The term insertion time is used to describe the length of time which is required to rearrange a subcritical mass of fissile

Fissile

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

 material into a prompt critical mass

Critical mass

A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...

. This is one of the three main requirements in a nuclear weapon design

Nuclear weapon design

Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...

 to create a working fission atomic bomb. The need for a short insertion time with plutonium

Plutonium

Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

 is the reason why the implosion method was chosen for the first plutonium bomb, while with uranium

Uranium

Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

-235 it is possible to use a gun design.

The basic requirements are:

• To start with a subcritical system
• To create a super prompt critical
  Prompt critical
  In nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapid, exponential increase in the number of fission events...
  
   system
• 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
  Neutron
  The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
  
   in the fissile material through spontaneous fission
  Spontaneous fission
  Spontaneous fission is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units , spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses...
  
   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.
  • Alpha emitters such as polonium
    Polonium
    Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
    
     or plutonium-238 can be rapidly combined with beryllium
    Beryllium
    Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
    
     to create a neutron source.
  • Boosted fission weapon
    Boosted fission weapon
    A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...
    
     where a D-T reaction is caused by the heating of the implosion. This creates the neutrons required for the start up of the device.
  • Neutrons can be generated using an electrostatic discharge tube, this tube uses the D-T reaction.