Quintessa

PITZER

Quintessa has developed software to convert the format of the Yucca Mountain Project thermodynamic database.

In 2008, Quintessa was commissioned by NWMO to convert the database format from its native EQ3/6 format into PHREEQC format. The associated work is documented in a Quintessa technical memorandum. The PHREEQC formatted version of the database can be obtained by contacting Steven Benbow. As noted in the Quintessa technical memorandum, there are known limitations with the database and any application must be undertaken with care to ensure that the database is used in an appropriate manner.

The Yucca Mountain database includes Pitzer data to parameterise the activity coefficients of aqueous species. Traditional approaches to defining activity coefficients, such as the Debye-Huckel or Davies approaches, are only applicable to dilute aqueous solutions with ionic strengths up to about 0.8, while the Pitzer approach is valid for more saline solutions.

The importance of the Pitzer approach for high ionic strength solutions can be appreciated from the illustration, which shows how the formation water pH varies as CO2 is injected into an Na-Cl brine with ionic strength of 6. It can be seen that the Pitzer approach predicts much lower pH and consequently more undersaturated conditions with respect to calcite than conventional databases.

The Yucca Mountain database is stored in EQ3/6 format. To enable NWMO to use the Yucca Mountain project database in their preferred geochemical modelling code, PHREEQC, Quintessa developed software to convert the database from its native EQ3/6 format into PHREEQC format. The software was developed in accordance with Quintessa's ISO 9001:2000 approved software procedures, which also meet the requirements of the TickITplus scheme.