Critical Mass

For a chain reaction to happen the mass of the fissionable material must be greater than a certain minimum value. This minimum value is known as the critical mass and is when the surface area to mass ratio is too small.

If mass < critical mass: more neutrons are escaping than are produced. Stops

If mass = critical mass: number of neutrons escaping = number of neutrons produced. Steady

If mass > critical mass: more neutrons are produced than are escaping. Meltdown