The Use of Surface Analytical Techniques to Measure the Loadings of Uranium and Plutonium Sorbed Simultaneously from Sol
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THE USE OF SURFACE ANALYTICAL TECHNIQUES TO MEASURE THE LOADINGS OF URANIUM AND PLUTONIUM SORBED SIMULTANEOUSLY FROM SOLUTION ONTO ROCKS J A BERRY, H E BISHOP, M M COWPER, P R FOZARD and J W McMILLAN AEA Technology, 220 Harwell, Oxfordshire, UK
ABSTRACT
Small polished blocks of granite, diorite and dolerite were immersed in solutions containing uranium and plutonium at equal initial concentration. The samples were analysed by the advanced surface analytical techniques of secondary ion mass spectrometry (SIMS) and nuclear microprobe analysis. The results show that both actinides sorb onto the same minerals in the three rocks. However, SIMS data show that significantly more uranium was sorbed than plutonium. INTRODUCTION
The movement of radionuclides from an underground radioactive waste repository through the geosphere, will be significantly retarded by sorption onto minerals in the rocks along groundwater flowpaths. Of particular importance is the behaviour of long-lived radioelements such as uranium and plutonium. The quantification of sorption behaviour onto minerals along fiowpaths is important in the performance assessment of a proposed radioactive waste repository. However, in flowpaths surrounding a repository, the groundwaters may contain a "cocktail" of radioelements that could compete with each other for sorption sites. An assessment of competition between radioelements such as uranium and plutonium is important in performance assessment models. In the literature, many data regarding the extent of radioelement sorption onto geological materials, have been obtained by standard batch sorption (crushed rock) or intact rock sample techniques 1 ,2,3 , which measure a simple distribution (RD or KD) between the radioelement remaining in solution and the radioelement sorbed onto the solid phase. However, this approach generally does not identify the minerals of importance and their mechanistic role in sorption processes. By immersing well-characterised intact rock samples in solutions containing the radioelements of interest, the advanced surface analytical techniques of nuclear microprobe analysis (Rutherford backscattering (RBS) and particle induced X-ray emission (PIXE)), and secondary ion mass spectrometry (SIMS) can be used to obtain definitive information regarding the minerals in a rock that are important for radioelement sorption from solution. The data obtained, such as whether sorption is confined to the mineral surface or if there is diffusion of the radioelement into the mineral lattice, provide information on the sorption mechanism and may be used to aid intrepretation of results from routine sorption experiments. Individually, nuclear microprobe analysis and SIMS are powerful techniques for locating and measuring radioelements sorbed on rock surfaces. However, using the two techniques in parallel, their complementary qualities allowed quantitative analysis of radioelements such as uranium and plutonium to be measured simultaneously. The poor elemental selectivity of RBS for high atomic number elem
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