TOUGH2-GRS: Movement of groundwater, gases and radionuclides
With TOUGH2-GRS, GRS researchers have further developed a simulation software that can be used to simulate the hydraulic processes in the repository system in safety analyses. The hydraulic processes include, for example, the flow of groundwater and the transport of radionuclides dissolved in it. In addition, the TOUGH2-GRS code also calculates the movement of gases.
How do water and gas get into the repository?
Water from rain and water bodies penetrates into deep rock strata. Groundwater is usually not found in intact salt deposits since salt is impermeable to water. The situation is different with claystone. Here, water is a component of the host rock and is naturally found in the rock pores. Claystone can nevertheless effectively retain dissolved radionuclides because it is very dense and has retentive chemical properties. With the TOUGH2-GRS code, these retention processed are also modelled.
Gases can occur during the corrosion of waste containers or during the decomposition of organic substances, which are often a component of intermediate- and low-level radioactive waste. Gas formation is strongest in the first 10,000 years after waste emplacement. TOUGH2-GRS is able to predict the effect of the gases produced in the repository. This prediction is important for assessing the safety of a repository since high pressures inside the repository caused by gases can impair its retention function.
Gases and groundwater can also flow together in the rock pores; a process referred to by scientists as two-phase flow and which is also modelled in TOUGH2-GRS. This makes it possible, for example, to investigate whether polluted water is displaced by the resulting gases and thus reaches the biosphere more quickly.
Application of TOUGH2-GRS
TOUGH2-GRS is typically applied for the performance of safety analyses in the context of disposal. The simulation results from TOUGH2-GRS can be used to demonstrate long-term safety in the site selection procedure. GRS has already used TOUGH2-GRS in various projects. One example is the ZIESEL project (two-phase flow in a saline repository using the Morsleben repository ERAM as an example).
Development history of the simulation code
The source code TOUGH2 (“Transport Of Unsaturated Groundwater and Heat”) was developed at the Lawrence Berkeley National Laboratories in the USA. There, the code is not only used for the safety assessment of repositories but also in other fields of work, such as in the oil and gas industry.
GRS has been using and extending the code for process and long-term safety analyses for repositories since 1991. Over the years, GRS has extended the code by various components. The TOUGH2 source code further developed by GRS is referred to as TOUGH2-GRS. The newly developed version TOUGH2-MP-GRS can be run on several processors (computer clusters) to calculate large model areas faster.
- Source code: TOUGH2, TOUGH2-MP of the Lawrence Berkeley National Laboratory, California, USA
- Use with EOS module: EOS7
- Further developments of GRS: regarding TOUGH2-GRS, TOUGH2-MP-GRS
- Field of application: laminar flow of liquids and gases in porous media, transport of radionuclides and heat
- Programming language: Fortran
Extensions by GRS:
- compaction of crushed salt backfill in converging rock salt cavities
- corrosion of seals by fluids
- gas formation and water consumption due to metal corrosion
- time-dependent changes in seal permeability
- gas transport through opening of microcracks
- changes in the properties of the main gas component
- pressure limitation due to gas infiltration
- decay, advection, diffusion, anion exclusion for radionuclide chains
- new functional dependencies of permeability and capillary pressures
- improved speed of equation solvers
- plausibility checks for input
Project highlights Disposal
The main objective of THEREDA is the creation of a comprehensive, internally consistent thermodynamic reference database for geochemical modelling, with the work focusing on the special conditions in the near-field of a repository for radioactive waste in Germany.