The use of advanced surface-analytical techniques t o 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 t o quantify the extent of sorption of actinides via Rutherford back-scattering (RBS); data on the minerals o n which sorption had occurred were provided by particleinduced X-ray emission. Both surface and su b-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, w i t h 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 o n t o minerals can be routinely determined w i t h sensitivities d o w n to 1 ng cm-*. The data obtained are used to identify the minerals in a rock that are important for actinide sorption.

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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) have been used in parallel to determine qualitatively and quantitatively the distribution of uranium and plutonium sorbed onto a variety of rocks and their constituent minerals. This identified which minerals are important in the sorption of uranium and plutonium for a given rock. Data on uranium and plutonium loadings on minerals are presented. Strath Halladale granite from the Altnabreac district, Caithness, Scotland [7]:This granite is composed primarily of quartz, alkali feldspar (orthoclase), plagioclase feldspar and biotite. However, the rock has been subjected to low grade regional metamorphism resulting in the alteration of the feldspars to secondary phases such as dioctahedral micas (muscovite, sericite) and clays. Chlorite (replacing biotite) and epidote are common accessory phases. Ballachulish diorite from Glen Coe, Scotland [8]:The diorite sample is a phase one grey quartz diorite from the Ballachulish igneous complex. It is composed primarily of oligoclase-andesine plagioclase feldspar with poikilitic alkali feldspar (microcline) and quartz. Biotite was the main mafic phase. Common accessory minerals included ilmenite, iron oxides, apatite, zircon and sphene. Both plagioclase and biotite showed signs of alteration to sericite and chlorite respectively. Whin Sill dolerite from High Force, Teeside, England [9]:This rock is a well characterized quartz dolerite. The sample used was composed of labradorite plagioclase feldspar, alkali feldspar, augite and quartz. Biotite and skeletal ilmenite are the main mafic phases. In places the sample had been extensively altered to chlorite (replacing augite and biotites) and secondary clays (replacing feldspars). St Bees sandstone from Cumbria, England [10]: Two sandstone samples were used during the study. One (SBS1) was characterized by a vein of ferroan dolomite (Ca(Mg,Fe)(C0 3 ) 2 ) cutting across the sample. The second (SBS2) was characterized