铀尾矿渗滤液中铀在地下水中的迁移模拟

选择某铀尾矿库作为研究对象,采集尾矿坝中的铀尾矿样品、含水层砂土样品及隔水层粘土样品,研究U在关键地层土壤样品上的等温吸附规律及降雨条件下U的浸出过程,得到U在关键地层的吸附迁移参数及源项释放规律,并运用FEFLOW6.2软件建立铀尾矿评价区地下水三维模型,模拟U的迁移行为及浓度分布。实验结果表明U在砂土与粘土上的吸附符合线性等温吸附模型,分配系数Kd分别为20.41 L/kg、45.92 L/kg;实验周期内酸雨淋浸与去离子水淋浸条件下U浸出平衡浓度分别为0.83 mg/L、0.79 mg/L,浸出率分别为46.07%、20.92%。模拟结果表明经过30年的迁移,U污染晕峰值浓度为0.595 mg/L,峰值浓度迁移距离为36.44 m;经过50年的迁移,U污染晕峰值浓度为0.440 mg/L,峰值浓度迁移距离为42.93 m。     

Equation of state of uranium oxide

The total and partial pressures over liquid UO₂ have been measured and calculated up to 5000K. A review of previous work is given. The equation of state of UO₂ as the main constituent of the fast breeder oxide fuel is required up to at least 5000K in order to estimate the energy release in a loss of flow (LOF) driven hypothetical core disruptive accident (HCDA) of the liquid metal fast breeder reactor (LMFBR).Two models, a macroscopic “mixture” model and a microscopic “defect” model have been developed to determine the oxygen potential of UO₂₀₀ up to 5000 K.A combination of mass spectrometric, Langmuir probe and high tension diode studies, applied for the first time to the laser vaporization process, revealed large quantities of ions emitted directly from the surface, and resolved previous discrepancies between measured and calculated vapour pressures by an enhanced rate of evaporation due to ion emission. As shown theoretically intrinsic ion emission can contribute to the net evaporation rate only if the resulting positive space charge can be neutralised. It is proposed that this can be accomplished by the presence of “hot” electrons in the plasma.The recommended equilibrium total pressure over liquid UO_2.00, valid between the melting point and 5000K, is $\text{log p (MPa) = ? 2.717 ? 20131/T + 1.925}\text{log T}$.     

Variable energy positron beam study of Xe-implanted uranium oxide

Doppler broadening of annihilation gamma-line combined with a slow positron beam was used to measure the momentum density distribution of annihilating pair in a set of sintered UO₂ samples. The influence of surface polishing, of implantation with 800-keV ¹³⁶Xe²⁺ at fluences of 1 × 10¹⁵ and 1 × 10¹⁶ Xe cm^?2, and of annealing were studied by following the changes of the momentum distribution shape by means of S and W parameters. The program used for this purpose was VEPFIT. At the two fluences in the stoichiometric as-implanted UO₂, formation of Xe bubbles was not detected. The post-implantation annealing and over-stoichiometry in the as-implanted sample caused Xe precipitation and formation of Xe bubbles.     

Direct electrochemical reduction of solid uranium oxide in molten fluoride salts

The direct electrochemical reduction of UO₂ solid pellets was carried out in LiF-CaF₂ (+2 mass.% Li₂O) at 850 °C. An inert gold anode was used instead of the usual reactive sacrificial carbon anode. In this case, oxidation of oxide ions present in the melt yields O₂ gas evolution on the anode. Electrochemical characterisations of UO₂ pellets were performed by linear sweep voltammetry at 10 mV/s and reduction waves associated to oxide direct reduction were observed at a potential 150 mV more positive in comparison to the solvent reduction. Subsequent, galvanostatic electrolyses runs were carried out and products were characterised by SEM-EDX, EPMA/WDS, XRD and microhardness measurements. In one of the runs, uranium oxide was partially reduced and three phases were observed: nonreduced UO₂ in the centre, pure metallic uranium on the external layer and an intermediate phase representing the initial stage of reduction taking place at the grain boundaries. In another run, the UO₂ sample was fully reduced. Due to oxygen removal, the U matrix had a typical coral-like structure which is characteristic of the pattern observed after the electroreduction of solid oxides.     

Equilibrium and nonequilibrium molecular dynamics simulations of heat conduction in uranium oxide and mixed uranium-plutonium oxide

The thermal conductivity of nuclear fuels such as UO_2+x and (U,Pu)O_2−x has been calculated by the molecular dynamics (MD) simulation in terms of oxygen stoichiometric parameter x, temperature and Pu content. In the present study, the MD calculations were carried out in both equilibrium (EMD) and nonequilibrium (NEMD) systems. In the EMD simulation, the thermal conductivity was defined as the time-integral of the correlation function of heat fluxes according to the Green-Kubo relationship. Meanwhile, in the homogeneous NEMD, it was given by the ratio of the time-averaged heat flux to the perturbed external force subjected to each particle in the simulated cell. NEMD, as compared with EMD, gave somewhat precise results efficiently. Furthermore, both MD calculations showed that the thermal conductivity of these oxide fuels decreased with increase of temperature and defects, i.e. excess oxygen or vacancy, and was rather insensitive to Pu content for the stoichiometric fuel.     

Diode-like ion-track asymmetric nanopores: Some alternative methods of fabrication

We present two fabrication methods which allow production of ion-track nanopore membranes with pronounced geometrical asymmetry and diode-like ionic conductivity. The asymmetry of the pores is provided by self-assembly of surfactant molecules at ion-track entrances on one side of the ion-irradiated polymer foil. On the other side of the membrane, the effect of surfactant is excluded using a pre-treatment with ultraviolet radiation or a surfactant-free etching solution. Highly tapered pore profiles are obtained and characterized using SEM and FESEM. In electrolyte solutions, the asymmetric pores thus fabricated exhibit a high rectification of electric current. The nanopores with a large angle aperture can be of interest for resistive-pulse technique applications and for the atomic beam projection nanolithography.     

Magnetic and optical properties of cobalt nanowires fabricated in polycarbonate ion-track templates

Cobalt nanowires with diameter 75 nm were synthesized in ion track-etched membranes by electrochemical deposition. Scanning electron microscopy displays cylindrical wires with smooth and homogeneous contours. X-ray diffraction studies indicate that the wires possess a face centered cubic structure and a preferred growth orientation along the [1 1 0] direction. The wires exhibit magnetic anisotropy, which is observed and is ascribed to shape anisotropy. The optical extinction spectrum shows a band which probably originates from a surface plasmon resonance.     

某铀尾矿库区地下水238U迁移模拟

将铀尾矿库核素迁移问题概括为均质多孔介质中稳定的二维水流条件下的三维溶质迁移问题,利用Visual Modflow软件(MT3D)采用全隐式差分法对某铀尾矿库放射性核素迁移进行了模拟研究,模拟值结果与浓度实测值较吻合,揭示了尾矿库放射性核素时空迁移规律。     

Binomial distribution function for intuitive understanding of fluence dependence of non-amorphized ion-track area

We propose the binomial distribution function is a useful function to describe the fluence dependence of overlapped and non-overlapped area of ion-tracks created by high-energy heavy ions. The validity of the function has been proven by simple computer simulation assuming that ion-tracks are introduced at random positions of two-dimentional grid. In order to test the applicability of the function for describing accumulation behavior of non-amorphized ion-tracks, asymmetric X-ray diffraction peak observed for CeO₂ irradiated with 200-MeV ¹⁹⁷Au ions has been analyzed. The asymmetric peak observed after the irradiations can be explained by the sum of the original peak attributed to the matrix and the new peak partly attributed to non-overlapped area of ion-tracks. It has been found that the binomial distribution function is useful for explaining the fluence dependence of the non-overlapped area of ion-tracks.     

Properties of uranium vacancies in uranium monocarbide

From the excess resistivity obtained by quenching from temperatures between 1300 and 1600 °C a formation energy of 1.7 eV is determined. The quenchedin resistivity recovers with an activation energy of 2.2 eV in the temperature region between 400 and 600 °C. These two values are attributed to the formation and migration activation energies of uranium vacancies, respectively. It is further shown that uranium monocarbide remains single-phase, if the deviation from stoichiometry does not exceed more than a few tenths of one percent and if gases (mainly oxygen) are in solid solution.     

A model for the behavior of thorium uranium mixed oxide kernels in the pelletizing process

A behavior model of nuclear fuel kernels in the pelletizing process was developed to predict the microstructure of (Th,5%U)O₂ sintered pellets. Methods, equipments and components were developed in order to measure the density, the specific surface area and the crushing strength of the kernels and produce fuel pellets. It enables a correlation between the kernels properties and the microstructure, density and open porosity that were obtained in the fuel pellet produced with these kernels. It was possible to obtain a mathematical expression that allows one to calculate, from the kernel density and specific surface, the density that will be obtained in the fuel pellet for each compactation pressure value. The investigation showed which kernels properties are desired to obtain fuel pellets that satisfy the quality requirements for a stable performance in a power reactor. This model has been validated by experimental results and fuel pellets were obtained with an optimized microstructure that satisfies the fuel specification for an in-pile stable behavior.     

Processing of uranium oxide and silicon carbide based fuel using polymer infiltration and pyrolysis

Ceramic composite pellets consisting of uranium oxide, UO₂, contained within a silicon carbide matrix, were fabricated using a novel processing technique based on polymer infiltration and pyrolysis (PIP). In this process, particles of depleted uranium oxide, in the form of U₃O₈, were dispersed in liquid allylhydridopolycarbosilane (AHPCS), and subjected to pyrolysis up to 900 °C under a continuous flow of ultra high purity argon. The pyrolysis of AHPCS, at these temperatures, produced near-stoichiometric amorphous silicon carbide (a-SiC). Multiple polymer infiltration and pyrolysis (PIP) cycles were performed to minimize open porosity and densify the silicon carbide matrix. Analytical characterization was conducted to investigate chemical interaction between U₃O₈ and SiC. It was observed that U₃O₈ reacted with AHPCS during the very first pyrolysis cycle, and was converted to UO₂. As a result, final composition of the material consisted of UO₂ particles contained in an a-SiC matrix. The physical and mechanical properties were also quantified. It is shown that this processing scheme promotes uniform distribution of uranium fuel source along with a high ceramic yield of the parent matrix.     

热液型铀矿床可能的铀源——以俄罗斯斯特列措夫铀矿田为例

位于俄罗斯外贝加尔地区的斯特列措夫铀矿田为世界上最大的火山岩型铀矿田,也是俄罗斯最大的铀资源所在地。研究表明,斯特列措夫诸多铀矿床铀源主要源于组成破火山口地层大约1/3的强烈蚀变的流纹岩和由海西期黑云母花岗岩构成的破火山口基底。华南地区为我国重要的热液型铀矿床产地,斯特列措夫铀矿田铀源的研究对我国华南地区开展铀矿找矿勘探工作和成矿研究具有一定的指导意义。     

Chemical behavior of Mo99 formed by the irradiation of uranium oxide with slow neutrons

In this paper we examine the problem of extracting Mo⁹⁹ from irradiated samples of U₃O₈ and UO₂, fired in vacuum and in a stream of argon from 400 to 1200 °. The relation of the per cent of Mo⁹⁹ extracted to temperature and time of firing was demonstrated.The activation energy for removing Mo⁹⁹ from uranium oxide grains for some sections of the curves was calculated on the basis of the relations obtained between the logarithm of the value of Mo⁹⁹ extraction and the inverse absolute temperature.