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What is a Nuclear Meltdown?

Japanese officials and nuclear experts have said they cannot rule out the possibility of a nuclear meltdown at a Japanese nuclear power plant that was badly damaged by last week's earthquake and tsunami. Here is a quick guide to the nuclear process, what can go wrong, and how to prevent catastrophe.

-- Nuclear power is produced by harnessing the heat produced by the splitting of atoms inside uranium - a process known as fission.

-- Rods packed with uranium are submerged into water, and the heat produced by the nuclear reaction creates steam, which is used to power turbines that produce electricity.

-- The nuclear reaction can be controlled utilizing rods made of neutron-absorbing material, such as boron, essentially shutting down the fission process. But the rods still produce heat, even when control rods are in place, requiring a cooling system to maintain temperatures.

-- When the cooling system failed at Japan's Fukushima Dai-Ichi reactor Number 2, the fuel rods boiled through the available water and were for a period of hours exposed to the air.

-- If the rods get too hot, they can eventually melt, thus the term "meltdown."

-- In the event of a complete meltdown, the still-burning hot nuclear fuel could drip to the floor of the reactor. If the containment structure around the reactor is not strong enough, the fuel potentially could be exposed to the outside environment, and can have devastating consequences for nearby communities.

-- The world's worst nuclear power disaster was in Chernobyl, Ukraine in 1986. After an explosion at the plant, a cloud of radioactive dust spread for hundred of kilometers and was blamed for a surge of cancer deaths and birth defects. It has left some nearby towns uninhabitable to this day.

-- People also can be exposed to radiation poisoning through contaminated food and water. A recent U.N. study estimates the Chernobyl disaster caused 6,000 cases of thyroid cancer in children, largely through contaminated milk.

-- Workers at the Fukushima plant are pumping seawater, treated with boron, to try to cool the overheating reactor cores. This process, if successful, will completely shut down and destroy the reactor.

-- After the reactors are brought under control, nuclear technicians will either have to remove the spent fuel, or try to bury the remnants in a concrete "sarcophagus" that will prevent the excess radiation from leaking out, until they can be safely removed.