The Fukushima accident

Fukushima Daiichi nuclear power plant Source: Taro Hama @ e-kamakura/Moment/Getty Images

On 11 March 2011 at 14:46 local time, an earthquake with a magnitude of 9.0 (Richter scale) shook the north of the main Japanese island of Honshu. Not long after, a tsunami reached the northern east coast of the island. This had disastrous consequences for the people of the region.

Accident at the nuclear power plant Fukushima Daiichi

The earthquake disconnected the Fukushima Daiichi nuclear power plant from the public mains. The nuclear chain reaction taking place in reactor units 1 to 3, which were in operation at the time, was stopped by emergency shut-down.

Because of the tsunami following the earthquake, the emergency power supply in Units 1 to 4 also failed in the long term. As a result, these units lacked the energy supply for cooling the fuel elements in the reactor cores and the fuel element storage pools, which is also required after reactor shut-down.

Large parts of the emergency power supply in Units 5 and 6 also failed. An operational emergency diesel was used alternately for Units 5 and 6. Severe core damage in these units was thereby prevented.

In Units 1, 2, and 3 of the nuclear power plant, the nuclear cooling system and the cooling of the fuel element storage pools failed. This led to overheating of the reactor cores and melting of core material.

Release of radioactivity into the environment

The accident resulted in a significant release of radioactive materials into the environment. This also led to the classification of the accident at the Fukushima Daiichi nuclear power plant as level 7 “major accident” on the international reporting scale INES (International Nuclear and Radiological Event Scale).

Radionuclides of the elements iodine-131, tellurium-132, and caesium-134/137 were particularly relevant for the radioactive contamination of the environment (and humans) after the accident in Fukushima.

• Iodine-131 has a half-life of about 8 days (i.e. after 8 days, half of the iodine-131 has decayed). Tellurium-132 has a half-life of only three days; its decay produces radioactive iodine-132. Iodine-132 has a half-life of about 2 hours. As a result, radioactive iodine essentially disappeared from the environment after three months. This was also the case in Fukushima.
• Caesium-137 has a half-life of about 30 years and thus results in long-term environmental contamination. Caesium-134 was released in approximately the same amount as caesium-137 in the accident at the Fukushima Daiichi nuclear power plant but decays more rapidly because of its half-life of two years. Today, caesium-137 is still mainly responsible for the increased radiation in the area around the Fukushima nuclear power plant.

In order to avoid the further release of radioactive materials into the atmosphere from the destroyed reactors stabilization measures are carried out. The reactor units will be covered and the fuel rods of unit 1 to 4 will be removed.

In addition to the release into the atmosphere, radioactive materials (especially iodine-131, caesium-134, caesium-137, and strontium-90) were released into the water – mainly as contamination of the water fed in for emergency cooling. Large quantities of contaminated water have accumulated in the buildings through leaks in the safety containers. In March/April 2011, there was a discharge of heavily contaminated water into the sea. Even today, water – mainly groundwater – still penetrates the buildings from the outside.

The inflow of groundwater into the buildings has now been considerably reduced. A purification plant for the contaminated water that escapes from the building is also in operation. Radioactive caesium (and all other radionuclides except tritium) can thus be almost completely filtered out. However, the radionuclide tritium contained in the cooling water cannot be filtered out with the usual cleaning methods. (More about this: Fukushima - Ten years after the reactor accident (GRS)). Water that is not fed back into the reactors for cooling after treatment is therefore temporarily stored in various containers on the plant site. Some of the purified water may now also be discharged into the sea.

The BfS report “Die Katastrophe im Kernkraftwerk Fukushima nach dem Seebeben vom 11. März 2011: Beschreibung und Bewertung von Ablauf und Ursache” (in German only) provides detailed information on the course of the accident and the radiological consequences.

Civil protective measures

In order to minimise the health consequences of the Fukushima accident for people exposed to internal (inhalation of radioactive materials from the air and ingestion through food) and external (airborne radioactive substances and radionuclides deposited on the ground) radiation, approximately 120,000 people within a 40-kilometre radius of the Fukushima Daiichi nuclear power plant were evacuated after the reactor accident in March 2011 as a preventive measure or because of the high radiation.

Those who were evacuated were examined for external radiation exposure in order to identify and dispose of contaminated clothing, for example.

First the 2-kilometre radius (11 March, 20:50), then the 10-kilometre radius (12 March, 5:00 to 17:00), and finally the 20-kilometre radius around the reactor (12 March, 18:25) were evacuated. The population within a 30-kilometre radius was asked to remain indoors (15 March, 11:00 am). From April to June 2011, regions outside the 20-kilometre radius where doses of more than 20 millisieverts per year could have been expected were also evacuated. (For comparison: the annual natural radiation exposure in Germany is about 2–3 millisieverts.) The size of the original evacuation area has since been reduced by intensive decontamination measures.

In order to protect the population from ingesting radioactive substances with their food, the authorities in Japan prohibited the sale of contaminated food; even home-produced food from contaminated regions was not to be consumed. Today, almost no foodstuffs in Japan continue to exhibit radioactive contamination, only a few samples of wild boar, mushrooms and freshwater fish exceed the limits.