Technical Report NTB 02-16

NAGRA/PSI Chemical Thermodynamic Data Base 01/01

The Nagra/PSI Chemical Thermodynamic Data Base has been updated from version 05/92 to 01/01 to support an ongoing safety assessment of a planned Swiss repository for high-level radioactive waste. Database version 05/92 distinguished between “core data” and “supplemental data”. Core data are for elements commonly found as major solutes in natural waters. These data are well established and have not been changed to any significant degree in this update. Supplemental data comprise actinides and fission products, Mn, Fe, Si and Al. Our update from version 05/92 to 01/01 involved major revisions for most of the supplemental data. Altogether, more than 70% of our database contents have been updated.

Data for U, Np, Pu, Am and Tc recommended by the internationally recognised NEA TDB project were considered in our update. Our reasons for not accepting several NEA recommendations are documented in detail. Thermodynamic data for Th, Sn, Eu, Pd, Al, and solubility and metal complexation of sulphides and silicates were extensively reviewed. Data for Zr, Ni and Se were examined less rigorously as these elements are currently being reviewed in phase II of the NEA TDB project.

Our experiences from this two year team effort can be summarised as follows. (1) Detailed in-house reviews and critical appraisal of NEA recommendations greatly improved the chemical consistency and quality of the selected data, as shown e.g. by comparison of complexation constants for M(III) and M(IV) oxidation states of actinides and fission products. (2) On the other hand, we could discern major gaps in the data, especially missing carbonate complexes. (3) In some systems, e.g. ThO2 – H2O and UO2 – H2O, experimental data could not be described by a unique set of thermodynamic constants. There, a pragmatic approach based on solubility data was chosen to provide data for application to performance assessment.