CALCULATED COMPOSITIONS OF POREWATER AFFECTED BY A NUCLEAR WASTE REPOSITORY IN A TUFF GEOLOGIC ENVIRONMENT FROM 0 TO 10000 YEARS

Porewater compositions were estimated for an environment assuming that high-level radioactive waste has been stored for 10,000 years under g eologic conditions analogous to those at the Yucca Mountain site in Ne vada. The porewater compositions calculated with the EQ3/EQ6 geochemic al code are intended for use in preliminary performance assessments of borosilicate glass waste packages. The porewater compositions were ca lculated using water-rock interaction models that are loosely coupled with two time-temperature periods in the host rocks: a cooling period between 900 years and 3,000 years after repository closure and an isot hermal period from 3,000 years to 10,000 years. Significant changes in water composition are predicted to occur during the initial period of water-rock interaction; for example, the pH of the porewater increase s from 6.4 to 9.1. Constant porewater compositions are predicted durin g the isothermal period. The results suggest that major changes in por ewater composition will occur over a relatively short time frame and t hat these changes will presevere throughout the repository lifetime.