First results on 236U levels in global fallout

The global fallout 236U level in soil was deduced from measurements of 236U, 239+240Pu and 137Cs in surface soils which are solely influenced by global fallout. A total of 12 soil cores from the depths of 0–10, 0–20 and 0–30 cm were collected at a flat forest area in Japan. Concentrations of 239+240Pu and 238U were determined by α-particle spectrometry, while the 236U/238U ratio was measured with accelerator mass spectrometry (AMS). Consistent 236U/239Pu ratios between 0.212 and 0.253 were found. Using this ratio, the total global fallout of 236U on the earth is estimated to be as much as ca. 900 kg. This knowledge will contribute to the promotion of research on U isotopes, including 236U, for the fields of geo-resources, waste management and geochemistry.