural mixed-valence Np(V) and U(VI) compound was also synthesized.     

Oxidation behaviour of uranium and neptunium in stabilised zirconia

Yttria stabilised zirconia (YSZ) based (Zr,Y,U)O_2−x and (Zr,Y,Np)O_2−x solid solutions with 6 and 20 mol% actinide were prepared with Y/Zr ratios ranging from 0.2 to 2.0 to investigate uranium and neptunium oxidation behaviour depending on the oxygen vacancies in the defect fluorite lattice. Sintering at 1600 °C in Ar/H₂ yields a cubic, fluorite-type structure with U(IV) and Np(IV). Annealing (Zr,Y,U)O_2−x with Y/Zr=0.2 at 800 °C in air results in a tetragonal phase, whereas (Zr,Y,U)O_2−x with higher Y/Zr ratios and (Zr,Y,Np)O_2−x retain the cubic structure. XANES and O/M measurements indicate mixed U(V)-U(VI) and Np(IV)-Np(V) oxidation states after oxidation. Based on X-ray diffraction, O/M and EXAFS measurements, different oxidation mechanisms are identified for U- and Np-doped stabilised zirconia. In contrast to U, excess oxygen vacancies are needed to oxidise Np in (Zr,Y,Np)O_2−x as the oxidation process competes with Zr for oxygen vacancies. As a consequence, U(VI) and Np(V) can only be obtained in stabilised zirconia with Y/Zr=1 but not in YSZ with Y/Zr=0.2.     

Recovery of uranium from U3O8 irradiated with thermal neutrons

The extraction of uranium with different concentrations of HDEHP in n-hexane from varying concentrations of nitric acid solutions was investigated. Uranium in the organic phase was quantitatively stripped with a solution containing 8M H₂SO₄ and 6M HCl, the partition coefficient was determined by X-ray fluorescence and U. V. spectrophotometry. The assay of⁹⁵Zr,⁹⁵Nb,²³⁹Np and other gamma-emitters was made with a Ge(Li) detector coupled to a 4096 multichannel analyzer.