This paper describes the validity of two assumptions usually accounted for performance analyses of engineered barriers; nuclide release starts at 1, 000 yr after backfilling of packages, and the chemical environment around the package is kept under a reducing condition. Analyses by several computer codes such as TOUGH, PHREEQE and CHEMSIMUL were conducted to estimate the followings: the time at which the buffer material is fully saturated with water, geochemical conditions of the pore water, and hydrogen gas production due to package corrosion and to water radiolysis. Calculation results indicate, if bentonite is used as buffer material, that the buffer layer becomes saturated within several decades and that a reducing environment is accomplished due to mineral oxidation reactions in bentonite, with resulting in less corrosion of waste packages than expected from the previous experimental corrosion data. Hydrogen production due to package corrosion by the pore water is more dominant than that by steam corrosion or water radiolysis, and further discussion is still required for specifying the possible mechanical and chemical effects on the engineered barrier system due to produced hydrogen.