Light water, in the terminology of nuclear reactors, is ordinary water. Light water reactors are simpler and cheaper than heavy water reactors, and although they have the same power-generating capabilities, it is far more difficult to use them to produce weapons-grade plutonium, as the reactor must be shut down and the fuel rods replaced every 4 months because if it stays in any longer, the plutonium-240 concentration will get too high and poison the plutonium-239. A disadvantage of light water reactors is that they must use enriched uranium, while heavy water reactors can use natural uranium. Core of a nuclear reactor A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs in a split second). ... Water (from the Old English word wæter; c. ... A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water (as opposed to heavy water) as its neutron moderator. ... Heavy water reactors use heavy water as a neutron moderator. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Atomic mass (244) g/mol Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... Nuclear power station at Leibstadt, Switzerland. ... For information on radioactive toxins see Radiation poisoning A nuclear poison is a substances with a large neutron absorption cross-section in applications, such as nuclear reactors, when absorbing neutrons is an undesirable effect. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block ?, 7, f Appearance silvery white Atomic mass (244) g/mol Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Atomic mass 238. ...