Crack Propagation in Irradiated B4C Induced by Swelling and Thermal Gradients

Boron carbide pellets, used as the neutron absorber materials in fast breeder reactors, were observed to crack in two distinctly different ways. Radial thermal gradients are produced as a result of internal heat generation. In larger‐diameter pellets, these thermal expansion gradients are large and it was observed that pellet cracking occurred early in operation. It was also observed that swelling gradients were established at high burnup levels. These pellets were also cracked but in a different manner.