An active fracture model for unsaturated flow and transport in fractured rocks

A multidimensional, mountain-scale, thermal-hydrologic (TH) numerical model is presented for investigating unsaturated flow behavior in response to decay heat from the radioactive waste repository in the Yucca Mountain unsaturated zone (UZ), Nevada. The model, consisting of both two-dimensional (2-D) and three-dimensional (3-D) representations of the UZ repository system, is based on the current repository design, drift layout, thermal loading scenario, and estimated current and future climate conditions. This mountain-scale TH model evaluates the coupled TH processes related to mountain-scale UZ flow. It also simulates the impact of radioactive waste heat release on the natural hydrogeological system, including heat-driven processes occurring near and far away from the emplacement tunnels or drifts. The model simulations predict thermally perturbed liquid saturation, gas-and liquid-phase fluxes, and water and rock temperature elevations, as well as the changes in water flux driven by evaporation/condensation processes and drainage between drifts. These simulations provide mountain-scale thermally perturbed flow fields for assessing the repository performance under thermal loading conditions. Key Words: thermal-hydrologic processes in subsurface, thermal load, Yucca Mountain, fluid and heat flow in porous media, heat pipe, reservoir simulation, fractured unsaturated rock * Corresponding author. Tel.: +1-510-486-7291; fax: +1-510-486-5686, E-Mail address: YSWu@lbl.gov. 2 IntroductionIn the past decade, the 500-700 m thick Yucca Mountain unsaturated zone (UZ) has been extensively investigated as a potential subsurface repository for storing high-level radioactive wastes. While the site characterization has been mostly carried out for analyzing unsaturated flow and radionuclide transport in ambient, isothermal conditions [30,31], the inherent nature of nonisothermal flow and transport processes, created by repository heating from radioactive decay, has also motivated many research efforts to understand thermal-hydrologic (TH) behavior and its impact on the repository performance within the UZ. In particular, significant progress has been made in quantitative TH modeling studies at Yucca Mountain [9,10,6].Emplacement of heat-generating high-level radioactive waste in the UZ system of unsaturated welded and unwelded fractured tuff at Yucca Mountain will perturb the ambient condition and create complex multiphase fluid flow and heat-transfer processes.The physical phenomena associated with repository heating include conduction and convection heat transfer, phase change (boiling and condensation), two-phase flow of liquid and gas phases under variably saturated conditions, enhanced fracture-matrix interaction caused by rapid matrix drying and subsequent imbibition, diffusion and dispersion of vapor and gas, and vapor-pressure-lowering effects. These TH processes will last hundreds to thousands of years after waste emplacement, and will significantly redistribute the in-situ moisture content and alter the per...

Section: Numerical Model Gridsmentioning

confidence: 99%

A mountain-scale thermal–hydrologic model for simulating fluid flow and heat transfer in unsaturated fractured rock

Mukhopadhyay

Zhang

et al. 2006

“…Rock hydraulic properties needed as inputs into the model include fracture and matrix permeabilities, fracture and matrix porosities, fault aperture and permeabilities, van Genuchten (1980) parameters (for matrix, fractures, and the fault), and the parameter of the active fracture model (Liu et al, 1998), γ, for fractures. The fracture porosities for these two geological units were determined based on both fracture geometry and analyses of gas tracer test data .…”

Section: Calibration Of Seepage-rate Data and The Average Water-travementioning

confidence: 99%

“…These processes are generally complicated, owing to the complexity of fracture-matrix interaction, distinct differences in hydraulic properties between fractures and matrix, and nonlinearity involved in unsaturated flow (Liu et al, 1998). Model analyses of carefully designed field studies in natural fractured rocks are useful for improving our understanding of, evaluating modeling approaches to, and calibrating the relevant model parameters for these processes (e.g., Wang et al, 1999;Tsang and Birkholzer, 1999;Salve and Oldenburg, 2001;Liu et al, 2003a).…”

Section: Introductionmentioning

confidence: 99%

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

Liu

Salve

Wang

et al. 2004

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

“…Flow in the fractured system occurs within the network of fractures, within matrix blocks and between fractures and the matrix. Fingering liquid flow in fractures is modeled using the active fracture concept (Liu et al, 1998) in which fracture-matrix liquid flow resulting from the fingering flow is controlled through the active fracture parameter g .…”

Section: Conceptual Modelmentioning

confidence: 99%

Modeling thermal–hydrological response of the unsaturated zone at Yucca Mountain, Nevada, to thermal load at a potential repository

Haukwa

Bodvarsson

2003

This paper presents a numerical study on the response of the unsaturated zone (UZ) system of Yucca Mountain to heat generated from decaying radioactive wastes emplaced at the proposed repository. The modeling study is based on the current thermal-hydrological (TH) mountain-scale model, which uses a locally refined 2D north-south cross section and dual-permeability numerical approach. The model provides a prediction of the mountain-scale TH response under the thermal-load scenario of 1.45 kW/m, while accounting for future climatic changes and the effects of drift ventilation. The TH simulation results show that ventilation of the repository drifts has a large impact on thermal-hydrologic regimes and moisture-flow conditions at the repository. In both cases, with and without ventilation, the TH model predicts dry or reduced liquid saturation near the drifts for over 1,000 years, during which liquid flux through the drifts is reduced to either zero or less than the ambient flux. Without ventilation, the model predicts higher temperatures at the repository, but no major moisture redistribution in the UZ except in the areas very near the heated drifts.