Microbial gas-generation rates under expected WIPP conditions and their effects on repository performance - 075

Gas generation from microbial consumption of cellulosic, plastic, and rubber (CPR) materials in the Waste Isolation Pilot Plant (WIPP) could affect postclosure repository pressure and pH by the production of CO₂ and other gases. Acidic pH could increase actinide solubilities and thus increase radionuclide releases to the accessible environment. To prevent this, the DOE is emplacing sufficient MgO in the repository to sequester all CO₂ that could be generated. Therefore, an understanding of the range of possible microbial gas-generation rates and the products of these reactions is essential to predict repository performance. Microbial gas-generation rates for the consumption of papers, polyethylene, PVC, neoprene, and hypalon under expected WIPP conditions were determined from 10 years of experimental data gathered at Brookhaven National Laboratory (BNL). In these experiments, CPR materials were placed in serum bottles with and without microbial inocula and nutrients (N, P), and incubated at 30 deg. C under initially oxic or anoxic conditions. The inocula were prepared from microbial samples obtained from the WIPP repository, local salt lakes, and laboratory dust. A WIPP brine was added in such a way to produce either an inundated or humid environment. In experiments containing plastics and rubbers, a subset of these materials was first irradiated with accelerated electrons (low (∼600 Mrad) and high (∼5,000 Mrad) absorbed doses). Measurements were performed on samples for up to 10 years. Pressure (total gas production) and concentrations of various gases (CO₂, CH₄, N₂, N₂O, and H₂S) were monitored. Aqueous metabolite concentrations, microbial DNA, and microbial populations were analyzed. (authors)