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

fissile In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...

material into a prompt

critical mass In nuclear engineering, 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 (specifically, its nuclear fis ...

. This is one of the three main requirements in a

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 existing basic design types: * pure fission weapons, the simplest and least technically ...

to create a working fission

atomic bomb A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...

. The need for a short insertion time with

plutonium-239 Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main ...

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

uranium-235 Uranium-235 (235U or U-235) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile ...

it is possible to use a gun design. The basic requirements are: * To start with a

subcritical In nuclear engineering, 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 (specifically, its nuclear fissio ...

system * To create a super

prompt critical In nuclear engineering, prompt criticality describes a nuclear fission event in which criticality (the threshold for an exponentially growing nuclear fission chain reaction) is achieved with prompt neutrons alone (neutrons that are released immed ...

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 The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

in the fissile material through

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakd ...

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 is a chemical element with the symbol Po and atomic number 84. Polonium is a chalcogen. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic characte ...

plutonium-238 Plutonium-238 (238Pu or Pu-238) is a fissile, radionuclide, radioactive isotope of plutonium that has a half-life of 87.7 years. Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 ...

can be rapidly combined with

beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form m ...

to create a neutron source. ** Neutrons can be generated using an electrostatic discharge tube, this tube uses the D-T reaction.

References

{{DEFAULTSORT:Insertion Time Nuclear weapons Nuclear physics Nuclear technology