Editing Heavy water (section)

===Tritium production===
{{See also|Tritium#Deuterium}}
[[Tritium]] is the active substance in [[tritium illumination|self-powered lighting]] and controlled nuclear fusion, its other uses including [[autoradiography]] and [[radioactive label]]ing. It is also used in [[nuclear weapon design]] for [[boosted fission weapon]]s and [[tritium#Neutron initiator|initiators]]. Tritium undergoes [[beta decay]] into [[helium-3]], which is a stable, but rare, isotope of helium that is itself highly sought after. Some tritium is created in [[heavy water moderated reactor]]s when deuterium captures a neutron. This reaction has a small [[Neutron cross-section|cross-section]] (probability of a single neutron-capture event) and produces only small amounts of tritium, although enough to justify cleaning tritium from the moderator every few years to reduce the environmental risk of tritium escape. Given that helium-3 is a [[neutron poison]] with orders of magnitude higher capture cross section than any component of heavy or tritiated water, its accumulation in a heavy water neutron moderator or [[target (Physics)|target]] for tritium production must be kept to a minimum.

Producing a lot of tritium in this way would require reactors with very high neutron fluxes, or with a very high proportion of heavy water to [[nuclear fuel]] and very low [[neutron absorption]] by other reactor material. The tritium would then have to be recovered by [[isotope separation]] from a much larger quantity of deuterium, unlike production from [[lithium-6]] (the present method), where only chemical separation is needed.

Deuterium's absorption cross section for [[thermal neutron]]s is 0.52 milli[[barn (unit)|barn]] (5.2 × 10{{sup|−32}} m{{sup|2}}; 1 barn = 10{{sup|−28}} m{{sup|2}}), while those of [[oxygen-16]] and [[oxygen-17]] are 0.19 and 0.24 millibarn, respectively. {{sup|17}}O makes up 0.038% of natural [[oxygen]], making the overall cross section 0.28 millibarns. Therefore, in D{{sub|2}}O with natural oxygen, 21% of [[neutron capture]]s are on oxygen, rising higher as {{sup|17}}O builds up from neutron capture on {{sup|16}}O. Also, {{sup|17}}O may emit an [[alpha particle]] on neutron capture, producing radioactive [[carbon-14]].