查看更多>>摘要：Using molecular dynamics simulation, we studied the dynamic behavior and microscopic mechanisms of near-surface helium bubble in single crystal Cu subjected to shock loading. The whole process of bubble evolution includes three stages: shock compression, equilibration, and expansion-rupture-ejection. When shock wave reaches the bubble, it is compressed and an internal jetting is formed, leading to the increases of velocity and internal pressure of the bubble. The internal pressure then reduces slightly due to the relaxation of helium atoms. After the reflected rarefaction wave reaching the bubble, the internal pressure greatly decreases to several GPa and the volume expands a lot. Because of the pressure gradient between the bubble and the free surface, the velocity of the bubble increases again, squeezing the upper thin metal layer continuously and finally making it plastically fail or melt. Then the bubble ruptures, ejecting helium atoms and some Cu clusters. Impact strength has significant effects on this dynamic process. Higher impact velocity leads to stronger jet and more significant expansion and rupture of the bubble. The latter is attributed to the microstructure evolution of the surrounding metal atoms and the upper metal layer, for which plastic failure or melt is observed at higher impact velocity. The influences of helium bubble size and number ratio of He/vacancy are also investigated. We find that the internal jet is stronger and temperature rise and local melting of metal is more obvious for larger bubble, resulting in easier failure of the metal layer and thus more remarkable expansion and rupture of the bubble. Higher number ratio results in weaker internal jet but stronger expansion and rupture of the bubble. This is because more helium atoms in the bubble for higher ratio can impact on the upper metal layer, leading to easier failure of the metal layer.
查看更多>>摘要：The shadow corrosion phenomenon has been an unsolved issue for many decades now. Its unique occurrences were only observed in Boiling Water Reactors (BWRs). In the reactor setting, the use of Ni-based alloys or stainless steel near Zircaloy-2 cladding or fuel channel causes localized enhanced corrosion of the zirconium alloy at the contact point or where the two dissimilar metals are in close proximity. Under the radiation field, a galvanic coupling phenomenon has been identified as the governing mechanism for shadow corrosion. This form of corrosion, however, has not been successfully replicated outside of the reactor environment. In-situ proton irradiation-corrosion experiments were carried out to replicate a shadow on Zircaloy-2 with an electrically coupled Ni alloy. The precise control of proton flux and estimated hydrogen peroxide concentration, as well as comprehensive oxide thickness measurements, were used to develop an empirical model for shadow corrosion under neutron irradiation.
NSTL
Elsevier