The purpose of this study is to model the solubility, speciation, and transport of three actinides: uranium (U), neptunium (Np), and plutonium (Pu) by a geochemical modeling tool under possible repository environmental conditions upon waste package failure. The dissolution concentration of three actinides in the waste package, their aqueous speciation after dissolving in the unsaturated zone (UZ), and transport processes in the saturated zone (SZ) along the groundwater flow path at Yucca Mountain (YM) are simulated by geochemical modeling calculations using PHREEQC. This model report was prepared in fulfillment of Groundwater Characterization at Yucca Mountain, Task II: Surface Complexation and Solid Phase Dissolution, Subtask 6: Phenomena Modeling of Actinide speciation simulation (Research Foundation Task: ORD-RF-02). This subtask with evaluates of the sensitivity of the current models to solubility data and impact of actinide speciation data on the behavior and mobility of actinide species. For fundamental theory of three concerned actinide species on which the simulation built up, please see Technical Report, “Groundwater Characterization at Yucca Mountain, Task 2: Surface Complexation and Solid Phase Sorption” (Czerwinski, 2007). Uranium, neptunium, and plutonium have been considered to be the most important components in high-level radionuclide wastes (RW) after hundreds of years deposit. The geochemical model includes three sub-models: solubility model, aqueous speciation model, and transport model. Three actinides are programmed separately in each submodel. According to the definition of unqualified data, the third sub-model uses some unqualified data, so the modeling results of transport are considered to be Unqualified (UQ) and are not included in any conclusions. The scope of this model report is to describe the development of a geochemical model for simulating dissolution concentration of three actinides in the waste package, aqueous speciation in UZ directly below the repository, and transport in groundwater system. The three sub-models include: the solubility model, where water may have seeped into the repository from the surrounding rock, percolated into a failed waste package, and reacted with the waste form in the waste package. The second sub-model is the aqueous speciation model, where the dissolved radionuclide-bearing solution exits the waste package via advection and then mixes with pore water in the UZ. The third sub-model is the transport model, where the dissolved radionuclide minerals migrate through the UZ, enters the SZ beneath the groundwater table, and then reaches the 20 km alluvium aquifer down gradient to the southwest of YM. The transport sub-model involved some UQ data; however, the results that come from it are listed only in the Appendix. Source of data for this study are derived from YM Databases, including mineralogy, kinetics, and thermodynamics, which are valid for temperatures up to 100 o C and ionic strength up to 1 molal.
Actinide elements – Solubility; Groundwater; Nevada – Yucca Mountain; Neptunium – Solubility; Plutonium – Solubility; Radioactive waste canisters Radioactive waste repositories; Radioisotopes — Migration; Uranium – Solubility; Water chemistry
Earth Sciences | Geochemistry
Smiecinski, A. J.,
Stetzenbach, K. J.
Geochemical modeling of solubility and speciation of uranium, neptunium, and plutonium.
Available at: https://digitalscholarship.unlv.edu/yucca_mtn_pubs/66