The isotopic composition of natural uranium samples—Measurements using the new n( 233U)/ n( 236U) double spike IRMM-3636

Abstract

The new double spike isotope reference material IRMM-3636 was prepared gravimetrically using highly enriched 233U and 236U isotopic material. The n( 233U)/ n( 236U) ratio was adjusted to about 1:1 and certified with an extended relative uncertainty of 0.016% (coverage factor k = 2). By mixing the n( 233U)/ n( 236U) double spike with a sample and applying internal mass fractionation correction, the n( 235U)/ n( 238U) ratio of a given sample can be determined with the highest possible precision and accuracy. Due to the low relative abundance of 234U for the IRMM-3636 double spike (<0.00037%) measurements for n( 234U)/ n( 236U) of samples close to natural isotopic composition ( n( 234U)/ n( 236U) ≅ 5.4 × 10 −5) are also possible with a rather small correction and without significant increase in the uncertainty. The IRMM-3636 double spike was applied for measurements of natural uranium ore samples. The results for n( 235U)/ n( 238U) ratios were compared to earlier data obtained using UF 6 gas sources mass spectrometry and to the most recent IUPAC recommended value, published in 2003. The results show that the historically used “consensus value” of 1/137.88 (earlier IUPAC value) cannot be considered representative for natural uranium samples.