Preliminary Dose Estimation from the nuclear accident after the 2011 Great East Japan Earthquake and Tsunami (WHO, edited)

[Giuseppe Michieli]

[Source: World Health Organization, full page: (LINK). Edited.]

Preliminary Dose Estimation from the nuclear accident after the 2011 Great East Japan Earthquake and Tsunami


Authors: WHO

Publication details

Number of pages: 120
Publication date: 2012
Languages: English
ISBN: 9789241593662
Downloads: English pdf, 1.85Mb



Overview

The earthquake and tsunami in Japan on 11 March 2011 led to releases of radioactive material into the environment from the Fukushima Daiichi nuclear site. This report describes a preliminary estimate of radiation doses to the public resulting from this accident. These doses are assessed for different age groups in locations around the world, using assumptions described in the report. The dose assessment forms one part of the overall health risk assessment being carried out by WHO of the global impact of the accident at the Fukushima Daiichi nuclear power plant. The health risk assessment is the subject of a separate WHO report which will be published in Summer 2012.

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[Giuseppe Michieli]

Excerpt from the above mentioned report:

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5. Summary and conclusions

The radiation doses in this report represent a preliminary assessment of doses for the first year after the Fukushima accident based on data available to the panel up to September 2011. Doses are provided for different populations by age and geographical location.

Effective doses and equivalent doses to the thyroid have been estimated for three age groups: 1-year-old infants, 10-year-old children, and adults. These age groups were chosen to enable the characterization of the radiological impact on younger and more sensitive populations. The estimated doses are those received by a characteristic person living in the region or location of interest, and are mostly reported as bands of one order of magnitude, providing a level of accuracy commensurate with the preliminary nature of the assessment. The bands do not indicate the range of doses in the population in the area, which would have required distributions of input data not available to the panel within the time frame of this assessment1.

In general, the doses assessed are the committed doses for residents staying in a given region or location during the full first year after the accident. However, in a few locations in the most affected part of Fukushima prefecture, doses were calculated for the first four months after the accident as relocation took place at some time during the first few months.

On the basis of the input data used for this assessment, extrapolation of doses beyond the first year was not performed because of uncertainties that may influence long-term exposure, including future protective and remedial actions, that will further reduce radiation exposure (48, 49). The experience of the Chernobyl accident was that about 30% of the lifetime dose was delivered during the first year and about 70% during the first 15 years (31). On the basis of environmental activity concentration data, it can be expected that the fraction of the lifetime dose beyond the first year will be lower for the Fukushima Daiichi nuclear power plant accident than for the Chernobyl accident, due to the greater influence of the shorter-lived 134CS (half-life two years) compared to 137CS (half-life 30 years).

In summary, the key features of the assessed doses are as follows.

For the estimated effective doses:

■■ In Fukushima prefecture the estimated effective doses are within a dose band of 1−10 mSv, except in two of the example locations where the effective doses are estimated to be within a dose band of 10–50 mSv.

■■ In prefectures neighbouring Fukushima, the estimated effective doses re within a dose band of 0.1−10 mSv, and in all other prefectures the effective doses are estimated to be within a dose band of 0.1−1 mSv. In the rest of the world estimated effective doses are less than 0.01 mSv, and are usually far below this level.

■■ The exposure pathways that contribute most to effective dose vary with location and distance from the site. In the most affected regions the external dose from groundshine is important, but with increasing distance from the site the ingestion of food becomes the main contributor.

For the estimated thyroid doses:

■■ In the most affected area of Fukushima prefecture the estimated thyroid doses are within the dose band of 10−100 mSv, with the exception of one example location where estimated thyroid doses to adults are within a dose band of 1−10 mSv and another example location where the estimated thyroid doses to infants are within a dose band of 100–200 mSv.

■■ In the rest of Fukushima prefecture the estimated thyroid doses are within a dose band of 1−10 mSv to adults and 10−100 mSv to children and infants.

■■ In the rest of Japan the estimated thyroid doses are within a dose band of 1−10 mSv.

■■ In the rest of the world, estimated thyroid doses are less than 0.01 mSv, and are usually far below this level.

■■ The exposure pathways that contribute most to thyroid dose vary with location and distance from the site. In the most affected regions, inhalation from the cloud and the external dose from groundshine are important, but with increasing distance from the site the ingestion of food becomes the main contributor.

It can be concluded that the estimated effective doses outside Japan from the Fukushima Daiichi nuclear power plant accident are below (and often far below) the dose levels regarded by the international radiological protection community as very small. An annual dose of 0.01 mSv (10 μSv) corresponds to the radiological criterion for exemption of materials from need of regulatory control and for clearance of materials from any further control, as established in the International Basic Safety Standards for protection Against Ionizing Radiation and for the Safety of Radiation Sources (38). This level of dose is comparable to the average population dose resulting from 1.5 days of exposure to natural background radiation/natural sources of radiation (37).

It can also be concluded that low effective doses are estimated in much of Japan. To put this into context, and keeping in mind that dose limits do not apply in emergency exposure situations, it could be noted that estimated effective doses outside Fukushima and neighbouring prefectures are below the annual limit for public exposure in planned exposure situations (1 mSv) (10,38).

In the Fukushima prefecture and in neighbouring prefectures the estimated effective doses are below the internationally agreed reference level for public exposure due to radon in dwellings (annual effective dose of about 10 mSv (39)), except in two locations in the most affected part of Fukushima prefecture where the effective doses were estimated to be within a dose band of 10–50 mSv. To put this into context, the ICRP recommends reference levels for planned residual dose in emergency exposure situations (the dose that remains after protective actions have been taken) in the band of 20–100 mSv annual or acute effective dose (48).

In view of the time frame of the assessment, a number of assumptions were taken to calculate the doses, which mostly provide conservative estimates. In particular the panel was aware of some protective measures taken by the Government of Japan in relocating residents of certain areas (i.e. in the “deliberate evacuation area”). Because of lack of the necessary detailed information, it has been assumed that the relocation of residents in these areas took place at least four months after the accident, whereas a proportion of the population was relocated earlier.

Therefore, the doses estimated in the example locations considered for the most affected areas of Fukushima prefecture may be overestimated. Moreover, the protective effects of sheltering may not also have been fully taken into account due to lack of more detailed information. This being said, the dominant contributors to the total effective dose in these locations were the inhalation2 and external exposure pathways. While evacuation prior to the arrival of the radioactive cloud would be effective in reducing inhalation and external exposure from cloudshine, relocation after the radioactive release (e.g. at two months instead of four months as assumed) would not significantly reduce the overall dose as the early exposure pathways would remain unchanged.

For prefectures far away from Fukushima included under the scenario for the rest of Japan, food appears to be the main exposure source. In these locations, food was not monitored and it assumed that consumers only ate food coming from Fukushima and neighbouring prefectures. The doses are therefore clearly overestimated.

Comparison between the estimated doses for Japan in this report and those estimated from direct measurements of radionuclides in Japanese residents and travellers returning from Japan gives confidence that the estimated doses in this report do not underestimate the actual doses in Japan. The doses estimated from in vivo human measurements are similar, and in some cases lower, than those estimated in the assessment using environmental data, although a direct correlation between the quantities cannot be made.

As discussed, cautious assumptions have been made where data are lacking. However, the panel considers that the doses presented here are unlikely to be very significantly overestimated.

The Independent Expert Panel considers that the dose estimates are robust on the basis of knowledge and information on hand at the time of the study. The data used as input to this assessment are considered by the panel to be the most appropriate available on the timescale required (September 2011) and to be fit-for-purpose.

The long-term priority in radiological protection after a nuclear accident is to protect people with the highest exposures, and to reduce all individual exposures associated with the event to as low as reasonably achievable (30). This requires the knowledge of the dose distribution for the subsequent optimization of protection.

It has been reported that a number of remedial actions have been taken by the Government of Japan, municipal authorities and residents to lower radiation exposure (49). At the time of publication of this report, additional protective and remedial actions are being implemented3. Such measures that will further lower exposure can be accounted for in future studies, such as the upcoming two-year UNSCEAR assessment.4

In addition to the UNSCEAR study, a major initiative initiated in the Fukushima prefecture will inform future more detailed dose assessments. This unprecedented initiative, the Fukushima Health Survey, includes a survey to determine the whereabouts of every prefectural resident from the time of the March 11 nuclear accident onwards (a so-called “record of movement”) and to provide the basis for estimating the level of radiation exposure, which will assist in future health effect assessments.

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