Classification by moderator material

Used by thermal reactors:

• Graphite moderated reactors
• Water moderated reactors
• Heavy water reactors
• Light water moderated reactors (LWRs). Light water reactors use ordinary water to moderate and cool the reactors. When at operating temperatures if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate. Graphite and heavy water reactors tend to be more thoroughly thermalised than light water reactors. Due to the extra thermalization, these types can use natural uranium/unenriched fuel.
• Light element moderated reactors. These reactors are moderated by lithium or beryllium.
• Molten salt reactors (MSRs) are moderated by a light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF₂.
• Liquid metal cooled reactors, such as one whose coolant in a mixture of Lead and Bismuth, may use BeO as a moderator.
• Organically moderated reactors (OMR) use biphenyl and terphenyl as moderator and coolant.