Atomistic simulation of ion channeling in heavily doped Si:As

The structure of vacancy–As complexes (V–As_m, with m = 1, 4) is calculated by first-principles methods and implemented in Monte Carlo (MC) simulation of Rutherford backscattering-channeling (RBS-C) spectra. Using this procedure the atomic position of As, inferred from the model of the complex, is no longer treated as a free parameter of the simulation, as it usually occurs within MC fitting of RBS-C spectra. Moreover, physical effects which influence the backscattering and dechanneling yields of ions, such as the lattice distortion around the vacancy, are naturally taken into account in the simulation. We investigate the sensitivity of RBS-C to the different complex models and report an example of the application of the method to the case of heavily doped Si:As equilibrated at high temperature. The comparison of simulated and experimental angular scans shows that while the hypothesis of inactive As trapped in V–As₄ clusters is consistent with experimental observations, the presence of a significant amount of V–As clusters is unlikely.     

基于孔洞长大理论的多轴蠕变设计模型及其工程应用

多轴蠕变是高温构件失效的重要原因,工程设计中必须予以考虑.本文介绍了以孔洞长大理论为基础的多轴蠕变设计方法;探讨了孔洞长大的基本原理;讨论了基于该理论的5个常用模型;分析了这些模型在R5、ASME III、RCC-MR 及VGB-R 509L 中的应用,指出了高温构件多轴蠕变设计的困难,并对进一步的工作提出了建议.     

基于WEB的BES数据存储系统模型

讨论了基于WEB的数据存储系统模型的系统结构和基本方法。从BES数据结构分析、数据处理过程、系统结构设计、实验方案选择、关键技术以及图像处理等方面详细介绍了系统模型的实现过程，守成了从系统实验软硬件平台的建立到数据装入、数据库建立、数据存取、数据远程处理以及数据图形处理等功能。     

The development of validation methodology of multi-axial creep damage constitutive equations and its application to 0.5Cr0.5Mo0.25V ferritic steel at 590 °C

Research progress on the development of validation methodology for multi-axial creep damage constitutive equations and its specific application to 0.5Cr0.5Mo0.25V ferritic steel at 590 °C is presented. A set of new phenomenological multi-axial creep damage constitutive equations was proposed aiming at overcoming the deficiency of inconsistency between predicted rupture strains and observed ones. Based on these explicit consistent requirements, an improved validation methodology is proposed and applied to 0.5Cr0.5Mo0.25V ferritic steel at 590 °C. It shows that the predictions of this new set of constitutive equations are consistent with experimental observations. It also reveals a significant difference in creep curves between different sets of constitutive equations and the need for experimental data so that the coupling of damage and creep deformation can be further examined.     

Uncertainties associated with the reliability of thermal-hydraulic nuclear passive systems

ABSTRACT

The increasing adoption of passive safety based front-line systems in advanced nuclear power reactors due to their simplicity, cost competitiveness and autonomous nature makes it very essential to carefully consider the uncertainties associated with their behaviour and the phenomena linked to their operations. The passive safety systems (PSSs) are known to be characterized with several uncertainties in their modelling and operations. These uncertainties are usually more pronounced than those of their active systems counterparts due to the stochastic nature of the associated phenomena, insufficient knowledge of their physics of operation and inadequacy of relevant real/experimental data. This paper thus focused on the uncertainty issues of the thermal-hydraulic (t-h) PSSs which influence their reliability analysis. In addition, the inadequacies of previous research, current research challenges and likely future research directions on uncertainties associated with the models and phenomena applicable to the t-h PSSs adopted in advanced reactors were discussed. For the purpose of illustration, Weibull distribution was adopted for the failures associated with a generic passively cooled steam generator and Bayesian approach applied to account for phenomenological uncertainty. The output of the approach justified the need to account for epistemic uncertainty in reliability analysis of such systems.     

Techniques for depth heterogeneity identification in X-ray fluorescence

Two methods enabling depth heterogeneity to be recognized and taken into account in quantitative analysis have been elaborated and then applied to various samples. The first method involves predicting the Kα/Kβ (or L lines) ratios for all identified elements. The second method is based on comparing the spectra measured on different geometric arrangements. In particular, sample tilting makes identification of depth heterogeneities possible. If the sample is heterogeneous, the heights of some peaks in the X-ray spectra change according to the depth distribution of the corresponding elements. Monte Carlo simulations with the MCNP4C2 code can be used to predict these changes and interpret the experimental results.     

Effects of environment on annealing behavior of In+ implanted c-axis sapphire

Single crystal samples of 〈0001〉α-Al₂O₃ were implanted with 360 keV indium ions of doses of 1 × 10¹⁶ and 3 × 10¹⁶ ions/cm², at room temperature (RT). The implanted samples were annealed isothermally in air or in flowing high purity argon ambient at 900°C for 2 or 12 h. The damage and thermal annealing were evaluated using Rutherford Backscattering Spectrometry and Channeling (RBS-C) and X-ray Diffraction (XRD). Amorphization of sapphire was not observed, despite damage energy densities up to 105 dpa (displacements per atom obtained from TRIM code calculation) for the Al sublattice, which indicated that self-annealing was severe during In ion implantation of sapphire at RT. RBS-C measurement revealed differences in the annealing characteristics of the implanted indium ions that depended on the annealing environment. The XRD spectrum indicated the presence of the In₂O₃ phase in the subsurface region of the sample after annealing in air.     

Modified "DMC" technique for stretching DNA molecules

A modified “dynamic molecular combing”(DMC) technique used for stretching double-stranded DNA is reported.DNA molecules were stretched on the silanized mica surface by this technhique,its speed being precisely controlled with a computer,This approach combined the precise DNA stretching method with high resolution AFM imaging at nanometer scale,thus making it useful for DNA alignment manipulation and subsequent gene research.     

First results on ion micro-tomography at LIPSION

At the nanoprobe LIPSION ion micro-tomography can be used to determine the 3D distribution information of a sample’s mass density and elemental composition. For ion micro-tomography the two analytical techniques scanning transmission ion microscopy tomography (STIM-T) and particle induced X-ray emission tomography (PIXE-T) are combined. The required data are collected in two consecutive series of measurements, during which the sample is rotated by 180°/360° in small steps. Because all ions have to traverse the sample, the upper limit of the sample size is given by the range of the ions in the material. The tomogram is obtained using the discrete image space reconstruction algorithm (DISRA) by Sakellariou (1997) [1]. This algorithm iteratively corrects a sketchy initial tomogram estimated from the experimental reconstruction – obtained by backprojection of filtered projections (BFP) – and an a priori elemental composition. The necessary correction factors are calculated comparing the reconstruction of the experimental data with the reconstruction of simulated data. For the simulated data sets of STIM projections and PIXE maps are computed from the tomogram. These data sets are proceeded with the BFP algorithm to get simulated reconstruction data. Using the DISRA for ion micro-tomography, one can benefit from the high resolution of STIM-T by transferring it to the elemental distribution given by PIXE-T. This article presents first results of this technique applied on a phantom at the LIPSION facility.     

Channeling characterization of defects in silicon: an atomistic approach

We review here the possibilities opened by a recent development of the Monte Carlo binary collision approximation (MC-BCA) simulation of Rutherford backscattering spectrometry-channeling (RBS-C) spectra for the study of radiation damage in monocrystalline materials. The ion implantation of silicon has been chosen as a case study. Atomic-scale modeling of defect structures was used to determine the location of interstitial atoms in the host lattice. Among possible candidate defects, we have considered the elementary hexagonal, tetrahedral, 1 1 0-split interstitials, the Bond-defect and one type of tetra-interstitial cluster. For each defect model a large Si supercell was populated with a proper defect depth distribution and then it was structurally relaxed by the application of the classical EDIP potential. This model system was then given as an input to the MC-BCA simulation code and the spectra corresponding to nine different axial and planar alignments were calculated. For low defect concentration (a few atomic percent), the scattering yields are strongly dependent on the orientation and a distinct signature characteristic of the limited number of allowed interstitial positions in Si could be found. The comparison of simulations and experiments in the case of 180 keV self ion implantation allowed the identification of the dominant interstitial defect whose structural properties are represented by the split-1 1 0 interstitial. By increasing the concentration of defects (and their mutual interaction) the technique looses sensitivity and, at the same time, the contribution of lattice relaxation becomes important. Under these conditions, although the RBS-C response becomes similar to the one obtained from a random distribution of displaced atoms, the major structural features of a heavily damaged sample could be still observed.     

Electron backscatter diffraction of a plutonium-gallium alloy

An experimental technique has recently been developed to characterize reactive metals, including plutonium (Pu) and cerium, using electron backscatter diffraction (EBSD). Microstructural characterization of Pu and its alloys by EBSD had been previously elusive primarily because of the extreme toxicity and rapid surface oxidation rate associated with Pu metal. The experimental technique, which included ion-sputtering the metal surface using a scanning Auger microprobe (SAM) followed by vacuum transfer of the sample from the SAM to the scanning electron microscope (SEM), used to obtain electron backscatter diffraction Kikuchi patterns and orientation maps for a Pu-gallium alloy is described and the initial microstructural observations based on the analysis are discussed. The phase transformation behavior between the δ (face-centered cubic) and ε (body-centered-cubic) structures is explained by combining the SEM and EBSD observations.     

Role of the man–machine interface in accident management strategies

First, this paper gives a short general review on important safety issues in the field of man–machine interaction as expressed by important nuclear safety organisations. Then follows a summary discussion on what constitutes a modern Man–Machine Interface (MMI) and what is normally meant with accident management and accident management strategies. Furthermore, the paper focuses on three major issues in the context of accident management. First, the need for reliable information in accidents and how this can be obtained by additional computer technology. Second, the use of procedures is discussed, and basic MMI aspects of computer support for procedure presentation are identified followed by a presentation of a new approach on how to computerise procedures. Third, typical information needs for characteristic end-users in accidents, such as the control room operators, technical support staff and plant emergency teams, is discussed. Some ideas on how to apply virtual reality technology in accident management is also presented.     

Ion beam studies of InAs/GaAs self assembled quantum dots

Self assembled InAs/GaAs quantum dots (QD’s) emit in the wavelength range (1.3-1.55 μm) revealing an enormous potential to become the active elements of low threshold lasers and light emitting diodes for communication systems. However, the luminescence is dramatically quenched at room temperature (and even below) due to the defects in the GaAs matrix which open non-radiative recombination paths.In this study we combine Rutherford backscattering/channelling (RBS-C) and high resolution X-ray diffraction (HRXRD) to study the structural properties of the InAs/GaAs structures. The InAs/GaAs QD heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy. Channelling measurements reveal a good crystalline quality along the main axial directions with minimum yields in the range of 4-6% through the entire capping layer. An increase on the dechannelling rate was observed in the region where the InAs quantum dots were buried. The channelling results also give evidence for the presence of defects preferentially oriented. Detailed angular scans in a structure with a 28 nm cap allowed the study of the In orientation with respect to the GaAs matrix and a perfect alignment was found along the growth direction. The strain in the dots shifts the angular curves along the tilt directions.     

Admitting cold water into steam filled pipes without water hammer due to steam bubble collapse

Water hammer due to steam bubble collapse when cold water is admitted to vertical upward flowing, vertical downward flowing, and nominally horizontal pipes has been studied both experimentally and analytically. The work in horizontal pipes included a study of the effect of a slight downward inclination, a slight upward inclination, and the length of the pipe on the initiation of water hammer. Stability maps showing the combinations of filling velocities and liquid subcooling that cause water hammer and those which do not for each flow geometry were obtained from experiments. Analytical models were developed to predict those stability boundaries in the stability maps. All these models were tested with experimental data. Based on the verified models, a step-by-step approach for each flow geometry is presented for plant engineers and designers to follow in avoiding water hammer induced by steam bubble collapse.     

基于氦喷嘴带传输装置的微机数据获取系统

介绍一套特殊的核物理实验装置，它由氦喷嘴装置，带传输装置和微机数据获取与分析系统组成，氦喷嘴装置和带传输装置将反应物从反应室传送到探测器前面，同时给出一个同步信号来控制获取系统，获取系统准确识别控制信号，产生一个明显的标识符来标记不同的样品。本系统对短寿命远离稳定线纱的研究有独特的优势。     

Structural disorder in sapphire induced by 90.3 MeV xenon ions

In our previous work [15], we have evidenced, using RBS-C, two effects in the aluminium sublattice of sapphire irradiated with 90.3 MeV xenon ions: a partial disorder creation that saturates at ∼40% followed above a threshold fluence by a highly disordered layer appearing behind the surface. In this work, by RBS-C analysis of the oxygen sublattice, we have observed only one regime of partial disorder creation that saturates at ∼60% in tracks of cross-section double of that found for the aluminium sublattice. Complementary analysis by X-ray diffraction shows that the lattice strain increases with the fluence until a maximum is reached about 7.5 × 10¹² ions/cm². For higher fluences, strain decreases first indicating a little stress relaxation in the material and tends afterwards, to remain constant. This stress relaxation is found to be related to the aluminium sublattice high disorder.