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

Abstract

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...

Performance Assessment for Geological Disposal of Graphite Waste Containing TRISO Particles

“…[22][23][24] The drift shadow is a region in the unsaturated zone beneath the tunnel drift that is partially sheltered from downwardpercolating water, because the capillary action is not strong enough to draw water into the rock immediately below the drift~Fig. 9!.…”

“…Transport in this region is controlled mainly by diffusive processes, rather than by advection. [22][23][24] The radiocuclides will diffuse through this region until they reach a location at which they can be transported by advection through the fracture network in the unsaturated zone to the alluvium, at which point they will travel by advection to some downgradient location. The shortest diffusion path to a region of advective transport will be to consider that the contaminated water is released from an outside edge of the waste canister and is subsequently diffusively transported laterally to the edge of the tunnel drift~or equivalently to the edge of the drift shadow region!, a distance somewhat .2 m. In the remainder of the analysis, we consider that the length of diffusive transport in the near field is 2 m. …”

“…Although there are a number of different rock types in the proposed YMR, the current repository design locates ;70% of the drifts in the tsw35 hydrogeologic unit. 23,25 The supporting documentation for the TSPA-LA provides distributions for the essential physical parameters necessary to make the near-field calculations.…”

“…37 As discussed previously this diffusive transport will occur because the water contaminated with the radionuclides initially interfaces with the drift shadow region, a region in which advective transport is negligible. [22][23][24] The tortuosity correction factor in this is conservatively set to 1. Retardation effects due to sorption of radionuclides with the host rock are considered based on the mean of the distributions provided in the supporting documentation for the TSPA-LA.…”

Conceptual Model and Numerical Approaches for Unsaturated Zone Flow and Transport

“…This modification is motivated by field observations showing that under unsaturated conditions, not all fractures in a connected fracture network actually carry water flow (Liu et al 1998 [DIRS 105729]). The AFM has been reviewed and published by Liu et al (1998 [DIRS 105729]), and several papers (e.g., Houseworth et al 2003 The model validation criteria mentioned above allow for evaluating the AFM by showing whether model results based on the AFM are consistent with different field observations simultaneously when the relevant results are sensitive to the AFM parameters. Because carbon-14 data and fracture coating data are signatures of past flow and transport processes in the UZ, these data provide important insight into the flow and transport processes under ambient conditions and are directly related to the intended use of the AFM.…”