Irradiation behavior of boron carbide

Boron carbide is the prime candidate material for neutron absorbers in fast breeder reactors and the Fast Test Reactor. Important data required for design of control rods for these reactors concern swelling, gas release, and structural integrity of boron carbide under the expected operating conditions. Data for irradiations of boron carbide powders in a thermal reactor and powders and pellets in a fast reactor are presented and discussed in terms of expected performance in a fast reactor. The most important variable in determining irradiation behavior appears to be the mobility of the helium produced relative to the rate it is produced. The helium is not trapped in the lattice but precipitates. Denuded zones and absence of bubbles on grain boundaries suggest that helium diffuses rapidly along the grain boundaries. This is a major source of gas release.