How does a subcritical mass differ from a supercritical mass?

In nuclear physics, the distinction between subcritical and supercritical mass is defined by the ability of fissile material to sustain a chain reaction, measured by the multiplication factor, k.123

• Subcritical Mass: A mass of fissile material is considered subcritical if it is less than the critical mass required to sustain a chain reaction. In this state, the multiplication factor k is less than unity, meaning the chain reaction cannot be maintained.23
• Supercritical Mass: A mass is supercritical if it exceeds the critical amount. In this configuration, the multiplication factor k is greater than 1. This results in a divergent chain reaction where the number of fissions increases at each generation, releasing a significant amount of energy.23

The critical mass itself is not a fixed value; it depends on the material's type, density, and shape, as well as the presence of neutron reflectors that bounce escaping neutrons back into the core.2