A reactive transport model of neptunium migration from the potential repository at Yucca Mountain

• Published in: Journal of hydrology (Amsterdam) Vol. 209; no. 1; pp. 251 - 280
• Main Authors: Viswanathan, Hari S., Robinson, Bruce A., Valocchi, Albert J., Triay, Ines R.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.08.1998
• Subjects: Chemical elements; Chemical potential; Finite element method; Geochemistry; Hydrology; Joining; Mass transfer; Migration; Mountains; Neptunium; Propagation; Pulsation; Radioactive waste; Radioactive waste disposal; Repositories; Simulation; Unsaturated zone; Water tables; Yucca Mountain; Geochemistry; Unsaturated zone; Yucca Mountain; Neptunium; Radioactive waste disposal
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/S0022-1694(98)00122-X

Characterization and performance assessment studies for the potential high-level nuclear waste repository at Yucca Mountain have identified 2 3 7Np as a radionuclide of concern for the proposed repository. To predict the migration of neptunium after a repository breach, an understanding of the relevant hydrologic and geochemical processes is required. The hydrologic flow in the unsaturated zone at Yucca Mountain is dependent on the infiltration rate, the stratigraphy of the vadose zone, and the heat generated by the decaying radioactive waste. The geochemical processes that strongly affect 2 3 7Np migration include: solubility-limited release of 2 3 7Np from the near-field environment, aqueous speciation of neptunium into non-sorbing carbonate/hydroxy complexes and the sorbing NpO 2 + cation, sorption of neptunium onto the zeolitic tuffs via an ion exchange mechanism, and radioactive decay. The finite element heat and mass transfer (FEHM) code was used to investigate the coupled effects of chemical interactions and heat on neptunium transport from the potential repository to the water table. The selective coupling method is introduced to solve these reactive transport problems. The simulations indicate that in the absence of irreversible changes in the hydrologic and transport properties, the heat pulse does not significantly affect the migration of neptunium, as the time scale of heat pulse propagation is shorter than the time scales associated with neptunium release and migration. Water chemistry, particularly pH, calcium, and sodium concentration significantly affect the retardation of neptunium by the zeolitic rocks between the repository and the water table.