On 11th March 2011, a huge magnitude 9 earthquake struck North East Japan, triggering a large tsunami to hit the coast of Japan. It resulted in the death of 15,891 people and a further 2,500 people remain missing as the tsunami flooded 217 square miles of Japan. Within the flooded area lied the Fukushima nuclear power plant which was severely damaged during the event.
A 15m wave hit the power plant, where three of the six reactors were operational at the time. The reactors had stood up well the ferocity of the initial earthquake, but were shown to be vulnerable to the tsunami, which stopped the cooling systems from working. This led to the reactors overheating and ultimately experiencing a meltdown. The following day, radioactive material was beginning to leak from the site. Further material was released through vents to lower the pressure of the reactors. In a month, 770,000 tera Bq, about 20% of that which was released from Chernobyl (you can read about Chernobyl here), was released through vents. A backlog of material from reactor 3 which was connected to reactor 4 caused this reactor to eventually overheat and eventually explode on March 15th.
In the immediate aftermath of the meltdown, the government ordered the evacuation of those living within 3km of the nuclear power plant to leave their homes. As the severity of the incident was realised, the exclusion zone expanded to 20km on 15th March, and those living 20-30km away were advised to stay indoors. Some areas, such as Kawamata Town, Namie Town, and Iitate Village have since been reopened, after an initial evacuation.
Between 15th March and mid-July of 2011, an estimated 2.7 × 1016 Bq of caesium-137 was released into the ocean. The seas surrounding Japan have some of the strongest currents in the world which transported radioactive material much further than it would have otherwise reached. Contamination is ongoing, however, with groundwater travelling beneath the site carrying radioactive waste out to sea. Restrictions on fishing have been eased since the disaster although careful monitoring and a no fishing zone remains in the bay surrounding the plant.