Defect structure of neutron irradiated boron carbide

Boron carbide irradiated in thermal and fast reactor neutron spectra at 500 to 840°C up to 81% ¹⁰B burnup has been examined by transmission electron microscopy. Helium bubbles formed by agglomeration of helium from the ¹⁰B (n,α)⁷ Li reaction constitute the only visible form of irradiation-induced damage. The helium bubbles tend to have a disc-like morphology with the plane of the disc parallel to (111). Strong lattice strain fields around the bubbles evidenced a lack of thermal equilibrium. Bubble nucleation was influenced markedly by grown-in dislocations and stacking faults. Bubble coalescence, either by impingement or enhanced by bubble stress fields frequently gave rise to both trans- and intergranular microcracking. The significance of these observations on gas release and swelling of boron carbide are discussed briefly.