An experimental study of the dissolution rates of Nd-britholite, an apatite-structured actinide-bearing waste storage host analogue

The steady-state dissolution rates of Nd-britholite Ca9Nd(PO4)5SiO4F2 have been measured at 25 °C as a function of aqueous solution composition in open-system mixed flow reactors. Measured Nd-britholite dissolution rates based on Ca, P, and F release are found to be close to corresponding apatite dissolution rates; these 25 °C rates decrease from 10−8 to 10−12 mol/m2/s with increasing pH from 3 to 12. Measured Nd release rates are far slower than those of these other elements; Nd concentrations in solution appear to limited by precipitation of a secondary phosphate phase, which limits aqueous Nd concentrations to <10−12 mol/kg at near neutral conditions. Correspondingly, it appears likely that aqueous actinide release from analogous waste hosts will be similarly limited by the precipitation of sparingly soluble phases.