Predicting mineral alteration at the potential nuclear waste repository at Yucca Mountain, NV with reactive transport modeling

摘要

We present the results of a reactive transport modeling study that examines the sensitivity of mineral evolution to temperature, condensate properties and water flow conditions anticipated to occur in the potential Yucca Mountain nuclear waste repository site. We have investigated a key aspect of the thermal- hydrological repository system, the interaction of condensate water flowing through fractures at the site. Future simulations will include mineral changes at the boiling front, a process not considered in this study. Our simulations show that the principal mineral changes that occur are dissolution of the initial phases (mainly calcite, but also feldspars and clay) and formation of zeolites and hydrated aluminous phases. Major differences in mineral alteration occur over very short distances. Fracture mineral alteration is clearly coupled with the thermal hydrological environment. Fracture porosity is enhanced in those areas where condensate forms, an upper, lower temperature condensate zone and a lower, high temperature condensate zone. The locations where mineral precipitation occur and porosity decreases depend on the integrated residence time of the solution on the fracture surface. In general, mineral precipitation is expected to occur in the region -150 m above the water table. Whether the changes in porosity are significant enough to modify thermo-hydrological behavior remains to be evaluated.