- Fuel rods appear to be melting inside three over-heating reactors
- Experts class development as 'partial meltdown'
- Japan calls for U.S. help cooling the reactor
- 180,000 people have been evacuated amid meltdown fears
dailymail.co.uk | 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.
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 cool after the tsunami knocked out systems.
'Meltdown': The Fukushima Daiichi nuclear plant moments after it was rocked by a second explosion today. Officials later admitted that fuel rods are 'highly likely' to be melting in three damaged reactors
Fireball: A build-up of hydrogen in Unit Three of Fukushima ignites in a ball of fire that can be seen for miles
Smoke: An enormous cloud rises from the site, dwarfing the plant and raising fears of radiation problems
Extensive damage: Experts are now debating whether a radiation cloud could reach the West Coast
WHAT HAPPENS IN A NUCLEAR MELTDOWN ?
The Japanese reactors work by harnessing the energy of thousands of nuclear fuel rods, that are normally kept submerged in water to keep them cool.
But if the cooling system fails, the heat generated by the nuclear reaction increases uncontrollably.
If that continues for long enough, the nuclear fuel can melt, forming molten pools on the floor of the reactor at thousands of degrees celcius.
This is a meltdown.
These pools of molten fuel can melt through the reactor safety barriers - there is an inner and outer shield.
The worst case scenario is that the protective shield around the reactors is melted away, resulting in a serious leak of radioactive material.
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.
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