Flow and transport in the drift shadow in a dual-continuum model

Journal Article · Sun Sep 01 00:00:00 EDT 2002 · Journal of Contaminant Hydrology

OSTI ID:821017

Houseworth, J E; Finsterle, S; Bodvarsson, G S

The current concept for high-level radioactive waste disposal at Yucca Mountain is for the waste to be placed in underground tunnels (or drifts) in the middle of a thick unsaturated zone. Flow modeling and field testing have shown that not all flow encountering a drift will seep into the drift. The underlying reason for the diversion of unsaturated flow around a drift is that capillary forces in the fractures and matrix prevent water entry into the drift unless the capillary pressure in the rock decreases sufficiently to allow for gravity forces to overcome the capillary barrier. As a result of the capillary barrier effect, flow tends to be diverted around the drift, affecting the flow pattern beneath the drift. For some distance beneath the drift, water saturation and flux are reduced. This drift shadow zone is much more pronounced in the fractures than in the matrix due to dominance of gravity over capillary forces in the fractures. Moving downward, away from the drift, the shadow zone asymptotically re-equilibrates to the undisturbed flow conditions due to capillary forces. The behavior of radionuclide transport in this zone of reduced flow is investigated here because this will affect the amount of time required for radionuclides to penetrate the unsaturated zone. The delay of radio nuclide movement in the geosphere is one aspect of the potential repository system that could limit public exposure to radioactive waste. The behavior of flow and transport are calculated using a two-dimensional, drift-scale dual permeability model extending to nine drift diameters below the potential waste emplacement drift. The flow model is first compared with an analytical model for a single-continuum. Then the dual-continuum flow model is investigated with respect to drift-scale and mountain-scale property sets. Transport calculations are performed for a wide range of flow conditions and for different aqueous radionuclides and colloids. Findings indicate that transport times for dissolved or colloidal material released from a drift without seepage are several orders of magnitude longer than if the releases occurred in the undisturbed flow field. Furthermore, the calculations indicate that the transport rate for radionuclides released in the drift shadow is relatively insensitive to flow rates in the fractures, but is sensitive to the flow rate in the matrix.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Director. Office of Civilian Radioactive Waste Management. Yucca Mountain Site Characterization Office; Memorandum Purchase Order EA9013MC5X between Bechtel SAIC Company, LLC and LBNL (US)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 821017
• Report Number(s): LBNL-49868; JCOHE6; R&D Project: 81BB11; TRN: US200407%%137
• Journal Information: Journal of Contaminant Hydrology, Vol. 62-63; Other Information: Journal Publication Date: 2003; PBD: 1 Sep 2002; ISSN 0169-7722
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
COLLOIDS
FLOW MODELS
FLOW RATE
FRACTURES
HIGH-LEVEL RADIOACTIVE WASTES
ISOTOPES
PERMEABILITY
POSITIONING
RADIOACTIVE WASTES
RADIOISOTOPES
SIMULATION
TRANSPORT
WASTES
WATER SATURATION
YUCCA MOUNTAIN
UNSATURATED ZONE FLOW FRACTURED ROCK RADIONUCLIDE TRANSPORT YUCCA MOUNTAIN NUMERICAL MODELING.