Explain nuclear meltdown.

During nuclear fission, neutrons from split atomic nuclei create self-sustaining chain reactions as they collide with adjacent atomic nuclei. If not controlled, the reactions become runaway processes of positive feedback that release enormous amounts of energy. Inside a nuclear power plant, fission is controlled so that only one of the two or three neutrons emitted with each fission induces another event. The chain reaction thus maintains a constant output of energy at a controlled rate, rather than an explosively increasing output. In a reactor, neutrons are slowed down with a moderator (usually water or graphite), and excess neutrons are absorbed by control rods made of a metallic alloy. Meltdown occurs if the fission reaction proceeds out of control, increasing temperatures in the reactor. This happened in the meltdown at Three Mile Island in 1979, when mechanical failure and human error caused coolant water to drain from the reactor vessel, melting the metal surrounding the uranium fuel rods and releasing radiation.