Year
2002
Abstract
The measurement of isotopic ratios of fissile materials such as uranium and plutonium in the small amounts found typically in environmental samples places large demands on the analytical laboratories. The samples sent out in NUSIMEP-2 consisted of dried uranium nitrate which needed little processing before measurement but the NUSIMEP-3 samples consisted of low concentrations of uranium in saline solutions. These solutions needed chemical handling in most cases before measurement and were therefore a much more difficult test for the analysing laboratories. The uranium samples originated as isotopic mixtures of UF6 and the n(235U)/n(238U) ratios were certified by measurements in the gas phase. The minor isotopic ratios were certified subsequently by TIMS after transforming the uranium fluorides to nitrates in solution. The basic uranium solutions were then diluted in a series of steps finishing finally as saline solutions with concentrations of the salts matching sea and other saline waters and uranium concentrations of between 2 to 5 ppb. The blank uranium concentrations of the saline matrices and all diluting solutions were measured by ICP-MS using a highly enriched 233U spike (IRMM-058). The Ultra-Clean Chemical Laboratory at IRMM was extensively used to eliminate crosscontamination by other sources of uranium during the dilution and ampouling. The methods of preparation and certification of the four NUSIMEP-3 solutions are described and some results from participants shown.