Development of a Gas-Generation Model for the Waste Isolation Pilot Plant

Design-basis transuranic (TRU) waste to be emplaced in the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico may generate significant quantities of gas, which may affect the performance of the WIPP with respect to regulations for radioactive and/or chemically hazardous waste constituents. We are developing a model to predict gas generation in WIPP disposal rooms during and after filling and sealing. Currently, the model includes: (1) oxic and anoxic corrosion of steels and other Fe-base alloys, including passivation and depassivation; (2) microbial degradation of cellulosics with O2, NO3-, FeO(OH) , SO42-, or CO2as the electron acceptor; (3) α radiolysis of brine; (4) consumption of CO2and, perhaps, H2S by Ca(OH)2(in cementitious materials) and CaO (a potential backfill additive). The code simulates these processes and interactions among them by converting reactants (steels, cellulosics, etc.) to gases and other products at experimentally observed or estimated rates and plotting temporal reaction paths in three-dimensional phase diagrams for solids in the Fe-H2O-CO2-H2-H2S system.