[SY-B5] He impurities in boron carbide : structure, kinetics, and Raman signatures
Invited
Helium impurities, produced by nuclear reactions in the neutron absorber boron carbide, are a matter of concern because they form bubbles and eventually affect the mechanical properties of the absorbing elements in fast breeder sodium reactors. In order to shed light on the mechanisms of bubble formation we have investigated, using first principles methods, the insertion of He in boron carbide (with stoichiometry B4C) and the possible migration mechanisms. Whereas in absence of preexisting damage, at equilibrium, He is expected to occupy interstitial sites and diffuse via an interstitial mechanism [1], we show that, under irradiation where a supersaturation of vacancies is expected, He atoms can be trapped at vacancies and diffusion might be slowed down.
How to probe such kind of impurities? Raman spectroscopy is increasingly used to quantify damage in irradiated materials, including boron carbide. In order to help the interpretation of such experiments we calculated from first principles the first order Raman spectrum of B4C containing defects ; to circumvent the size effect we devised an inclusion procedure allowing us to obtain the dynamical matrix and the Raman tensor of a supercell containing a defect at a dilution lower than 0.1 atomic %. The results show that interstitial He impurities do not significantly perturb the spectrum, while substitutional ones do.
[1] A. Schneider, G. Roma, J.-P. Crocombette, V. Motte, D. Gosset, J. Nucl. Mater. 496, 157 (2017).
How to probe such kind of impurities? Raman spectroscopy is increasingly used to quantify damage in irradiated materials, including boron carbide. In order to help the interpretation of such experiments we calculated from first principles the first order Raman spectrum of B4C containing defects ; to circumvent the size effect we devised an inclusion procedure allowing us to obtain the dynamical matrix and the Raman tensor of a supercell containing a defect at a dilution lower than 0.1 atomic %. The results show that interstitial He impurities do not significantly perturb the spectrum, while substitutional ones do.
[1] A. Schneider, G. Roma, J.-P. Crocombette, V. Motte, D. Gosset, J. Nucl. Mater. 496, 157 (2017).