Evaluating actinide sorption to graphite with regards to TRISO repository performance

Graphite has the potential for inclusion in nuclear waste for disposal in waste repository settings. Implementation of High Temperature Gas-cooled Reactors contributes to this potential through use of TRISO fuel, if direct disposal of the graphite matrix surrounding the fuel is employed. The inclusion of the large mass and volume in the TRISO fuel waste form differs significantly from used light water reactor fuel waste forms, requiring new performance models to describe the behavior in a repository setting. The purpose of this study is to evaluate the potential for the graphite to improve actinide, specifically uranium and neptunium, retardation from the waste form.;A review of the literature exposed no specific data on neptunium interactions with graphite, so experimental study was employed. Uranium and neptunium sorption behavior was evaluated across a range of conditions through batch experiments. Solid/liquid ratios and temperature experiments were performed to evaluate possible effects on uranium sorption. Temperature was found to have a significant impact on measured sorption. Neptunium metal concentrations, pH range, counterion concentration experiments were performed for neptunium. Neptunium sorption appears to follow a linear isotherm, at the concentration range used. The partitioning to graphite was weakly influenced by pH, with a maximum Kd of 4.6 (ml/g). The ionic behavior showed that both the specific counterion, when switched from Cl- to ClO 4-, and concentration inhibits sorption,;The desorption kinetics were evaluated for neptunium using batch experiments, revealing close to negligible desorption once sorbed. Based on the results, even though low sorption occurs for neptunium, the negligible desorption allows graphite to significantly impact neptunium transport with respect to graphite mass. Surface complexation models were evaluated. Although a Triple Layer (TL) model was suggested for use, more data is needed (counterion influence) before implementation can be accomplished.