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For the first time, evidence for molten fuel found inside Unit 3.

On 11 March 2011, a tsunami caused by the Tohoku earthquake struck Tokyo Electric Power Company (TEPCO's) Fukushima Daiichi nuclear power station.  Reactor Units 4 to 6 were not working at the time as they were undergoing maintenance, while Units 1 to 3 had shut down automatically following the earthquake.  Waters from the tsunami flooded the diesel generators that should have pumped cooling water into the Units 1 to 5.  Only the generator for pumping coolant water into Unit 6 was left undamaged by the inrush of water.  As a result, Units 1 to 3 went into meltdown, while Unit 4 was damaged by an explosion caused by a build-up of hydrogen gas in the building.   
Corium, the name given to the lava-like mixture of fissile material created in a nuclear reactor core during a meltdown, is believed to have melted through the bottom of the pressure vessels in each of Units 1 to 3.
From 20 to 22 July, an underwater robot explored the ruined reactor core of Unit 3.  It found areas of solidified lava-like corium, which in places was up to a metre thick.  Melted fuel debris was first spotted by the robot on Friday.  Finding out about the state of things inside the reactor cores is crucial to work on the decommissioning of the site, which is likely to take decades to complete.  
Radiation levels have so far been too high for similar investigations to succeed in the reactors of Units 1 and 2.  Eventually, all of the debris in each of the reactors will need to be removed and stored safely.

Photo of International Atomic Energy Agency (IAEA) fact-finding team leader Mike Weightman examining Unit 3 on 27 May 2011 courtesy of  IAEA Imagebank.

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