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The influence of irradiation induced vacancies on the mobility of helium in boron carbide

Abstract : Boron carbide, used as a neutron absorber, undergoes nuclear reactionsproducing relevant quantities of He. The understanding of He kinetics at theatomic scale in the material is still in its infancy, in spite of decades ofexperimental work devoted to the characterization of He containing,irradiated, boron carbide samples. The interplay of He itself with intrinsicdefects created by irradiation on kinetics is still almost completely unknown.In this paper we present an exhaustive study of vacancies andsubstitutional helium impurities in boron carbide using Density FunctionalTheory. Analyzing the stability and mobility of these defects allows us to consider diffusion mechanisms other than the known interstitial mechanisms.We find that vacancies trap He interstitials, raising the activation energy of2D diffusion to approximately 2eV. The trapping mechanismis different according to the charge state of the vacancy in p-typeconditions, when vacancies are neutral or positive, He diffuses via adissociative mechanisms and is trapped in a substitutional position; in $n$-type conditions, negative vacancies trap He atoms traveling in an adjacent {111} plane by a charge transfer driven distorsion.No favorable vacancy assisted diffusion mechanism was identified forsubstitutional He atoms, except the dissociative one previously mentioned.Other possible vacancy diffusion mechanisms, which we also analyzed, arehindered by the high activation energy of vacancy self-diffusion.
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Kevin Gillet, Guido Roma, Jean-Paul Crocombette, Dominique Gosset. The influence of irradiation induced vacancies on the mobility of helium in boron carbide. Journal of Nuclear Materials, Elsevier, 2018, 512, pp.288-296. ⟨10.1016/j.jnucmat.2018.10.020⟩. ⟨cea-02339428⟩

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