Critical mass

This article is about nuclear fission reactions; not to be confused with Criticality (status).

A re-creation of the 1945 criticality accident using the Demon core: a plutonium pit is surrounded by blocks of neutron-reflective tungsten carbide. The original experiment was designed to measure the radiation produced when an extra block was added. The mass went supercritical when the block was placed improperly by being dropped.

In nuclear engineering, critical mass is the minimum mass of the fissile material needed for a sustained nuclear chain reaction in a particular setup. The critical mass of a fissile material depends upon its nuclear cross section, density, shape, enrichment, purity, temperature, and surroundings. It is an important parameter of a nuclear reactor core or nuclear weapon and is important in nuclear weapon design.

Critical size is the minimum size of the fissile material needed for a sustained nuclear chain reaction in a particular setup. If the size of the reactor core is less than a certain minimum, too many fission neutrons escape through its surface and the chain reaction is not sustained. A perfect sphere, which has the lowest surface-area-to-volume ratio, gives the minimal critical size.