MEASUREMENT OF THE SORPTION OF ACTINIDES ON MINERALS USING MICROANALYTICAL TECHNIQUES

The use of advanced surface-analytical techniques to study the sorption of the actinides uranium and plutonium on to rocks and their consistuent minerals, in the context of radioactive waste disposal, is described. Nuclear microprobe analysis was used to quantify the extent of sorption of actinides via Rutherford back-scattering (RBS); data on the minerals on which sorption had occurred were provided by particle-induced X-ray emission. Both surface and sub-surface concentrations of actinides were measurable. Secondary-ion mass spectrometry (SIMS) was used to measure qualitatively the distribution of sorbed actinides and their penetration rates into minerals. The equipment used at Harwell is described. Complementary use of both techniques in parallel is highly advantageous; RBS is used to quantify actinide surface loadings, with limited lateral and depth resolution, but, allied to SIMS, which has excellent spatial resolution, samples can be analysed both quantitatively and with high spatial resolution. Concentrations of uranium and plutonium sorbed on to minerals can be routinely determined with sensitivities down to 1 ng cm-2. The data obtained are used to identify the minerals in a rock that are important for actinide sorption.