Via The Telegraph:
"There was a huge explosion" between 6:00 am (2100 GMT Monday) and 6:15 am at the number-two reactor of Fukushima No.1 nuclear power plant, a Tokyo Electric Power Co (TEPCO) spokesman said.
The government also reported apparent damage to part of the container shielding the same reactor at Fukushima 250 kilometres (155 miles) northeast of Tokyo, although it was unclear whether this resulted from the blast.
Chief Cabinet Secretary Yukio Edano told reporters the suppression pool of the number-two nuclear reactor appeared to have been damaged.
This is the bottom part of the container, which holds water used to cool it down and control air pressure inside.
"But we have not recorded any sudden jump in radiation indicators," Edano added.
Earlier a cloud of radioactive dust billowed from the Fukushima Daiichi power plant after it suffered its second explosion in three days.
Government officials admitted that it was "highly likely" the fuel rods in three separate reactors had started to melt despite repeated efforts to cool them with sea water. Safety officials said they could not rule out a full meltdown as workers struggled to keep temperatures under control in the cores of the reactors.
The Fukushima crisis now rates as a more serious accident than the partial meltdown at Three Mile Island in the US in 1979, and is second only to the 1986 Chernobyl disaster, according to the French nuclear safety authority. After insisting for three days that the situation was under control, Japan urgently appealed to US and UN nuclear experts for technical help on preventing white-hot fuel rods melting.
The International Atomic Energy Agency (IAEA) said it was "unlikely" that the accident would turn into another Chernobyl, but failed to rule it out completely.
For those of you unfamiliar with the Chernobyl accident, it was a man-made catastrophe caused by engineers who ran a dangerous test on a reactor system later discovered to contain numerous design flaws and malfunctioning components, while the reactor was in an unstable condition. The test involved shutting off the steam to the power station's main turbine generators in order to determine whether the "freewheeling" (spinning under their own inertia, without the force of the steam) turbines could still produce enough power to keep the reactor's water pumps operating, while the backup power system was brought online.
When the steam lines to the turbines were shut off, the water circulation in the reactor slowed down. This resulted in a burst of neutron energy that overheated the reactor. Then the control rods malfunctioned and could not be reinserted fully. The overheated water formed steam pockets, which eventually over-pressurized the reactor, causing it to explode.
The explosion exposed the overheated reactor components to outside air, and they immediately erupted in a huge fireball. And since the reactor had no shielding, highly radioactive dust, smoke, and debris from the explosion and fire contaminated the surrounding area for hundreds of miles. With nothing to control it, the overheated core melted. Investigators also found evidence of a brief uncontrolled chain reaction within the core, which probably intensified the fire and resulting core meltdown.
While the situation at Fukushima is dire and very serious, in reality it bears little resemblance to Chernobyl. As far as we know, before the earthquake and tsunami, the power station was in good repair and was operating within normal parameters. Its operation and safety protocols conformed to the most current international standards for nuclear reactors. It seems highly unlikely that safety-obsessed Japanese engineers would have run their plant as carelessly as the Soviets ran Chernobyl. And unlike the Soviet Union, which went to great efforts to conceal accidents and downplay problems, Japanese officials are now utilizing resources from around the world in order to deal with this incident.
We are also seeing numerous reports of "soaring radiation levels" in the wake of the reactor explosions at Fukushima. But this is about as detailed as the reporting gets:
Afterward, officials in Ibaraki, a neighboring prefecture just south of the area, said up to 100 times the normal levels of radiation were detected Tuesday. While those figures are worrying if there is prolonged exposure, they are far from fatal.
But what are the "normal levels"? My common sense tells me that this actually means levels have only gone from "really, really, really, really low" up to "really, really, really low". Again, this is worth keeping an eye on, but no where near a panic-level emergency.