Irradiation Creep Mechanisms in Metals: A Combined Numerical and Experimental Perspective

The metallic materials used in nuclear reactors undergo irreversible deformation during operation when exposed to fast neutron flux. This deformation can occur under simultaneous irradiation and applied stress, a phenomenon known as irradiation creep, but also when no stress is applied, in which case it is referred to as irradiation swelling and growth.

These phenomena are well characterized on a macroscopic scale through tests conducted in experimental reactors. However, from a microscopic perspective, many uncertainties remain regarding the precise processes at the scale of point defects and dislocations. Numerous elementary mechanisms have been proposed theoretically to explain irradiation creep, but it has not been possible to clearly identify the mechanism that is actually active in-reactor.

To study the fundamental irradiation creep mechanisms, original studies were conducted, at room temperature, on model materials such as pure copper and pure aluminum. This research combined micromechanical testing, nanoscale observations using in situ transmission electron microscopy and multiscale numerical simulations (molecular dynamics, object kinetic Monte Carlo simulations).

First, micromechanical tests conducted on copper under ion beam have been able to evidence irradiation creep deformation. Then, in-situ tensile tests conducted on copper under ion beam have allowed to discover a new irradiation creep mechanism that has been confirmed using molecular dynamics simulations. It implies the initial pinning of dislocation on irradiation defects and its unpinning by displacement cascades.

Second, in situ straining under electron irradiation of pure aluminum have allowed the systematic study of the effect of an applied stress on dislocation loop nucleation and growth. It was shown that the applied stress has only an effect on loop nucleation and not on loop growth. This points the relevance of a stress induced preferential nucleation mechanism but discards the well-known stress induced preferred absorption mechanism. This result has been confirmed by both object kinetic Monte Carlo and molecular dynamics simulations.

This last result has been extended to the effect of an applied stress on dislocation climb under irradiation suggesting the SIPA mechanism is not relevant to explain irradiation creep deformation. These fundamental studies have already led to a better understanding of the radiation-induced creep mechanisms.

About the presenter

Fabien Onimus’ current research focuses on irradiation damage in zirconium alloys and other metals such as copper and aluminum. He is particularly interested on the effect of irradiation on the mechanical behavior and elementary deformation mechanisms after irradiation, but also under irradiation. Under his supervision, an original multi-scale approach is being implemented, using both experiments and numerical simulations, from the atomic scale up to the scale of mechanical test specimens, not to mention the dislocation and polycrystalline scales. Dr. Fabien Onimus studied Fundamental Physics and Materials Science at Paris-Saclay University (France), at the University of Manchester (UK) and Ecole Centrale Paris (France). After graduating from Ecole Centrale Paris in 2000, he obtained his PhD in 2003 (Ecole Centrale Paris), in collaboration with CEA (French Atomic Energy Commission), on the effects of irradiation in zirconium alloys used in the nuclear industry. He obtained his accreditation to supervise research in 2015. He is currently a Research Director at CEA and Professor at the French National Institute for Nuclear Science and Technology.