Radionuclide Transport Models Under Ambient Conditions

Water Resources Research

2003

[1] Chlorine 36 was generated from nuclear tests in the 1950s and 1960s and has been used to identify fast flow paths at Yucca Mountain, the proposed repository for high-level nuclear waste , 1998]. Bomb pulse 36 Cl, brought into the subsurface by infiltrating rainwater, presumably resides along fracture surfaces because of the extremely low rock matrix permeability. However, leaching a rock sample to extract this salt inevitably extracts pore water chloride (Cl) and rock matrix chloride, thereby making it difficult to obtain reproducible measurements or detect the specific bomb pulse signatures. Complexities introduced by these sources of older chloride include dilution of bomb pulse 36 Cl/Cl ratios for samples from strata with a high Cl concentration, variations in measured ratios as a function of leaching time, rock chip size, and the differing effects of active leaching from those of passive leaching. This work provides both a conceptual model and a mathematical solution for the leaching processes and examines the role of sample leaching in the 36 Cl studies of Yucca Mountain rocks. An analytical solution is derived for the diffusion of Cl and 36 Cl in composite media (rock matrix and water) to accommodate variable diffusivity. This solution is subsequently used to develop a leaching model that takes into account bomb pulse signal, matrix pore water, and relatively hard to leach components (isolated fluid inclusion and mineral boundary salts). The model is then applied to samples from stratigraphic units at Yucca Mountain to obtain leachate concentrations from different setup methods (protocols), including duration, chip size, and gravitational settling of the water-rock mixture. The model results show that the probability of detecting a 36 Cl/Cl bomb pulse signal is severely diminished under longer leaching times and smaller rock fragment sizes and that leaching times of 1 to 10 hours are most likely to be successful in detecting a bomb pulse signal. Bomb pulse 36 Cl/Cl ratios are more likely to be observed if pore water Cl concentrations were initially low prior to the introduction of bomb pulse carrying water.

Section: Resultsmentioning

confidence: 99%

Implications of halide leaching on ³⁶Cl studies at Yucca Mountain, Nevada

Sonnenthal

Water Resources Research

2003

“…Boving and Grathwohl (2001) compiled m values for different types of rocks and found that m is generally >2 in materials of low porosity (≤0.2). By comparing measured local‐scale matrix diffusion coefficients and the corresponding molecular diffusion–coefficient values in free water, Moridis and Hu (2000) concluded that m = 2 is a good approximation for the rock matrix in the unsaturated zone of Yucca Mountain. The laboratory data cited by Becker and Shapiro (2000) suggest that m = 2.93 (obtained using measured φ = 1.5%) is valid for the crystalline rock matrix under consideration.…”

Section: Data From Literaturementioning

confidence: 99%

Scale Dependency of the Effective Matrix Diffusion Coefficient

Liu

Zhang

2004

Vadose Zone Journal

It has been recognized that matrix diffusion is an important process for retarding solute transport in fractured rock. Based on analyses of tracer transport data from a number of field tests, we demonstrate that the effective matrix-diffusion coefficient may be scaledependent and generally increases with test scale. A preliminary theoretical explanation of this scale dependency is also presented, based on the hypothesis that solute travel paths within a fracture network are fractals.2

“…In an effort to incorporate this flow behavior into the continuum approach, Liu et al (1998) developed an AFM that assumes only a portion of fractures in a connected unsaturated fracture network contribute to liquid water flow. Because the AFM can provide a large range of flow behaviors in unsaturated fractures (including fast flow behavior), it has been used extensively for modeling large‐scale flow and transport in the unsaturated zone of Yucca Mountain (e.g., Moridis and Hu, 2000; Wu et al, 2000; Wu et al, 2002; Liu et al, 2003b; Zhou et al, 2003; Buscheck et al, 2002; Xu et al, 2003).…”

mentioning

confidence: 99%

The Active Fracture Model: Its Relation to Fractal Flow Patterns and an Evaluation Using Field Observations

Liu

Zhang

2003

Vadose Zone Journal

The active fracture model (AFM) (Liu et al., 1998) has been widely used in modeling flow and transport in the unsaturated zone of Yucca Mountain, Nevada, a proposed repository of high‐level nuclear wastes. This study presents an in‐depth evaluation of the AFM, based on both theoretical arguments and field observations. We first argue that flow patterns observed from different unsaturated systems (including the unsaturated zone of Yucca Mountain) may be fractals. We derive an interesting relation between the AFM and the fractal flow behavior, indicating that the AFM essentially captures this important flow behavior at a subgrid scale. Finally, the validity of the AFM is demonstrated by the favorable comparison between simulation results based on the AFM and 14C age and fracture coating data collected from the unsaturated zone at Yucca Mountain. These data sets independently provide important insight into flow and transport processes at the Yucca Mountain site. Potential future improvements of the AFM include expanding it to consider film flow and multifractal concepts.