Modeling and analysis of the migration of HTO and²³⁷Np in a fractured granite core at the Äspö hard rock laboratory

Abstract

Migration characteristics of HTO and 237 Np were analyzed through experiments performed with a fractured granite core in the Chemlab2 probe at the Äspö hard rock laboratory in Sweden. These experimental data covered the elution of HTO and 237 Np as well as information of retained actinides onto the fracture surfaces. The flow field was simulated with a two-dimensional variable aperture channel model. The mass transport process was described with a randomwalk method. Three types of basic migration processes were considered : advection, sorption and matrix diffusion. By a combination of these processes, specific effects of each process on transport were evaluated. The retardation of the neptunium were analysed quantitatively by the model. The results were compared to the experimental findings in terms of the elution curves and of distribution patterns on the fracture surface. The elution curves of HTO were used to determine the hydraulic properties in the fractured core such as the extent of a dispersion effect according to the flow rates. After termination of the migration test, the rock core was opened, and the retained radioactivities onto the fracture surfaces were measured to investigate an extend of 237 Np retardation and the retardation mechanism. A retardation factor was obtained by comparing the simulated results to the distribution of sorbed tracers along the flow path. The validity of sorption data from batch tests was also discussed for estimating migration of sorbing tracers.

Neptunium(V) transport in granitic rock: A laboratory scale study on the influence of bentonite colloids

“…6 the breakthrough of chloride can be seen to be governed by the flow velocity. As expected, the breakthrough occurs earlier for the faster flow rate, followed by a slight tailing at both flow velocities, which can be attributed to dispersion in the column as previously shown by Park et al (2012). The modeled breakthrough curves are in good agreement with the measured ones.…”

“…a difference of a factor of approximately 20. Other factors such as a slow sorption kinetic for neptunium(V) uptake by granite, as shown in Park et al (2012) could play a role in addition to a high reversibility of neptunium(V) sorption under flowing water conditions as was shown for neptunium(V) uptake by montmorillonite in our previous study (Elo et al, 2017). Slow sorption kinetics is supported by the results obtained at pH = 8, where more than 10 times higher sorption distribution coefficient is obtained for the 0.3 mL/h flow rate in comparison to the 0.8 mL/h one.…”