Molecular dynamics simulation of cascade-induced ballistic helium resolutioning from bubbles in iron Roger E. Stoller ⇑ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6114, USA article info Article history: Available online 21 November 2012 abstract Molecular dynamics simulations have been used to assess the ability of atomic displacement cascades to eject helium from small bubbles in iron. This study of the ballistic resolutioning mechanism employed a recently-developed Fe–He interatomic potential in concert with an iron potential developed by Ackland and co-workers. The primary variables examined were: irradiation temperature (100 and 600 K), cascade energy (5 and 20 keV), bubble radius (0.5 and 1.0 nm), and He-to-vacancy ratio in the bubble (0.25, 0.5 and 1.0). Systematic trends were observed for each of these variables. For example, ballistic resolutioning leads to a greater number of helium atoms being displaced from larger bubbles and from bubbles that have a higher He/vacancy ratio (bubble pressure). He resolutioning was reduced at 600 K relative to 100 K, and for 20 keV cascades relative to 5 keV cascades. Overall, the results indicate a modest level of He removal by ballistic resolutioning. The results may be particularly relevant to fusion irradiation conditions which produces high levels of helium by transmutation. They can be used to provide initial guidance in selection of a ‘‘resolution parameter’’ that can be employed in kinetic models to predict the bubble size distribution that evolves under irradiation. Published by Elsevier B.V. 1. Introduction The evolution of gas-stabilized bubbles in irradiated materials is inherently a dynamic process, involving a balance of growth and shrinkage mechanisms. Simplistically, the bubble volume will grow in units of atomic volume by absorbing vacancies and shrink by either emitting vacancies or absorbing interstitials. Addition or emission of gas atoms will also change the volume, with the rela- tive change dependent on the ratio of gas atoms to vacancies in the bubble. The kinetics of bubble evolution due to reactions with point defects and mobile gas atoms has been extensively investi- gated using various models of cluster dynamics [1–7]. However, the potentially important mechanism of dynamic ejection of gas due to elastic collisions with energetic knock-on atoms has re- ceived much less attention, even though it has been discussed in the literature for many years [8–14]. Such resolutioning could limit bubble growth and promote continuous bubble nucleation by ejecting gas atoms back into the material matrix [5,6]. Mechanisms which may influence bubble evolution are of particular interest to fusion research because the higher-energy neutrons produced by deuterium–tritium fusion lead to higher helium generation rates under these conditions. Ejection of gas atoms from bubbles has been proposed to result from one of two mechanisms. The first is referred to as track reso- lutioning, and is primarily discussed in the context of nuclear fuel in which a highly energetic (up to 100 MeV), heavy fission frag- ment deposits its kinetic energy in a small volume. Typically, it is assumed that bubbles within this volume will be dissolved due to the high energy density in the fission track and all (or most) the gas atoms contained in these bubbles will be returned to the matrix. Ballistic resolutioning, which is illustrated in Fig. 1 [15], oc- curs as the result of direct collisions between an energetic particle and individual gas atoms in a bubble. In this case, the energetic particle could be a fission fragment, a matrix atom recoiling from a collision with a high energy neutron, or (much less frequently) the neutron itself. Essentially the same process has been investi- gated with respect to precipitate resolutioning [16,17]. Modern computational tools make the ballistic resolutioning mechanism well suited for investigation by molecular dynamics (MD). For example, Parfitt and Grimes conducted an MD study of ballistic resolutioning for the case of helium bubbles in UO 2 [18]. In addi- tion to the two mechanisms just mentioned, they identified what they called a ‘‘damage assisted resolution’’ mechanism in which helium was thermally incorporated into disordered regions of the lattice adjacent to the initial bubble late in the cascade process. More recently, Govers et al. [19] expanded the work from Ref. [18] and included an investigation of track resolutioning via a thermal spike mechanism. 0022-3115/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnucmat.2012.11.015 ⇑ Address: Oak Ridge National Laboratory, Bldg. 4100, MS-6114, Oak Ridge, TN 37831-6114, USA. Tel.: +1 865 576 7886; fax: +1 865 241 3650. E-mail address: rkn@ornl.gov Journal of Nuclear Materials 442 (2013) S674–S679 Contents lists available at SciVerse ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat