Standard molar Gibbs free energy of formation of PbO(s) over a wide temperature range from EMF measurements

The EMF of the following galvanic cells,

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Kanthal,Re,Pb,PbOCSZO₂ (1 atm.),Pt

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Kanthal,Re,Pb,PbOCSZO₂(1 atm.),RuO₂,Pt

were measured as a function of temperature. With O₂ (1 atm.), RuO₂ as the reference electrode, measurements were possible at low temperatures close to the melting point of Pb. Standard Gibbs energy of formation, Δ_fG⁰_mβ-PbO was calculated from the emf measurements made over a wide range of temperature (612–1111 K) and is given by the expression: Δ_fG⁰_mβ-PbO±0.10 kJ=−218.98+0.09963T. A third law treatment of the data yielded a value of −218.08 ± 0.07 kJ mol⁻¹ for the enthalpy of formation of PbO(s) at 298.15 K, Δ_fH⁰_mβ-PbO which is in excellent agreement with second law estimate of −218.07 ± 0.07 kJ mol⁻¹.     

Dynamic response of target electrons upon low and medium energy ion impact

We found that differential scattering cross sections for medium and low energy He⁺ and Ne⁺ impact on high Z-atoms were significantly enhanced compared with those calculated from the inter-atomic potential based on the Hartree–Fock–Slater atomic model coupled with the bare nuclear charge of a projectile. The enhanced scattering cross sections determined experimentally are reproduced well by a simple model that the center of gravity of target electrons is shifted toward the projectile during a large-angle collision. The shift from the target nucleus is expressed as a function of inter-nuclear distance in terms of a dipole moment (Z₁ and Z₂: atomic numbers of projectile and target, : polarizability, e: electron charge). The effective polarizability β (≡Z₁/Z₂) is expressed as a function of ion velocity v [10⁷ cm/s], in the form β = 0.079 exp[−0.46v].     

赣杭构造带某铀矿区地下水中铀的存在形式研究北大核心CSCD

为研究铀矿区地下水化学性质对铀的存在形式的影响,本文以赣杭构造带某铀矿区地下水为研究对象,在对9个典型采样点地下水化学成分分析的基础上,采用数理统计软件SPSS 18.0和地球化学模拟软件PHREEQC及llnl.dat数据库,探究了研究区内地下水水化学特征及U的存在形式。结果表明:本研究区地下水水化学类型以HCO_(3)-Na与HCO_(3)-Na·Ca为主,U含量与Ca^(2+)和Mg^(2+)浓度体现出较强正相关性,与SO_(4)^(2-)的相关性次之;地下水中U元素主要以六价为主,几乎占100%,主要存在形式依次为UO_(2)(CO_(3))_(2)^(2-)、UO_(2)(CO_(3))_(3)^(4-)、UO_(2)CO_(3)、UO_(2)(OH)2、UO_(2)(OH)_(3)^(-)、UO_(2)OH^(+)等6种,其中UO_(2)(CO_(3))_(2)^(2-)占绝对优势,整体以碳酸铀酰形式为主,这也与研究区地下水酸碱性相对应。     

铀在北山地下水中的种态分布及溶解度分析

为了解铀酰离子在北山地下水中的吸附、扩散和迁移行为,利用地球化学计算软件PHREEQC,采用由OECD/NEA发布的最新铀的热力学数据,计算了铀在我国高放废物地质处置库重点研究区甘肃北山地下水中的种态分布,并分析了围岩中存在的方解石对铀溶解度的影响。计算结果表明,在北山地下水组成不变的前提下,在偏酸性条件下,铀主要以UO₂F⁺、UO₂SO₄、UO₂²⁺、UO₂F₂和UO₂(SO₄)₂^2-的形式存在,而在中性至弱碱性条件下,主要以 UO₂(CO₃)^4-₃、UO₂(CO₃)₂^2-、UO₂(OH)₃^-和UO₂(OH)₄^2-的形式存在。我国计划建造的高放废物处置库的设计深度为地下500～1000m,其水岩体系一般呈弱碱性。在这样的弱碱性水岩体系中,以阴离子形式存在的铀酰配合物具有较强的可移动性。当地下水的pH=7.56时,在Eh<24mV的条件下,铀主要以沥青铀矿的形式存在,而在更高的Eh条件下,则主要以UO₂²⁺与CO₃^2-和OH^-形成的阴离子配合物的形式存在。当地下水与空气接触时,O₂的存在会使Eh升高,此时铀的主要存在种态为UO₂²⁺及其各种配合物。当围岩体系中存在方解石时,在pH<8.0的条件下,铀在地下水中的溶解度会显著提高,而在更高pH条件下,方解石对铀的溶解度无明显影响。     

螯合剂对铀在小鼠体内分布的影响研究

铀在生物体内的化学形态及分布是评价铀毒性的基础。文章通过尾静脉注射给药方式研究螯合剂对染铀小鼠体内铀分布的影响。通过建立包含主要体液金属离子、小分子配体及UO₂²⁺与螯合剂的热力学平衡模型,采用数值模拟方法研究UO₂²⁺在血液中的形态。实验结果表明：DTPA易与UO₂²⁺形成［(UO₂)₂(OH)DTPA］^2-,对小鼠血液中的UO₂²⁺有明显的促排作用,但在促排的同时对小鼠的肾脏和骨骼有损伤;抗坏血酸对小鼠体内各组织器官的铀促排几乎无作用,但形成的［(UO₂)₂(OH)Ascorbate］²⁺会阻碍肾脏中U(Ⅵ)的代谢;NTA对小鼠体内各组织器官铀的促排作用小,并会促使体内出现固相(UO₂)₃(PO₄)₂·4H₂O,对骨骼有较强的损伤作用,不适合作为铀的促排剂。     

Electronic excitation effects in CeO2 under irradiations with high-energy ions of typical fission products

In order to understand the formation mechanism of a crystallographic re-structuring in the periphery region of high-burnup nuclear fuel pellets, named as “rim structure”, information on the accumulation process of radiation damage and fission products (FPs), as well as high-density electronic excitation effects by FPs, are needed. In order to separate each of these processes and understand the high-density electronic excitation effects, 70–210 MeV FP ion (Xe^10–14+, I⁷⁺ and Zr⁹⁺) irradiation studies on CeO₂, as a simulation of fluorite ceramics of UO₂, have been done at a tandem accelerator of JAEA-Tokai and the microstructure changes were determined by transmission electron microscope (TEM). Measurements of the diameter of ion tracks, which are caused by high-density electronic excitation, have clarified that the effective area of electronic excitation by high-energy fission products is around 5–7 nm  and the square of the track diameter tends to follow linear function of the electronic stopping power (S_e). Prominent changes are hardly observed in the microstructure up to 400 °C. After overlapping of ion tracks, the elliptical deformation of diffraction spots is observed, but the diffraction spots are maintained at higher fluence. These results indicate that the structure of CeO₂ is still crystalline and not amorphous. Under ion tracks overlapping heavily (>1 × 10¹⁵ ions/cm²), surface roughness, with characteristic size of the roughness around 1 μm, is observed and similar surface roughness has also been observed in light-water reactor (LWR) fuels.     

Post-irradiation examination of uranium-doped rock-like oxide fuels

Post-irradiation examinations of rock-like oxide fuels were performed in JAERI to evaluate irradiation behavior and geochemical stability. Five kinds of fuels were prepared using 20% enriched U instead of Pu; a single-phase fuel of an yttria-stabilized zirconia containing UO₂ (U-YSZ), two particle-dispersed type fuels of U-YSZ particles + MgAl₂O₄/Al₂O₃ powder, two homogeneously blended type fuels of U-YSZ powder + MgAl₂O₄/Al₂O₃ powder. The fuels were irradiated in JRR-3 for about 100 days and estimated irradiation conditions were as follows; linear power was 15 kW m⁻¹, thermal neutron fluence was 7 x 10²⁴ m⁻² and fuel temperatures at the surface were 740–1130 K. From the results of non-destructive examinations, the stainless steel cladding surfaces were partially discolored by oxidation and no remarkable deformation of the pins was observed. Significant pellet fragmentation was not observed in spite of the crack formation as observed in irradiated LWR UO₂ fuels. Nonvolatile FPs were observed only in pellets but volatile Cs moved partly to the plenum. From these examinations, no significant difference in macroscopic irradiation behavior was distinguished among 5 fuels.     

Structure and kinetic studies of U(VI)-benzamidoxime complex in non-aqueous solutions by H- and C-NMR

The structural and kinetic studies of U(VI) complex with benzamidoxime(Hba) as ligand in CD₃COCD₃ have been studied by means of ¹H and ¹³C NMR. The Hba molecule was found to coordinate to UO₂²⁺ in the form of anionic benzamidoximate (ba), and the number of ba coordinated to UO₂²⁺ was determined to be 3 by analyzing the chemical shift of ¹³C NMR signal for Hba in the presence of UO₂²⁺. The exchange rate constants(k_ex) of ba in [UO₂(ba)₃] were determined by the NMR line-broadening method. The kinetic parameters were obtained as follows: k_ex(25°C) = 3.1 × 10³s⁻¹, ΔH^≠ = 35.8 ± 3.5 KJ mol⁻¹, and ΔS^≠ = −65 ± 13.7 J K⁻¹ mol⁻¹. The UV-visible absorption spectra of solutions containing UO₂²⁺ and Hba were also measured. The molar extinction coefficient of the complex was found to be extremely large compared with those of UO₂(L)₅²⁺ (L = unidentate oxygen donor ligands) complexes. This is due to the strong electron withdrawing of UO₂²⁺ from Hba and suggests that an interaction between UO₂²⁺ and Hba is very strong. Such a high affinity of monomeric amidoxime to UO₂²⁺ reasonably explains the high adsorptibility of amidoxime resin to U(VI) species, and is considered to result in the high recovery of U(VI) species from sea water using amidoxime resin.     

Heavy ion irradiation effects in the rare-earth sesquioxide Dy2O3

Polycrystalline pellets of the rare-earth sesquioxide Dy₂O₃ with cubic C-type rare-earth structure were irradiated with 300 keV Kr²⁺ ions at fluences up to 5 × 10²⁰ Kr/m² at cryogenic temperature. Irradiation-induced microstructural evolution is characterized using grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). In previous work, we found a phase transformation from a cubic, C-type to a monoclinic, B-type (C2/m) rare-earth structure in Dy₂O₃ during Kr²⁺ ion irradiation at a fluence of less than 1 × 10²⁰ Kr/m². In this study, we find that the crystal structure of the top and middle regions of the implanted layer transform to a hexagonal, H-type (P6₃/mmc) rare-earth structure when the irradiation fluence is increased to 5 × 10²⁰ Kr/m²; the bottom of the implanted layer, on the other hand, remains in a monoclinic phase. The irradiation dose dependence of the C-to-B-to-H phase transformation observed in Dy₂O₃ appears to be closely related to the temperature and pressure dependence of the phases observed in the phase diagram. These transformations are also accompanied by a decrease in molecular volume (or density increase) of approximately 9% and 8%, respectively, which is an unusual radiation damage behavior.     

The thermal conductivity of UO2 vapor

The thermal conductivity, λ of a saturated vapor over UO_1.96 is calculated in the temperature range 3000–6000 K. The calculation shows that the contribution to λ from the transport of reaction enthalpy dominates all other contributions. All possible reactions of the gaseous species UO₃, UO₂, UO, U, O, and O₂ are included in the calculation. We fit the total thermal conductivity to the empirical equation λ = exp(a+ b/T+cT+dT² + eT³), with λ in cal/(cm s K), T in kelvins, a = 268.90, B = − 3.1919 × 10⁵, C = −8.9673 × 10⁻², d = 1.2861 × 10⁻⁵, and E = −6.7917 × 10⁻¹⁰.     

Interface mixing induced by swift heavy ions at metal-oxide/silicon interfaces

It was observed previously that ceramic/ceramic bilayers were very sensitive with respect to the electronic stopping power S_e, i.e. strong interface mixing, scaling with , occurred if a threshold S_ec was exceeded. The threshold seemed to be determined by the higher track formation threshold of two constituents forming the bilayer. Although no track formation has been observed in crystalline Si even for Uranium projectiles, interface mixing was observed previously for some Si-multilayers.

In this paper we report on the interface mixing of NiO, Fe₂O₃, TiO₂ on Si due to irradiation with 90–350 MeV Ar-, Kr-, Xe- and Au-ions at 80 K at fluences up to 9E15 ions/cm². Interface mixing, analyzed by means of Rutherford Backscattering Spectrometry (RBS), is found for these bilayers, too. But the threshold for intermixing is significantly higher compared to the ceramic/ceramic bilayers. This observation could be an evidence for the threshold being determined by the Si-layer. In contrast to NiO/Si and Fe₂O₃/Si, where an usual random walk mixing Δσ² = kΦ was observed, the interface broadening Δσ² for TiO₂/Si is found to scale nonlinearly with the ion fluence, which indicates that mixing is driven by a chemical solid-state reaction. At higher fluences plateaus form at the low energy Ni-edge of the RBS spectra. The plateaus indicate phase formation. X-Ray diffraction spectra does not show any evidence for new crystalline phases.     

Resonant coherent excitation of 2s electron of Li-like Fe ions to the n = 3 states

We observed resonant coherent excitation of the 2s electron to the n = 3 states of 83.5 MeV/u Li-like Fe²³⁺ ions planar-channeling in the plane of a Si crystal. A survival fraction of the Li-like ions was measured as a function of the angle between the incident beam and the [0 0 1] axis. Clear resonance dips corresponding to the transitions of a 2s electron to all the n = 3 states were observed. The transition of each resonance dip was identified by comparing with spectroscopic data. The resonance dips at the transition energies corresponding to the optically forbidden 2s_1/2–3s_1/2, 2s_1/2–3d_3/2 and 2s_1/2–3d_5/2 transitions were observed as well as the resonance dips at transition energies corresponding to the optically allowed 2s_1/2–3p_1/2 and 2s_1/2–3p_3/2 transitions.     

Structural study on uranyl ion in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ionic liquid

The structure of uranyl ion in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ionic liquid (BMINfO) has been studied with ¹H- and ³⁵Cl-NMR, Raman, and UV-visible spectroscopy. In the ¹H-NMR spectrum of the BMINfO solution prepared by dissolving UO₂(ClO₄)₂·5 6H₂O, the signal of H₂O coordinated to UO₂²⁺ was observed at 6.64 ppm at 50°C (free H₂O in BMINfO: 3.1 ppm at 50°C), suggesting that the uranyl species exists as the aquo complex, [UO₂(H₂O)_n]²⁺. The signal of the coordinated H₂O disappears with heating at 120°C for 3 h under vacuum. This indicates the dehydration from [UO₂(H₂O)_n]²⁺. On the other hand, the ³⁵Cl-NMR signal of ClO₄⁻ as the counter anion of UO₂²⁺ was observed at 1011 ppm (vs. Cl⁻ in D₂O) regardless of heating. This indicates that no ClO₄⁻ ion is in the first coordination sphere of UO₂²⁺. Furthermore, the UV-visible absorption spectra showed that the characteristic absorption bands due to UO₂²⁺ were sharpened with the dehydration. This means the simplification of the structure around UO₂²⁺. These results described above suggest that UO₂²⁺ in BMINfO has no ligand in its equatorial plane after the dehydration, i.e. UO₂²⁺ exists as a bare cation in this system.     

Mechanism of adatom formation on Ag(1 1 0) studied by combined quasi-elastic He atom scattering and low energy ion scattering

Two complementary techniques, i.e. low-energy ion scattering in the neutral impact collision ion scattering spectroscopy (NICISS) mode and energy resolved He atom scattering (HAS) are combined to study the adatom formation mechanism on Ag(1 1 0). The NICISS spectra have been collected between 300 and 900 K along the 0 0 1, and azimuthal directions while HAS measurements of the diffuse elastic peak has been performed along the directions between 190 and 800 K. Both techniques are sensitive to surface disorder and HAS data have been corrected by using the NICISS results in particular on the anharmonicity estimated through a two atoms scattering model. In fact, above 500 K surface anharmonicity is clearly detected and accompanies the proliferation of thermal induced defects. Without this anharmonic correction the adatom formation energy E_a is too low and similar to the surface diffusion barrier. Instead, including the anharmonicity the estimate, E_a = (0.38 ± 0.03) eV, is in excellent agreement with the predictions of molecular dynamics simulations.     

Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization

Brannerite, ideally MTi₂O₆, (M=actinides, lanthanides and Ca) occurs in titanate-based ceramics proposed for the immobilization of plutonium. Standard enthalpies of formation, ΔH⁰_f at 298 K, for three brannerite compositions (kJ/mol): CeTi₂O₆ (−2948.8 ± 4.3), U_0.97Ti_2.03O₆ (−2977.9 ± 3.5) and ThTi₂O₆ (−3096.5 ± 4.3) were determined by high temperature oxide melt drop solution calorimetry at 975 K using 3Na₂O · 4MoO₃ solvent. The enthalpies of formation were also calculated from an oxide phase assemblage (ΔH⁰_f-ox at 298 K): MO₂ + 2TiO₂=MTi₂O₆. Only UTi₂O₆ is energetically stable with respect to an oxide assemblage: U_0.97Ti_2.03O₆ (ΔH⁰_f-ox=−7.7±2.8 kJ/mol). Both CeTi₂O₆ and ThTi₂O₆ are higher in enthalpy with respect to their oxide assemblages with (ΔH⁰_f-ox=+29.4±3.6 kJ/mol) and (ΔH⁰_f-ox=+19.4±1.6 kJ/mol) respectively. Thus, Ce- and Th-brannerite are entropy stabilized and are thermodynamically stable only at high temperature.     

Formation of uranium mononitride by the reaction of uranium dioxide with carbon in ammonia and a mixture of hydrogen and nitrogen: II. Reaction rates

Kinetics of the carbothermic synthesis of UN from UO₂ in an NH₃ stream and a mixed 75% H₂ + 25% N₂ stream were studied in the temperature range of 1400–1600°C by X-ray analysis and weight change measurement of the sample. The weight change was divided into two parts; i.e. weight loss due to carbothermic reduction of UO₂ and weight loss due to removal of carbon by hydrogen. The former followed the first-order rate equation −1n(1 − ₀) = k₀t, and the latter the rate equation of phase boundary reaction 1 − (1 − _c)^1/3 = k_ct. The apparent activation energy of the former was in the range of 320–380 kJ/mol. The value of the latter in an NH₃ stream was 175–185 kJ/mol, which was smaller than that in a mixed 75% H₂ + 25% N₂stream (285 kJ/mol). In this method, the rate of the removal of carbon by hydrogen determines that of the formation of high purity UN.     

Phase transitions in U3O8−z: I, heat capacity measurements

The heat capacity of U₃O_8−z with various O/U ratios was measured in the range from 250 to 750 K, and λ-type heat capacity anomalies were found in each sample. The transition temperatures were 487 and 573 K for UO_2.663, 490 and 576 K for UO_2.656 and 508, 562 and 618 K for UO_2.640. The entropy changes of the transitions were 0.44 and 0.39 J K⁻¹mol⁻¹ for UO_2.663, 0.58 and 0.47 J K⁻¹mol⁻¹ for UO_2.656 and 0.62, 0.51 and 0.25 J K⁻¹mol⁻¹ for UO_2.640, increasing as O/U decreases. The enthalpy change due to the transition varied linearly with the transition temperature except for UO_2.640, showing the presence of the same mechanism of phase transition among the samples with various O/U ratios. The mechanism of the phase transition was discussed on the assumption that the transition is originated from the order-disorder rearrangement of U⁵⁺ and U⁶⁺ with a consequent displacement of atoms, similarly to the case of U₄O_9−y.     

Radiation enhanced diffusion of implanted platinum in silicon guided by helium co-implantation for arbitrary control of platinum profile

The in-diffusion of platinum into a low-doped n-type float zone silicon guided and enhanced by radiation damage produced by co-implantation of helium ions was investigated. The implantation of 1 MeV platinum ions at different doses ranging from 5 × 10¹¹ to 5 × 10¹² cm⁻² was used to produce a finite source for platinum diffusion. Single and multiple energy implantation of helium ions with energies 7, 9, 11 and 13 MeV introducing different profiles of radiation defects were applied to enhance and shape the diffusion of platinum atoms performed by 20 min annealing at 725 °C in vacuum. The distribution of in-diffused platinum was studied by monitoring the acceptor level of substitutional platinum (E_C − E_T = 0.23 eV) by deep level transient spectroscopy. Results show that the helium co-implantation significantly enhances platinum diffusion and allows its control up to the depths of hundreds of micrometers. The resulting Pt_s distribution is given by the profile of radiation damage produced by helium ions while the amount of in-diffused Pt_s can be controlled by the dose of platinum implantation. It is also shown that an extra annealing at 685 °C performed prior to helium implantation substantially increases the amount of in-diffused platinum.     

Environmentally acceptable nuclear fuel cycle development of a new reprocessing system

The aim of the present study is to establish a new reprocessing system for spent nuclear fuel, which would overcome the environmental problems in the nuclear fuel cycle. In order to achieve this, the following subjects have to be conquered: recoveries of high ratios of uranium and trans uranium elements from spent nuclear fuel, separations of strong radioactive elements, such as Sr and Cs, and assurance of the extreme safety during operation. The last subjects might be of particular importance in order to avoid any potential danger. Therefore, in the present system all processes were performed under mild aqueous conditions. Experiments were carried out for a simulated spent fuel solution, which was calculated from the ORIGEN CODE containing uranium and 17 major elements. The system consists of the following processes: 1. dissolution of spent UO₂ fuel involving off-gas treatment of I and Ru; 2. neutralization of the dissolved fuel solution with NaHCO₃---Na₂CO₃ mixed solution to be slightly basic at pH about 9 followed by the separation of precipitated fission products by centrifugation; 3. separation of Cs by a precipitation method using tetraphenylborate ion; 4. recovery of U, Np and Pu as precipitates of hydrolyzed compounds from alkaline solution; 5. separation of Am and Cm from lanthanide elements. The concentration of residual uranium in the final solution was measured to be ppm order, indicating that the decontamination factor of U was as large as 10⁴. Other hexa-valent actinide ions, NpO₂²⁺ and PuO₂²⁺, also have extremely large stability constants for the complex formation with carbonate ion, and are expected to behave similarly with UO₂²⁺. In conclusion, the present reprocessing system enables us to recover U, Pu and Np from spent nuclear fuel by means of a simple precipitation method in much higher ratios compared with other reprocessing methods, to separate hazardous Cs and Sr from high-level waste, and to exclude any potential danger owing to chemical processes under mild aqueous conditions.     

Neutralization rate for slow Ar ions in front of KCl(0 0 1)

Charge state distributions of reflected ions are measured when 5 keV Ar^q+(q = 0−2) ions are incident on a clean KCl(0 0 1) surface at grazing angle, θ_i. Although the charge state distribution does not depend on the incident charge state at larger θ_i, significant dependence of the charge state distribution on incident charge state is observed at smaller θ_i. The ionization of Ar⁰ is completely suppressed at θ_i < 20 mrad, while large neutralization probability is observed for Ar⁺ incidence. These features allow us to derive the position-dependent neutralization rate of Ar⁺ in front of KCl(0 0 1). The obtained neutralization rate decreases exponentially with distance from the surface as it is usually assumed.