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 Table of Contents 
NEWS AND INFORMATION
Year : 2016  |  Volume : 39  |  Issue : 2  |  Page : 113-116  

A brief summary of: UNSCEAR White paper on the “Developments since the 2013 report on levels and exposure due to the Accident at Fukushima Daiichi Nuclear Power Station”


Associate Editor, RPE, Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Web Publication13-Sep-2016

Correspondence Address:
D D Rao
Associate Editor, RPE, Internal Dosimetry Section, Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.190398

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How to cite this article:
Rao D D. A brief summary of: UNSCEAR White paper on the “Developments since the 2013 report on levels and exposure due to the Accident at Fukushima Daiichi Nuclear Power Station”. Radiat Prot Environ 2016;39:113-6

How to cite this URL:
Rao D D. A brief summary of: UNSCEAR White paper on the “Developments since the 2013 report on levels and exposure due to the Accident at Fukushima Daiichi Nuclear Power Station”. Radiat Prot Environ [serial online] 2016 [cited 2022 Jan 19];39:113-6. Available from: https://www.rpe.org.in/text.asp?2016/39/2/113/190398

A committee of United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) had assessed radiation exposure of the public, workers, and nonhuman biota that resulted from the March 11, 2011, accident at the Fukushima Daiichi nuclear power station (FDNPS), and submitted in the form of “2013 Fukushima report.” This report consisted of information disclosed or published in peer reviewed journals before the end of October 2012. Subsequently, much relevant information has been published or become available. The committee intended to make use of this information and update the findings and refine the assessments so as to provide a scientific appraisal of the new material. In view of this white paper, on the assessments incorporating the published information from October, 2012 to December, 2014 is brought out.

The technical content of the report consists of six chapters for different categories, and each of the chapter is divided into three sections as A, B, and C, in addition to chapters on Introduction and Evaluation of new information.

  1. Recapitulation of 2013 Fukushima report
  2. Findings of review of new publications
  3. Potential implication of new publications.


The committee identified a large number of new publications and systematically appraised about eighty, of which more than half have corroborated one or another of the major assumptions made by the committee in its 2013 report. None of them challenged the report's major assumptions or affected its main findings.

The chapter-wise highlights are as follows.


  Updates on Radionuclide Releases to Atmosphere Top


A:The estimates of the two most important radionuclides of 131 I and 137 Cs ranged from 100 to 500 petabecquerels (PBq) and 6–20 PBq, respectively. The averages of the published estimates were about 10% and 20%, respectively, of the corresponding releases to the atmosphere estimated for the Chernobyl accident. The radionuclides released to the atmosphere were dispersed over mainland Japan, and then were deposited on the ground by means of (a) dry deposition, and (b) wet deposition with rain and snow. The releases of radionuclides 131 I and 137 Cs in the source term were 120 and 8.8 PBq, respectively.

B: None of the publications appraised materially affected the main findings or challenged the major assumptions of the 2013 Fukushima report, and several publications demonstrated that inverse or reverse modeling can reasonably be applied to reconstruct the source term. The latest estimate has been derived using improved atmospheric and oceanic transport, dispersion and deposition models, as well as new information on measured levels of radiation dose or radionuclides in the environment. The total releases of 131 I and 137 Cs estimated were 151 and 14.5 PBq, respectively. These differences in total releases are relatively small in the context of the ranges of releases (100–500 PBq for 131 I and 6–20 PBq for 137 Cs).


  Updates on Radionuclide Releases to Water Top


A: The direct discharge and releases from FDNPS to the ocean mainly occurred during the 1st month following the accident, and that the continuing releases were insignificant in the perspective of doses to the public. The direct releases were about 10–20 PBq for 131 I and 3–6 PBq for 137 Cs. In addition, due to the deposition from the atmosphere was about 60–100 PBq for 131 I and 5−8 PBq for 137 Cs. The committee had concluded that measured levels of 137 Cs in seawater near the FDNPS site declined rapidly from a peak of 68,000 Bq/L on 7 April 2011 and were generally below 200 Bq/L by the end of April, after which the rate of decrease was much smaller. Concentrations decreased rapidly with distance from the coast: at 15 and 30 km offshore from the FDNPS site, they were about 100 and 1000 times lower, respectively, than near the FDNPS site.

B: The estimates of direct discharges and releases to the marine environment and of indirect input from deposition from the atmosphere have not been significantly challenged by recent publications. It was suggested that deposition to the ocean surface from the atmosphere could have been underestimated because no measurements were available to constrain this term in atmospheric dispersion models. In the initial period, the levels in most samples outside a radius of 30 km from FDNPS were reported as not detectable (less than about 6–9 Bq/L), whereas the new more sensitive analysis has identified the variations of 137 Cs concentration at the surface, intermediate, and bottom depths, with detectable levels between 0.1 and 1 mBq/L. At a larger scale, numerical models have indicated that the levels of 137 Cs in seawater had decreased to the preaccident levels 2.5 years after the accident.


  Updates on Evaluation of Doses for the Public Top


A: For the assessment of doses to the public from external exposure, it used models with parameter values mostly derived from European studies after the Chernobyl accident. In 2013 Fukushima report, population-averaged deposition densities of radionuclides for Japanese districts or prefectures, derived by combining measurements of radionuclide deposition densities with data on the age compositions and typical occupancy factors were used. For assessment of doses to the public from internal exposure from inhalation, radionuclides in the passing radioactive plume were only considered, while inhalation of resuspended radionuclides considered insignificant. Intakes of radionuclides in food and drinking water in the 1st year following the accident had been assessed using the database of food and drinking water measurements carried out in Fukushima Prefecture and other prefectures of Japan. Measurements of radionuclides in people, such as whole-body-counter (WBC) and thyroid measurements provide a direct source of information on internal exposure. However, at the time of preparation of the 2013 Fukushima report, the number of thyroid measurements had been limited (about 1100 persons), and these data had been able to be used only to corroborate doses to the thyroid in a few settlements estimated using numerical models.

B: Of the 12 publications appraised, none materially affected the main findings of the 2013 Fukushima report, while 10 provided confirmation of the main assumptions in whole or part.

External exposure

A correlation between outdoor dose rate (ambient dose equivalent rate from deposited radionuclides) and personal dose rate (personal dose equivalent rate due to external exposure) was demonstrated for use in reconstructing doses for groups and individuals. A large series of individual measurements by optically stimulated luminescent dosimeter (OSLD) of dose to more than 1000 students from external exposure over a 2 years period, as well as forecasts of doses for the next 10 years, was presented. It noted significant progress in a number of areas that will contribute to enhancing the quality and reliability of estimated doses from external exposure. These include the further clarification of patterns of external exposure; the validation of dose estimates with individual measurements; and the development of a national probabilistic model for estimating doses from external exposure. It has also been noted that there was little new information (at least not reported in peer-reviewed publications) available in other areas, including: the migration of caesium radioisotopes in various environments; the determination of shielding parameters for Japanese buildings; the assessment of the effectiveness of urban decontamination; and long-term dose rate measurements in inhabited areas.

Internal exposure

Early WBC measurements provided the most reliable retrospective assessment of dose from internal exposure due to early inhalation and/or ingestion of caesium radionuclides. WBC measurement data were converted to committed effective dose assuming early acute intake, regardless of whether this was by ingestion or inhalation. The resulting doses from internal exposure to caesium radionuclides were about three times lower than those estimated in the 2013 Fukushima report using the database on measurements in foodstuffs and inhalation modeling. One of the studies reported 565 measurements by WBC and the committed effective dose to adult residents was <0.1 mSv for 380 members and 0.1–0.2 mSv for 180 members and all were <0.4 mSv. The estimated doses from internal exposure to residents in three areas within 20–50 km of FDNPS resulting from intakes of caesium radionuclides during the summer of 2012 by ingestion (based on analysis of meal duplicates) and by inhalation (based on analysis of air dust) are reported. The results demonstrated that, in the summer of 2012, the doses from internal exposure (<0.1 mSv) were two orders of magnitude lower than the doses from external exposure (1–10 mSv) as measured by personal OSLD.

A low ingestion rate of caesium radionuclides by residents of Fukushima Prefecture in the winter of 2011–2012 and the summer of 2012 was reported from the analysis of meal duplicates collected from 200 families. Among 200 meals analyzed, only 12 were found to have average concentrations of more than 1 Bq/kg. For the highest measured caesium radionuclide levels in food, the assessed annual effective dose from ingestion did not exceed 0.1 mSv.


  Updates on Evaluation of Doses for Workers Top


A: By the end of October 2012, the Tokyo Electric Power Company (TEPCO) had reported statistics on doses to about 25,000 workers at the FDNPS site, most of whom were employed by contractors. The average effective dose to FDNPS workers over the first 19 months after the accident had been about 10 mSv. About 34% of the workforce had received effective doses over this period above 10 mSv while 0.7% of the workforce (173 individuals) had received effective doses more than 100 mSv. The highest reported effective dose was 679 mSv for the TEPCO worker who also had received the highest reported committed effective dose due to internal exposure (590 mSv). The independent assessments of the doses due to internal exposure for 12 workers (out of a total of 13) who had committed effective doses due to internal exposure higher than 100 mSv had confirmed that they had received absorbed doses to the thyroid due to inhalation of 131 I in the range of 2–12 Gy.

B: Of the seven publications appraised, all have provided some confirmation of one or other of the report's major assumptions. Results of measurements of 131 I in the thyroid of 560 FDNPS workers which provide committed effective dose estimates that are consistent with those presented in the 2013 Fukushima report. Results of dicentric chromosome assay for 12 workers showed that estimated whole body absorbed doses were less than 300 mGy for all 12 workers and that no acute radiation syndrome effects were expected for the workers examined. There was a need for the reevaluation of doses because of significant discrepancies between doses from internal exposure assessed by TEPCO and those assessed by contractors of TEPCO, resulting from differences in the methods adopted for dose assessment. New guidance from the Japanese Ministry of Health, Labour and Welfare (MHLW) on the standard methods to be used resulted in a revision of committed effective doses for a few hundred workers. As a result of this reevaluation, the numbers of workers found to have received committed effective doses in the dose ranges 50−100 mSv and over 100 mSv increased by a few percent. MHLW estimated the contribution from 132 Te to the committed effective dose, given the uncertainties, to be approximately 10% of the contribution from 131 I. Effective doses for 1536 workers have been re-assessed by the new standard method. Increase in the estimates of the committed effective dose range from 1 mSv to 90 mSv. As a result, there have been further small increases (by one or two) in the numbers of workers with committed effective doses above 100 mSv, and with doses between 50 and 100 mSv. Most of the increases occurred in workers with committed effective doses <50 mSv.


  Updates on Health Implications for Workers and Public Top


A:The health risks resulting from the Fukushima accident were expected to be far lower than those for the Chernobyl accident, owing to the substantially lower doses received by the public and workers. No deterministic effects from radiation exposure had been observed among the public, and none had been expected. No discernible radiation-related increases in rates of leukemia or breast cancer (two of the most radiogenic cancer types), nor in other types of solid cancer besides possibly thyroid cancer, had been expected. The sensitive ultrasound-based thyroid screening of those under 18-year-old at the time of the accident had been expected to detect a large number of thyroid cysts and solid nodules, “that would not normally have been detected without such intensive screening.” However, similar or even slightly higher rates of cysts and nodules were found in the prefectures of Aomori, Yamanashi, and Nagasaki that had not received significant radionuclide deposition from the accident. Among FDNPS emergency workers, deterministic effects had been considered unlikely. 2−3 excess cancers could be inferred over the lifetime among workers with doses greater than 100 mSv (mainly from external exposure), although it is unlikely that such increased incidence of cancer due to irradiation would be discernible. The most important health effects that had been observed among the general public and workers were considered to be on mental health and social well-being.

B: None of the ten publications appraised challenge the assumptions or findings of the 2013 Fukushima report; instead they served to strengthen or complement those findings. A study reported that (as of March 2014) 51 out of the 287,056 individuals (177 per million) in the Fukushima Health Management Survey (FHMS) screening program had a diagnosis of thyroid cancer. The study of thyroid screening results in areas where people were not exposed to radiation resulting from the accident found rates of thyroid nodules and cysts that were nominally somewhat higher than the FHMS study. Taking all of the screening studies in nonexposed areas indicates a prevalence of thyroid cancer of 380 per million compared to 177 per million in the FHMS.


  Updates on Evaluation of Doses and Effects for Nonhuman Biota Top


A: Estimated radiation doses due to the accident to nonhuman biota through the application of suitable models. The corresponding estimates of effects due to the radiation exposure had then been inferred through generic evaluations of dose–effects relationships. Exposures of both marine and terrestrial nonhuman biota following the accident had been, in general, too low for acute effects to be observed.

B: Of the twenty publications appraised, nine provided confirmation of the major assumptions in whole or part of 2013 report. Several new publications provided information on radionuclide activity levels in nonhuman biota and on processes influencing transfer. The results from these studies appeared to have no major repercussions on the findings of the 2013 Fukushima report, although synthesis of the newly published data could help refine the transfer models originally applied. This document is available on the UNSCEAR website at www.unscear.org for further reading.



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