Results 1 - 10 of 29674
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[en] In the U.S., spent nuclear fuel (SNF) is stored in dry storage cask systems for long-term interim storage. The systems commonly used consist of welded stainless steel containers enclosed in ventilated cement or steel overpacks. These containers may be required to perform their waste isolation function for many decades, and failure by chloride-induced stress corrosion cracking (SCC) due to deliquescence of deposited salts is a major concern, particularly for near-marine storage sites. This paper presents a probabilistic performance assessment model to evaluate the probability of canister failure (through-wall penetration) by SCC. The model first assesses whether environmental conditions for SCC—the presence of an aqueous film—are present at canister weld locations (where tensile stresses are likely to occur) on the canister surface. Geometry-specific storage system thermal models and weather data sets representative of U.S. SNF storage sites are implemented to evaluate location-specific canister surface temperature and relative humidity (RH). As the canister cools and aqueous conditions become possible, the occurrence of corrosion is evaluated. Corrosion is modeled as a two-step process: first, pitting is initiated, and the extent and depth of pitting is a function of the chloride surface load and the environmental conditions (temperature and RH). Second, as corrosion penetration increases, the pit eventually transitions to a SCC crack, with crack initiation becoming more likely with increasing pit depth. Once pits convert to cracks, a crack growth model is implemented. The SCC growth model includes rate dependencies on both temperature and crack tip stress intensity factor, and crack growth only occurs in time steps when aqueous conditions are predicted. The model suggests that SCC is likely to occur over potential SNF interim storage intervals; however, this result is based on many modeling assumptions. Sensitivity analyses provide information on the model assumptions and parameter values that have the greatest impact on predicted storage canister performance, and provide guidance for further research to reduce uncertainties. (author)
[en] This paper presents the design of a modified split Hopkinson pressure bar (SHPB) where partial lateral confinement of the specimen is provided by the inertia of a fluid annulus contained in a long steel reservoir. In contrast to unconfined testing, or a constant cell pressure applied before axial loading, lateral restraint is permitted to develop throughout the axial loading: this enables the high-strain-rate shear behaviour of soils to be characterised under conditions which are more representative of buried explosive events. A pressure transducer located in the wall of the reservoir allows lateral stresses to be quantified, and a dispersion-correction technique is used to provide accurate measurements of axial stress and strain. Preliminary numerical modelling is utilised to inform the experimental design, and the capability of the apparatus is demonstrated with specimen results for a dry quartz sand. (paper)
[en] The multi-band template analysis (MBTA) pipeline is a low-latency coincident analysis pipeline for the detection of gravitational waves (GWs) from compact binary coalescences. MBTA runs with a low computational cost, and can identify candidate GW events online with a sub-minute latency. The low computational running cost of MBTA also makes it useful for data quality studies. Events detected by MBTA online can be used to alert astronomical partners for electromagnetic follow-up. We outline the current status of MBTA and give details of recent pipeline upgrades and validation tests that were performed in preparation for the first advanced detector observing period. The MBTA pipeline is ready for the outset of the advanced detector era and the exciting prospects it will bring. (paper)
[en] Any leak in one of the ITER actively cooled components would cause significant consequences for machine operations; therefore, the risk of leak must be minimized as much as possible. In this paper, the strategy of examination to ensure leak tightness of the ITER internal components (i.e. examination of base materials, vacuum boundary joints and final components) and the hydraulic parameters for ITER internal components are summarized. The experiences of component tests, especially hot helium leak tests in recent fusion devices, were reviewed and the parameters were discussed. Through these experiences, it was confirmed that the hot He leak test was effective to detect small leak paths which were not always possible to detect by volumetric examination due to limited spatial resolution. (paper)
[en] Melt motion simulations of recent ASDEX Upgrade experiments on transient-induced melting of a tungsten leading edge during ELMing H-mode are performed with the incompressible fluid dynamics code MEMOS 3D. The total current flowing through the sample was measured in these experiments providing an important constraint for the simulations since thermionic emission is considered to be responsible for the replacement current driving melt motion. To allow for a reliable comparison, the description of the space-charge limited regime of thermionic emission has been updated in the code. The effect of non-periodic aspects of the spatio-temporal heat flux in the temperature distribution and melt characteristics as well as the importance of current limitation are investigated. The results are compared with measurements of the total current and melt profile. (paper)
[en] Highlights: • Optimal thermodynamic parameters of two-phase ejector refrigeration system. • Empirical correlation for primary evaporator temperature. • Calculation of optimal condenser and evaporator dimensions. • Comparison of calculated values with experimental study. - Abstract: Air-conditioning is necessary for the comfort of passengers in commercial buses. However, installing an air-conditioning system can add extra load on the engine and result in extra fuel cost. Therefore, an improvement in the air-conditioning system can lower the fuel consumption of the buses and reduce the size of the evaporator and the condenser. It is known that using two-phase ejector as an expansion valve in the air-conditioning system can improve the system performance. This study offers a model to predict the optimal thermodynamic parameters for a two-phase ejector refrigeration system for buses using R134a under various operating conditions. An empirical correlation is derived to determine the optimal thermodynamic parameters of the system. The effect of evaporation and condensation temperatures on the heat transfer surface area are discussed and graphically illustrated. Moreover, an experimental study to validate the developed model has been carried out in a midibus air-conditioning system. The study findings revealed that the heat transfer surface area can be reduced by about 4% and 55% in the condenser and evaporator, respectively.
[en] Highlights: • A micro-radial radioisotope thermoelectric generator is manufactured and tested. • The simulated performance of the RTG are compared with the experimental value. • Performance characteristics were determined in different sizes and numbers. • The designed RTG is expected to be a reliable space power supply for MEMS. - Abstract: To satisfy the flexible power demand of the low power dissipation devices in the independent space electric system, a micro-radial milliwatt-power radioisotope thermoelectric generator (RTG) was prepared and optimized in this research. The overall geometrical dimension of the RTG in the experiment was 65 mm (diameter) × 40 mm (height). The RTG, which was built and tested using simulated radioisotope source, eventually obtained an open-circuit voltage of 92.72 mV, an electric power of 149.0 μW, and an energy conversion efficiency of 0.015% at the ambient temperature of 293.15 K and heat source power from 0.1 W to 1 W. On the basis of the structure used in the experiment, the length and cross-sectional area of the thermoelectric leg and the number of thermoelectric modules were effectively optimized through the COMSOL Multiphysics. With the optimized length of 35 mm and cross-sectional area of 1.2 mm2, the RTG with four thermoelectric modules achieved a 15.8 mW output power under 1 W heat source power. The maximum conversion efficiency calculated using COMSOL code increased to 1.58%. According to the optimized electrical output, the micro-radial RTG is expected to be a reliable space power supply for micro components and could satisfy the low power requirements of space missions.
[en] Graphical abstract: The effects of CO2/R41 as an azeotropy refrigerant on system performance are analyzed under various working conditions. The results show that CO2/R41 mixture is a good choice to substitute for pure CO2 in the studied systems because of its low optimal high pressure and high system COP. - Highlights: • The CO2/R41 mixture is an azeotropy refrigerant applied in refrigeration systems. • Comparing with pure CO2, CO2/R41 mixture can decrease the optimal high pressure. • Comparing with pure CO2, CO2/R41 mixture can increase the system COP. - Abstract: In this study, blend of CO2 with R41 is evaluated for application in three different systems including a refrigerated cabinet, an air-source heat pump water heater and a water-source heat pump water heater. The effects of CO2/R41 mixture as an azeotropy refrigerant on system performance are analyzed under various working conditions. The results show that CO2/R41 mixture may be a good choice to substitute for pure CO2 in the studied systems because of its stable chemical properties, low optimal high pressure, high system COP, low compression ratio, low discharge temperature, high refrigerating capacity per unit, and high heating capacity per unit. Furthermore, the exergy efficiencies of the three studied systems are improved to more than 23% with the proposed CO2/R41 (0.5/0.5) mixture refrigerant.
[en] Highlights: • Wetness fraction of the low pressure cylinders is identified. • Condenser thermodynamic characteristics under varying working conditions are investigated. • The objective function of the condenser pressure optimization is presented. • The set value of the mass flow rate of the circulating water is obtained with the manipulation strategy. - Abstract: The operation conditions of the cold-end system have significant impact on the unit thermal economy. However, due to the difficulty of online determination of some key parameters, the condenser pressure optimization has been a challenging task for a long time. This paper proposes an online applicable approach to optimize the condenser pressure with variable speed pumps, taking the mass flow rate of the circulating water as the manipulating variable, to achieve better thermal economy. After the exhaust steam wetness fraction is online identified, the condenser thermodynamic characteristics under varying working conditions are investigated based on the effectiveness and steady-state energy balance of the condenser. By maximizing the net power benefit, defined as the difference between the unit power increment and the pump power consumption increment, the optimal mass flow rate of the circulating water is derived. To validate the approach, pseudo-online simulations are conducted with the history data from an ultra-supercritical unit. The retention time during which the set value of the mass flow rate of the circulating water remains constant is studied in context with the implementation of the manipulation strategy under on-site scenario. Simulation results reveal the energy-saving potential of condenser pressure optimization with the proposed approach.
[en] Highlights: • Chaotic flow in a 2D multi-turn PHP was investigated. • Non-linear temperature oscillations were analyzed. • Optimal filling ration and minimum thermal resistance were obtained. - Abstract: Numerical study has been conducted for the chaotic flow in a multi-turn closed-loop pulsating heat pipe (PHP). Heat flux and constant temperature boundary conditions have been applied for heating and cooling sections respectively. Water was used as working fluid. Volume of Fluid (VOF) method has been employed for two-phase flow simulation. Volume fraction results showed formation of perfect vapour and liquid plugs in the fluid flow of PHP. Non-linear time series analysis, power spectrum density, correlation dimension and autocorrelation function were used to investigate the chaos. Absence of dominating peaks in the power spectrum density was a signature of chaos in the pulsating heat pipe. It was found that by increasing the filling ratio and evaporator heating power the correlation dimension increases. Decreasing of the autocorrelation function with respect to time showed the prediction ability is finite as a result of chaotic state. An optimal filling ratio of 60% and minimum thermal resistance of 1.62 °C/W were found for better thermal performance of the pulsating heat pipe. It is notable that two dimensional simulations in current study lead better understanding of the mechanism and validating the numerical method for full three dimensional modeling.