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Ionisierende Strahlung

Environmental consequences of the Fukushima accident: Radiological situation in Japan

  • After the reactor accident in Fukushima, released radionuclides contaminated the environment.
  • With regard to radiation exposure, the short-lived radioactive isotopes of iodine and tellurium were of importance during the first days and weeks after the accident and, on the longer term, the radionuclides of the element caesium (caesium-134 and caesium-137).
  • A map shows the dispersion of caesium-137 deposited on the ground. As a result of the radionuclides' radioactive decay and official decontamination measures, the dose values decrease continuously.
  • The Japanese authorities intend to achieve the possible additional dose due to deposited radioactive particles being no more than 1 mSv per year.

The radioactive material released into the atmosphere as a result of the reactor accident in Fukushima was distributed locally, regionally and globally. As a result it was deposited in oceans and on the earth’s surface. The distribution depended mainly on the meteorological conditions at the moment of release, such as wind and precipitation.

Radiation exposure to the population occured

  • in the first weeks after the accident through inhalation and external irradiation of the radioactive substances in the air, 
  • later on only from the radionuclides deposited on the ground and the ingestion of radionuclides with food.

Relevant radioactive substances

Mainly three radioactive substances are responsible for the radiation exposure:

  • the radioactive isotopes of iodine and tellurium in the first few days and weeks after the accident; these radionuclides have meanwhile decayed. Due to their short half-life of several days they can no longer be detected.
  • In the long run radiation exposure was mainly caused by the radionuclides of the element cesium (cesium-134 and cesium-137).

Distribution of caesium-137 deposited on the ground

Caesium-137 deposited on the ground in kilobecquerel per square metre, based on helicopter measurements performed at the beginning of November 2011. Cäsium-137 auf dem BodenCaesium-137 deposited on the ground in kilobecquerel per square metre, based on helicopter measurements performed at the beginning of November 2011. Source: Japan Ministry for the Economy.Trade and Industry, METI

The map on the right shows the distribution of caesium-137 deposited on the ground. This map is based on data collected by helicopter measurements at the beginning of November 2011.

A soil contamination of 1,000 kilobecquerel per square metre of caesium-137 (yellow colored areas) corresponds to a radiation exposure of approximately

  • 13 millisievert in the first year (effective dose, no contribution due to contaminated food and inhalation of radioactive substances),
  • 6 millisievert in the second, and
  • 4 millisievert in the third year.

This calculation includes the contribution by all other radionuclides, the population’s average time spent outdoors and the reduction of the annual dose values.

Official decontamination measures by the authorities, the dose values decrease continuously.

Radiation exposure of population

Dose values in Japanshow / hide

Based on deposited radionuclides, the World Health Organization (WHO) estimated the following radiation exposure to the population:

  • In the restricted area within a radius of 20 kilometres from the reactor, and in highly exposed areas in the Northwest up to a distance of about 45 kilometres, the estimated dose values in the first year after the accident amounted to between 10 and 50 millisievert (effective dose, in addition to the natural radiation). The population living in these areas was evacuated after the accident, the contribution to the thyroid dose for children and adults in these areas is estimated at 10 to 100 millisievert, in a particular location up to 200 millisievert (for children).
  • Other highly impacted regions in the eastern part of the Fukushima prefecture showed dose rates (effective dose) of 1 to 10 millisievert (for example in Fukushima-City).
  • In all other Japanese prefectures, such as Tokyo, the effective dose due to the accident did not exceed 1 millisievert (0.1 to 1 millisievert).

These calculations also take into account the consumption of contaminated food.

Estimation of individual radiation exposure in the Fukushima prefectureshow / hide

In September 2011, a large-scale study began in Fukushima Prefecture. The aim was to assess the individual radiation exposure of all residents of the Fukushima prefecture in the first four months after the accident. All residents, approximately 2 million people, were questioned as to their whereabouts and how long they stayed at certain places during and after the accident. The radiation exposure was estimated for approximately 350,000 residents on the basis of these questionnaires (only external exposure, i.e. the ingestion or inhalation of radioactive substances was not taken into consideration): 95 per cent of the persons were exposed below 2 millisievert, just under 5 per cent were exposed between 2 and 10 millisievert and about 120 persons received between 10 and 25 millisievert.

Since then, the annual dose values have been decreasing significantly. This is due to

  • the radioactive decay of the deposited radionuclides,
  • the migration of the radionuclides into deeper soil layers (thus reducing the radiation exposure on the surface), and
  • weathering of radionuclides from surfaces such as roofs or streets.

Decontamination of radioactive polluted areas after the Fukushima accident

Dekontamination areas Dekontamination areasDecontamination areas: yellow – areas with an external radiation of 1 to 20 millisievert per year; red – areas with annual radiation doses exceeding 20 millisievert, in some places even 50 millisievert. Source: Japan Ministry for the Economy, Trade and Industry, METI

In March 2011, about 160,000 people in an area of up to 40 kilometres from the Fukushima-Daiichi nuclear power plant had to leave their homes because of the increased radiation. Since then, the Japanese authorities have been decontaminating the area with immense effort. Gradually the evacuees can return to their communities and homes.

Decontamination measures depend on the amount of the external radiation. A Decontamination Act was implemented in Japan on 1 January 2012, entitled "Act on Special Measures Concerning the Handling of Radioactive Pollution".

Roadmap to decontamination

The Japanese Ministry of the Environment published a roadmap with respect to the decontamination of particular areas where external radiation exceeded the value of 0.23 microsievert (µSv) per hour. This value leads to an additional dose of one millisievert per year. More than one hundred municipalities in eight prefectures were affected.

The Japanese authorities want to ensure that the possible additional dose due to deposited radionuclides does not exceed 1 mSv per year. In comparison in one year, a person receives a dose of about 1 mSv from natural external radiation.

Decontamination measures

Decontamination of areas around Fukushima with less than 20 millisievert per yearshow / hide

The local administration is responsible for the decontamination of the yellow-coloured areas in the figure on the right, the so-called "Intensive Contamination Survey Areas", where levels of external radiation of up to 20 millisievert were measured. Here, the Japanese government grants financial and technical support. As of today, vast areas have already been decontaminated.

Decontamination of areas with more than 20 millisievert per yearshow / hide

Decontamination of all areas with an external radiation exceeding 20 millisievert, the so-called "Special Decontamination Areas", is managed by the Japanese government. The main objective is to reduce the annual dose to less than 20 millisievert. As soon as this value is reached, the evacuated population will be allowed to return to their homes.

In many municipalities, decontamination work is finished; e.g. in the community of Tamura City in June 2013 and in Naraha with previously more than 7,000 inhabitants in March 2014, decontamination has already been completed successfully. In spring 2017 evacuation recommendations were also lifted for Namie and Minamisoma.

The partially blocked route of the Joban Expressway was reopened to traffic in February 2014. This important highway in northern Japan passes a few kilometers in front of the crippled nuclear power plant. After April 2011 people needed official permission to be allowed to pass.

However, to a large extent, only elderly people return home. Especially young families with children remain in their new homes. This is partly due to the environmental radiation which is above the natural level, the lack of jobs in the aftermath of the earthquake and tsunami and the still insufficient infrastructure.

Areas with more than 50 millisievert per yearshow / hide

Areas with an annual dose of more than 50 millisievert have been declared exclusion zones. They are mainly situated within a radius of 20 kilometers around the damaged nuclear power plant. They may only be entered with a special permit, protective clothing and a dosimeter. The population will probably not be able to return to these areas for a long time. Before the accident at the Fukushima nuclear power plant occurred, about 25,000 persons lived here.

Interim storage of contaminated soil and organic waste

The clean up of streets, decontamination of buildings and roofs or fruit trees is performed with the help of, for example, high-pressure cleaners or by manual work. The top soil is cleared away and leaves are collected.

Huge amounts of contaminated soil including forest soil and agricultural land as well as organic waste such as leaves and branches are placed into plastic bags and stored intermediately in situ. Planning for final storage has only just begun.

Japan: Limit values for the consumption of contaminated food

In order to minimize the radiation exposure caused by the consumption of contaminated foodstuffs, Japan introduced limit values. These limit values were also adopted by the European Union for imported products from Japan. Foodstuffs with caesium-134/137 concentrations exceeding 100 becquerel per kilogram, for example, are not allowed to be sold. Since the accident, foodstuff is monitored in Japan. If the admissible limit values are exceeded, the respective products are withdrawn from the market.

Limit values rarely exceeded

Several hundreds of thousands of radionuclide measurements for over 500 different foodstuffs from all Japanese prefectures have been published by now. Shortly after the accident at the beginning of the monitoring procedure it was found that almost 1 per cent of the samples exceeded the limit values. Meanwhile only very few food products exceed the limit values. Highly contaminated wild boars are a new problem now. Their population increased rapidly in the restricted zone and they feed on, for example, contaminated mushrooms.

Radiation exposure due to contaminated food

Based on the measurement results in the Fukushima prefecture and presuming typical consumption behavior, the World Health Organization estimated the ingestion dose for the population. The Fukushima radionuclides caused less than 1 millisievert in the first year after the accident, and less than 0.1 millisievert in the second year. Due to the small amounts of contaminated foodstuff and the decrease of contamination values, the additional dose continues to decrease (In comparison people living in Germany ingest an annual average of 0.3 millisievert via natural radionuclides.).

State of 2018.03.07

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