Editing Nuclear weapon design (section)

===Fusion===
{{Main|Nuclear fusion}}
Fusion produces neutrons which dissipate energy from the reaction.<ref>"neutrons carry off most of the reaction energy", Glasstone and Dolan, ''Effects'', p. 21.</ref> In weapons, the most important fusion reaction is called the D-T reaction. Using the heat and pressure of fission, hydrogen-2, or deuterium (<sup>2</sup>D), fuses with hydrogen-3, or tritium (<sup>3</sup>T), to form helium-4 (<sup>4</sup>He) plus one neutron (n) and energy:<ref name="fusionmath"/>
:::<math>{}^2\mathrm{D} + {}^3\mathrm{T} \longrightarrow {}^5\mathrm{He}^{*} \longrightarrow {}^4\mathrm{He} + n + 17.6\ \mathrm{MeV} </math>

[[File:Deuterium-tritium fusion.svg|right|200 px]]
The total energy output, 17.6 MeV, is one tenth of that with fission, but the ingredients are only one-fiftieth as massive, so the energy output per unit mass is approximately five times as great. In this fusion reaction, 14 of the 17.6 MeV (80% of the energy released in the reaction) shows up as the kinetic energy of the neutron, which, having no electric charge and being almost as massive as the hydrogen nuclei that created it, can escape the scene without leaving its energy behind to help sustain the reaction – or to generate x-rays for blast and fire.{{Citation needed|date=June 2021}}

The only practical way to capture most of the fusion energy is to trap the neutrons inside a massive bottle of heavy material such as lead, uranium, or plutonium. If the 14 MeV neutron is captured by uranium (of either isotope; 14 MeV is high enough to fission both <sup>235</sup>U and <sup>238</sup>U) or plutonium, the result is fission and the release of 180 MeV of fission energy, multiplying the energy output tenfold.{{Citation needed|date=June 2021}}

For weapon use, fission is necessary to start fusion, helps to sustain fusion, and captures and multiplies the energy carried by the fusion neutrons. In the case of a neutron bomb (see below), the last-mentioned factor does not apply, since the objective is to facilitate the escape of neutrons, rather than to use them to increase the weapon's raw power.{{Citation needed|date=June 2021}}