Experimental determination of intragranular helium diffusion rates in boron carbide (B4C)

Abstract

Boron carbide, B4C, is widely used as a neutron absorber in nuclear reactors. Since neutron absorption by 10B leads to 4He production, it appears necessary to study He behaviour and its possible effects on the B4C ceramic. In this study, the diffusion characteristics of ion-implanted He in B4C (500 keV, fluences from 1 x 10+13 to 2 x 10+15 He.cm−2) were investigated mainly by Thermo-Desorption Spectroscopy (TDS) from 600 °C up to B4C melting point. The experiments were done on dense B4C samples having large grains (30–60 μm) to render grain boundaries effects on He outward diffusion ineffective and thus to access intragranular He diffusion kinetics. From controlled temperature ramp experiments, it was notably observed that He release was realized in two main stages. A first He population was able to exit the material at moderate temperatures by interstitial diffusion. Then a second population was quantitatively released only over 1150 °C. This was attributed to He atoms that, in their initial interstitial diffusion course at moderate temperatures from their implantation sites, got trapped in defect aggregates and/or He bubbles. As the nucleations of both these traps are expected to be related to helium and irradiation defect concentrations, the ratio of the two He populations was indeed found to be correlated with the implantation fluence. From the obtained He release curves, the apparent activation energies (Ea) of He intragranular diffusion in B4C was determined (2.6–3.1 eV) in the 800–1100 °C temperature range. This value appears slightly higher than the ones determined at lower temperatures, hinting that a change in diffusion mechanism may occur around 800 °C. The apparent Ea of He detrapping from He bubbles (∼2.5 eV) and from defect aggregates (∼4 eV) were also determined for temperatures within 1200–1500 °C.