MEASUREMENT OF THE SORPTION OF ACTINIDES ON MINERALS USING MICROANALYTICAL TECHNIQUES

The use of advanced surface-analytical techniques to study the sorptio n of the actinides uranium and plutonium on to rocks and their consist uent minerals, in the context of radioactive waste disposal, is descri bed. Nuclear microprobe analysis was used to quantify the extent of so rption of actinides via Rutherford back-scattering (RBS); data on the minerals on which sorption had occurred were provided by particle-indu ced X-ray emission. Both surface and sub-surface concentrations of act inides were measurable. Secondary-ion mass spectrometry (SIMS) was use d to measure qualitatively the distribution of sorbed actinides and th eir 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 e xcellent spatial resolution, samples can be analysed both quantitative ly and with high spatial resolution. Concentrations of uranium and plu tonium sorbed on to minerals can be routinely determined with sensitiv ities down to 1 ng cm-2. The data obtained are used to identify the mi nerals in a rock that are important for actinide sorption.