PREPARATION OF LARGE-SIZE DRIED SPIKE IRMM-1027G: HOW WELL CAN WE CERTIFY THE FINAL CONCENTRATIONS?

The application of large sized spikes for isotope dilution measurements of concentrated solutions of uranium and plutonium, such as found in dissolved nuclear fuels, is accepted now as the method of choice. Experience, in particular at the on-site laboratories at La Hague and Sellafield, has shown that reliable measurements by IDMS can be made of the uranium and plutonium isotopic amount contents in such solutions with these spikes. About 1000 spikes are produced and certified at IRMM per year. During the past years the methods of production have been refined to take into account the needs of the analysts at the reprocessing plants. Very high demands are made on the spikes and the amount in each spike vial must be certified for U and Pu with an expanded, combined relative uncertainty of less than 0.1% (k=2). For this reason, the solutions are made by dissolving pure metals. In this way the lowest uncertainties can be achieved, even though the amount of certified material in the form of metal in the world is limited. In the campaign reported here, a discrepancy was found between the values of the Pu content expected from the mass of the metal (MP2) dissolved and that found by verification IDMS measurements performed at IRMM and at ITU on a sub-set of the spikes. The discrepancy has led to a large number of independent IDMS measurements being performed and finally the Pu amount content was certified using these measurements. Following this experience, th