The development and use of T2GGM: a gas modelling code for the postclosure safety assessment of opg's proposed l&ilw deep geologic repository, Canada

As part of the postclosure safety assessment of Ontario
Power Generation’s (OPG’s) proposed Deep Geologic
Repository (DGR) for Low and Intermediate Level Waste
(L&ILW) at the Bruce site, Ontario, a Gas Generation Model
(GGM) has been developed and used to model the detailed
generation of gas within the DGR due to corrosion and
microbial degradation of the organics and metals present.
The GGM is based on a kinetic description of the various
microbial and corrosion processes that lead to the generation
and consumption of various gases. It takes into account the
mass-balance equations for each of the species included in the
model, including three forms of organic waste (cellulose, ionexchange
resins, and plastics and rubbers), four metallic waste
forms and container materials (carbon and galvanised steel,
passivated carbon steel, stainless steel and nickel-based alloys,
and zirconium alloys), six gases (CO2, N2, O2, H2, H2S, and
CH4), five terminal electron acceptors (O2, NO3
-, Fe(III), SO4
2-,
and CO2), five forms of biomass (aerobes, denitrifiers, iron
reducers, sulphate reducers, and methanogens), four types of
corrosion product (FeOOH, FeCO3, Fe3O4, and FeS), and water.
The code includes the possibility of the limitation of both
microbial and corrosion reactions by the availability of water.
The GGM has been coupled with TOUGH2 to produce
T2GGM; a code that models the generation of gas in the
repository and its subsequent transport through the geosphere.
T2GGM estimates the peak repository pressure, long time
repository saturation and the total flux of gases from the
geosphere.
The present paper describes the development of T2GGM
and the numerical modelling work undertaken to calculate the
generation and build-up of gas in the repository, the two-phase
exchange of gas and groundwater between the repository and
the surrounding rock, and between the rock and the surface
environment. The results have been used to inform the safety
assessment modelling.