Neptunium Solubility in the Near-Field Environment of a Proposed Yucca Mountain Repository

Abstract

ABSTRACTFor representing the source-term of a proposed repository at Yucca Mountain, NV, the performance assessment (PA) approach evaluates the disequilibrium degradation of the waste forms to capture a bounding rate for radionuclide source-term availability and use solubility constraints that are more representative of longer-term, equilibrium processes to limit radionuclide mass transport from the source-term. These solubility limits capture precipitation processes occurring either as the waste forms alter, or in the near-field environment as chemical conditions evolve. A number of alternative models for solubility controls on dissolved neptunium concentrations have been evaluated. These alternatives include idealized models based on precipitation of simple, discrete neptunium phases and more complex considerations of trace amounts of neptunium being incorporated into secondary uranyl phases that form during waste form alteration. Thermodynamic constraints for neptunium under oxidizing conditions indicate that tetravalent neptunium solids (e.g., NpO2) are more stable relative to pentavalent phases (e.g., Np2O5), and therefore have solubilities that set lower dissolved concentrations of neptunyl species. Within solution, the pentavalent neptunyl ion (NpO2+) and its complexes dominate. Data on solids and solutions from slow flow-through (dripping) tests on spent nuclear fuel (SNF) grains indicate that neptunium is tetravalent in the SNF and that, over the ∼9 years duration of the tests, the dissolved neptunium concentrations are near to, or below, calculated NpO2 solubility. Currently, the NpO2 solubility model is applied at the waste forms within waste packages because the degradation of SNF and corrosion of alloys ensures active reduction reactions. The more conservative Np2O5 solubility model is applied in environments distal to the waste forms (i.e., the invert below waste packages) where reduction of dissolved pentavalent neptunium is less certain. Consideration of Np incorporation into secondary uranyl phases suggests that both of these idealized models will provide conservative estimates of neptunium release to the geosphere.