Anomalous Uranium Isotope Ratio in Atmospheric Deposits in Japan

Origin of enriched uranium contained in Japanese atmospheric deposits

“…The monthly uranium depositions in 1964 at Fukuoka are extremely large compared with those in 2000, a cause for which is larger mass depositions in 1964. The amount of enriched uranium deposited in Fukuoka in these days is thus smaller than in the 1960s, while the 235 U/ 238 U ratios in the 1960s and in 2000s are comparable ( Figure 2) [4].…”

“…This finding is associated with the existence of enriched uranium-bearing hot particles originating from nuclear test explosions. In contrast, the 235 U/ 238 U ratio in the acid-insoluble part of the deposit in 2000 is higher than the natural ratio, suggesting the existence of enriched uranium fused with silicate minerals [4]. Based on the temporal variation of the specific activity of 234 Th in atmospheric deposits collected in Osaka, Japan, Matsunami and Mamuro [13] suggested that uranium from nuclear test explosions had been deposited in Japan in the 1970s within two months after the explosions.…”

“…The 235 U/ 238 U ratios in those samples, however, had never been measured. In 2006, we started the study on uranium isotope ratios recorded in the Japanese atmospheric deposits and found the anomalous 235 U/ 238 U ratios in the atmospheric deposits collected at Fukuoka located in the southwestern Japan, where the Asian mineral dusts event (Kosa) is observed frequently in springtime [4]. That was the first report on anomalous 235 U/ 238 U ratios obtained by direct measuring of atmospheric deposits, clarifying that mineral dusts containing enriched uranium have been transported to Japan by the prevailing westerly since 1960s until today continuously, although an atmospheric nuclear test has never been conducted since 1982.…”

“…The 235 U/ 238 U isotope ratios in the Fukuoka deposits in the month of March between 1964 and 2000[4]. N( 235 U/ 238 U) denotes the normalized value of 235 U/ 238 U ratio in deposits against that in sea water.…”

An aerosol particle containing enriched uranium encountered in the remote upper troposphere

“…Releases of enriched uranium and other radionuclides into the atmosphere have been reviewed by Salbu and Lind (2011). Besides major single releases such as the Chernobyl reactor failure or the burnup of the Cosmos satellite reactors (Krey et al, 1979;Leifer et al, 1987), longer-term sources of uranium such as windblown dust from nuclear test sites have been postulated to increase the 235 U content of atmospheric uranium (Kikiwada et al, 2009). Apart from isolated events, most of the uranium in the remote atmosphere is not enriched.…”