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Molecular dynamic simulation of helium diffusion within a ring dislocation inside a stoichiometric UO2 nanocrystal was carried out. Two different sets interaction potentials were used. One of the sets was characterized by a significantly stronger binding of helium to the crystal lattice. Temperature dependencies of the diffusion coefficient along the dislocation line were obtained. It was shown that the strengthening in the binding of helium led both to a decrease in the diffusion coefficient of the atom and to an increase in the temperatures at which the model dislocation was annealed. [ABSTRACT FROM AUTHOR]

A molecular dynamics simulation of helium diffusion in solid CaF2, SrF2, and BaF2 was carried out using the empirical pair potentials reproducing the experimental strong binding of helium to cations in these crystals. The model diffusion coefficients and activation energies are determined. [ABSTRACT FROM AUTHOR]

Molecular dynamic simulation of helium diffusion within a ring dislocation inside stoichiometric UO2 nanocrystal was carried out. During the computational experiments (2.7 ns) in the temperature range from 525 to 1000 K, the dislocation was stable and the helium atom was contained inside the dislocation core. The magnitudes of the diffusion coefficient varied from 4.1·10–6 to 6.6·10–5 cm²/s. The diffusion process could be characterized by the constant activation energy ED = (0.25 ± 0.05) eV. [ABSTRACT FROM AUTHOR]

The goal of the overall program is to develop a dynamic numerical model of helium refrigerators and the associated cooling systems based on commercial simulation software. The aim is to give system designers a tool to search for optimum control strategies during the construction phase of the refrigerator with the help of a plant “simulator”. In a first step, a simple Brayton refrigerator has been investigated, which consists of a compressor, an after-cooler, a counter-current heat exchanger, a turboexpander and a heat source. Operating modes are “refrigeration” and “liquefaction”. Whereas for the steady state design only component efficiencies are needed and mass and energy balances have to be calculated, for the dynamic calculation one needs also the thermal masses and the helium inventory. Transient mass and energy balances have to be formulated for many small elements and then solved simultaneously for all elements. Starting point of the simulation of the Brayton cycle is the steady state operation at design conditions. The response of the system to step and cyclic changes of the refrigeration or liquefaction rate are calculated and characterized. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]