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Abstract

Choosing buffer materials with the required engineering and sorption characteristics for radwaste disposal is very important. This work investigates the relationships between engineering characteristics (plastic index; PI) and sorption properties (distribution ratio; Rd) for two mixed buffer materials that are composed of bentonite/quartz sand and bentonite/laterite, to provide an overall functional evaluation of buffer materials. Se is the nuclide of interest, and both synthetic groundwater (GW) and seawater (SW) were employed in batch sorption experiments. Deionized water (DIW) was used in engineering property tests. SW and GW were also used to evaluate the effects on PI. The results indicate that PI was proportional to the bentonite content of the tested buffer materials, independently of the solution used. The coagulation and flocculation effects lead to the ordering PIDIW > PIGW > PISW. The sorption of Se increased with the bentonite content in bentonite/quartz sand mixtures, and the sorption fraction increased with the laterite content in bentonite/laterite mixtures. These findings demonstrate that the sorption of Se on soil follows the order, laterite soil > bentonite > quartz sand, regardless of whether GW or SW was used. Hence, adding laterite to the mixture improved the sorption of the anionic form of Se. A mixed material with a PI value of approximately 50 to 70 in GW and SW effectively balances the engineering needs with the desired chemical properties. A bentonite content of 50% and the addition of both laterite and quartz sand may be optimal for buffer materials, and is thus worthy of further study. This work further indicates that the PI values of both mixtures followed the additivity rule in SW and GW, but not in DIW. The Rds value of Se indicated the good additivity of the bentonite/laterite mixtures in both SW and GW and of the bentonite/quartz sand mixtures in SW, but not of the bentonite/quartz sand mixtures in GW. These results provide useful information on the choice of the composition of the buffer material, and for use in the evaluation of the overall Rd and PI of the mixture from the Rd and PI of each buffer component.

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