Historical changes of 236U/238U and 235U/238U isotopic ratios in Tokyo Bay from the 1960s to the 2000s.

We examine the historical changes of 236U/238U and 235U/238U in a sediment core collected in Tokyo Bay and elucidate the anthropogenic sources of uranium in the 1960s-2000s. Uranium-236 was detected in samples deposited in the 1960s-2000s, and the 236U/238U ratio of the sediment core shows peak values in the 1970s. The 235U/238U isotopic ratios in samples deposited in the early 1960s are almost identical to that of natural uranium, implying that the 236U might have originated from global fallout. A decrease in 235U/238U was observed in the late 1960s-2000s, suggesting that depleted uranium from nuclear fuel reprocessing increased the 236U/238U ratios in the sediment. The 236U/238U values in sediments from the 1980s-2000s were lower than those in the 1970s but considerably higher than those in the 1960s, suggesting that the main source of depleted uranium still remains around Tokyo Bay. Our results demonstrated that the depleted uranium released in the 1970s should be considered as an important end-member when using uranium isotopic ratios as environmental tracers in closed aquatic environments around industrial cities.

Temporal change of 236U/238U and 235U/238U isotopic ratios in atmospheric deposition in Tokyo and Akita from 1963 to 1979.

We examine the temporal changes of 236U/238U and 235U/238U in atmospheric deposition from samples collected in Tokyo and Akita from 1963 to 1979 and elucidate the spatial distribution and historical changes of the anthropogenic sources of uranium in Japan. The 236U/238U ratio of atmospheric deposition in Tokyo peaked in 1963 and again during the 1970s, while the corresponding 235U/238U ratios of atmospheric deposition during the second peak period were lower than that of natural uranium. The 236U/238U ratios of atmospheric deposition in Akita samples peaked in 1963. The 235U/238U ratios in Akita samples were almost identical to that of the natural uranium ratios. These results suggest that the peak of 236U/238U in 1963 corresponds to what is recognized as representative for global fallout. The increase of 236U/238U and the decrease of 235U/238U observed simultaneously in the 1970s indicate that depleted uranium has subsequently been released into the environment around Tokyo. The cumulative deposition density of 236U for atmospheric fallout samples collected in Tokyo from 1968 to 1979 is an order of magnitude larger than that of the global fallout, suggesting that the depleted uranium in the 1970s is a major component of 236U in Tokyo and should be considered as an end-member when using 236U as an environmental tracer in the industrial city. This knowledge can facilitate future research using 236U as an effective environmental tracer.