Geochemistry of Natural Components in the Near-Field Environment, Yucca Mountain, Nevada

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.

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Geochemistry of Natural Components in the Near-Field Environment, Yucca Mountain, Nevada

MRS Proceedings ◽

10.1557/proc-985-0985-nn13-10 ◽

2006 ◽

Vol 985 ◽

Author(s):

Zell E. Peterman ◽

Thomas A. Oliver

Keyword(s):

Pore Water ◽

Nuclear Waste ◽

Near Field ◽

Major Elements ◽

Yucca Mountain ◽

Water Soluble ◽

Nuclear Waste Repository ◽

Chemical Compositions ◽

Seepage Water ◽

Field Environment

AbstractThe natural near-field environment in and around the emplacement drifts of the proposed nuclear waste repository at Yucca Mountain, Nevada, includes the host rock, dust, seepage, and pore water. The chemical compositions of these components have been analyzed to provide a basis for assessing possible chemical and mineralogical reactions that may occur after nuclear waste is emplaced. The rock unit hosting the proposed repository has a relatively uniform chemical composition as shown by samples with a mean coefficient of variation (CV) of 9 percent for major elements. In contrast, compositional ranges of underground dust (bulk and water-soluble fractions), pore water, and seepage water are large with mean CVs ranging from 28 to 64 percent for major constituents.

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