Papers by Keyword: Displacement Cascades

Abstract: Using molecular dynamics method to study the cascade collision for the case of PKA(Primary Knock-on Atom) atoms at different distance from the grain boundary(GB) of iron chromium alloy. It is found that the PKA atoms at the GB will produce a large size cluster (size from 11 to 409 ) consisting of interstitial and vacancies, and many small clusters (number from 5 to 50). The size and number of the cluster depend heavily on PKA energy, while depend weakly on temperature. The PKA atom at distance of 1nm from the GB, sometimes produces large size defect clusters both inside and outside the GB region. When the PKA atom is at 1nm, 2nm and even 3nm, 4nm from the GB, the GB will effectively absorb the interstitial atoms. It is found that the atomic ratio of Cr-interstitial to total interstitial produced at the GB region is much less than one at outside of GB region.

Abstract: UO₂ transforms into a superionic conductor at temperatures in excess of 2000 K with oxygen ions becoming mobile and exhibiting collective diffusive dynamics. While the response of UO₂ to irradiation is of current interest, the possible impact of superionic characteristics on defect dynamics and recovery following radiation has not been explored yet. In the current work, we use atomistic simulations to elucidate the short-time dynamical response of stoichiometric UO₂ subjected to low energy radiation knocks. We observe that the oxygen ions exhibit a collective behavior that is characterized by frequent hopping across their native lattice sites and forming quasi-one-dimensional string-like structures, which are typical of the superionic state. Approximately, a quarter of the displaced oxygen ions dynamically recover through concerted string-like displacements. Our simulations thus suggest a plausible correlation between defect recovery of irradiated UO₂ and the characteristic superionic hopping mechanism among the oxygen ions.

Abstract: This paper describes the computer simulation of irradiation growths induced by neutron irradiations in zirconium using a combination of Molecular Dynamics (MD) and Kinetic Monte Carlo (KMC) methods. First, we performed the MD simulation of the displacement cascade on a defect cluster to study the interaction between the defect cluster and the displacement cascade. The MD simulations provide a lot of information on the amount of the defect production and the subsequent morphological change in the defect cluster. The results are used to make simple models that describe the nature of the displacement cascade overlap on the defect clusters. The models are then implemented into the KMC simulation code to extend the length- and time-scale of the simulation, which allows us to evaluate directly the defect cluster accumulations during a long-term irradiation. The irradiation growth strain resulting from the defect cluster accumulations is simply evaluated, and compared to an available experimental data. The comparison suggests that the displacement cascade overlap plays an important role on the irradiation growth, and, consequently, the KMC method with the simple models must be appropriate for the simulations of the irradiation growth.

Abstract: In this work, the point defect production in α-Zr at 600K by displacement cascade with Primary Knock-on Atom (PKA) energy up to 20KeV has been investigated by molecular dynamics (MD) simulations. Especially, the influence of subcascades formation on the point defect and cluster production in 20keV cascades has been investigated in detail. The subcascade formation was seen in 36 cases of 20keV cascade simulations out of 50 cases. As the damaged areas are divided into many small areas, the maximum size of point defect cluster tends to decrease when the displacement cascade is settled. On the other hand, subcascades formation has not had an influence on the number of Frenkel pairs in final state of the 20keV cascades comparatively.