Measurements of long-lived $^{121m}$Sn and $^{126}$Sn nuclides in Low and Intermediate Level Nuclear Waste
Résumé
In France, nuclear wastes are managed by the National Radioactive Waste Management Agency (ANDRA). Several repository sites have been built in order to accommodate nuclear waste packages. One is dedicated to wastes containing short-lived radionuclides with radioactivity at Low and Intermediate Level. Criteria for 143 radionuclides have been defined by ANDRA which guarantees the safety of the facility. Among this long list, Sn-121m and Sn-126 have to be declared as soon as their activities are over 10E-3 Bq/g. Both tin isotopes are fission products obtained according to thermal fission yields of 0.00003 % and 0.06 %, respectively. Sn-121m can also be produced by activation of tin present as an additive in zircaloys and as an impurity in inconels and steels used in the nuclear industry. Sn-121m decays with a half-life of 55 years whereas Sn-126 is a very long-lived radionuclide with a half-life estimated at around 10E5 years. Because of the long half-life and the unknown impact of high energy gamma emissions of Sn-126, ANDRA allows a very low Sn-126 activity limit (2.7 Bq/g) in waste packages. Due to their potential low activities in nuclear wastes, both tin nuclides need to be separated from the matrices and concentrated through chemical separations prior to any measurement. From a literature review, it can be emphasized that only a few radiochemical procedures have been published. So, the LASE (Operator Support Analyses Laboratory) laboratory of CEA Saclay (France) has developed a selective radiochemical method for their measurements in nuclear wastes. The procedure starts with a microwave digestion or alkaline fusion with the addition of a natural tin carrier and a 113Sn radioactive spiker. The following steps are constituted of ion exchange separations combined with co-precipitations and a highly selective ethylation step. The final purified fractions thus obtained are suitable for gamma-spectrometry analysis of Sn-121m and Q-ICPMS measurements of Sn-126. The protocol yield is evaluated by gamma measurement of 113Sn. The overall separation yield varies from 45 % to 70 % depending on matrix, with a detection limit of about 20 Bq/g for Sn-121m and 0.1 Bq/g for Sn-126, respectively. The presentation will first focus on the optimization and the validation of the procedure. The second part of the work will be addressed to its application to various types of intermediate level nuclear matrices (activated steels and ion-exchange resins).
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