Question

Why is a critical mass of fissile material needed to initiate a fission chain reaction?

The nucleus of a fissile atom will not fission unless it is bombarded by more than one fast-moving neutron.

Every neutron released by the initial fission reaction must collide with another fissile nucleus to sustain the chain reaction.

The number of neutrons used to initiate a fission reaction must be greater than the number of neutrons released.

A neutron ejected in an initial fission reaction must have a high probability of colliding with another fissile nucleus.