Editing Heavy water (section)

===Neutron moderator===
Heavy water is used in certain types of [[nuclear reactors]], where it acts as a [[neutron moderator]] to slow down neutrons so that they are more likely to react with the [[fissile]] [[uranium-235]] than with [[uranium-238]], which captures neutrons without fissioning.
The CANDU reactor uses this design. Light water also acts as a moderator, but because light water absorbs more [[neutron]]s than heavy water, reactors using light water for a reactor moderator must use [[enriched uranium]] rather than natural uranium, otherwise [[Critical mass|criticality]] is impossible. A significant fraction of outdated power reactors, such as the [[RBMK]] reactors in the USSR, were constructed using normal water for cooling but [[graphite-moderated reactor|graphite as a moderator]]. However, the danger of graphite in power reactors (graphite fires in part led to the [[Chernobyl disaster]]) has led to the discontinuation of graphite in standard reactor designs.

The breeding and extraction of plutonium can be a relatively rapid and cheap route to building a [[nuclear weapon]], as chemical separation of plutonium from fuel is easier than [[isotopic separation]] of U-235 from natural uranium.
Among current and past [[nuclear weapons states]], Israel, India, and North Korea<ref>{{Cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/TRS407_scr/D407_scr1.pdf|title=Heavy Water Reactors: Status and Projected Development}}</ref> first used plutonium from heavy water moderated reactors burning [[natural uranium]], while China, South Africa and Pakistan first built weapons using [[highly enriched uranium]].

The [[Uranverein|Nazi nuclear program]], operating with more modest means than the contemporaraneous Manhattan Project and hampered by many leading scientists having been driven into exile (many of them ending up working for the Manhattan Project), as well as continuous infighting, wrongly dismissed graphite as a moderator due to not recognizing the effect of impurities. Given that [[isotope separation]] of uranium was deemed too big a hurdle, this left heavy water as a potential moderator. Other problems were the ideological aversion regarding what propaganda dismissed as "[[Jewish physics]]" and the mistrust between those who had been enthusiastic Nazis even before 1933 and those who were ''[[Mitläufer]]'' or trying to keep a low profile. In part due to allied sabotage and commando raids on [[Norsk Hydro]] (then the world's largest producer of heavy water) as well as the aforementioned infighting, the German nuclear program never managed to assemble enough uranium and heavy water in one place to achieve [[Critical mass|criticality]] despite possessing enough of both by the end of the war.

In the U.S., however, the first experimental atomic reactor (1942), as well as the [[Manhattan Project]] Hanford production reactors that produced the plutonium for the [[Trinity test]] and [[Fat Man]] bombs, all used pure carbon (graphite) neutron moderators combined with normal water cooling pipes. They functioned with neither enriched uranium nor heavy water. Russian and British plutonium production also used graphite-moderated reactors.

There is no evidence that civilian heavy water power reactors—such as the CANDU or [[Atucha I nuclear power plant|Atucha]] designs—have been used to produce military fissile materials. In nations that do not already possess nuclear weapons, nuclear material at these facilities is under [[IAEA]] safeguards to discourage any diversion.

Due to its potential for use in [[nuclear weapon]]s programs, the possession or import/export of large industrial quantities of heavy water are subject to government control in several countries. Suppliers of heavy water and heavy water production technology typically apply [[IAEA]] (International Atomic Energy Agency) administered safeguards and material accounting to heavy water. (In Australia, the ''Nuclear Non-Proliferation (Safeguards) Act 1987''.) In the U.S. and Canada, non-industrial quantities of heavy water (i.e., in the gram to kg range) are routinely available without special license through chemical supply dealers and commercial companies such as the world's former major producer [[Ontario Hydro]].