The Japanese nuclear reactor hit by the tsunami went into ‘meltdown’ today, as officials admitted that fuel rods appear to be melting inside three damaged reactors.

That means there is a risk that molten nuclear fuel can melt through the reactor’s safety barriers and cause a serious radiation leak.

There have already been explosions inside two over-heating reactors at the Fukushima Dai-ichi nuclear plant, and the fuel rods inside a third were partially exposed as engineers desperately fight to keep them under control after the tsunami knocked out emergency cooling systems.

Japanese chief cabinet secretary Yukio Edano said it was ‘highly likely’ that the fuel rods inside all three stricken reactors are melting.

Some experts class that a partial meltdown of the reactor, but others would only use that term for when molten nuclear fuel melts through a reactor’s inner chamber – but not through the outer containment shell. 

As fuel rods melt, they form an extremely hot molten pool at the bottom of the reactor that can melt through even the toughest of containment barriers.

Japan is fighting to avoid a nuclear catastrophe after the tsunami. There was a hydrogen explosion at the reactor in Unit Three of the power station earlier today, in which eleven workers were hurt by the blast that was felt 25 miles away.

The reactor at Unit One of Fukushima exploded on Saturday, blowing several walls away but engineers said the core was still contained. The fuel rods in the reactor in Unit Two of the plant were partially exposed from their coolant today – which also increases the risk of meltdown.

Engineers have been fighting to keep the reactors under control after the tsunami knocked out emergency coolant systems on Friday.

Earlier engineers were frantically trying to cool radioactive materials at all the reactors with seawater but had halted the process, which resulted in a rise in radiation levels and pressure.

Plant managers knew an explosion was a possibility as they struggled to reduce pressure inside the reactor containment vessel in Unit Three, but apparently felt they had no choice if they wanted to avoid a complete meltdown.

In the end, the hydrogen in the released steam mixed with oxygen in the atmosphere and set off the blast, which was felt 25 miles away.

The plant’s operator Tokyo Electric Company said radiation levels at the reactor were still within legal limits.

Japan’s Chief Cabinet Secretary Yukio Edano said the Unit Three reactor’s inner containment vessel holding nuclear rods is intact, allaying some fears of the risk to the environment and public.

The government had warned that a further explosion was possible because of the build-up of hydrogen in the building housing the reactor.

More than 180,000 people have been evacuated from the area.

The International Atomic Energy Agency (IAEA) said Japanese authorities reported that radioactivity levels ‘at the site boundary’ of another nuclear power plant at Onagawa had returned to normal.

Earlier a state of emergency had been declared after the high levels of radiation were detected at the nuclear power complex.

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