Tritium concentrations in the atmospheric environment at Rokkasho, Japan before the final testing of the spent nuclear fuel reprocessing plant

摘要

This study aimed at obtaining background tritium concentrations in precipitation and air at Rokkasho where the first commercial spent nuclear fuel reprocessing plant in Japan has been under construction. Tritium concentration in monthly precipitation during fiscal years 2001 -2005 had a seasonal variation pattern which was high in spring and low in summer. The tritium concentration was higher than that observed at Chiba City as a whole. The seasonal peak concentration at Rokkasho was generally higher than that at Chiba City, while the baseline concentrations of both were similar. The reason for the difference may be the effect of air mass from the Asian continent which is considered to have high tritium concentration. Atmospheric tritium was operationally separated into HTO, HT and hydrocarbon (CH3T) fractions, and the samples collected every 3 d—14 d during fiscal year 2005 were analyzed for these fractions. The HTO concentration as radioactivity in water correlated well with that in the precipitation samples. The HT concentration was the highest among the chemical forms analyzed, followed by the HTO and CH3T concentrations. The HTand CH3T concentrations did not have clear seasonal variation patterns. The HT concentration followed the decline previously reported by Mason and Ostlund with an apparent half-life of 4.8 y. The apparent and environmental half-lives of CH3T were estimated as 9.2 y and 36.5 y, respectively, by combining the present data with literature data. The Intergovernmental Panel on Climate Change used the atmospheric lifetime of 12 y for CH4 to estimate global warming in its 2007 report. The longer environmental half-life of CH3T suggested its supply from other sources than past nuclear weapon testing in the atmosphere.