The Fukushima Disaster happened after a major earthquake took place following a 15 meter tsunami that killed thousands of people and disabled all the power supply and cooling of three Fukushima Daiichi reactors. All these cause a nuclear disaster on March 11th, 2011. All three cores largely melted in the first three days. This accident was rated “7 on the INES scale, due to high radioactive releases over days 4-6, eventually a total of some 940 PFq.” The earthquake in East Japan that had a magnitude of 9.0 at 2.46 on Friday 11 March 2011 had a very big impact. It did considerable damage in the region and the following tsunami made the mess much bigger. Eleven reactors were operating at the same time the earthquake hit and they all shut down automatically. None of the fukushima reactors were working at that time but the unit four became a problem five days after the disaster. The reactors proved that they were very vulnerable to the tsunami. The entire site was flooded by the 15-metre tsunami.
It was a catastrophic failure resulted in a meltdown of three of the plant’s nuclear reactors. The second day after the disaster happened the plant started to release big amount of radioactive materials. It became the largest nuclear incident since the Chernobyl disaster. It began to release around 10-30 percent of radiation slowly growing. In August 2013, it was stated that the “massive amount of radioactive water is among the most pressing problems that are affecting the cleanup process, which is expected to take decades.” Some of these have been released into the sea. Some people evacuated the area and this caused a huge amount of pollution that has been expanding further than Japan.
Source
http://www.world-nuclear.org/info/safety-and-security/safety-of-plants/fukushima-accident/
http://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster
http://www.walesonline.co.uk/news/uk-news/japan-fights-back-year-after-2044537