Abstract
The chemical compositions of atmospheric deposits collected in Fukuoka, Japan, in 1964 and 2000, which are known to have high 235U/238U uranium isotope ratio were determined. High uranium contents relative to that of continental crust were observed for the deposition samples in 1964 together with the high 235U/238U ratios, which indicated that the origin of uranium is fallout materials due to the large-scale 1961- 1962 nuclear explosions conducted by US and former USSR. The deposits in 2000 were mainly composed of dusts from the East Asia. This suggested that uranium with a high 235U/238U ratio in the deposits in 2000 has been transported as a component of mineral dust particles contaminated with nuclear debris from nuclear test sites from the East Asian deserts and arid regions by the prevailing wind.
Highlights
INTRODUCTIONEspecially 235U/238U ratio, do not fractionate substantially in the nature (within ca. ±1‰) except those from natural nuclear reactors [1]
Uranium isotope ratios, especially 235U/238U ratio, do not fractionate substantially in the nature except those from natural nuclear reactors [1]
In 2006, we started the study on uranium isotope ratios recorded in the Japanese atmospheric deposits and found the anomalous 235U/238U 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]
Summary
Especially 235U/238U ratio, do not fractionate substantially in the nature (within ca. ±1‰) except those from natural nuclear reactors [1]. Warneke et al [2] reported high 235U/238U ratios for the Alpine ice core samples and archived herbage samples of IACR-Rothamsted, UK compared to the natural ratio in the 1950s and 1960s. Their observation suggests that uranium having anomalous isotope ratios had been emitted by nuclear test explosions and had spread all over the world as fallout. That was the first report on anomalous 235U/238U 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, an atmospheric nuclear test has never been conducted since 1982. We discuss the origin of such mineral dusts and enriched uranium contained in them based on the chemical compositions of the atmospheric deposits
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