Update: It's possible that a meltdown may already have occurred at one nuclear power plant. As AP wrote 4 minutes ago:
An official with Japan's nuclear safety commission says that a meltdown at nuclear power plant affected by the country's massive earthquake is possible.
Ryohei Shiomi said Saturday that officials were checking whether a meltdown had taken place at the Fukushima Dai-ichi power plant, which had lost cooling ability in the aftermath of Friday's powerful earthquake.
Japanese nuclear authorities said that there was a high possibility that nuclear fuel rods at a reactor at Tokyo Electric Power's Daiichi plant may be melting or have melted, Jiji news agency reported.
Experts have said that if the fuel rods have been damaged, it means that it could develop into a breach of the nuclear reactor vessel and the question then becomes one of how strong the containment structure around the vessel is and whether it has been undermined by the earthquake.
In addition, there are problems at three Japanese nuclear power plants.
The Fukushima plant is leaking radiation, and a nuclear expert says that things are getting worse, and "Fukushima has 24 hours to avoid a core meltdown scenario". (See Tyler Durden's report).
"The situation is still several stages away from Three Mile Island when the reactor container ceased to function as it should," said Tomoko Murakami, leader of the nuclear energy group at Japan's Institute of Energy Economics
Two other Japanese nuclear reactors are now in trouble as well. Two other Japanese nuclear reactors are now in trouble as well [UPDATE: It is now up to 5 nuclear reactors].
As MSNBC notes:
Coolant systems failed at three quake-stricken Japanese nuclear reactors Saturday, sending radiation seeping outside one and temperatures rising out of control at two others.
Radiation surged to around 1,000 times the normal level in the control room of the No. 1 reactor of the Fukushima Daichi plant, Japan's Nuclear and Industrial Safety Agency said. Radiation — it was not clear how much — had also seeped outside, prompting widening of an evacuation area to a six-mile radius from a two-mile radius around the plant. Earlier, 3,000 people had been urged to leave their homes.
Tokyo Electric Power Co. said Saturday that the temperatures of its No.1 and No.2 reactors at its Fukushima Daini nuclear power station were rising, and it had lost control over pressure in the reactors.
About an hour after the plant shut down, however, the emergency diesel generators stopped, leaving the units with no power for important cooling functions.
Hours after the evacuation order, the government announced that the plant will release slightly radioactive vapor from the unit to lower the pressure in an effort to protect it from a possible meltdown.
Good luck to the Japanese scientists bravely trying to avert catastrophe. As MSNBC notes:
Japan has a "tremendous amount of technical capability and resources" to respond to the issue ....