Defect Production In Cascades Research Articles

Recent Progress Toward an Integrated Multiscale-Multiphysics Model of Reactor Pressure Vessel Embrittlement

ABSTRACTThe continued safe operation of nuclear reactors and their potential for lifetime extension depends on ensuring reactor pressure vessel integrity. Reactor pressure vessels and structural materials used in nuclear energy applications are exposed to intense neutron fields that create atomic displacements and ultimately change material properties. The physical processes involved in radiation damage are inherently multiscale, spanning more than 15 orders of magnitude in length and 24 orders of magnitude in time. This paper reports our progress in developing an integrated, multiscale-multiphysics (MSMP) model of radiation damage for the prediction of reactor pressure vessel embrittlement. Key features of the fully integrated MSMP model include: i) combined molecular dynamics (MD) and kinetic lattice Monte Carlo (KMC) simulations of cascade defect production and cascade aging to produce cross-sections for vacancy, self- interstitial and vacancy-solute cluster size classes for times on the order of seconds; ii) an integrated reaction rate theory and thermodynamic code to predict the evolution of nanostructural and nanochemical features for times on the order of decades; iii) a micromechanics model to calculate the resulting mechanical property changes. This paper will focus on the combined use of MD and KMC to simulate the long-term rearrangement (aging) of defects in displacement cascades and thus, produce late-time production cross-sections for vacancy and vacancy cluster features.

Migration and coalescence of Xe nanoprecipitates in Al induced by electron irradiation at 300 K

Effects of 1 MeV electron irradiation on Xe precipitates in Al, formed by ion implantation, have been observed in situ by high-voltage transmission electron microscopy. Individual Xe precipitates undergo melting and recrystallization, migration which leads to coalescence, and shape changes. These processes are driven by the production of defects without either cascade defect production or the introduction of additional Xe atoms. Precipitate migration is due to an irradiation-induced surface diffusion process on the Xe/Al interfaces. Coalescence of close precipitates is enhanced by directed motion as a result of the net displacement of Al atoms out of the volume between them.

Atomic-scale modeling of radiation damage by SAS

Stochastic annealing simulations provide a way of exploring the evolution of displacement damage created by irradiation over large time and distance scales while retaining explicit information on the spatial distribution of individual defects. In this article, the various stages of defect production in displacement cascades and the models that are applicable at each stage are discussed. The role of stochastic annealing simulations as a link between molecular dynamics and reaction-rate theory is illustrated by simulations of defect production in cascades in copper.

Molecular dynamics and binary collision modeling of the primary damage state of collision cascades

Quantitative information on defect production in cascades in copper obtained from recent molecular dynamics (MD) simulations is compared to defect production information determined earlier with a model based on the binary collision approximation (BCA). The total numbers of residual defects, the fractions of them that are mobile, and the sizes of immobile clusters compare favorably, especially when the termination conditions of the two simulations are taken into account. A strategy is laid out for integrating the details of the cascade quenching phase determined by MD into a BCA-based model that is practical for simulating much higher energies and longer times than MD alone can achieve. The extraction of collisional phase information from MD simulations and the correspondence of MD and BCA versions of the collisional phase is demonstrated at low energy.

Efficiency of defect production in cascades

In copper and silver, the defect production by energetic ions has been investigated for projectile masses between 1 and 209 in the energy range between 10/sup 4/ and 10/sup 6/ eV. Thin-film residual-resistivity measurements were used to determine the defect production below 10K. Calculations of the number of Frenkel pairs produced by various projectiles as a function of energy were performed, based on Lindhard's cross sections and a modified Kinchin-Pease expression. A comparison of measured and calculated damage rates shows that the defect-production efficiency decreases with ion mass for light projectiles, whereas at masses >20 the cascade efficiency remains rather constant and assumes a value of xi approximately 0.3 for both Ag and Cu. The decrease in defect efficiency indicates a transition from isolated Frenkel-pair formation to the production of energetic displacement cascades.

Effect of cascade energy density on the defect cluster type in heavy-ion-irradiated gold

For the quantification of defect production by energetic displacement cascades in metals, a knowledge of the types and numbers of defects created is required. In Au a considerable fraction of the defects generated in such cascades is present in large enough clusters to be observed by transmission electron microscopy (TEM) and analysed as to their structure, sizes, and types (interstitial or vacancy clusters). Heavy ion bombardments have provided an efficient means to study the defect production in cascades as a function of cascade energy. An important problem concerns the question whether visible interstitial clusters are formed in individual displacement cascades in addition to the vacancy clusters that are formed by collapse of the depleted zones. Two conditions may enhance the formation of interstitial clusters formed in cascades:1) An increase in displacement density as produced by overlapping two low energy cascades is expected to lead to a higher interstitial concentration in close vicinity of the depleted zones.