Solubility constraints on uranium concentrations in groundwaters of the Tono uranium deposit, Japan

A key concept underpinning most safety assessments of geological disposal systems for high-level nuclear wastes is that the maximum aqueous concentrations of actinide elements released from the waste will be limited by the solubility of the corresponding tetravalent oxide or hydrous oxide. The validity of this concept for U is evaluated in the present study using hydrochemical and mineralogical data from the Tono uranium (U) deposit in Gifu Prefecture, Japan. In addition, the Ion-term stability of U mineralization in the Tono deposit is assessed in terms of plausible range of U solubilities in paleogroundwaters migrating through the deposit. A thermodynamic evaluation of the aqueous-speciation and solubility behaviour of U indicates that U concentrations in the coexisting groundwaters greatly exceed the solubility of both uraninite [UO2(c)] and coffinite [USiO4(c)]. The dissolved U concentrations appear instead to be controlled by the solubility of amorphous hydrous U(IV) oxide [UO2(am)]. which is metastable with respect to uraninite. Mineralogical and microbiological investigations suggest that pH-E-h conditions similar to those observed presently in groundwaters of the lower Toki Lignite-bearing Formation (pH approximate to 8-10; E-h approximate to -250--410 mV) have been buffered. possibly since the present geological structure formed about 5-0.7 Ma, by microbial sulphate reduction and precipitation/dissolution of calcite. If so, corresponding U concentrations controlled by UO2(am) solubility are calculated to have been in the range of 10(-6.1) and 10(-8.7) mol kg(-1) depending on the possible environmental conditions, i.e., E-h, pH and carbonate concentration.