" For 10 points, name this process in which large atomic nuclei break down into smaller parts, which was used in the atomic bombs dropped on Japan.
This process is modeled in an equation that uses the four factors, including thermal utilization and resonance escape velocity. The number of particles required to sustain this process is the "effective neutron multiplication factor," k.  Beryllium is used as a neutron reflector to reduce this phenomenon's necessary supercritical mass. Breeder reactors facilitate this process, which can sustain itself in a "chain reaction." For 10 points, name this process in which large atomic nuclei break down into smaller parts, which was used in the atomic bombs dropped on Japan.
This process is modeled in an equation that uses the four factors, including thermal utilization and resonance escape velocity.
{nuclear fission} [prompt on nuclear chain reaction; do not accept “nuclear fusion” or “fusion”]