No abstract

Sensitivity analyses based on an illustrative Performance assessment case study of a disposal concept for nuclear fuel waste have shown that radioisotopes of Se and Sn could have a significant effect on cumulative radioactive dose if they were to be transported through the geosphere without retardation. Static batch Sorption methods, coupled with 2" factorial experimental designs, were used to determine the extent to which Se and Sn can be sorbed by granite and goethite as a function of total dissolved sohds concentration, [TDS], natural fulvic acid concentration as dissolved organic carbon, [DOC], pH and, for the studies with Se, the Se concentration, [Se]. Aqueous speciation and the Saturation indices of solubility-controlling solid phases were estimated using the speciation code HARPHRQ with the HATCHES thermodynamic database.The experimental results indicated that Se Sorption on granite was low and not affected by changes in [DOC] or [TDS]. Increased [Se] and increased pH decreased sorption. For Se Sorption on goethite, the pH ränge was narrow but indicated that sorption decreased as pH increased. Increased [TDS] and [Se] lowered sorption on goethite, but changes in [DOC] had no effect on sorption. For Sn, increased pH, [TDS] and [DOC] decreased sorption on granite. For Sn sorption on goethite, increased [DOC] resulted in decreased sorption, but differences in [TDS] and pH had little consistent effect on sorption.fect the sorption of dissolved contaminants by geological materials [12][13][14], The experiments documented here were done to augment the database of sorption coefFicients for Se and Sn and to investigate the effect of various geochemical parameters on sorption by granite and goethite. Materials and methods SolidsThe solids used in these sorption studies were granite, obtained from the Lac du Bonnet bathoHth, Manitoba, and goethite, obtained from Biwabik, Minnesota. These materials were chosen for study as granite is being considered as a host rock tape for disposal of radioactive waste in many countries, and goethite is a common alteration mineral in water-bearing fractures. Both solids were crushed and wet-sieved to recover the 106-180 |im particle size for use in the sorption studies. The specific surface area of the granite was 0.18 m^ • g"' and that of the goethite was 2.43 m^ • g-', as determined by the BET method [15] using Nj. Both materials were analyzed by powder X-ray diffraction (XRD) and X-ray fluorescence (XRF) to determine their mineralogy and chemical composition. The XRD analyses of the goethite indicated that up to 25% quartz was present as an impurity in this material. Albite, K-feldspar, biotite and quartz were identified in the granite by XRD analysis. The chemical compositions of the soHds, as determined by XRF analyses, are given in Table 1.

Four researchers were asked to provide independent modeled estimates of the solubility of a radionuclide solid phase, specifically Pu(OH)4, under five specified sets of conditions. The objectives of the study were to assess the variability in the results obtained and to determine the primary causes for this variability.In the exercise, modelers were supplied with the composition, pH and redox properties of the water and with a description of the mineralogy of the surrounding fracture system A standard thermodynamic data base was provided to all modelers. Each modeler was encouraged to use other data bases in addition to the standard data base and to try different approaches to solving the problem.In all, about fifty approaches were used, some of which included a large number of solubility calculations. For each of the five test cases, the calculated solubilities from different approaches covered several orders of magnitude. The variability resulting firom the use of different thermodynamic data bases was in most cases, far smaller than that resulting from the use of different approaches to solving the problem.