Editing Nuclear weapon design (section)

===Tritium production===
An essential nuclear reaction is the one that creates [[tritium]], or hydrogen-3. Tritium is employed in two ways. First, pure tritium gas is produced for placement inside the cores of boosted fission devices in order to increase their energy yields. This is especially so for the fission primaries of thermonuclear weapons. The second way is indirect, and takes advantage of the fact that the neutrons emitted by a supercritical fission "spark plug" in the secondary assembly of a two-stage thermonuclear bomb will produce tritium ''[[in situ]]'' when these neutrons collide with the lithium nuclei in the bomb's lithium deuteride fuel supply.

Elemental gaseous tritium for fission primaries is also made by bombarding [[isotopes of lithium|lithium-6]] (<sup>6</sup>Li) with [[neutron]]s (n), only in a nuclear reactor. This neutron bombardment will cause the lithium-6 nucleus to split, producing an alpha particle, or [[helium]]-4 (<sup>4</sup>He), plus a triton (<sup>3</sup>T) and energy:<ref name="fusionmath"/>
:::<math>{}^6\mathrm{Li} + n \longrightarrow {}^4\mathrm{He} + {}^3\mathrm{T} + 5\ \mathrm{MeV} </math>

But as was discovered in the first test of this type of device, [[Castle Bravo]], when [[Isotopes_of_lithium|lithium-7]] is present, one also has some amounts of the following two net reactions:
:{{sup|7}}Li + {{sup|1}}n → {{sup|3}}T + {{sup|4}}He + {{sup|1}}n
:{{sup|7}}Li + {{sup|2}}H → 2 {{sup|4}}He + {{sup|1}}n + 15.123 MeV

Most lithium is <sup>7</sup>Li, and this gave Castle Bravo a yield 2.5 times larger than expected.<ref>Parsons, Keith M.; Zaballa, Robert A. (2017). Bombing the Marshall Islands: A Cold War Tragedy. Cambridge University Press. pp. 53–56. {{ISBN|978-1-108-50874-2}}</ref>

The neutrons are supplied by the nuclear reactor in a way similar to production of plutonium <sup>239</sup>Pu from <sup>238</sup>U feedstock: target rods of the <sup>6</sup>Li feedstock are arranged around a uranium-fueled core, and are removed for processing once it has been calculated that most of the lithium nuclei have been transmuted to tritium.

Of the four basic types of nuclear weapon, the first, pure fission, uses the first of the three nuclear reactions above. The second, fusion-boosted fission, uses the first two. The third, two-stage thermonuclear, uses all three.