No abstract

Data scatter found in a database of Cs sorption coe‹cients (K d ) was investigated to demonstrate the usage of the mechanistic sorption model. A large diŠerence found between the data given by the batch sorption and diŠusion experiments was of great interest and the analyses of the pH dependence of the K d by adopting either PHREEQC or the revised version of MINEQL had been carried out to determine the reason for the disagreement among the sorption data. Both of them were able to treat the ion exchange reaction and the surface complexation simultaneously. In particular, the latter was a largely modiˆed form of the original version suitable for the compacted form. The same parameter set was used in the analyses even under extremely diŠerent experimental conditions. The results indicated that the K d for both cases could be reproduced by the model calculation and the perturbation of environmental condition such as the Na concentration, and the pH has a strong in‰uence on the data stability. The results also suggested that the scatter of the K d of Cs could be understandable and minimized through the analysis by adopting the mechanistic sorption models.

Scatter of data found in a sorption database may bring about possible risk dilution in the result drawn by the probabilistic safety analysis unless inappropriate data were carefully examined and excluded if necessary. To reduce such scatter, the advantage of data analysis by adopting theoretical sorption models was demonstrated. The sorption coe‹cients (K d ) of various combinations of bentonite, minerals and rocks with elements in the database were examined atˆrst to extract the data sets that have enough information for the analysis using the mechanistic sorption models such as the ion exchange and the surface complexation. The K d of Cs on bentonite was selected as an example and the factors dominating the data scatter were found to be the concentrations of Cs and coexisting ions. The procedure to estimate the parameters that were not given in the database, such as the selectivities of ion exchange reactions among Cs and coexisting ions, and the cation exchange capacity were also suggested to assist in the model analysis. Although the K d of Cs varied in several orders of magnitude, the scatter could be reduced within the same order of magnitude after appropriate interpretation of the reason for the scatter and data selection.