The choice of the Callovo-Oxfordian formation in eastern France for construction of a proposed repository for high-level, long-lived radioactive waste (HLW) is based primarily on the low hydraulic conductivity of the clay-rich host rock. This property is also intrinsically linked to a low capacity of the rock to evacuate the significant amounts of hydrogen gas generated over time by processes such as anoxic corrosion of metallic materials and radiolysis of organic waste. The effects of hydrogen production on the behavior and safety performance of the disposal system components must be evaluated for the operating and postclosure periods of the repository. In order to do this, numerical simulations using TOUGH2-MP were performed on a vitrified waste (HLW) disposal cell and its access drift, for the operating period. The objective was to investigate generation and transfer of hydrogen within and outside the disposal cell, coupled with the desaturation of the access drift near field due to the combined action of drift ventilation and the coupled behavior of dry air and hydrogen within the disposal cell. Particular attention was focused on the form of hydrogen (expressed or dissolved), total gas pressure buildup, degree of gas saturation, gas transport pathways, gas concentrations, and gas exchanges between the disposal cell and the access drift.

Simulation results show the validity of the conceptual assumption based on anoxic conditions in the useful part of the disposal system. The major part of the hydrogen comes to the access drift during the operating phase. Internal boundaries between interface zones and concrete lining are preferential pathways for the gas transfer.