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Fractured crystalline rocks are under consideration by several countries 9 as host formations for high-level nuclear waste repositories. The redox evolution in 10 these host rock formations is an important issue for the stability and safety of these 11 disposal sites. If, for example, during a glaciation/deglaciation event, oxygen-rich 12 glacial meltwater penetrates to the depth of the planned repository, some of the 13 engineered barriers would be adversely affected. Moreover, oxidizing conditions 14 would increase the solubility and mobility of many radionuclides. Reactive trans-15 port simulations, which are typically used to assess the redox buffering capacity of 16 these host rock formations, are computationally demanding and, thus, calculations 17 for e.g. the evaluation of oxygen penetration are usually carried out over simplified 18 geometries and the heterogeneity of the site, both physical (e.g. variability in the 19 groundwater flow field and the kinematic porosity) and mineralogical (e.g. vari-20 ability in the abundance of Fe(II)-bearing minerals), is usually represented in a 21 simplified fashion. Here, we show how a recently developed numerical framework, 22 combined with High Performance Computing (HPC) technologies, allows the full 23 geometrical, physical and mineralogical complexity of the site under study to be 24 efficiently included in reactive transport calculations. A synthetically-generated 25 realistic three-dimensional fractured medium is used to assess oxygen penetration 26 patterns and their dependency on both the hydrogeological conditions and the

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