ANALYSIS ON EVOLVING ENVIRONMENTS OF ENGINEERED BARRIERS OF HIGH-LEVEL RADIOACTIVE-WASTE REPOSITORIES DURING THE 1ST 1000 YEARS

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