GEOS 5311 Lecture Notes: Numerical Modeling Example, Yucca Mountain, Nevada Dr. T. Brikowski Spring 2010 file:yuccamtn.tex,v (1.11, January 26, 2010), printed March 30, 2011
Introduction 1
Introduction: YMNWR An important recent example of large-scale hydrologic investigation and numerical modeling is the Yucca Mountain Nuclear Waste Repository in Nevada. Designed to house the nation s high-level nuclear waste, mostly produced by nuclear power plants Waste will be in the form of ceramic cylinders Total radiation content will be 10-100 times that currently in the ground at the Nevada Test Site (of course I can t find the original data for this) EPA designated licensing criteria for the repository, among which are: 2
no movement of radionuclide from the repository for 1000 yrs no movement of radionuclide to the accessible environment for 10,000 maximum radiation dose 15 millirem/yr from all pathways (current US average exposure is 360 mr/yr, mostly from space) Yucca Mountain was chosen as a candidate site based on its thick unsaturated zone [Winograd, 1981] and remoteness (Fig. 1). See also USGS website Clear need for predictive modeling, preceeded by extensive laboratory and field investigations (Site Characterization Project) 3
FYI for now Yucca Mountain project has been abandoned 4
Location and Topography of Yucca Mountain Figure 1: [Location and topography, Yucca Mountain. After Flint et al. [Fig. 1, 2001]. 5
Unsaturated Hydrology 6
Benefits of Unsaturated Repository hydraulic conductivity - the ability to conduct fluids, is generally orders of magnitude lower for unsaturated rocks fluid phase is disconnected, making travel times for contaminants much longer simpler engineering (theoretically less water present for chemical reaction) 7
Complications of Unsaturated Zone See also cartoon of Yucca-specific processes (Fig. 2) hydraulic conductivity is moisture- and texture-dependent conductivity depends on the history of moisture changes in the rock variation in water content can completely change the nature of bulk conductivity of the rock 8
Complications at Yucca Mountain Figure 2: Cartoon of unsaturated zone processes, Yucca Mountain. After Flint et al. [Fig. 16, 2001]. 9
Investigation/Model Results 10
Conceptual Model The consensus view on how the system should work (see Fig. 3): very dry environment (3 cm mm yr ) yr precip.) minimal recharge (5 then fractured rock (welded tuffs) should have very low conductivity, and non-welded tuff higher moisture content and conductivity top welded tuff horizons should be zones of accumulation ( perching ) of groundwater regional dip will guide this water to vertical barriers/conduits (fault zones) 11
Early Conceptual Model Figure 3: Initial conceptual model of Yucca Mountain unsaturated zone [originally published in Scott and Bonk, 1984]. After Flint et al. [Fig. 4, 2001]. 12
Early Results As part of DOE s Site Characterization Plan lab and field studies, and predictive models of unsaturated and unsaturated zones were made: Early Model Results very elaborate unsat zone model, based on Pruess et al. [TOUGH2, 1999] IFD multi-phase modeling package (Fig. 5) predicted travel time from surface to repository (1 km deep) was 1000 yrs combined unsaturated/saturated zone travel times for dissolved radionuclides were on the order of 100,000 years Unsaturated Water Chemistry 13
early shallow studies indicated rapid infiltration, at least along washes [Yang, 1987] construction of the initial underground tunnel ( ESF) revealed 50 year-old water throughout the repository level! (Fig. 4) 14
ESF Isotope Data Figure 4: Observed modern tracers at ESF level [originally published in Wolfsberg et al., 1999]. Both tritium ( 3 H) and 36 Cl are products of atmospheric testing of atomic weapons, and were not present before 1945. After Flint et al. [Fig. 17, 2001]. 15
Unsaturated Performance Model Grid Figure 5: Current unsat zone performance model grid (IFD). After Flint et al. [Fig. 18, 2001]. 16
Alternative Conceptual Models Figure 6: Alternative conceptual models for the unsaturated zone and their representation as characteristic curves. After Flint et al. [Fig. 10, 2001]. 17
State of the Art at Yucca Mtn 18
Current Issues The current focus at Yucca Mountain has shifted somewhat must characterize infiltration events that lead to rapid fracture flow (e.g. Fig. 7) find a way to reasonably model this process on a scale of tens-hundreds of meters revise conceptual model to incorporate lack of natural deflection of infiltration (Fig. 8) modify repository design to deflect infiltration pulses (these aren t deflected by the rock itself, as first thought) 19
Transient Fracture Flow Concept Figure 7: Cartoon of transient fracture flow. Water injected along the top (borehole) travels by time-varying paths, affecting the conditions measured at a hypothetical observation point in the rock (red circle). After Faybishenko et al. [Fig. 18, 2003]. 20
Current Conceptual Model Figure 8: Current (2000) Yucca Mountain conceptual model. Main difference is lack of horizontal offset of infiltration waters in non-welded tuff (PTn). After Flint et al. [Fig. 19, 2001]. 21
Bibliography 22
Faybishenko, B., G. S. Bodvarsson, and R. Salve, On the physics of unstable infiltration, seepage, and gravity drainage in partially saturated tuffs, J. Contam. Hydrology, 62-63, 63 87, 2003. Flint, A. L., L. E. Flint, G. Bodvarsson, E. M. Kwicklis, and J. Fabryka-Martin, Evolution of the conceptual model of unsaturated zone hydrology at Yucca Mountain, Nevada, J. Hydrology, 247, 1 30, 2001. Pruess, K., C. Oldenburg, and G. Moridis, Tough2 user s guide, version 2.0, Report LBNL-43134, Lawrence Berkeley Nat. Lab, Berkeley, CA, 1999. Scott, R. B., and J. Bonk, Preliminary geologic map of yucca mountain, nye county, nevada, with geologic sections, Open-File Reports, OFR 84-494, 1984. Winograd, I. J., Radioactive waste disposal in thick unsaturated zones, Science, 212, 1457 64, 1981. Wolfsberg, A. V., J. T. Fabryka-Martin, K. S. Campbell, S. S. Levy, and P. H. Tseng, Use of chlorine-36 and chloride data to evaluate fracture flow and transport models at yucca mountain, in Proceedings of International Symposium, Dynamics of Fluids in Fractured Rocks, Concepts and Recent Advances, Lawrence Berkeley National Laboratory, Berkeley, CA, 1999. Yang, I. C., Chemical evidence of preferred water flow paths in unsaturated fractured tuffs, yucca mountain, nevada, EOS, 68, 316, 1987.