unlike the other five reactor units, reactor 3 ran on mixed core, containing both uranium fuel and mixed uranium and plutonium oxide, or mox fuel (with the core comprising ~6% mox fuel), during a loss of cooling accident in a subcritical reactor mox fuel will not behave differently from uox fuel. the key difference between plutonium-239 and uranium-235 is that plutonium emits fewer delayed neutrons than uranium when it undergoes fission.