Molecular dynamics simulations of irradiation cascades in alpha-zirconium under macroscopic strain Sali Di a , Zhongwen Yao a,⇑ , Mark R. Daymond a , Fei Gao b a Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON, Canada K7L 3N6 b Pacific Northwest National Laboratory, Richland, WA 99352, USA article info Article history: Received 13 July 2012 Received in revised form 22 January 2013 Accepted 23 January 2013 Available online 10 February 2013 Keywords: Displacement cascade Strain field Zirconium Molecular dynamics abstract Numerous computer simulation studies have been performed on the radiation damage of zirconium. In contrast to most of the work in the literature which has focused on the effects of temperature and recoil energy on defect production and defect clustering, we have developed a computational model to consider the influence of elastic strain field on the formation of defects and their clusters, as strain is commonly present in a real reactor environment. In this work, irradiation induced displacement cascades in alpha- zirconium experiencing a macroscopic strain have been studied by molecular dynamics (MD) simulations using a many-body interatomic potential. The external strain mainly affects the size of defect clusters rather than the total number of defects. The sizes of interstitial and vacancy clusters respond differently to the external strain conditions. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The CANDU (Canadian Deuterium Uranium) pressurized heavy water nuclear reactor has some distinguishing features compared with the pressurized water reactor (PWR) design. The CANDU reac- tor uses deuterium-oxide (heavy water) as the moderator and cool- ant. In contrast to the enriched uranium fuel required for PWRs, natural uranium fuel can be used in the CANDU design. Multiple pressure tubes, together with the calandria tubes, act as the pres- sure boundaries in the CANDU, compared to the single steel pres- sure vessel of a PWR. Currently, the pressure tubes are produced from Zr-2.5Nb alloy; calandria tubes are made from Zircaloy-2; and Zircaloy-4 is used for fuel cladding. Zirconium alloys are widely used in the CANDU reactors due to the low neutron absorp- tion cross section of zirconium, the high neutron efficiency of the CANDU design is why natural uranium fuel can be used. However, the harsh neutron irradiation in the reactor can in- duce macroscopic deformation and mechanical property degrada- tion of zirconium alloys [1]. Moreover, deuterium ingress due to the corrosion of zirconium alloys can lead to the presence of hy- dride precipitates, and cracks can also be initiated and propagated in the alloy when those hydrides are large enough by the delayed hydride cracking process [2]. Numerous experimental [3,4] and simulation studies [5–7] have been carried out to investigate the irradiation damage of zirconium alloys. Previous simulation studies, especially molecular dynamics (MD) simulations of the displacement cascade in zirconium, did not consider the potential effect of strain on the irradiation- induced defects [5–7]. During operation, the pressure tube experiences moderately high tangential and axial stresses. In addition, flaws and stress concentrations exist in the structure, lo- cally raising the stress. Furthermore, the deuterium precipitates have significant strain fields surrounding them (up to 5% in one crystallography direction) due to the associated local volume expansion [8]. Therefore, understanding the impact of strain on irradiation-induced structures has importantly practical conse- quences. It has been noticed that strain effects have only been studied by MD in bcc alpha iron and fcc copper [9,10]. In this paper, external strains will be applied to investigate the impact of strain on defect production and defect clustering in hcp zirconium. 2. Method The most important factor in MD simulation is the potential since it determines the interaction of atoms in the MD simulation. There are various methods that have been developed to create a reliable potential, including the embedded atom method (EAM) potential [11], F-S potential [12], Modified-EAM [13], Tersoff po- tential [14] and lattice inversion methods potential [15,16]. All these potentials are intended to reproduce the physical properties of the system as close as possible to the experimental value or to ab initio results. 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.01.048 ⇑ Corresponding author. Tel.: +1 613 533 3435; fax: +1 613 533 6610. E-mail address: yaoz@me.queensu.ca (Z. Yao). Nuclear Instruments and Methods in Physics Research B 303 (2013) 95–99 Contents lists available at SciVerse ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb