In recent years, the authors and co-workers have developed a 3D finite element model for coupled two-phase hydro-mechanical problems (THGM) in fractured rock masses. In this model, zero-thickness interface elements are used to explicitly represent the THGM behaviour of discontinuities (fractures, joints, bedding planes, etc.) in the rock mass. The model has been implemented in the finite element code DRAC5, which is equipped with fracture-based interface elements and MPI parallel capabilities [1]. Originally, the two-phase model was verified with elastic interfaces and used the well-known empirical water retention curve (WRC) of Van Genuchten including a simple reduction factor on the WRC parameters based on the fracture opening [2]. In this presentation, an alternative approach is proposed, based on the exponential WRC for porous medium proposed by Rodríguez [3] which relies on the use of the pore size distribution curves where the pore structure of the material is taken into account. The main assumptions are that (a) for a virgin discontinuity the “initial” water retention curve is the same as that of intact rock or porous medium, and that (b) the effect of the mechanical opening of the fracture may be introduced via a reduction factor on the WRC parameters. The new water retention curve for interfaces also takes into account the loss of tensile strength and the model considers the possibility of mechanical closure so that in a limit state of very high compression, when the fracture opening would vanish totally, the water retention curve parameters would recover their initial values [4]. The results of an isothermal hydrogen injection and subsequent extraction in an initially fully-saturated fracture are presented. A linear damage-frictional mechanical law is considered for the interfaces. The results show perfect agreement with the closed-form solution. REFERENCES [1] Garolera, D., “Zero-thickness interface elements in petroleum geomechanics: sand production and hydraulic fracture” PhD thesis, Universitat Politècnica de Catalunya (2017). [2] L. Barandiarán, j. Liaudat, C. M. López and I. Carol. Numerical modelling of two-phase flow in fractured rock masses using zero-thickness interface elements. Presentations and videos to 16th International Conference on Computational Plasticity (COMPLAS 2021) 2021. Pages 1-12. https://www.scipedia.com/sj/complas2021 [3] Rodríguez, M., “Análisis numérico termi-mecánico y termo-hidrométrico del hormigón expuesto