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.