Thermodynamic Database for NH-Species of Radioactive Elements

Ochs, M.*; Lothenbach, B.*; Christl, I.*; Yui, Mikazu; Shibata, Masahiro ; Jintoku, Takashi

In the original list of ligands to be considered in the JNC-TDB, NH had not been included, but NH, complexes were evaluated for Pd due to their importance for such soft acid metals. Presently, NH- complexes are considered to be important for the Japanese TRU disposal program, because nitrate contained in this type of waste could possibly be reduced to NH under repository conditions. This report presents thermodynamic data that are particularly critical for evaluating the safety functions of the repository near field, but are also of general importance for performance analysis. An extensive data search and initial evaluation for the interaction of NH with Pb, Sn, Bi, Nb, and Sb were carried out, but yielded no useful results. Therefore, no thermodynamic data can be recommended for NH-species of these elements. However, the compiled information allowed a rough comparative evaluation of stability constants in the system NH - HO - Metal, which indicates that NH is not likely to be a critical ligand for Sn(IV), Bi, Nb, and Sb(V) in dilute solutions. For Pb, complexation with NH may become relevant at elevated NH concentrations. Our evaluation further indicated that NH could be a relevant ligand for Ni and Co. Their interaction with NH had been studied to some degree. Subsequently, a first, traceable compilation of original experimental data was carried out for Ni and Co. However, in most case, the possible complex formation of Ni with the anions of the electrolyte and OH has not been considered in the calculations of the different constants. It remains to be studied in detail whether these interactions can be neglected or corrected for in all cases. A brief attempt was made to carry out some calculations for Ni in order to roughly estimate possible effect of the presence of NH on a repository performance. The calculations carried out clearly show that very high NH ...