Natural and Anthropogenic 236U in Environmental Samples

When thermal neutrons are captured by 235U, there is about a 15% probability that 236U (t1/2 = 2.3×107 a) is produced (85% leads to fission). While anthropogenic 236U is therefore present in spent nuclear fuel at levels of 236U/U up to 10-2, estimates for the natural ratio in the pre-anthropogenic environment range from 10-14 (with a few mg/kg U in rock) to 10-10 (uranium mineral). Since the implementation of 236U measurements at VERA in 2002, we have investigated 236U/U isotopic ratios in various environmental samples and 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 with high uranium content in Bad Gastein, Austria. We verified our method by dilution series both for small isotopic ratios and for small samples sizes, suggesting a detection limit below 5×10-12 for samples of 0.5 mg U. Chemistry blanks of ~2×107 atoms 236U per sample limit the sensitivity for smaller samples. Whereas the river sediments from Garigliano river (Southern Italy) were close to this limit, an anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria. Additionally, we normalized our natural in-house standard Vienna-KkU against a certified reference material (IRMM REIMEP-18 A). The paper combines newly measured and previously published data to provide a survey of anthropogenic and natural 236U in the environment, and compares the results with calculated estimates.

STEIER P.;  BICHLER Max;  FIFIELD L. Keith;  GOLSER Robin;  KUTSCHERA Walter;  PRILLER Alfred;  QUINTO Francesca;  RICHTER Stephan;  SRNCIK M.;  TERRASI Philippo;  WACKER Lukas;  WALLNER A.;  WALLNER G.;  WILCKEN Klaus M.;  WILD Eva Maria; 

2008-07-04

ELSEVIER SCIENCE BV

JRC43107

0168-583X,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC43107,   

10.1016/j.nimb.2008.03.002,