Water is the most important reacting agent that directly controls radionuclide release from a nuclear waste repository to a human-accessible environment. In this paper, we present a water balance model to calculate the amount of water that can accumulate inside or percolate through a breached waste package in Yucca Mountain repository environments as a function of the temperature and relative humidity in the surrounding waste emplacement drift, the rate of water dripping from seepage, the area of breaches on the waste package, and the extent of waste degradation. The model accounts for sheet flows created as water drips fall onto the waste package surface, water vapor diffusion across waste package breaches, and water vapor equilibrium with unsaturated porous corrosion products. Preliminary model simulation results indicate that a breached waste package may maintain a large part of its barrier capability, and probably <1% of the total seepage flux impinging on the waste package surface can enter the package. Vapor diffusion of water through the breaches can be as important as liquid water flow into the waste package. Waste degradation reactions can consume a significant fraction of water entering the waste package. The water saturation inside waste packages will be low (<0.5), and the advective water flux out of a waste package will be small (with the mean value <0.5 [script l]/yr per package) over a wide range of seepage rates considered (1 to 1000 [script l]/yr). Furthermore, the ionic strength of in-package water will remain relatively high for the first 10000 yr, which will likely destabilize colloid suspensions and limit colloid releases.