Below is a Video of the explosion from the BBC …
This is serious – Nuclear materials may have been able to escape, Japanese officials fear a meltdown. We can see a massive blast at one of the buildings of the Fukushima 1 plant, about 250km (160 miles) north-east of Tokyo.
A huge cloud of smoke billows out and large bits of debris are flung far from the building.
Japan’s NHK TV showed before and after pictures of the plant. They appeared to show that the outer structure of one of four buildings at the plant had collapsed after the explosion.
The Tokyo Electric Power Co, the plant’s operator, said four workers had been injured.
It is not yet clear in exactly what part of the plant the explosion occurred or what caused it.
Chief Cabinet Secretary Yukio Edano said experts were trying to determine the level of radiation at the site.
Cooling systems inside several reactors at both the Fukushima plants stopped working after Friday’s earthquake cut the power supply.
Japan’s nuclear agency said on Saturday that radioactive caesium and iodine had been detected near the number one reactor of the Fukushima 1 plant.
The agency said this may indicate that containers of uranium fuel inside the reactor may have begun melting.
Air and steam, with some level of radioactivity, has been released from several of the reactors at both plants in an effort to relieve the huge amount of pressure building up inside.
Mr Kan said the amount of radiation released was “tiny”.
Thousands of people have been ordered to evacuate the area within a 10-km radius of the plant. BBC correspondent Nick Ravenscroft said police stopped him 60km from the Fukushima 1 plant.
Analysts say a meltdown would not necessarily lead to a major disaster because light-water reactors would not explode even if they overheated.
But Walt Patterson, of the London research institute Chatham House, said “this is starting to look a lot like Chernobyl”.
He said it was too early to tell if the explosion’s aftermath would result in the same extreme level of radioactive contamination that occurred at Chernobyl.
The explosion was most likely caused by melting fuel coming into contact with water, he told the BBC.