The term insertion time is used to describe the length of time which is required to rearrange a subcritical mass of
fissile material into a prompt
critical mass. This is one of the three main requirements in a
nuclear weapon design to create a working fission
atomic bomb. The need for a short insertion time with
plutonium-239 is the reason the implosion method was chosen for the first plutonium bomb, while with
uranium-235 it is possible to use a gun design.
To make the change between these two states in a length of time (insertion time) which is shorter than the time between the random appearance of a
neutron in the fissile material through
spontaneous fission or by other random processes.
Also at the right moment in time, neutrons must be injected into the fissile material to start up the fission process. This can be done by several methods.