Variations of 129I in the atmospheric fallout of Tokyo, Japan: 1963–2003

Abstract

Atmospheric fallout samples collected from Tokyo between 1963 and 2003 were analyzed using accelerator mass spectrometry (AMS) in order to determine 129I/127I ratios and to examine the deposition rate of 129I and its secular variation in Tokyo. The 129I/127I ratios in the atmosphere during 1963–1977 ranged from 1 × 10−8 to 2 × 10−8. This is roughly 4 orders of magnitude higher than pre-atomic levels, possibly due to atmospheric nuclear weapons tests. The calculated monthly atmospheric deposition rates of 129I differed from those produced by nuclear fallout of 90Sr and 137Cs, indicating that the variations in 129I deposition are not influenced exclusively by either nuclear bomb testing or by the Chernobyl accident. After 1978, high 129I depositions (up to 0.13 mBq/m2/month) were observed. The 129I depositions started to increase markedly at the latter half of the 1970s. The secular variation of the estimated annual 129I deposition in Tokyo showed a close relationship between the annual atmospheric discharge of 129I from the Tokai Reprocessing plant. Therefore, the atmospheric fallout collected from Tokyo after the late 1970s is influenced primary by the 129I discharge from the Tokai Reprocessing plant.