An aftershock has struck near a nuclear plant in Japan – hours after an explosion and radiation leak there sparked fears a meltdown was under way.
The 6.4 magnitude tremor occurred close to the sites of two nuclear power plants in Fukushima, where the walls and a roof of one reactor were destroyed in a blast.
Plumes of smoke were sent billowing into the sky and several workers at the Daiichi power station – also known as Fukushima 1 – were thought to be injured.
Officials said the container of reactor unit 1 had not been damaged.
Four Daiichi workers were taken to hospital after being exposed to radiation at the plant and officials were planning to distribute precautionary doses of iodine to residents – used to help prevent thyroid cancer.
“They are working furiously to find a solution to cool the core… they have begun to inject sea water into the core,” Mark Hibbs, a senior associate at the Nuclear Policy Program for the Carnegie Endowment for International Peace, said.
“That is an indication of how serious the problem is and how the Japanese had to resort to unusual and improvised solutions to cool the reactor core.”
The nearby Daini power station – Fukushima 2 – also suffered a loss of control of pressure in one containment vessel but a spokesman said the reactor pressure remained stable.
The Fukushima plants are operated by Tokyo Electric Power Company (Tepco).
The Japanese government doubled the Daiichi evacuation radius to 12 miles and urged residents of the region to stay indoors, turn off air conditioning units and not to drink tap water.
The International Atomic Energy Agency (IAEA) said that the evacuation zone at the Daini nuclear site was extended to six miles from the previous radius of 1.8 miles.
“The Japanese have always been very trusting of the authorities but there has been panic buying,” Fukushima resident Graham Chave told Sky News.
“I am 30 miles away but people are concerned and worried that the evacuation zone is going to be expanded and wondering where they will go.”
Radioactivity of 1,000 times the normal level in the control room at the Daiichi plant and eight times the normal level in the area immediately outside the site have since fallen.
It came after Japan’s Nuclear Safety Commission said the plants, 150 miles north of the capital Tokyo, may be experiencing meltdown.
Sky News’ Holly Williams, in Japan, said: “The government is saying they are prepared for any eventuality.
“But it seems like the authorities themselves are not entirely sure what happened.”
Williams said that the stoic Japanese people have been shaken by the double disaster of earthquake and tsunami.
“Most people on the face of it seem quite calm,” Williams said.
“But when you speak to them, those that have been struck personally by this, they are jittery.”
Ian Hore-Lacey, of the World Nuclear Association (WNA), said he believed the blast had been caused by a hydrogen build-up.
“If the hydrogen has ignited, then it is gone, it doesn’t pose any further threat,” he said.
He added the explosion may not necessarily have caused radiation leakage and told Sky News safety measures at modern nuclear plants should prevent harm to surrounding residents.
“If there were any meltdown in those reactors, it would be largely, if not entirely, contained,” he said.
“Any western reactor is built inside a containment structure, which is normally about 1.2 metres (3ft 11in) thick of reinforced concrete. They are designed to contain the worst conceivable accident.
“You can’t get a nuclear explosion at a nuclear power plant. That’s quite impossible, because they’re run with fuel that’s only enriched to about 5% at the most.”
But he said even if there was a meltdown, it would not affect humans within a six-mile radius.
The 8.9 magnitude quake has left at least a 1,300 people dead and triggered a tsunami that devastated parts of Japan and set off warnings as far away as California and Chile.
Pressure at the faulty Daiichi reactor may have risen to 2.1 times the designed capacity, the country’s trade ministry said.
Media reports also said the radiation level was rising in the turbine building, forcing Tepco said it released a small amount of filtered radioactive vapour into the atmosphere to reduce the pressure.
Chief cabinet secretary Yukio Edano said: “Residents are safe… we want people to be calm.”
Japan informed the IAEA that the quake and tsunami cut the supply of off-site power to the Daiichi plant and diesel generators intended to provide back-up electricity to the cooling system.
If power is lost, the options to cool the core are limited.
If the core overheats, then the fuel would become damaged and a molten mass could melt through the reactor vessel, releasing a large amount of radioactivity into the containment building surrounding the vessel.
It is unclear if the quake has undermined the containment building, which might allow radioactivity to leak out.
Power supply systems that would provide emergency electricity for the plant were being put in place, the WNA said.
The reactors shut down due to the earthquake account for 18% of Japan’s nuclear power-generating capacity.
Nuclear power produces almost a third of the country’s electricity. Many reactors are located in earthquake-prone zones such as Fukushima and Fukui on the coast.
The IAEA estimates that around a fifth of nuclear reactors around the world are currently operating in areas of significant seismic activity.