Gas Migration in Geological Facilities

Many of the radioactive waste disposal concepts need to consider the generation, fate and transport of gas. Gas can be generated through a variety of processes and the presence of significant quantities of gas can alter the hydraulic evolution of the repository as well as providing a potential mechanism for gaseous radionuclides to be transported over significant distances. In QPAC the multi-phase flow module provides a capability to model the movement of water, multiple gas phases, water vapour and the dissolution of gases in the water.

One project where this capability has been used recently is in Quintessa's support to the Nuclear Decommissioning Authority's participation in the EC FORGE project. FORGE is an integrated, multi-disciplinary project that examined: gas flow mechanisms; the long-term integrity of seals especially for gas flow along interfaces; the role of the Excavation Damaged Zone (EDZ) as a conduit for preferential flow; and laboratory to field up-scaling.

In particular, Work Package 1 (WP1) was concerned with the development and modelling of benchmark problems. The aim of WP1 was less a comparison of numerical codes than an attempt to answer some key PA-related questions such as what are the relative roles of the EDZ, interfaces and backfill for gas migration at the disposal facility scale. The first benchmark case was concerned with gas generation and transport in and around a disposal cell shown schematically below. The processes in the QPAC model included the time dependent release of gas and the subsequent multi-phase flow of water and gas throughout the system.

Module and Cell Scale

The figures below show the modelled system together with gas saturations after 1000 years, colour coded on a logarithmic scale, demonstrating the potential importance of interface flow. Work was carried out on key issues including upscaling, mechanical coupling and the modelling of interface flows.  A key aspect of the QPAC work has been to understand how simple models can be made (focussing on processes and spatial discretisation), while retaining the behaviours of the model.

Building on this work, two further benchmarks have been considered; a 'module' of 100 disposal cells (see Figure above) and a 'whole repository' model consisting of 10 modules and the surface access engineering.  QPAC has been applied successfully to produce stable, sensible results for these much larger models and through use of innovative discretisation approaches, have managed to retain explicit representation of individual disposal canisters, even at the whole-repository scale.

Gas SaturationsGas Saturation