Technical Report NTB 00-10

SANTA-CHEM A Nagra-JNC co-developed hyperalkaline water-rock interaction code Code description and applications

The initial aim with SANTA-CHEM was to develop a code where the chemical system had to be wholly specified in the input files via a set of species and mineral reactions. The idea was that, by requiring more effort at the stage of describing the chemical system, problems arising from "rubbish in" resulting in "rubbish out" might be avoided. It was further hoped that it would help develop better understanding and chemical judgement amongst users. User-friendliness and simplicity of input were also required so that it would be straightforward to use the code, especially for scoping calculations, which often require repeated running of calculations with only minor modifications to the input.

A further aim was to have a coupled chemical – transport code in which the chemical system was given 'priority' over a simple transport system, so that the chemistry could be explored without hindrance from a complex and time consuming transport model.

The transport model considers one dimensional advective flow with diffusion. It is important to note the following points:

  • Flow is assumed to occur in fractures.
  • The rock porosity is assumed to be concentrated in the fractures, hence
  • Rock matrix porosity is ignored.
  • An initial Darcy velocity is specified by the user, along with the hydraulic gradient.
  • Darcy velocity may be considered fixed for a calculation (but this can result in unrealistically high particle velocities if the porosity becomes small) or allowed to vary as porosity changes.
  • Hydrodynamic dispersion is not included in the current version of SANTA-CHEM.
  • For simplicity, in the current version of SANTA-CHEM, the same diffusion coefficient is assumed to apply to all diffusing species.

The chemical model is constructed in terms of components, (aqueous) species and minerals, and includes the following:

  • Instantaneous equilibrium is assumed between all components and species in solution.
  • Mineral dissolution reactions may achieve equilibrium within a single time step or can be controlled by user-defined kinetic rate constants so that equilibrium is approached over several time steps.
  • The rate of mineral precipitation reactions is considered to be sufficiently fast that they can be assumed to reach equilibrium within a single time step.
  • The rate of mineral dissolution can be dependent or independent of surface area.
  • All reactions used in a calculation, both aqueous species and minerals, must be explicitly defined by the user along with species stability constants, mineral solubility products and kinetic rate constants, where appropriate.
  • It is assumed that all reactions take place at 25º C.
  • The extended Davies equation is used for the ionic strength.
  • Mineral and solution reactions are written without water being included. This implies that the activity of water is always 1.0, and that water is always present in excess.

This report describes the SANTA-CHEM programme, including governing equations and solution techniques. Appendices include descriptions of the input files needed to run the code, along with definitions of all the input parameters, and examples of the output files with explanation, and test cases for transport and examples of calculations for chemical systems.