Japanese officials are cautioning that a nuclear meltdown may occur at the Fukushima Daiichi nuclear power plant near the town of Okuma. According to Japan’s Jiji Press, some of the reactor’s nuclear fuel rods were briefly exposed to the air after the reactor’s water levels dropped through evaporation. A fire engine is currently pumping water into the reactor and the water levels are recovering, according to an operator of the Tokyo Electric Power Co. (TEPCO), which operates the plant. A TEPCO spokesman said the company believes the reactor is not melting down or cracking and that workers are currently attempting to raise the water level.
If a meltdown takes place — essentially the core of the reactor overheating and damaging the fuel rods themselves — it would be the first since the Chernobyl disaster in 1986 and the Three Mile Island incident in 1979.
The Fukushima Daiichi power plant was shut down automatically on March 11 due to the magnitude 8.9 earthquake that hit Japan. The on-site diesel backup generators also shut down about an hour after the event, leaving the reactors without power and thus without the ability to cool down the core. Japanese officials were operating the cooling system via battery power and were flying in batteries by helicopter to keep the temperature regulated.
An unchecked rise in temperature could cause the core to essentially turn into a molten mass that could burn through the reactor vessel. This may lead to a release of an unchecked amount of radiation into the containment building that surrounds the reactor. This building could be breached if enough pressure builds, or, in this case, if the containment building was already breached through the earlier effects of the earthquake.
At the moment, it would appear that Japanese authorities are still trying to contain the reaction inside the reactor. That indicates that the core has not completely melted and that the reaction has not yet gotten out of hand. However, the situation could quickly become uncontrollable and the added water being pumped into the reactor could rapidly evaporate if the temperatures rise too quickly to be cooled off.(Source http://www.stratfor.com)