The Situation at Fukushima

A major earthquake on 11 March 2011 caused a 15-metre tsunami to strike the Fukushima Daiichi nuclear power plant on Japan's Tohoku coast, disabling the power supply and heat sinks, thereby triggering a nuclear accident. The reactors involved were boiling water units of a 1960s design owned and operated by Tokyo Electric Power Company and supplied by GE, Toshiba and Hitachi. Reactors 1-4 came into commercial operation 1971-78. Without cooling water, the cores of units 1, 2 and 3 overheated and largely melted in the first three days. Hydrogen generated by this high-temperature process caused explosions in the upper service floors of reactor buildings at units 1 and 3. Unit 4 had not been operating, but was affected by a hydrogen explosion due to gas back-flow from unit 3. All four reactors are written off. Two other reactors at the plant were not involved in the accident. The major accident was rated at Level 7 on the International Nuclear Event Scale due to high radioactive releases to air in the first few days. The bulk of releases occurred with the explosions, while a leak of contaminated water to sea continued for two months. Further releases of radioactivity to the air were brought to insignificant levels before the end of 2011, although much radioactivity remains dispersed on the ground in the surrounding area.
  Effects on people
Significant amounts of radioactivity were released, but prompt evacuation from the immediate area made sure that no member of the public received enough exposure to cause harm. Some 160,000 people were evacuated from their homes and only in 2012 were some allowed limited return. Certain areas are still off limits but the Japanese government has announced it is ready to lift the evacuation order on the first nearby town in April. Radiation was never expected to have any measureable effect on the health of the population and this was confirmed in 2013 by an estimation from the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) that no person in Fukushima prefecture would be exposed, through the environment or their food, to more than 10 mSv in their entire lifetime. This is one tenth of the level at which health effects are known to become more likely, and therefore no measureable increase in cancer rates is expected. The government continues to monitor the health of all Fukushima residents. Stress, worry and the social problems of relocation have been repeatedly identified as the only likely causes of ill health.
  Effects on the sea, fishing and food
Groundwater travels naturally from the land to the sea and, in doing so is believed to mingle with heavily contaminated water in the basements of the power plant buildings. This continues to sea and a major effort is underway to identify the routes it is taking and manage groundwater to reduce this to the maximum extent possible. A silt fence has long been in place to prevent contamination reaching the open sea and the diluting effects of ocean currents mean that radioactivity cannot be detected in seawater beyond the plant harbour. Radioactive material continues to run off from the land through rivers to the sea and can be found in certain species of fish. However, all food from affected areas has been strictly monitored since the accident and prevented from sale if in excess of highly conservative standards.
  Current situation

It is presumed that the remains of the reactor cores (molten corium or fuel debris) are within the buildings and stably cooled by water circulation. A large water treatment plant was built to cope with the fact that this water becomes contaminated by the core materials in the destroyed reactors. Also there is considerable storage capacity built at the site to hold decontaminated water and avoid its release. Management of extensive water storage at various levels of radioactivity is becoming a challenge that has been given much media attention. Nitrogen is being injected into all three reactors to ensure inert atmosphere there and prevent any chance of further hydrogen explosions. Nuclear fuel in storage pools is being stably cooled and is believed not to have been significantly damaged. Removal of fuel from the storage pool in unit 4 began in November 2013.
  Main activity
The priority is removal of used fuel from the four storage pools at the top of the reactor buildings and this is well underway at unit 4. In about ten years Japanese technicians expect to be ready to begin removal of the melted core material from inside units 1-3. The four reactors will be decommissioned in 30-40 years, which is typical for any nuclear facility.

 Source : World Nuclear News