Navigation and service

Ionising radiation

Environmental Radioactivity - Medicine - Occupational Radiation Protection - Nuclear Hazards Defence

Ionisierende Strahlung

Environmental consequences: Radiological situation in Japan

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

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

Relevant radioactive substances

Three radioactive substances are essentially responsible for the radiation exposure:

  • The radioactive isotopes of iodine and tellurium in the first few days and weeks after the accident.
  • In the long run after several weeks the radiation exposure was mainly caused by the radionuclides of the element cesium (caesium-134 and caesium-137).

Dose values in Japan

Based on deposited radionuclides the World Health Organization (WHO) estimated the followin 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 amounted in the first year after the accident 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 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 prefecture

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 sites 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, 5 per cent were exposed between 2 and 10 millisievert and about 120 persons got between 10 and 25 millisievert.

Since then the annual dose values decrease 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
  • the weathering of radionuclides from surfaces such as roofs or streets.

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

Distribution of caesium-137 deposited on the ground

The map illustrated 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 coloured 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 any other radionuclides, the population’s habitual places of stay and the reduction of the annual dose values.

Decontamination of radioactively polluted areas after the Fukushima accident

In March 2011, about 160,000 people had to leave their homes because of the increased radiation. They lived within a radius of 40 kilometres from the Fukushima-Daiichi nuclear power plant. Since then, the Japanese authorities decontaminate the area with immense effort. Gradually the evacuees can return to their communities and homes.

Measures to decontaminate polluted areas

Decontamination measures depend on the amount of the external radiation. To remove the long-lived radionuclides deposited on the grounds and surfaces in the north-east of the Fukushima prefecture, a Decontamination Act was passed into force in Japan on 1 January 2012, which is entitled “Act on Special Measures Concerning the Handling of Radioactive Pollution”.

In January 2012, the Japanese Ministry of the Environment also 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. This situation could be found in more than one hundred municipalities in eight prefectures.

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

Decontamination of areas with less than 20 millisievert per year

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. They are granted financial and technical support by the Japanese government.

Decontamination of areas with more than 20 millisievert per year

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

The decontamination in the municipality of Tamura City in June 2013 and in Naraha with more than 7,000 inhabitants were already successfully completed. The evacuation recommendations were lifted. The partially blocked route of the Joban Expressway was reopened to traffic in February 2014. This important highway in northern Japan passes a few kilometres in front of the crippled nuclear power plant. From April 2011 on passway was only possible with official permission.

However, to a large extent, only elderly people return. 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 the tsunami and the still unsufficient infrastructure.

Areas with more than 50 millisievert per year

Areas with an annual dose of more than 50 millisievert have been declared exclusion zones. They are mainly situated within a radius of 20 kilometres around the damaged nuclear power plant. They may only be entered with a special permit, wearing protective clothing and carrying a dosimeter. The population will probably not be able to return to these areas for a long period of 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 partly performed with the help of 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 just begun.

Japan: Introduction of limit values for the consumption of contaminated food

In order to restrict 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, is 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 only rarely exceeded

Several hundreds of thousands of radionuclide measurements for over 500 different foodstuffs from all Japanese prefectures have been published by now. 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. Now, a new problem are highly contaminated wild boars. Their population increased rapidly in the restricted zone and they eat a lot of 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 of people. The Fukushima radionuclides caused less than 1millisievert in the first year after the accident, furthermore, less than 0.1 millisievert in the second year. In comparison people living in Germany ingest an annual average of 0.3 millisievert via natural radionuclides.

State of 2016.12.15

How do you rate this article?

© Bundesamt für Strahlenschutz