Study of He and Li diffusion in B4C – Role of damage

In the context of fast neutron reactors, boron carbide is a ceramic material with excellent properties for controlling nuclear reactor power. Thanks to its high absorption cross sections of 10B, this material provides an excellent neutron absorber. As a consequence of the 10B(n,α) nuclear reactions, significant quantities of helium and lithium are produced (up to about 1022 cm^-3), that modify significantly the chemical and structural composition of the initial material. We will discuss the diffusion behaviour of these light elements through a fundamental approach that uses ion beams. The two isotopes ³He (to simulate ⁴He) and ⁷Li were introduced by ion implantation using the ion implanter of the SCALP Orsay facility at IJCLab (Paris-Saclay University, France) at depths ranging between 0.6 and 1.3 μm in sintered B4C pellets of different grain sizes and with maximum atomic concentrations at range maximum in between 0.03 and 3%. Au post-irradiation experiments performed at the JANNuS Saclay facility, were used to create ballistic damage in a control way. The isotope distribution profiles were determined before and after thermal annealing in the 500-1000°C temperature range by using either the ³He (²H, ⁴He)¹H nuclear reaction or Secondary Ion Mass spectrometry (SIMS). These two techniques, thanks to their excellent depth resolution, allow determining apparent diffusion coefficients through the evolution of the profiles, and estimating activation energy values from the Arrhenius law. Raman spectroscopy analysis allows to evaluate the disorder recovery of the material in the various steps. The behaviour of these elements in B₄C will be shown. Concerning He diffusion, it is shown that the He bubble nucleation is enhanced by the irradiation induced-defects that trap helium atoms. Lithium thermal diffusion appears heterogeneous as a function of depth up to a given concentration threshold which suggests the formation of complexes that trap lithium.

The different diffusion and trapping mechanisms as a function of the implanted ions, as well as the implantation and annealing conditions and the material grain size will be discussed with respect to the ballistic damage.