Radionuclide Transport Models Under Ambient Conditions

Moridis, George J.; Hu, Qinhong

doi:10.2172/837084

“…For simplicity, our model assumes the uniformly distributed infiltration rate along the infiltration plot to be consistent with the use of the uniform property distribution in the site-scale model of the unsaturated zone of Yucca Mountain. One consideration in our modeling study is to evaluate approaches used in the site-scale model (Wu et al, 2002;Moridis and Hu, 2000). Previous field infiltration tests in the unsaturated zone of Yucca Mountain (David Hudson, 2000, unpublished results) showed that for a given water pressure, infiltration rates could reach constant values quickly.…”

Section: Field Observationsmentioning

confidence: 99%

“…Although some physical explanations for this enhancement are available, as discussed in Section 4.3, more detailed studies of the associated mechanisms are needed to confirm these explanations. The current site-scale model for unsaturated zone of Yucca Mountain (Wu et al, 2002;Moridis and Hu, 2000) does not consider the effects of this enhancement.…”

Section: Implication For Radionuclide Transport In the Unsaturated Zomentioning

confidence: 99%

Field investigation into unsaturated flow and transport in a fault: model analyses

Liu

¹

,

Salve

²

,

Wang

³

et al. 2004

Journal of Contaminant Hydrology

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Results of a fault test performed in the unsaturated zone of Yucca Mountain, Nevada, were analyzed using a three-dimensional numerical model. The fault was represented as a vertical fracture and the surrounding rock was treated as a dual-continuum system in the model. Model calibration against the seepage and water travel velocity data suggests that the lithophysal cavities connected to fractures can considerably enhance the effective fracture porosity and therefore retard water flow in fractures. Comparisons between simulation results and the tracer concentration data also indicate that the matrix diffusion is an important mechanism for solute transport in unsaturated fractured rock. We found that an increased fault-matrix interface area was needed for matching the observed tracer data, which is consistent with previous studies. The study results suggest that the current site-scale model for the unsaturated zone of Yucca Mountain may underestimate radionuclide transport time within the unsaturated zone, because effects of an increased fracture-matrix interface area and the lithophysal cavities are not considered in the current site-scale model.2

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“…The two colloid sizes selected span the expected range of waste-form colloid sizes (Moridis and Hu, 2000). Under the oxidizing conditions expected in the unsaturated zone, the aqueous form of technetium is the negatively charged pertechnetate ion (TcO 4 -), which is nonsorbing.…”

Section: Transport Parametersmentioning

confidence: 99%

“…Therefore, the only distinctive property of colloids used here for colloid transport is a reduced diffusion coefficient. Colloid diffusion coefficients are estimated for the fracture and matrix continua using the Stokes-Einstein equation (Moridis and Hu, 2000). The tortuosity for colloids is assumed to be the same as the value found for technetium.…”

Section: Transport Parametersmentioning

confidence: 99%

“…The driftshadow transport calculations presented here are for technetium, neptunium, plutonium, and two 9 sizes of waste form colloids (6 nm and 450 nm). These radionuclides were chosen because of their significance in dose calculations for total system performance assessment (McNeish, 2000).The two colloid sizes selected span the expected range of waste-form colloid sizes (Moridis and Hu, 2000). Under the oxidizing conditions expected in the unsaturated zone, the aqueous form of technetium is the negatively charged pertechnetate ion (TcO 4 -), which is nonsorbing.…”

mentioning

confidence: 99%

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

Houseworth

¹

,

Finsterle

²

,

Bodvarsson

³

2003

Journal of Contaminant Hydrology

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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 * Corresponding author. Fax: +1-510-486-6115; e-mail: JEHouseworth@lbl.gov 2 for radionuclides to penetrate the unsaturated zone. The delay of radionuclide 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.Key Words: Unsaturated zone flow, fractured rock, radionuclide transport, Yucca Mountain, numerical modeling. Introduction Hydrogeology at Yucca MountainYucca Mountain consists of alternating layers of welded and nonwelded volcanic tuffs. The potential repository horizon lies in the unsaturated zone, approximately 300 m below the ground surface and 300 m above the water table. This horizon consists of welded tuffs composed of a 3 dense porous rock matrix, which is highly fractured. Fracture permeability in the welded tuffs is on the order of darcies (~10 -12 m 2 ), and the rock matrix permea...

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Impact of the implementation of MPI point-to-point communications on the performance of two general sparse solvers

Amestoy

¹

,

Duff

²

,

L’Excellent

³

et al. 2003

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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 * Corresponding author. Fax: +1-510-486-6115; e-mail: JEHouseworth@lbl.gov 2 for radionuclides to penetrate the unsaturated zone. The delay of radionuclide 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.Key Words: Unsaturated zone flow, fractured rock, radionuclide transport, Yucca Mountain, numerical modeling. Introduction Hydrogeology at Yucca MountainYucca Mountain consists of alternating layers of welded and nonwelded volcanic tuffs. The potential repository horizon lies in the unsaturated zone, approximately 300 m below the ground surface and 300 m above the water table. This horizon consists of welded tuffs composed of a 3 dense porous rock matrix, which is highly fractured. Fracture permeability in the welded tuffs is on the order of darcies (~10 -12 m 2 ), and the rock matrix permea...

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Radionuclide Transport Models Under Ambient Conditions

Cited by 7 publications

References 29 publications

Field investigation into unsaturated flow and transport in a fault: model analyses

Field investigation into unsaturated flow and transport in a fault: model analyses

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

Impact of the implementation of MPI point-to-point communications on the performance of two general sparse solvers

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