The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) on 9 March released its 2020 Report, “Levels and effects of radiation exposure due to the accident at the Fukushima Daiichi Nuclear Power Station: Implications of information published since the UNSCEAR 2013 Report”. UNSCEAR Chair Gillian Hirth noted: “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.” Twelve UNSCEAR Member States and one observer contributed with over 30 experts to the 2020 evaluation.

The new report summarises all relevant scientific information (peer reviewed literature and monitoring data) available up to the end of 2019. These data relate to the levels and effects of radiation exposure due to the accident at the Fukushima Daiichi nuclear power station (FDNPS). The report aims to summarise all scientific information and provide an appraisal of the implications of this information for the UNSCEAR 2013 Report, the findings of which it broadly confirms.

Main findings of that report were:

  • Cancer rates would remain stable;
  • There was a theoretical increased risk of thyroid cancer among the most exposed children;
  • There would be no impact on birth defects/hereditary effects;
  • There would be no discernible increase in cancer rates for workers;
  • There would be a temporary impact on wildlife.

Between 2013 and 2020 UNSCEAR arranged follow-up activities to keep abreast of additional information published in the scientific literature. Three White Papers ( 2015, 2016, 2017 ) were published after periodic reviews and evaluations of the implications of new scientific developments. The results generally confirmed the assumptions and findings of the 2013 Report.

In the last decade, a significant amount of new information emerged on exposure estimates, hence the 2020 report. UNSCEAR said this had made it possible “to perform an improved and more robust evaluation of the levels and effects of radiation due to exposure from the accident”. In the 2020 report, the areas considered are:

  • Radionuclide releases to atmosphere, dispersion and deposition;
  • Radionuclide releases to water, dispersion and deposition;
  • Evaluation of doses for public;
  • Evaluation of doses for workers;
  • Health implications for workers and public; and,
  • Evaluation of doses and effects for non-human biota.

UNSCEAR in 2013 concluded that the total releases to the atmosphere of Iodine-131 (I-131) and Caesium-137 (Cs-137) were 100-500 peta becqerel (PBq) for I-131 [equivalent to 2.7-13.5 million curies] and 6-20 PBq for Cs-137. The ranges correspond to about 2-8% of the total inventory of I-131 and about 1-3% of the total inventory of Cs-137 in the three Fukushima Daiichi reactors (units 1–3) from which the releases occurred at the time of the accident. Releases of less volatile radionuclides (e.g. strontium-90 and plutonium 239) were negligible. About 80% of the total amount released was dispersed over and deposited on to the Pacific Ocean. Its revised estimates put the total releases of I-131and Cs-137 to the atmosphere at 120 and 10 PBq respectively.

Releases of radionuclides into the Pacific Ocean “occurred directly and indirectly and still continue, albeit at decreasing rates”. They comprise:

  • direct releases in the first three months (in leakage and deliberate release of water containing radionuclides) of about 10-20 PBq of I-131 and about 3-6 PBq of Cs-137Cs;
  • deposition of radionuclides on to the ocean surface following their release of about 60-100 PBq of I-131 and 5-8 PBq of Cs-137;
  • direct release of about 60 terabecquerel (TBq) of Cs-137 in ground water draining from the site up to October 2015, when measures were taken to reduce releases, and about 0.5 TBq a year thereafter; and
  • continuing indirect releases of about 5-10 TBq of Cs-137Cs a year via rivers draining catchment areas where radioactive material was deposited.

The material released to the Pacific Ocean was rapidly dispersed and diluted in seawater: by 2012, the concentrations of Cs-137, even in the coastal waters off the FDNPS site, were little above pre-accident levels. Concentrations in marine foods have declined rapidly: 41% of samples caught off the Fukushima coast in 2011 exceeded the Japanese Government’s long-term limit, decreasing to 17% in 2012, and, from the beginning of 2015, to just four samples out of 9,000.

As to transfer of radionuclides released to the atmosphere through terrestrial and freshwater environments, radiocaesium has been found to be more strongly bound to soils in Japan than in many soils affected by the Chernobyl accident, resulting in its reduced transfer to crops and downward migration into soil, “with implications for doses via ingestion and from external radiation from deposited radionuclides”. Concentrations of radionuclides 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 Government limit.

The improved models used in the 248-page report, based on additional monitoring data and more comprehensive information about people’s actual diet and behaviour in Japan, allowed UNSCEAR to update its dose estimates. The report indicated that updated dose estimates to members of the public have either decreased, or are comparable with previous estimates. “The Committee therefore continues to consider that future health effects directly related to radiation exposure are unlikely to be discernible.”

UNSCEAR also assessed the incidence of thyroid cancer that could be inferred from estimated radiation exposure and concluded that this is not likely to be discernible, in any of the age groups considered, including children and those exposed in utero to radiation. “The Committee believes that, on the balance of available evidence, the large increase, (relative to that expected), in the number of thyroid cancers detected among exposed children is not the result of radiation exposure. Rather, they are the result of ultrasensitive screening procedures that have revealed the prevalence of thyroid abnormalities in the population not previously detected.”

In addition, UNSCEAR found no credible evidence of excess congenital anomalies, stillbirths, preterm deliveries or low birthweights related to radiation exposure in the general public. As to workers, it concluded that an increase in the incidence of cancers is unlikely to be discernible for leukaemia, and total solid cancers (including thyroid cancer).

Evaluating information on the transfers of released radioactive material through the terrestrial, freshwater and marine environments, UNSCEAR said concentrations of Caesium-137, even in the coastal waters off the FDNPS site, were little above the levels prevailing before the accident. “The Committee continues to consider that regional impacts on wildlife populations with a clear causal link to radiation exposure resulting from the FDNPS accident is unlikely, although some detrimental effects in some plants and animals have been observed in areas of enhanced radiation levels.” Radionuclide concentrations in most monitored foodstuffs declined rapidly following the accident.

However, further studies could be useful on regional impacts on wildlife populations and the impacts of radiation exposure on non-human biota under field conditions that are able to take account of higher levels of biological organization, within natural environments, and elements of ecosystem function and structure.