Editing Neutron moderator (section)

== Non-graphite moderators ==

Some moderators are quite expensive, for example [[beryllium]], and reactor-grade heavy water. Reactor-grade heavy water must be 99.75% pure to enable reactions with unenriched uranium. This is difficult to prepare because heavy water and regular water form the same [[chemical bond]]s in almost the same ways, at only [[kinetic isotope effect|slightly different speeds]].

The much cheaper light water moderator (essentially very pure regular water) absorbs too many neutrons to be used with unenriched natural uranium, and therefore [[uranium enrichment]] or [[nuclear reprocessing]] becomes necessary to operate such reactors, increasing overall costs. Both enrichment and reprocessing are expensive and technologically challenging processes, and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material, causing [[Nuclear proliferation|proliferation]] concerns.

The CANDU reactor's moderator doubles as a safety feature. A large tank of low-temperature, low-pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss-of-coolant accident conditions. It is separated from the fuel rods that actually generate the heat.  Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high "[[neutron economy]]". Unlike a light water reactor where adding water to the core in an accident might provide enough moderation to make a subcritical assembly go [[Criticality (status)|critical]] again, heavy water reactors will decrease their reactivity if light water is added to the core, which provides another important safety feature in the case of certain accident scenarios. However, any heavy water that becomes mixed with the emergency coolant light water will become too diluted to be useful without isotope separation.