Papers by Keyword: Helium

Abstract: Molecular dynamics (MD) simulation was used to explore how models of W-He respond to irradiation induced damage. Displacement cascades up to 10 keV recoil energy were simulated for W-Σ17 and W-Σ17-He models. The pre-existing He bubbles within and around the grain boundary region have a major effect on the number and distribution of surviving Frenkel pairs. Frenkel pairs increased as the energy of the primary knock-on atom (PKA) increased across all models. Models containing pre-existing He bubbles showed a significant reduction in the number of surviving vacancies/SIAs compared to those without He bubbles. A large portion of point defects accumulate at the grain boundary which acts as a sink for defects during the recrystallization phase. The presence of He bubbles within or near the grain boundary region facilitates the defects generation, absorbs residual point defects, and form clusters. When He bubbles are located around the grain boundary, the number of surviving vacancies/SIAs decreased by 23% to 60% compared to models without He bubbles. However, for models with He bubbles located within the grain boundary structure, a much more extensive reduction occurred compared to models without He bubbles, which is between the range of 76% to 92%.

Abstract: The effects of helium concentration and displacement damage on microstructural evolution at low dpa and low helium concentration were mainly investigated in specimens of austenitic stainless steel 316FR or SUS304 and a high chromium martensitic steel (HCM12A). The 316FR and HCM12A specimens were implanted uniformly with helium at 823 K up to 30 appm-He or 50 appm-He by 50 MeV cyclotron accelerator using energy degraders. After the helium implantation, the microstructures were examined by a transmission electron microscopy and positron annihilation lifetime measurements. Irradiation hardening behaviors were analyzed using SUS304 and HCM12A steels at 823 K implanted with He ion up to 100 appm with different He/dpa ratios in the HIT ion irradiation experiments and the hardening behaviors were examined by nano indentation method. In the irradiation and annealing specimens, these mechanical properties and microstructures were examined to understand the effects of helium production, displacement damage and annealing on microstructural development, and kinetic Monte Carlo (kMC) simulations were also performed to understand the microstructural development, and the results were compared with the results of TEM observation and positron annihilation lifetime measurements. Important some differences in the microstructural developments such as cavity formation and growth between austenitic stainless steel and martensitic steel were observed in low dpa and low helium concentration conditions.

Abstract: Kinetics of the siderite ore roasting in the air, helium and hydrogen flows has been studied in a gasometrical unit with continuous mass variation logging. We have derived the expression for determination of an apparent degree of calcination and identified its dependence on the size of the prill, the heat treatment duration, and gas-phase composition. Using a generalized chemical kinetics equation, we have obtained a formula for calculation of the decomposition period for siderite ore samples. It has been found that calcination rate increases with the temperature rise, irrespective of the sample size and atmospheric composition. Calcination process has been studied at low temperatures. We have demonstrated that it is feasible to describe the process of siderite ore thermal dissociation by a first-order kinetics equation. We have obtained the expression to calculate the duration of this process depending on different parameters. Using a generalized chemical kinetics equation, we have obtained a formula for checking the expressions that describe the experimental data. We have studied kinetics of the reduction of roasted ore samples at various temperatures using different sizes of the samples. The obtained results have been applied for optimization of the design values and operating conditions of the siderite ore roasting in shaft furnaces. These will also be used for designing a shaft furnace consisting of a calcination zone, reduction zone (metallization zone) and metallized product cooling zone, which will increase iron content in the end-product to 65-70%.

Abstract: A manufacturing process was developed using an electric through-type furnace for capillary tubes used for single-use injection syringes. The process flow diagram consisting of a number of sequential steps and tube heat treatment conditions in protective atmosphere of pure hydrogen providing also for the tube purging with inert gas were considered. The electric furnace installed capacity and heating element dimensions were found as a result of the thermotechnical calculations. Industry research was carried out with a view to optimize the annealing process of capillary tubes in the electric furnace. A choice of material for the muffle fabrication was justified. Temperature fields inside the muffles were evaluated. The optimal flow of protective gas and the maximum allowable flow of purging gas in the form of nitrogen and argon were determined. Mechanical properties were studied for tubes (stocks), welded at the medical goods fabrication plant in Tumen and fabricated at the Pervouralsky Novotrubny Plant. Test data were obtained for optimal rates of tube movement in the furnace, allowing production of capillary tubes, acceptable for medical needle fabrication after annealing. It was demonstrated that capillary tube heat treatment, ensuring the required condition of both outside and inside surfaces, as well as required mechanical properties, is possible with the use of nitrogen as protective (instead of hydrogen) and purging gases. At that probability of better tube surface condition significantly grows.

Abstract: Molecular dynamics (MD) simulations, which treat atoms as point particles and trace their individual trajectories, are always employed to investigate the transport properties of a many-body system. The diffusion coefficients of atoms in solid can be obtained by the Einstein relation and the Green-Kubo relation. An overview of the MD simulations of atoms diffusion in the bulk, surface and grain boundary is provided. We also give an example of the diffusion of helium in tungsten to illustrate the procedure, as well as the importance of the choice of interatomic potentials. MD simulations can provide intuitive insights into the atomic mechanisms of diffusion.

Abstract: The evolution of the helium-related defect in polycrystalline tungsten was studied by Doppler Broadening Positron Annihilation Spectroscopy and Scanning Electron Microscope as functions of annealing temperature and implantation fluence. The experimental results showed that the defect type was not changed at lower annealing temperature, and the decline of S parameter manifested that the lower temperature annealing led the decrease of defect concentration. In addition there existed the same defect type with the increment of the implantation fluence, thus the rise of S parameter implied the increase of the defect concentration.

Abstract: The microstructure of He⁺-irradiation induced defects in tungsten was studied as functions of annealing temperature by means of scanning electron microscope (SEM) and Doppler broadening positron annihilation spectroscopy (DB-PAS). The results show that the S parameter become higher after helium implanted in tungsten. When the annealing temperature increases to 923 K, plenty of large size of holes and new types of defects could be found in this sample.

Abstract: The most common fracture mechanism of nuclear reactor internals is irradiation-assisted stress corrosion cracking (IASCC). Its susceptibility at relatively low dose is dominated by conventional mechanisms such as radiation-induced segregation and radiation hardening. However, the aging of the nuclear fleet combined with the increase of their life-span reveals other mechanisms that could play an important role on IASCC susceptibility. A large amount of helium (He) can be accumulated in reactor internal components of pressurized water reactors (PWR) after long term operation. This occurrence could significantly increase (or even dominate) the IASCC susceptibility at high doses. He has been homogeneously implanted in an especially designed miniaturized specimen at 300°C up to 1000 appm. Slow strain rate tests (SSRT) results in high temperature air and in simulated PWR conditions indicate that homogenized, as-implanted He does not have a significant effect on IASCC up to 1000 appm under these test conditions.

Abstract: Ab initio calculations of electronic structures of Zr–H and Zr–He systems have been done. The influence of hydrogen or helium impurities on the electron density distribution of the host metal has been considered. Extremely inhomogeneous redistribution of the metal valence charge density within the first coordination sphere of the impurity was found. The character of the observed anisotropy depends on the impurity type.

Abstract: This paper presents the development of a semi-automated blimp platform for aerial photography. The blimp can be tele-operated from a ground station and is capable of autonomously holding its heading and altitude when the pilot releases control. The blimp consists of a helium-filled envelope to achieve the buoyancy and a pair of propellers to drive the blimp in the desired direction. A basic control algorithm and sensing system are also proposed to control and maintain the heading and altitude of the blimp.