NE Seminar: “Insight on High-dose Radiation Damage Buildup by Multistage Molecular Dynamics Simulations”

Date(s) - 03/31/2022
1:55 pm - 2:55 pm



Kai Nordlund, Ph.D.

University of Helsinki

Dr. Kai Nordlund is a professor of computational materials physics and a vice-rector of the University of Helsinki. As vice-rector, he puts particular emphasis on high-quality education, student and staff wellbeing and promotion of the highest level science. Living in a family where four languages are spoken routinely, multilingualism and ensuring equal opportunities for foreign staff and students are close to his heart. He received his Ph.D. in physics in 1995 at the University of Helsinki, and after postdoc positions at the University of Illinois and Academy of Finland was appointed full professor at his alma mater in 2003. In addition to being vice-rector, he is leading together with three other senior scientists a 25-person research group doing quantum mechanical, classical and mesoscale atomistic simulations of radiation and other non-equilibrium effects in all classes of materials. As of 2022, he has published more than 560 refereed publications, and his h-index exceeds his age.



Materials in nuclear fission and fusion reactors are subject to high doses of radiation damage due to the neutron irradiation environment.  A key question for understanding the response of materials to high dose irradiation is, “What are the atomistic mechanisms underlying the evolution?” For a long time, molecular dynamics (MD) simulations were considered to be unsuitable for modeling this at realistic temperatures, because of the limited time scale the method can handle. Due to this, point defects have barely any time to move between radiation events.

However, already around 2000, we found that the experimental amorphization dose of common semiconductors like Si and Ge can be reproduced within a factor of two even by MD simulations that overestimate the radiation flux by many orders of magnitude [1]. Encouraged by this, around 2015 we started modeling the buildup of damage also in metals by running numerous cascades repetitively in the same MD simulation cells. Already the first results showed good qualitative agreement with experiments on radiation damage in Ni and high entropy alloys[2], and soon afterward we could show also good quantitative agreement in the same materials [3]. The latter was enabled by a newly developed method for modeling Rutherford Backscattering/channeling spectra directly from MD-generated atom coordinates  [4]. The basic reason why MD simulations that vastly overestimate radiation fluxes still can get a reasonable agreement with experimental damage buildup, seems to be simply that after even a moderate dose, most damage is in clusters or dislocations that move much less than simple point defects. This conclusion is strongly supported by the most recent results on radiation buildup in W, where experimental hydrogen retention can be well reproduced by a combination of regular MD cascade simulations and a defect creation-relaxation algorithm (CRA) [5].
[1] J. Nord, K. Nordlund, and J. Keinonen, Phys. Rev. B 65, 165329 (2002)
[2] F. Granberg, K. Nordlund, M. W. Ullah, K. Jin, C. Lu, H. Bei, L. M. Wang, F. Djurabekova, W. J. Weber, , and Y. Zhang, Phys. Rev. Lett. 116, 135504 (2016),
[3] S. Zhang, K. Nordlund, F. Djurabekova, F. Granberg, Y. Zhang, and T. S. Wang, Mater. Res. Lett. 5, 433 (2017)
[4] S. Zhang, K. Nordlund, F. Djurabekova, Y. Zhang, G. Velisa, and T. S. Wang, Phys. Rev. E 94, 043319 (2016)
[5] D. R. Mason and F. Granberg and M. Boleininger and T. Schwarz-Selinger and K. Nordlund and S. L. Dudarev, Phys. Rev.
 Mater. 9 (2021) 095403