Realistic modeling approach for tracer migration and retention in fractured crystalline rocks from the Grimsel Test Site

Abstract:

Radionuclide (RN) transport in fractured crystalline rocks can be conceptualized by a dual-porosity model where RNs are transported by advective water flow through a fracture and are retarded by diffusion and sorption into the surrounding rock matrix. In order to develop a realistic model and reliable parameters for long-term safety assessments of geological disposal, it is necessary to understand and quantify the effects of heterogeneities found around the fractures on RN transport processes. The different types and scales of heterogeneities that must be considered for the RN transport in the natural fracture systems include: (1) heterogeneous distribution of mineral and pore in the rock matrix, (2) heterogeneity in mineral and pore distribution near the fracture surface, (3) heterogeneous flow distribution in the complex channel structures found along fracture openings.

This paper presents a comprehensive approach developed for coupling laboratory tests, microscopic observations and modeling in order to understand and quantify tracer transport processes occurring in natural fracture, using different types of fractured granodiorite sample from the Grimsel Test Site (GTS), Switzerland. Laboratory tests including through-diffusion, batch sorption and flow-through tests using five tracers with different retention properties indicated that tracer retention was consistently in the sequence of HDO ≈ Se < Cs < Ni < Eu. Microscale heterogeneities around the fracture were clarified and quantified by coupling X-ray computed tomography and electron probe microanalysis. Realistic model incorporating heterogeneities around the fracture, and their properties such as porosity, sorption and diffusion parameters, provided a much better interpretation for breakthrough curves of all tracers, measured in flow-through tests. Mechanistic understanding and detailed modeling considering the effects of heterogeneities around a natural fracture should improve confidence for the safety assessment in fractured crystalline rocks.

*This work was part of “The project for validating near-field assessment methodology in geological disposal” supported by the Ministry of Economy, Trade and Industry of Japan.