The evolution kinetics of radiation-induced defects in a material has a direct impact on changes in its microstructure and consequently on its mechanical properties. This makes the quantitative prediction of this kinetics and the phenomena governing it a major challenge for the nuclear industry.

This challenge can now be taken up by intercoupling computer simulation techniques operating on different scales. This is what is meant by multi-scale simulation; the numerical results obtained on one time and space scale were taken and used as input data for modeling on the next higher scale:
- first of all, ab initio² computer simulations rooted in quantum mechanics described the structure and migration of defects and defect clusters. These simulations, which call for considerable computing resources, were performed by drawing intensively on the capabilities of the CCRT (research and technology computing center) set up on the CEA's Bruyères-le-Châtel site.
- the second stage consisted in taking these elementary properties and reconstructing, on the basis of a kinetic model³, the evolution of defects and their effects on the macroscopic properties of an irradiated iron sample one micron (a thousandth of a millimeter) in size, over a period of about one hour.

The simulations were compared with indirect experimental measurements⁴. The excellent agreement obtained demonstrates the realism of this multi-scale model, which highlights the role played by the hitherto unsuspected migration of small interstitial and vacancy clusters. They challenge the interpretation of several earlier experiments and simulations and open the way for the quantitative simulation of more complex irradiated materials such as industrial steels. They will be used in interpreting the mechanical behavior of the ferritic steels used as structural materials in existing nuclear fission plants, as well as those proposed for future fusion plants.



Provided that enough energy is transmitted, bombarding materials with high-energy particles can displace atoms within the crystal lattice via a ballistic collision phenomenon. As they are struck, the atoms can be displaced by several interatomic distances before stopping in the interstitial position, leaving behind them gaps known as vacancies. These elementary defects - interstitials and vacancies - are mobile. They can migrate in the material and gather into clusters that grow to form a defect microstructure. It is this microstructure that will affect certain properties of the material - particularly its mechanical properties - by interacting, for example, with dislocations.

1. These results have been obtained within the context of the European "Perfect" project for predicting the effects of irradiation on nuclear reactor components.
2. i.e. with no adjustable parameter.
3. The method used is the kinetic Monte Carlo model, which offers a statistical reconstruction of the sequencing over time of the various reactions between defects as a function of their probability of occurrence.
4. in this case, temperature-dependent changes in resistivity.