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The effects of irradiation on 3C-silicon carbide (SiC) and amorphous SiC (a-SiC) are investigated using both in situ transmission electron microscopy (TEM) and complementary molecular dynamics (MD) simulations. The single ion strikes identified in the in situ TEM irradiation experiments, utilizing a 1.7 MeV Au3+ ion beam with nanosecond resolution, are contrasted to MD simulation results of the defect cascades produced by 10–100 keV Si primary knock-on atoms (PKAs). The MD simulations also investigated defect structures that could possibly be responsible for the observed strain fields produced by single ion strikes in the TEM ion beam irradiation experiments. Both MD simulations and in situ TEM experiments show evidence of radiation damage in 3C-SiC but none in a-SiC. Selected area electron diffraction patterns, based on the results of MD simulations and in situ TEM irradiation experiments, show no evidence of structural changes in either 3C-SiC or a-SiC. [ABSTRACT FROM AUTHOR]

Understanding radiation damage in crystalline systems at the atomic scale is essential for the development of multi-scale predictive models for advancing nuclear science and engineering applications. State-of-the-art techniques used for investigating irradiation effects include molecular dynamics (MD) simulations, which can provide attosecond resolution of damage cascades over picosecond time scales, and in situ transmission electron microscopy (TEM), which can provide millisecond resolution in real-time. In this work, MD simulations and in situ TEM ion beam irradiation of crystalline TiO2 with 46 keV Ti1− ions are performed and results are compared. The MD results show that the ratio of the titanium to oxygen defects evolves during the radiation cascade. The vacancies are produced mostly in the core, while self-interstitials are concentrated at the periphery of the cascade. Cluster analysis of the MD results confirms the formation of a void (or a cluster of vacancies) that contains as much as ≈10 000 vacancies in the ballistic phase, compared to <1000 after annealing. The radial distribution functions and the simulated selected area electron diffraction patterns at the peak of the ballistic phase confirm the existence of a short-range order and medium-range order throughout the simulation. However, the long-range order reemerges after annealing of the cascade event in agreement with the in situ TEM ion beam irradiation experiments. The MD simulations and the experiments show no indication of amorphization. [ABSTRACT FROM AUTHOR]

Simulations are performed using the reverse non-equilibrium molecular dynamics (rNEMD) method and the Stillinger-Weber (SW) potential to determine the input parameters for achieving ±1% convergence of the calculated thermal conductivity of silicon. These parameters are then used to investigate the effects of the interatomic potentials of SW, Tersoff II, Environment Dependent Interatomic Potential (EDIP), Second Nearest Neighbor, Modified Embedded-Atom Method (MEAM), and Highly Optimized Empirical Potential MEAM on determining the bulk thermal conductivity as a function of temperature (400–1000 K). At temperatures > 400 K, data collection and swap periods of 15 ns and 150 fs, system size ≥6 × 6 UC2 and system lengths ≥192 UC are adequate for ±1% convergence with all potentials, regardless of the time step size (0.1–0.5 fs). This is also true at 400 K, except for the SW potential, which requires a data collection period ≥30 ns. The calculated bulk thermal conductivities using the rNEMD method and the EDIP potential are close to, but lower than experimental values. The 10% difference at 400 K increases gradually to 20% at 1000 K. [ABSTRACT FROM AUTHOR]

The performance of very high temperature reactor plants with direct and indirect closed Brayton cycles (CBCs) is compared and the effects of the molecular weight of the CBC working fluid on the number of stages and sizes of the axial flow, single shaft compressor and turbine are investigated. The working fluids considered are helium (4 g/mole), He-Xe, and He-[N.sub.2] binary mixtures (15 g/mole). Also investigated are the effects of using low and high pressure compressors with intercooling, instead of a single compressor, and changing the reactor exit temperature from 700[degrees]C to 950[degrees]C on the plant thermal efficiency, the CBC pressure ratio, and the number of stages in and size of the turbomachines. For plants with direct CBCs, the effect of cooling the reactor pressure vessel with He bled off at the exit of the compressor is also investigated. The present analyses are performed for a reactor thermal power of 600 MW, shaft rotation speed of 3000 rpm, and intermediate heat exchanger temperature pinch of 50[degrees]C. [DOI: 10.1115/1.3126264]

The effects of orientations of porous graphite and smooth copper surfaces on saturation nucleate boiling and critical heat flux (CHF) of FC-72 dielectric liquid and of liquid subcooling on nucleate boiling in the upward facing orientation are investigated. The saturation CHF on porous graphite is much higher than on copper and as much as 53% of that in the upward-facing orientation, compared to only 18% on copper.

Transient Langmuir probe measurements of the plasma density, electron temperature and plasma potential during slow quenching of a Cs-Ba tacitron at two different discharge conditions were performed. After the application of a negative voltage pulse on the grid, the grid aperture sheaths initially grew then shrank temporarily because of a rise in the ion flux from the grid-collector region, then grew again until the discharge eventually extinguished. Discharge extinguishing happened when the effective ionization cross section became decoupled from the continually rising electron temperature.