2-Page Summary for Neptunium solubility in the Near-field Environment of A Proposed Yucca Mountain Repository
The total system performance assessment (TSPA) for the proposed repository at Yucca Mountain, NV, includes a wide variety of processes to evaluate the potential release of radionuclides from the Engineered Barrier System into the unsaturated zone of the geosphere. The principal processes controlling radionuclide release and mobilization from the waste forms are captured in the model to assess the dissolved concentrations of radionuclides in the source-term. The TSPA model of the source-term incorporates the far-from-equilibrium dissolution of, for example, spent nuclear fuel (SNF) to capture bounding rates of radionuclide availability as the SNF degrades. In addition, for individual radionuclides, the source-term model evaluates solubility constraints that are more indicative of longer-term, equilibrium processes that can limit the potential mass transport from the source term in those cases. These solubility limits represent phase saturation and precipitation processes that can occur either at the waste form as it alters, or at other locations in the near-field environment (e.g., within the invert) if chemical conditions are different. Identification and selection of applicable constraints for solubility-limited radionuclide concentrations is a primary focus in formulating the source-term model for the TSPA. Neptunium is a long-lived radionuclide that becomes a larger fraction of the potential dose as radioactive decay of other radionuclides proceeds. To delineate appropriate long-term source-term controls on dissolved neptunium concentrations, a number of alternative models have been defined. The models are based on data both collected within the Yucca Mountain Project and taken from published literature, and have been evaluated against independent data sets to assess their applicability. The alternative models encompass ones based on precipitation of neptunium within its own separate oxide phases (i.e., ''pure'' Np-phases), and those where neptunium is incorporated into the secondary (tertiary, quaternary, etc.) uranyl phases forming as alteration products of SNF (secondary phases). The constraints on dissolved neptunium concentrations from ''pure'' Np-phase models provide more elevated bounds compared to the values derived from models of trace incorporation of Np into secondary uranyl phases. Such secondary phase models depend on a larger set of processes and variables compared to more idealized solubility models for Np-oxides, and therefore, secondary phase models generally will have more complex bases to define adequately the expected behavior.
- Research Organization:
- Yucca Mountain Project, Las Vegas, NV (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- OSTI ID:
- 838904
- Resource Relation:
- Other Information: PBD: 29 Mar 2005
- Country of Publication:
- United States
- Language:
- English
Similar Records
Secondary Uranium-Phase Paragenesis and Incorporation of Radionuclides into Secondary Phase
Direct Investigations Of The Immobilization Of Radionuclides In The Alteration Phases Of Spent Nuclear Fuel
Related Subjects
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
DECAY
DISSOLUTION
NEPTUNIUM
NUCLEAR FUELS
OXIDES
PERFORMANCE
PRECIPITATION
RADIOACTIVITY
RADIOISOTOPES
SATURATION
SOLUBILITY
SOURCE TERMS
TRANSPORT
WASTE FORMS
YUCCA MOUNTAIN