April 2011 – Projected External Doses for Fukushima Fallout Zone

Author: No Comments Share:

EXECUTIVE SUMMARY

1 INTRODUCTION ………………………………………………………………………… 6
2 FIRST-YEAR EXTERNAL DOSE ASSESSMENTS FROM THE FUKUSHIMA ACCIDENT. …… 7
2.1 FIRST-YEAR EXTERNAL DOSE ESTIMATE CARRIED OUT BY IRSN 28 DAYS AFTER THE ACCIDENT ……….. 7
2.2 FIRST-YEAR EXTERNAL DOSE ESTIMATE CARRIED OUT BY UNITED STATES DOE/NNSA 38 DAYS
AFTER THE ACCIDENT ……………………………………………………………………………………….. 10
2.3 FIRST-YEAR EXTERNAL DOSE ESTIMATE CARRIED OUT BY THE JAPANESE “MINISTRY OF
EDUCATION, CULTURE, SPORTS, SCIENCE AND TECHNOLOGY” (MEXT) 44 DAYS AFTER THE
ACCIDENT. …………………………………………………………………………………………………… 11
2.4 FIRST-YEAR EXTERNAL DOSE ESTIMATE FROM THE MAPPING OF CESIUM DEPOSITS (MEXT) …………. 14
3 IMPACT OF EVACUATION MEASURES ON PROJECTED EXTERNAL DOSES …………… 20
3.1 IMPACT OF EVACUATION MEASURES ON POPULATION SIZES ACCORDING TO THE PRESCRIBED
REFERENCE LEVELS ………………………………………………………………………………………….. 20
3.2 IMPACT OF EVACUATION MEASURES ON PROJECTED EXTERNAL DOSES ACCORDING TO
IMPLEMENTATION TIMES …………………………………………………………………………………….. 22
4 CONCLUSIONS …………………………………………………………………………. 24


EXECUTIVE SUMMARY

On April 8, 2011, 28 days after the nuclear accident at the Fukushima nuclear power plant (NPP), IRSN published on its website the first worldwide map of doses likely to be received by the Japanese population as a result of external irradiation occurring the 1st year following the accident. This map was derived from dose rate data collected by the US DoE/NNSA based on airborne measurements and published on their website on April 7, 2011.

The IRSN map revealed significant external doses in a northwest zone from the NPP, about 20 km in width and 50 to 70 km in depth. Other dose maps were then produced and published by DoE/NNSA on April 18, 2011 and more recently by the Japan “Ministry of Education, Culture, Sports, Science and Technology” (MEXT), on the 44th day after accident. These dose maps were consistent with the first dose assessment carried out by IRSN and show dose values of the same order of magnitude (difference less than a factor of 2.5).

On the 56th day after the accident, MEXT published the first maps of caesium depositions. They revealed high values comparable with the most contaminated areas of Chernobyl, even beyond the initial 20 km-radius evacuation zone around the Fukushima plant. A new dose assessment was carried out by IRSN on the 66th day after the accident to estimate projected doses due to external exposure from radioactive deposits, for exposure durations of 3 months, 1 year and 4 years before evacuation.

The estimated projected doses reach particularly significant values, some of them even above 200 mSv, which are no longer in the range of “low doses” according to UNSCEAR definition. Moreover these dose levels do not take into account neither the doses received from other pathways such as immersion within the plume and inhalation of particles in the plume during the accident nor the doses already received or to be received from ingestion of contaminated foodstuffs. The total effective doses to be received (external + internal) could be much higher according to the type of deposit (dry or wet), diet and source of food.

The number of Japanese people living in the most contaminated areas outside the initial 20 km radius evacuation zone around the Fukushima plant (874 km2 with caesium 134+137 deposits higher than 600,000 Bq/m2) was estimated to 70,000 people including 9,500 children of 0-14 years in age. This significant number reaches about 26% of that of Chernobyl (270,000 people) for a surface area only 8.5% of that of Chernobyl (10,300 km2).


IRSN have also studied:

the impact of the selection of a dose reference level, within the range of 20 to 100 mSv recommended by ICRP in emergency situations, on the number of people to be evacuated; averted doses for these populations resulting from an evacuation according to 3 different scenarios: evacuation 3 months, 1 year or 4 years after the accident.

The level of projected external doses in upcoming years – up to 4 Sv lifetime dose in the most contaminated areas (30 million Bq/m2 of caesium-137 + 134) – requires the implementation of protective actions such as evacuation of population.

According to the ICRP recommendations in emergency situations, the selection of the highest protective reference level, i.e. 20 mSv, would avert external doses above this level for 15,000 to 20,000 people.

If the Japanese authorities decide to take an even more protective reference level, for example 10 mSv for the 1st year, the averted external doses for the affected populations (70,000 people) would be much higher if the evacuation is quickly prescribed. An evacuation one year after the accident would result in a 59% decrease of the projected external dose for this population; evacuation three months after the accident would result in an 82% decrease.

This policy for preventing the risk of developing long-term leukaemia and radiation-induced cancer has been clearly understood by the Japanese authorities as shown in the map of population evacuation beyond the initial zone of exclusion of 20 km brought to the IRSN knowledge on May 16,
i.e. the 66th day after the accident.

The prescribed evacuation area seems to meet the 20 mSv reference level – the most protective dose value within the range recommended by ICRP in an emergency situation. This decision made by the Japanese authorities proves retrospectively the relevance of the IRSN’s radiological assessment map – the first to have been published worldwide, 28 days after the accident.


Introduction

The radiological consequence of the nuclear accident in Fukushima was not estimated in the days following the accident due to the lack of reliable data from Japan about the composition of release, the environmental measurements and the individual monitoring performed in the affected population by the radioactive fallout in the neighbourhood of the nuclear plant.

The only health-related information available at that time was concerning the evacuation of the Japanese population within a radius of 20 km around the plant and the sheltering of the population in a 20-30 km area as well as the instructions from U.S. authorities to evacuate their nationals within a radius of 80 km, a precautionary measure that was rather tricky to interpret.

Airborne dose rate measurements performed by the US DoE/NNSA1 and published on their website on April 7, 2011 enabled IRSN to publish on its own website, on April 8, 2011, the first map of doses likely to be received by the population via external exposure during the 1st year following the accident in the fallout zone to the northwest of the plant. Other maps were then published by the DoE/NNSA and more recently by the “Ministry of Education, Culture, Sports, Science and Technology” (MEXT) of Japan.

The purpose of this report is to provide insight on all radiological assessments performed to our knowledge to date and the impact of population evacuation measures to be taken to minimize the medium and long-term risks of developing leukaemia or other radiation-induced cancers.

This report only considers the external doses already received as well as the doses that may be received in the future from fallout deposits, regardless of doses received previously from the radioactive plume.

It should be noted that as regards the risk of developing thyroid cancer in children and adolescents, doses to this organ have already been received. As a result, the impact of protective measures, such as the administration of stable iodine within hours after exposure at the latest, can no longer be considered.


Conclusions

Since the accident in Fukushima has occurred several dose assessments have been carried out by IRSN, US DoE and Japan MEXT. The results are of the same order of magnitude; a high degree of consistency between these radiological assessments and the magnitude of cesium-137 and cesium- 134 deposits is observed.

Projected external doses for the people living lifetime in contaminated areas include significant values, even beyond low doses according to UNSCEAR definition, i.e. less than around 200 mSv. The number of people involved could also be high, around 70,000 including 9,500 children of 0-14 years
in age (13.7% of the total Japanese population in 2005).

These dose levels do not take into account other exposure pathways, such as immersion within the plume and internal contamination resulting from inhalation of particles in the plume, as well as internal doses already received or to be received from contaminated food ingestion.

The total effective doses (external + internal) could be increased considerably according to the type of deposit (dry or wet), diet and source of food.

The level of external doses projected in upcoming years – up to 4 Sv lifetime in the most contaminated areas (30 million Bq/m2 of cesium-137 + 134) – requires the implementation of protective actions such as evacuation of population.

According to the ICRP recommendations in radiation emergency situations, the selection of the highest protective reference level, i.e. 20 mSv, would avert external doses above this level for 15,000 to 20,000 people.

If the Japanese authorities decide to take an even more protective reference level, for example 10 mSv for the 1st year, the averted external doses for the affected populations (70,000 people) would be much higher if the evacuation is quickly prescribed. An evacuation one year after the accident
would result in a 59% decrease of the projected external dose for this population; evacuation three months after the accident would result in an 82% decrease.

This policy for preventing the risk of developing long-term leukemia and radiation-induced cancer has been clearly understood by the Japanese authorities as shown in the map of population evacuation beyond the initial zone of exclusion of 20 km reported to the IRSN knowledge on May 16 i.e. the 66th day after the accident (Figure 10). The prescribed evacuation area seems to meet the 20 mSv reference level – the most protective dose value within the range recommended by the ICRP in an emergency situation.

This decision made by the Japanese authorities proves retrospectively the relevance of the IRSN’s radiological assessment map – the first to have been published worldwide, 28 days after the accident.


[toggle_simple title=”Related articles” width=”600″]

[/toggle_simple]

Previous Article

March 23rd, 2011 – Fukushima – The Radiological Impact

Next Article

March 18th, 2011 – Design Basis Floods Determined from Area-Discharge Envelopes – Not comfortable with level of conservatism