The interaction of helium and displacement damage in inertial confinement fusion reactors
Abstract Helium is thermodynamically insoluble in metals and tends to form cavities if the conditions are appropriate for its migration. The processes of helium migration and interaction with point defects are strongly influenced by the level and the mode of displacement damage production. In this paper, we present an application of the rate theory for helium cavity formation in Inertial Confinement Fusion Reactors (ICFR's). A new computer code “EXPRESS” has been developed for the solution of the time-dependent rate equations describing the evolution of the space-averaged damage state of an ICFR first wall. An optimal algorithm was developed for the long-term solution of the rate equations (10 5 pulses) with modest computational requirements. It is shown that the timefluctuations in point defect and helium concentrations due to ICF-type pulsing lead to an overall reduction in the swelling of ICF materials.
Atomistic simulations to study the effects of helium bubbles on crack tip behavior in single crystal Ni
Safety of nuclear reactors is an issue that constantly hovers in the mind of the designers. Helium embrittlement in nickel-based alloys poses challenges hitherto unfathomed in the nuclear industry. Experimental and theoretical studies have revealed that helium bubbles have a detrimental effect on mechanical properties of nickel. However, the exact mechanism governing helium-induced embrittlement has not been properly explored. In the present work, the effect of various configurations of helium bubbles on the mechanical properties of nickel crystal has been investigated using atomistic simulations. The effect of various orientations of the crack plane and direction in the face centered cubic crystal of Ni was studied in conjunction with helium bubbles. In the three orientations of the crack, deformation was governed by Shockley and stair rod type dislocation in two of the orientations, while twinning and Lomer Cottrell locks govern the deformation in the third orientation. Despite the fact that helium bubbles reduce the strength of a single crystal of Ni containing a centrally embedded crack, an increase in the number of helium atoms has a negligible effect of the toughness of the material. Switching between the spatial coordinates of a helium bubble in front of the crack tip significantly affects the strength of the single crystal Ni. The results indicate that consideration of the geometry of the crystal along with modulation of helium clusters is an effective means to understand various defects in Ni crystals.
The peculiar thermo-structural behavior of the anionic lipid DMPG.
Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG), around 100 mM ionic strength, are known to exhibit a thermal behavior similar to that of the largely studied lipid dimyristoyl phosphatidylcholine (DMPC), which undergoes a gel to liquid crystalline phase transition at 23 degrees C, well characterized by differential scanning calorimetry (DSC), and other methods. However, at low ionic strength, DMPG has been shown to present a large gel-fluid transition region, ranging from 18 to 35 degrees C. This intermediate phase is optically transparent and characterized by a continuous change in membrane packing. Structural properties of the DMPG gel-fluid transition region will be discussed, based on results obtained by several techniques: electron spin resonance (ESR) of spin labels at the membrane surface and intercalated at different depths in the bilayer; light scattering; DSC; small angle X-ray scattering (SAXS); and fluorescence spectroscopy of probes in the bilayer.