On 11 March 2011, at 14.46 local time, a 9.0-magnitude earthquake occurred near Honshu, Japan, creating a devastating tsunami that left a trail of death and destruction in its wake. The earthquake and subsequent tsunami, which flooded over 500 square kilometres of land, resulted in the loss of more than 20,000 lives and destroyed property, infrastructure and natural resources. They also led to the worst civil nuclear disaster since the one at Chernobyl in 1986. The loss of off-site and on-site electrical power and compromised safety systems at the Fukushima Daiichi nuclear power station led to severe core damage to three of the six nuclear reactors on the site; this resulted in the release, over a prolonged period, of radioactive material into the environment.

In May 2011, UNSCEAR embarked upon a two-year assessment of the levels and effects of radiation exposure from the accident. It reported its findings to the General Assembly in October 2013 (A/68/46), and a detailed publication titled "Levels and effects of radiation exposure due to the nuclear accident after the 2011 great east-Japan earthquake and tsunami" with the supporting scientific data and evaluation was published online in April 2014.

The UNSCEAR 2020/2021 Report is a new evaluation that summarizes additional information, measurements and results since the UNSCEAR 2013 Report. It is titled “Levels and effects of radiation exposure due to the nuclear accident at the Fukushima Daiichi Nuclear Power Station (FDNPS): Implications of information published since the UNSCEAR 2013 Report”. It is available in English and Japanese.

This report summarizes all the scientific information available (up to the end of 2019) relating to the levels and effects of radiation exposure due to the accident at the Fukushima Daiichi Nuclear Power Station and appraises it in the context of the information and conclusions of the UNSCEAR 2013 Report. The main objectives of the report are:

• To summarize all of the information available since 2012 and assess its implications for the UNSCEAR 2013 Report.
• To validate and revise estimates of doses to the public, based on more detailed analyses of the available information.
• To set out an improved appraisal of the uncertainties.
• To better address issues and objectives not fully addressed in the UNSCEAR 2013 Report.

It covers the following topics:

• Radionuclide releases to atmosphere, dispersion and deposition;
• Radionuclide releases to, and dispersion and deposition in, the marine environment;
• Transfer of radionuclides in terrestrial and freshwater environments;
• Assessment of doses to the public;
• Assessment of doses for workers;
• Health implications for workers and public;
• Assessment of doses and effects for non-human biota.

About 30 experts and 13 independent critical reviewers prepared the UNSCEAR 2020/2021 Report.  It involved a comprehensive review of over 500 scientific articles, which were all checked for quality and peer reviewed. In addition, the Committee used monitoring data from the literature and also from Minamisoma city and Naraha town. The Committee reviewed and adopted the report at the end of 2020. The report and supporting material are available here. The printed report can also be ordered from the United Nations shop.

Based on available data since October 2012 the UNSCEAR 2020/2021 Report broadly confirms the major findings and conclusions of the UNSCEAR 2013 Report.

No adverse health effects among Fukushima residents have been documented that could be directly attributed to radiation exposure from the accident, nor are any expected to be detectable in the future.

Using the greater amount of information now available, the report includes:

• More realistic estimates of the levels of radiation exposure of members of the public;
• Quantitative assessment of the uncertainties in the estimated levels of exposure and of the ranges of exposures as well as average exposures among the groups of the population.

It also confirms that, 10 years on, the levels of radiation exposure from the accident, in all but the most highly contaminated areas, have reduced to levels that are below the radiation exposure from natural background.

Since the UNSCEAR 2013 Report, no adverse health effects among Fukushima residents have been documented that could be directly attributed to radiation exposure from the accident, nor are expected to be detectable in the future.

Exposure to radiation could lead to an increased incidence of disease in the exposed population. However, for example, with cancer, it is not generally possible to distinguish by observation or testing whether or not the disease of a specific patient has been caused by the radiation exposure. The Committee has therefore assessed the risks resulting from radiation exposure following the accident by estimating whether any increased incidence of a particular disease, calculated theoretically from the estimated doses, would be detectable compared to the normal statistical variability in the baseline incidence of the disease in that population. The Committee’s conclusion is that its revised estimates of dose are such that future radiation-associated health effects are unlikely to be detectable.

The Committee found no credible evidence of excess birth defects, stillbirths, premature births or low birthweights related to radiation exposure. Increases in the incidence of cardiovascular and metabolic conditions have been observed among those evacuated following the accident but are probably associated with concomitant social and lifestyle changes and are not attributable to radiation exposure.

The Committee also concluded that no detectable excesses of thyroid and other types of cancer that are sensitive to radiation, such as leukaemia or breast cancer, were likely because of the generally low levels of radiation exposure in the Fukushima Prefecture population.

The Committee also found that, 10 years on, the levels of radiation exposure for the accident, in all but the most highly contaminated areas, have reduced to levels that are below the radiation exposure from natural background.

Excess psychological distress occurred in the aftermath of the combined earthquake, tsunami and Fukushima Daiichi Nuclear Power Station accident.  However, the report does not address other health consequences, such as mental health or financial impacts, which are beyond the Committee’s mandate.

After the accident, the greatest concern was whether the resulting exposure to radiation would lead to an increase in rates of thyroid cancer. Although a substantial number of thyroid cancers have been detected among exposed children, the Committee believes that, on the balance of available evidence, the (relative to expected) large increase in thyroid cancers is the result of ultrasensitive screening procedures that have revealed the prevalence of thyroid abnormalities not previously recognized in the population, and is not a result of radiation exposure.

The average effective dose to the more than 20,000 emergency workers involved in mitigation and other activities at the Fukushima Daiichi Nuclear Power Station site from March 2011 to the end of March 2012 was about 13 mSv. About 36% received an effective dose more than 10 mSv, while 0.8% (174 workers) were assessed to have received more than 100 mSv in this period.

Annual effective doses have been considerably lower since April 2012, with average annual effective doses declining from about 6 mSv in the year to the end of March 2013 to about 2.5 mSv in the year to the end of March 2020. No worker has received an annual effective dose of more than 50 mSv since April 2013.

The UNSCEAR 2020/2021 Report determined that an increase in the incidence of cancers is unlikely to be discernible amongst workers for leukemia, total solid cancers, or thyroid cancer.

The 2020/2021 UNSCEAR Report, annex B makes a detailed comparison of the main impacts of the Fukushima and Chernobyl accidents in terms of doses and effects on people and non-human biota.

The radiological impact of the accident at Fukushima Daiichi Nuclear Power Station was much less than that at Chernobyl Nuclear Power Station for reasons that include:

• The releases from the Fukushima Daiichi Nuclear Power Station of radioiodine and radiocaesium - the more radiologically significant radionuclides - were only about 10% of the releases from Chernobyl.

• Only about 20% of the releases to atmosphere from Fukushima were dispersed over the Japanese land mass, with the remainder dispersed over and deposited on the Pacific Ocean with much lower impact.

• The Japanese Government imposed timely and effective monitoring and restrictions to limit exposure from food and drinking water; whereas, in some areas of the Former Soviet Union, restrictions were delayed resulting in very high doses to the thyroids of those affected.

• As a result, thyroid doses to evacuees from Fukushima were about 100 times lower than those from Chernobyl, and thyroid doses to those not evacuated and living in more affected areas around Fukushima Daiichi Nuclear Power Station were several tens of times lower than those living around the Chernobyl nuclear power station.

• A substantial fraction of the 19,000 thyroid cancers observed (up to 2016) among people who were children or adolescents at the time of the accident were attributable to radiation exposure from the Chernobyl accident. Increase in thyroid cancer incidence observed following the Fukushima accident are most likely the result of the sensitive thyroid screening programme and not radiation exposure.

• In Fukushima, because the clay type soil, radiocaesium more tightly bound to the soil and did not migrate so easily. This was not the case in Chernobyl after the 1986 accident.

The Marine Environment

The Committee reviewed the movement of radioactive material released as a result of the accident into the marine environment. By 2012, the concentrations of ¹³⁷Cs, in the coastal waters off the Fukushima Daiichi Nuclear Power Station site, were little above the levels prevailing before the accident. Concentrations of ¹³⁷Cs in marine foods have declined rapidly: 41% of samples caught off the coast of Fukushima Prefecture in 2011 exceeded the long-term limit established by the Japanese Government, decreasing to 17% in 2012, and, from the beginning of 2015, to just four samples out of 9,000 (0.05%).

Terrestrial and Freshwater Environment

A large body of Japan-specific information has been accumulated on the levels and transfer of radionuclides released to the atmosphere through terrestrial and freshwater environments. Monitoring programmes that began immediately after the accident enabled timely restrictions to be applied to prevent foodstuffs being marketed where the radionuclide concentration exceeded regulation values and limits established by the Japanese Government. Radionuclide concentrations in most monitored foodstuffs declined rapidly following the accident. Since 2015, no samples of livestock and crop products, and less than a few per cent of most monitored wild food and freshwater fish products, have exceeded the limit established by the Japanese Government. In addition, a small number of monitored agricultural food samples (less than a few per cent) exceeded the Codex Alimentarius guideline levels for international trade in 2011, and no samples have exceeded them from 2012 onwards.

The large-scale remediation projects implemented by the Japanese national Government and municipal administrations, as well as natural processes and radioactive decay, have further reduced concentrations of radiocaesium in terrestrial and freshwater environments.

Wildlife (non-human biota)

The Committee continues to consider that regional impacts on wildlife populations with a clear causal link to radiation exposure resulting from the Fukushima accident would have been unlikely, although detrimental effects on individual organisms might have been possible, and some effects have been observed in plants and animals in the absence of any wide-scale group impacts.

Radionuclide concentrations in most monitored foodstuffs have declined rapidly following the accident. Since 2015, no samples of livestock and crop products, and less than a few per cent of most monitored wild food and freshwater fish products, have exceeded the limit established by the Government of Japan.

In 2011, the year of the accident, 41% of samples of marine foods caught off the coast of Fukushima Prefecture exceeded the long-term limit established by the Japanese Government. By 2012, they had decreased to 17%, and, from the beginning of 2015, only four samples out of 9,000 exceeded the established limits.

In addition, a small number of monitored agricultural food samples (less than a few per cent) exceeded the Codex Alimentarius guideline levels for international trade in 2011, and no samples have exceeded them from 2012 onwards.

Looking to the future, the 2020/2021 Report concludes that:

• Timely monitoring (e.g., whole body counting, thyroid measurements, personal dosimetry) of representative groups of workers and the public at the earliest opportunity after an accident would greatly enhance the quality and informativeness of any assessment of doses to workers and the public following a radiological or nuclear accident. If appropriate human measurements are not made in the immediate aftermath of an accident, doses to people can only be assessed using models together with other measurements that may be available, for example, in the facility where the accident occurred and/or in the wider environment. Experience has shown that the use of such models often leads to conservative estimates of doses.

• Care is needed over the widespread use and interpretation of sensitive ultrasound thyroid screening following radiation exposure as a result of events such as the Fukushima accident. There is compelling evidence that sensitive ultrasound screening detects many more cases of abnormalities and cancer in the thyroid than would be detected following the presentation of clinical symptoms. The consequential over-diagnosis of thyroid cancers, many of which may never result in clinical symptoms, has the potential to cause considerable anxiety among some of those screened and to lead to unnecessary treatment, the detrimental effects of which may outweigh those of the radiation exposure itself, especially if the thyroid doses are relatively low.

According to the Governing principles for the Committee's work representatives, alternates and advisers are nominated annually by Governments on the basis of their scientific qualifications and experience and are to perform scientific evaluations in accordance with established scientific procedures and values. They are expected to, inter alia, have in-depth knowledge and experience with regard to a broad range of relevant scientific and technical issues related to radiation science, stay abreast of scientific developments and foster effective support nationally. They are also to act independently and to discharge their duties without bias in relation to external pressures or interests that may be in conflict with the Committee’s procedures and tasks. Furthermore, they are to act with integrity and to respect matters of intellectual property.

All experts involved in UNSCEAR evaluations are nominated by the national representatives, and endorsed by the Bureau. Every expert signs a declaration of no conflict of interest (including observers from international organizations and critical reviewers). The expert groups report annually on their work to the Committee, including review of peer reviewed literature in a transparent manner.

The draft reports are reviewed by independent critical reviewers, before submission for endorsement by the Bureau and approval by the Committee. The approved scientific reports are then presented by the UNSCEAR Chair to the UN General Assembly and published as annexes of the UN General Assembly reports.

The main sources of funding for UNSCEAR is the UN regular budget. Since 2007 it is supplemented by an UNSCEAR trust fund, with voluntary contributions from Member States. Overall, the main contribution from Member States to the scientific work of the Committee is through provision of in-kind experts, exchange of data (e.g. monitoring data, global surveys) - mainly expertise sharing.