A Pitzer ion-interaction model for concentrated aqueous solutions was added to the reactive multiphase flow and transport code TOUGHREACT. The model is described and verified against published experimental data and the geochemical code EQ3/6. The model is used to simulate water-rock-gas interactions caused by boiling and evaporation within and around nuclear waste emplacement tunnels at the proposed high-level waste repository at Yucca Mountain, Nevada. The coupled thermal, hydrological, and chemical processes considered consist of water and air/vapor flow, evaporation, boiling, condensation, solute and gas transport, formation of highly concentrated brines, precipitation of deliquescent salts, generation of acid gases, and vapor-pressure lowering caused by the high salinity of the concentrated brine.