Simulation of neptunium migration as a function of redox conditions and clay mineralogy

Migration of neptunium, i.e. ²³⁷Np, a minor component of high-level nuclear waste, is in focus regarding the safety of nuclear waste disposal sites due to its long half-life and radiotoxicity. Opalinus Clay, the preferred host rock in Switzerland and also an option in Germany, is regarded as a reference formation with diffusion as the dominant transport process additionally retarded by sorption. One-dimensional diffusion simulations are conducted with PHREEQC to quantify neptunium migration. Sorption on clay minerals is integrated using surface complexation models. Numerical simulations are based on two diffusion experiments with the same setup, but one order of magnitude difference in transport parameters attributed to the core samples which stem from different rock facies. In the laboratory experiments, Np(V) was applied via a synthetic oxygen free pore water with neutral pH. The redox conditions are controlled by pyrite, while the pH of the system is buffered by calcite. The experiment of Fröhlich et al. (Radiochimca Acta, 2013, 101, 553-560) defined the numerical setup applied to the experiment of Wu et al. (Environmental Science & Technology, 2009, 43, 6567-6571). Our results indicate that neptunium migration is mainly controlled by pyrite oxidation and the associated reduction of Np(V) to Np(IV), while clay mineral quantities have only a minor impact. Since neptunium speciation and hence migration is very sensitive to oxidation and reduction reactions, redox conditions need to be accurately controlled and monitored in the laboratory if transport parameters are determined to be applied in the context of safety assessments.