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. 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 sea life, fishing and food

The diluting effects of ocean currents mean that radioactivity cannot be detected in seawater and is only an issue on the seabed in the immediate vicinity of the plant. Radioactive material continues to run off from the land through rivers to the sea and can be found in certain species of fish. All food from affected areas is strictly monitored and prevented from sale if in excess of highly conservative standards brought in during 2012.

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. Contamination of groundwater has been determined on site with this presumed to be due to mixing with contaminated water in building basements. Radiation monitoring in the sea gives no evidence that contamination in groundwater, or any leaks from storage, have reached the seafront. A silt fence is in place to prevent contamination reaching the open sea.

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.

Main activity

The priority is removal of used fuel from the four storage pools at the top of the reactor buildings, then later removal of the melted core material from inside units 1-3, which is targeted to begin in about ten years. The four reactors will be decommissioned in 30-40 years.

	Unit 1		Unit 2
	A cover has been built over unit 1 to protect it from the weather and ensure no further airborne releases. Equipment is being installed within to remove debris and fuel from the storage pool.		Work is going on in the reactor building in preparation for designing equipment to enable engineers to determine the status of the torus for the suppression chamber structure which is thought to be damaged.
	Unit 3		Unit 4
	Debris is being removed from the top of the building and from the used fuel storage pool. A covering structure has been planned.		The most heavily damaged building, the structure of its fuel storage pool has already been reinforced and debris has been cleared from the service floor. Now a more substantial over-structure is being fitted with gear to enable unloading of fuel from its storage pool, commencing in December 2013.