福岡県における環境放射能調査 (1963年度～1990年度)

抄録

This report summarizes the analytical results of deposition, soil and total diet in regard to ⁹⁰Sr and ¹³⁷Cs concentrations in the area of Fukuoka prefecture during the period of fiscal year 1963-1990 as well as measurements of the γ-ray dose rate in the same area during the period of fiscal year 1970-1990.
The annual fallout peaked value in fiscal year 1963 showing 639MBq/km² for ⁹⁰Sr and 839MBq/km² for ¹³⁷Cs and then decreased on an exponential curve until fiscal year 1985, reflecting the effect of Chinese nuclear tests. ¹³⁷Cs fallout at a level of 34.4MBq/km² was detected upon the accident at the Chernobyl nuclear power plant in 1986. However, it began to decrease in the following year and had shown a lower annual level since fiscal year 1989 than those which were detected before the accident. The results of soil analysis showed a sign of accumulation of both ⁹⁰Sr and ¹³⁷Cs, indicating a longterm effect. The radioactivity of the two nuclides in total diet showed a sloping down curve similar to that of the deposition. The γ-ray dose rate was in the range 51.3-101nGy/h, averaging 70.7±5.24nGy/h. The dose equivalent due to external exposure to ¹³⁷Cs in the soil was estimated to be 14.5μSv/year on average and that due to internal exposure to ⁹⁰Sr and ¹³⁷Cs was 4.7μSv/year on average. After the survey, it was found that the ⁹⁰Sr and ¹³⁷Cs concentration levels in the fallout in Fukuoka prefecture were lower than that of the national average and the mean dose equivalent due to both internal and external exposure to the two nuclides was estimated to be 18.0μSv/year. This value is within the error limit of 0.495±0.0367mSv/year, which represents an annual dose equivalent due to the γ-ray dose rate, and is less than 1% of the dose of 2.4mSv/year, which represents the mean dose due to the total natural radiation in the world.
It is also 1.8% of 1.0mSv/year, that represents the dose equivalent limit based on the ICRP recommendation. This level of an additional radioactivity was deemed to have virtually no effect on human health.