Editing Tritium (section)

=== Nuclear weapons ===
Tritium is an important component in nuclear weapons; it is used to enhance the efficiency and yield of [[fission bomb]]s and the fission stages of [[hydrogen bomb]]s in a process known as "[[boosted fission weapon|boosting]]" as well as in [[Neutron generator|external neutron initiator]]s for such weapons.

==== Neutron initiator ====
These are devices incorporated in [[nuclear weapon]]s which produce a pulse of neutrons when the bomb is detonated to initiate the [[fission reaction]] in the fissionable core (pit) of the bomb, after it is compressed to a [[critical mass]] by explosives.  Actuated by an ultrafast switch like a [[krytron]], a small [[particle accelerator]] drives [[ion]]s of tritium and deuterium to energies above the 15&nbsp;[[Electronvolt|keV]] or so needed for deuterium-tritium fusion and directs them into a metal target where the tritium and deuterium are [[Adsorption|adsorbed]] as [[hydride]]s. High-energy [[fusion neutron]]s from the resulting fusion radiate in all directions. Some of these strike plutonium or uranium nuclei in the primary's pit, initiating a [[nuclear chain reaction]]. The quantity of neutrons produced is large in absolute numbers, allowing the pit to quickly achieve neutron levels that would otherwise need many more generations of chain reaction, though still small compared to the total number of nuclei in the pit.

==== Boosting ====
{{Main|Boosted fission weapon}}
Before detonation, a few grams of tritium–deuterium gas are injected into the hollow "[[pit (nuclear weapon)|pit]]" of fissile material. The early stages of the fission chain reaction supply enough heat and compression to start deuterium–tritium fusion; then both fission and fusion proceed in parallel, the fission assisting the fusion by continuing heating and compression, and the fusion assisting the fission with highly energetic (14.1-[[Electronvolt|MeV]]) neutrons. As the fission fuel depletes and also explodes outward, it falls below the density needed to stay critical by itself, but the fusion neutrons make the fission process progress faster and continue longer than it would without boosting. Increased yield comes overwhelmingly from the increased fission. The energy from the fusion itself is much smaller because the amount of fusion fuel is much smaller. Effects of boosting include:
* increased yield (for the same amount of fission fuel, compared to unboosted)
* the possibility of [[variable yield]] by varying the amount of fusion fuel
* allowing the bomb to require a smaller amount of the very expensive fissile material
* eliminating the risk of predetonation by nearby nuclear explosions
* not so stringent requirements on the implosion setup, allowing for a smaller and lighter amount of high explosives to be used

The tritium in a [[warhead]] is continually undergoing radioactive decay, becoming unavailable for fusion. Also, its [[decay product]], helium-3, absorbs neutrons. This can offset or reverse the intended effect of the tritium, which was to generate many free neutrons, if too much helium-3 has accumulated. Therefore, boosted bombs need fresh tritium periodically. The estimated quantity needed is {{convert|4|g}} per warhead.<ref name=ieer/> To maintain constant levels of tritium, about {{convert|0.20|g}} per warhead per year must be supplied to the bomb.

One [[mole (unit)|mole]] of deuterium-tritium gas contains about {{convert|3.0|g}} of tritium and {{convert|2.0|g}} of deuterium. In comparison, the 20&nbsp;moles of plutonium in a nuclear bomb consists of about {{convert|4.5|kg}} of [[plutonium-239]].

==== Tritium in hydrogen bomb secondaries ====
{{see also|Nuclear weapon design}}
Since tritium undergoes radioactive decay, and is also difficult to confine physically, the much larger secondary charge of heavy hydrogen isotopes needed in a true [[hydrogen bomb]] uses solid [[Lithium hydride#Lithium deuteride|lithium deuteride]] as its source of deuterium and tritium, producing the tritium ''in situ'' during secondary ignition.

During the detonation of the primary [[fission bomb]] stage in a thermonuclear weapon ([[History of the Teller–Ulam design|Teller–Ulam staging]]), the [[Thermonuclear weapon|sparkplug]], a cylinder of {{sup|235}}U/{{sup|239}}Pu at the center of the fusion stage(s), begins to fission in a chain reaction, from excess neutrons channeled from the primary.  The neutrons released from the fission of the sparkplug split [[lithium-6]] into tritium and helium-4, while lithium-7 is split into helium-4, tritium, and one neutron. As these reactions occur, the fusion stage is compressed by photons from the primary and fission of the {{sup|238}}U or {{sup|238}}U/{{sup|235}}U jacket surrounding the fusion stage. Therefore, the fusion stage breeds its own tritium as the device detonates. In the extreme heat and pressure of the explosion, some of the tritium is then forced into fusion with deuterium, and that reaction releases even more neutrons.

Since this fusion process requires an extremely high temperature for ignition, and it produces fewer and less energetic neutrons (only fission, deuterium-tritium fusion, and {{nuclide|lithium|7}} splitting are net neutron producers), [[lithium deuteride]] is not used in boosted bombs, but rather for multi-stage hydrogen bombs.