Radiogenic uranium in paragenetic mineral associations

Isotope composition of U in minerals of two mineral associations based on aeschynite and pyrochlore was studied. Although the ratio of U isotopes in the mineral associations as a whole is equilibrium (or close to equilibrium), the distribution of the radiogenic ²³⁴U isotope between separate parts of these associations is essentially nonuniform. Two models of the disturbance of the radioactive equilibrium are discussed: dependence of the ²³⁴U/²³⁸U ratio on the U concentration in the mineral (Adloff-Roessler model) and transfer of ²³⁴Th recoil atoms from one phase to another (Sheng-Kuroda model). It is impossible within the framework of any of the models to consistently account for the observed distribution of the radiogenic U between different mineral phases. For the quasi-closed mineral system based on aeschynite, a model of the redistribution of the radiogenic U under the action of natural solutions mainly within the mineral association is suggested; for the open system involving pyrochlore, the effects of natural leaching are explained taking into account the valence and chemical state of uranium in the minerals.     

Nuclear spectrometric determination of uranium isotopes without use of radiochemical yield monitors

A method is described for the determination of uranium isotopes in solid samples without the use of radiochemical yield monitors. The analytical procedure employs direct determination of ²³⁸U via the 63.3 keV gamma emission of its first daughter, ²³⁴Th. The measurement of this emission requires that corrections must be made for the gamma-ray attenuation and for contribution due to the 63.9 keV ²³²Th photoemission for samples rich in natural thorium. Isotopic ratios are determined by alpha-particle spectrometric assay of the uranium fraction purified by an anion-exchange technique. The method provides reliable analytical measurements for samples containing widely different amounts of natural uranium and thorium.     

Radiogenic 234U and 210Po in humus acids of dyctionemic shale

Radiochemical analysis is made of different fractions of humic and fulvic acids separated from organic matter of graptolitic argillite (dyctionemic shale) of the Baltic basin. The relation between the decalcination of humates and fulvates and the isotopic composition of U is studied. The tightness of binding to humus substances, including humin, is different for ²³⁸U and ²³⁴U, which is probably due to preferential stabilization of the radiogenic uranium in the oxidation state +4. Based on the experiments on precipitation of humates at pH 0–2, it is suggested that the thiol groups of humic acids are involved in chemical binding of ²¹⁰Po. Experiments on partial decalcination of humates in 1 M HNO₃ revealed stronger binding of the radiogenic Po as compared to ^208,209Po. The results were interpreted in terms of the known models.     

Radionuclides in ground waters from observation holes in the Shelter local area

The volume activity of ³H, ⁹⁰Sr, ¹³⁷Cs, ²³⁴U, ²³⁵U, ²³⁸U, ²³⁸Pu, ^239+240Pu, and ²⁴¹Am in ground waters from observation holes 1-G-6-G in the north section of the Shelter local area of the Chernobyl Nuclear Power Plant (CNPP) was measured. The distribution of radionuclides in the suspension fractions of the ground waters was evaluated. The main contribution to the pollution of ground waters with uranium is due to natural uranium isotopes: ^234,235,238U. The activity ratios of ²³⁸Pu, ^239+240Pu, and ²⁴¹Am in ground waters are similar to those in the spent fuel of 4th CNPP block.     

Reaction of ozone with uranium(IV) oxalate in water

Decomposition of aqueous suspensions of uranium(IV) oxalate under the action of an ozone–oxygen mixture was studied. The process occurs in two steps. In the first step, the U(IV) oxidation with the formation of oxalic acid uranyl solutions prevails. The second step involves decomposition of oxalate ions and hydrolysis of uranyl ions. An increase in temperature accelerates the transformation of uranium(IV) oxalate into uranium(VI) hydroxide compounds. In solutions containing KBr or UO₂Br₂, the following reaction occurs: O₃ + Br^– → O₂ + BrO^–. The arising hypobromite ions and hypobromous acid oxidize uranium(IV) oxalate extremely efficiently. The possible mechanism of ozonation of aqueous uranium(IV) oxalate suspensions is discussed.     

Uranium(IV) Complexes Formed in Extraction with Tributyl Phosphate from Nitric Acid Solutions

Speciation of U(IV) in tributyl phosphate (TBP) solutions prepared by extraction of U(IV) from 2 M HNO₃ was studied. The electronic spectra showed that in the solutions containing from 3 to 60% TBP a mixture of disolvate U(NO₃)₄(TBP)₂ and hydrates with hypothetical formula (TBP) _m ...[U(NO₃) _k · (H₂O) _n ]^(4-k)+ (k = 3 or 4) is formed. Within the 70-100% concentration range, the hexanitrate complex (TBP) _n ...2H₅O₂(H₂O) _p ⁺...[U(NO₃)₆]^{2 -} also appears. In undiluted TBP, as the concentration of uranyl nitrate increases, first the hexanitrate complex and then hydrated complexes of U(IV) gradually disappear. At uranium concentration more than 300 g l^-1, only U(IV) tetranitrate disolvate exists in the organic phase.     

The development of a method for the preparation of large amounts of 234Th-purified 238U-oxide

High purity ²³⁸U-oxide in amounts up to 60 g is required for capture cross-section measurements. The material needs to be purified for the ²³⁴Th daughter of ²³⁸U in order to reduce the natural γ background. It has been shown that 60 g of U₃O₈ can be purified by cation-exchange and retransformed in U₃O₈ in about 20 h. When the target is ready for use the ²³⁴Th content is 2% of the saturation value.     

Dose effect for South Serbians due to 238U in natural drinking water

The use of depleted uranium ammunition in South Serbia during the 1999 Kosovo conflict raised a great deal of public concern in the Balkans. Radioactivity levels of 238U in 20 wells and lake water samples were checked from the viewpoint of internal radiation exposure for South Serbian subjects. We have measured 238U concentration using inductively coupled plasma mass spectrometry, whereas thermal ionisation mass spectrometry has been used for the measurement of isotope ratios, e.g. 234U/238U and 235U/238U. The concentration of uranium in water samples varies in the range 1.37-63.18 mBq/L. 234U belongs to the 238U natural radioactive decay series, and at secular equilibrium, the abundance ratio, 234U/238U, corresponds to the ratio of their half-lives. The 234U/238U activity ratio varies in the range 0.88-2.2 and 235U/238U isotope ratio varies from 0.00698 to 0.00745. These findings indicate that uranium in water was a mixture of natural and anthropogenic origin. The annual effective dose due to 238U was estimated to be in the range 9.2 x 10(-5)-2.1 x 10(-3) mSv.     

Sorption of U(VI) onto layered double hydroxides and oxides of Mg and Al,prepared using microwave radiation

Layered double hydroxides of Mg and Al, containing CО₃^2– ions in the interlayer space (LDH-Mg-Al-CО₃), and layered double oxides of Mg and Al (LDO-Mg-Al) were prepared using microwave radiation (MWR). The use of MWR allows not only acceleration of the synthesis of both LDH and LDO, but also preparation of compounds with high kinetic characteristics of the U(VI) sorption. The degree of U(VI) sorption (α) from 10^–2 M aqueous U(VI) solutions at a sorption time of 4 h and V/m = 50 mL g^–1 exceeds 99.0%. In sorption from more concentrated (10^–1 M) aqueous U(VI) solutions under similar conditions, α on all the samples does not exceed 37.5%.     

Uranium isotopes in public drinking water and dose assessment for man in Poland

238U, 234U and 235U were determined in tap water from municipal water pipes that drew their supply from surface water or ground water in various locations in Poland. Average activity concentrations of 238U, 234U and 235U in tap water from surface water were 9.6 +/- 7.1, 12.8 +/- 9.7 and 0.41 +/- 0.31 Bq m(-3), respectively, whereas from ground water they were 4.5 +/- 6.0, 5.7 +/- 6.9 and 0.19 +/- 0.27 Bq m(-3), respectively. Activity concentrations of 234U were higher than 238U. Ratios of 234U/238U ranged from 1.07 to 2.60, indicating the lack of equilibrium between these isotopes. The average 235U/238U ratio was 0.043 +/- 0.008, being close to 0.046 for natural uranium. Average annual intake with water and food was 7.6 +/- 5.1 Bq for 238U and 9.5 +/- 6.6 Bq for 234U. Annual committed effective doses calculated from these intakes for adults were 0.34 +/- 0.23 and 0.47 +/- 0.32 microSv, respectively; 235U contributed to the total dose from the uranium isotopes by about 2%.     

Synthesis and crystal structure of Th(IV), U(IV), and Np(IV) tribromoacetates

Actinide(IV) tribromoacetates of the composition [An(CBr₃COO)₄(H₂O)₂]₂ (An = Th, U, Np) were synthesized and studied. Their structural feature is the formation of electrically neutral dimeric complexes. The surrounding of the An(IV) atoms in the dimers is formed by the oxygen atoms of six CBr₃COO^? anions and two water molecules; the coordination number of An(IV) is 9, and the coordination polyhedron can be described as distorted base-monocapped tetragonal antiprism. Four independent CBr₃COO^? anions in the structure are coordinated to the An(IV) atoms in different fashions: monodentate, bidentate chelate, and bidentate bridging. Hydrogen bonding links the dimers in infinite chains along [010] direction. The hydrogen bonding noticeably influences the geometric characteristics of the coordination surrounding of the An(IV) atoms.     

Dissolution behaviour of 238U, 234U and 230Th deposited on filters from personal dosemeters

Kinetics of dissolution of (238)U, (234)U and (230)Th dust deposited on filters from personal alpha dosemeters was studied by means of a 26-d in vitro dissolution test with a serum ultrafiltrate simulant. Dosemeters had been used by miners at the uranium mine 'Dolní Rozínka' at Rozná, Czech Republic. The sampling flow-rate as declared by the producer is 4 l h(-1) and the sampling period is typically 1 month. Studied filters contained 125 +/- 6 mBq (238)U in equilibrium with (234)U and (230)Th; no (232)Th series nuclides were found. Half-time of rapid dissolution of 1.4 d for (238)U and (234)U and slow dissolution half-times of 173 and 116 d were found for (238)U and (234)U, respectively. No detectable dissolution of (230)Th was found.     

Procedure for Simultaneous Determination of Uranium and Transuranium Elements in Groundwater and Liquid Radioactive Wastes from the Shelter

An ion-exchange procedure for simultaneous determination of ²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U, ²³⁸Pu, ^{239 + 240}Pu, ²⁴¹Am, and ²⁴⁴Cm in groundwater and liquid radioactive wastes is described. The concentration and separation of U and Pu are performed on AV-17 anion exchanger in the chloride form from 9 M HCl. Am and Cm are separated from rare-earth elements by step-by-step elution from KU-2 cation-exchange resin in the NH ₄ ⁺ form with α-hydroxyisobutyric acid (pH 4.75). Finally, U, Pu, Am, and Cm are determined α-spectrometrically.     

The intercalation of graphite by uranium hexafluoride

Uranium hexafluoride is shown to react with graphite to yield an intercalation compound of nominal formula “C₁₃UF₆”. Wide line fluorine nuclear magnetic resonance demonstrates the presence not only of uranium (VI) fluoride but also of uranium (IV) fluoride. This assignment is confirmed by the observation of Curie-Weiss Law magnetic susceptibility, indicating approximately 10% of the uranium species to exist as U(IV). The presence of intercalated paramagnetic species, as in “C₁₃UF₆”, “C₁₃CrO₃”, and C₆FeCl₃, can obscure the properties of a fundamental conduction carrier electron spin resonance absorption in graphite/acceptor compounds.     

Effect of <Superscript>239</Superscript>Pu α-radiation on its extraction from nitric acid solutions with a 30% solution of tri-<Emphasis Type="Italic">n</Emphasis>-butyl phosphate in Isopar-M

Extraction of ²³⁹Pu from 4 M НNО₃ with an HNO₃-saturated 30% solution of tri-n-butyl phosphate (TBP) in Isopar-M in the cyclic mode was studied. As a result of four-month contact with an aqueous НNО₃ solution containing 0.11 M Pu(IV), the organic phase absorbed the total α-radiation dose of 380 kGy. The fraction of the residual Pu in the organic phase after stripping increases by a factor of 5–7 as the absorbed dose is increased from 32 to 263 kGy. The residual concentration of Pu in the organic phase after 45–50 stripping cycles (which corresponds to the irradiation dose with accelerated electrons of 120–150 kGy) increases by a factor of 3–4. The Pu distribution ratio in the extraction step in the cyclic mode varies only slightly.     

Assessment of uranium exposure from total activity and 234U:238U activity ratios in urine

Radiation workers at Atomic Weapons Establishment (AWE) are monitored for uranium exposure by routine bioassay sampling (primarily urine sampling). However, the interpretation of uranium in urine and faecal results in terms of occupational intakes is difficult because of the presence of uranium due to intakes from environmental (dietary) sources. For uranium in urine data obtained using current analytical techniques at AWE, the mean, median and standard deviation of excreted uranium concentrations were 0.006, 0.002 and 0.012 μg per g creatinine, respectively. These values are consistent with what might be expected from local dietary intakes and the knowledge that occupational exposures at AWE are likely to be very low. However, some samples do exceed derived investigation levels (DILs), which have been set up taking account of the likely contribution from environmental sources. We investigate how the activity and isotopic composition of uranium in the diet affects the sensitivity of uranium in urine monitoring for occupational exposures. We conclude that DILs based on both total uranium in urine activity and also (234)U:(238)U ratios are useful given the likely variation in dietary contribution for AWE workers. Assuming a background excretion rate and that the enrichment of the likely exposure is known, it is possible to assess exposures using (234)U:(238)U ratios and/or total uranium activity. The health implications of internalised uranium, enriched to <5-8 % by mass (235)U, centre on its nephrotoxicity; the DILs for bioassay samples at AWE are an order of magnitude below the conservative recommendations made by the literature.     

Synthesis and structure of dodecamolybdates of tetravalent cerium,thorium, uranium,neptunium, and plutonium

A series of isostructural compounds of the composition Na₇H[EMo₁₂O₄₂]·12H₂O, where E(IV) = Ce, Th, U, Np, or Pu, were synthesized and structurally characterized. In the [EMo₁₂O₄₂]^8– heteropolyanion (HPA), the central E(IV) atom is surrounded by six Mo₂O₉ groups, each constituted by two octahedra sharing a common face. The coordination polyhedron (CP) of the central atom is a weakly distorted icosahedron with the mean E(IV)–О bond lengths of 2.498, 2.529, 2.500, 2.490, and 2.488 Å for Ce, Th, U, Np, and Pu, respectively. In the structure of the compounds Na₇H[EMo₁₂O₄₂]·12H₂O, there are two crystallographically independent sodium atoms: Na(1) and Na(2). The oxygen surrounding of the Na(1) atom is formed by the terminal oxygen atoms of two heteropolyanions adjacent along [001], and its coordination polyhedron is an octahedron. The surrounding of the Na(2) atom (a six-vertex polyhedron) is formed by three terminal oxygen atoms of three Mo₂O₉ groups belonging to the same HPA and by three water molecules. The coordination polyhedra of the Na(2) atoms are linked with each other via common oxygen atoms of O_w(2) water molecules to form a chain “winding” around the 3₁ screw axis. The heteropolyanions and Na⁺ cations in the crystal form a framework constructed in a fashion characteristic of Dexter–Silverton type anions, with the coordination via three terminal oxygen atoms of three Mo₂O₉ groups. Excess negative charge of HPA is compensated by the proton localized on one of the six bridging O atoms. In the Mo₂O₉ doubled octahedra, the Mo–O bonds with the О atoms bonded to E(IV) and forming the edge of the common face are sensitive to the kind of the central atom.     

Determination of 234U/238U ratio: comparison of multi-collector ICPMS and ICP-QMS for water, hair and nails samples, and comparison with alpha-spectrometry for water samples

The (234)U/(238)U ratio in water, hair and nails samples was determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) and inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) and by alpha-spectrometry for the water samples only. A correlation of 0.99 was found between the two ICPMS methods and of 0.98 with alpha-spectrometry. The range of activity ratios was between 0.9 and 2.6 according to the MC-ICPMS measurements. The reproducibility of both ICPMS techniques was better than 4% for water samples containing 1 mug l(-1) of uranium and a (234)U/(238)U atom ratio of 54.9 x 10(-6). Sample preparation for the ICPMS consisted of dilution of water samples containing >10 microg l(-1) of uranium and measurement time was approximately 1 min, while alpha-spectrometry involved pre-concentration and separation of the uranium and counting times of 1,000 min.     

Dissolution of stoichiometric uraninite

A comparative study of dissolution of stoichiometric uraninite (UO₂), schoepite hydrate (UO₃·H₂O), and uranium(IV) hydroxide [U(OH)₄] was made. In the initial step, UO₂ is oxidized with oxygen dissolved in water to form an oxide that is richer in oxygen and therefore dissolves more readily. This is followed by formation on the uraninite surface of hydroxo complexes, which are stable under the conditions of their formation and form a passivating layer of a sort. In this case, the character of the dissolution process changes, and the transfer of the uranium hydroxo complexes formed to the solution becomes the limiting step. It follows from the comparison of the results obtained that, in the initial step of dissolution of stoichiometric uraninite under both dynamic and static conditions, schoepite is not an intermediate phase of the paragenetic sequence of mineral phase formation.     

Tc(VII)-catalyzed oxidation of U(IV) with nitric acid in solution of TBP in <Emphasis Type="Italic">n</Emphasis>-dodecane

Oxidation of U(IV) with nitric acid in 30% solution of TBP in n-dodecane is catalyzed by Tc ions; the rate-determining steps are 3U(IV) + 2Tc(VII) → 3U(VI) + 2Tc(IV) and Tc(IV) + Tc(VII) → Tc(V) + Tc(VI). Oxidation of U(IV) is inhibited by the reaction product, HNO₂, which partially binds Tc(IV) ions (TcO²⁺) in an inert complex. The overall rate equation of U(IV) oxidation is-d[U(IV)]/dt = k ₁[U(IV)][Tc][HNO₃]^?3 ? k ₄[U(IV)]²[HNO₂]²[HNO₃]^?1, where k ₁ = 4.8 ± 1.0 mol²1^?2 min^?1 and k ₄ = (2.4 ± 1.0) × 10⁵ 1² mol^?2 min^?1 at 25°C, [H₂O] = 0.4 M ([Tc] is the total Tc concentration in the reaction mixture). Water and U(VI) have no effect on the reaction rate.