JOINT MAE-CER SEMINAR

Using accelerated molecular dynamics simulations to understand helium kinetics in tungsten for fusion applications

Blas P. Uberuaga ​, LANL
May 24, 2017, 11:00am - 12:00pm, EBU-II 479

ABSTRACT:

Understanding microstructural evolution in irradiated materials requires characterizing fundamental atomic-­‐scale mechanisms. In the simplest of cases, one can simply explore, by hand, the relevant energy landscape. However, in the case of He implanted tungsten, the nature of defects is both vast and complex. For example, the mobility of defects responsible for bubble growth and, ultimately, the growth of mesoscale features such as fuzz, is very sensitive to their nature and to the number of He atoms and tungsten vacancies comprising the defect. While conventional molecular dynamics simulations can provide direct insight into atomic-­‐scale behavior, they are limited by the time scale that can be reached, typically fractions of a microsecond at best. This unavoidably leads to unrealistic simulation conditions, such as implantation fluxes that are orders of magnitude larger than even the conditions within ITER. Thus, while there is a fundamental need for atomic-­‐scale insight, the conventional tools are lacking.

Over the last 20 years, a new class of simulations methods has emerged to circumvent this limitation of molecular dynamics. The accelerated molecular dynamics (AMD) methods, based on solid statistical mechanics treatment of rare events, extend the time scale accessible to atomistic scale simulation. We apply these methods to the problem of He kinetics in tungsten, focusing on two distinct but complementary parts of the problem. We first consider the impact that the growth rate, necessarily high in conventional MD, has on the growth dynamics of He bubbles. We find that there are competing events that dictate the morphology of growing bubbles, events that are suppressed when the growth rate is too large. This leads to a cross-­‐over in behavior as a function of growth rate. Second, we examine the mobility of small vacancy-­‐helium complexes. Contrary to conventional wisdom, we find that over-­‐pressurized clusters can migrate through a repeated trap-­‐ mutation/reabsorption process that is very sensitive to the internal pressure of the defect. This unexpected mobility provides new avenues for the growth of bubbles and impacts the release of He from the surface, as observed in mesoscale simulations that account for such processes.  This work highlights the impact and unanticipated insights gained from extending the time scales of atomistic simulations beyond those of conventional MD.

BIO:

Blas Pedro Uberuaga has been a staff member in the Materials Science in Radiation and Dynamics Extremes group (MST-­‐8) in the Materials Science and Technology Division at Los Alamos National Laboratory since 2004. He received his PhD from the University of Washington in 2000, where he studied defect properties in semiconductors. Joining LANL as a postdoc with Arthur Voter in Theoretical Division, he then immersed himself in the accelerated molecular dynamics (AMD) techniques. In MST-­‐8, he has focused on radiation damage evolution in materials, with an emphasis on radiation damage in complex oxides and nanostructured materials. In particular, he has applied the AMD methods to understand the kinetic properties of defects responsible for the evolution of radiation damage in these types of materials.