Natural and anthropogenic 236U in environmental samples

The interaction of thermal neutrons with (235)U results in fission with a probability of similar to 85% and in the formation of (236)U (t(1/2) = 2.3 x 10(7) yr) with a probability of similar to 15%. While anthropogenic (236)U is, therefore, present in spent nuclear fuel at levels of (236)U/U up to 10(-2), the expected natural ratios in the pre-anthropogenic environment range from 10(-14) to 10(-10). At VERA, systematic investigations suggest a detection limit below (216)U/U = 5 x 10(-12) for samples of 0.5 mg U, while chemistry blanks of similar to 2 x 10(7) atoms (236)U per sample limit the sensitivity for smaller samples. We have found natural isotopic ratios in uranium reagents separated before the onset of human nuclear activities, in uranium ores from various origins and in water from a subsurface well in Bad Gastein, Austria. Anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria, whereas river sediments from Garigliano river (Southern Italy) were close to the detection limit. Finally, our natural in-house standard Vienna-KkU was calibrated against a certified reference material (IRMM REIMEP-18 A). (C) 2008 Elsevier B.V. All rights reserved.