Safe geological storage of highly active waste relies upon a series of barriers providing containment. In several repository concepts in magmatic [1] and sedimentary [2] rock, bentonite is used as a buffer or backfill due to its favorable hydraulic characteristics and sorptive properties. A sound understanding of the processes occurring in all barriers is crucial for formulating design and performance requirements. The focus of the current contribution is one such complex process occurring in the repository near-field, the re-saturation of the engineered barriers. This triggers bentonite swelling, which is a prerequisite for tight seals around canisters and in drifts. In the numerical simulation of the repository system for safety and performance analysis, the prediction of the swelling and homogenization behavior of bentonite is commonly handled using models based on the Theory of Porous Media (TPM) [3] and poromechanical theories based on Biot’s work [4]. In constitutive theory, usually a bijective relation between saturation and suction is given by different retention models such as that by van Genuchten [5]. Codes using such retention models are pushed to interpretative limits of application at lower water contents due to the nature of retention in these materials at high suctions. The origin and nature of this problem will be discussed and a new model with dual porosity and a separation of physically distinct water retention mechanisms will be presented as a possible solution. The focus of the presentation will be on the model formulation and verification using simple tests relevant in the repository context. Behavior naturally arising from the new model formulation, e.g. hysteresis in retention behavior, will be investigated and discussed. Aspects of thermodynamic consistency and the expandability of the model to include additional driving forces will be mentioned.