Fractures active model

Abstract Results from the four-year long heating phase of the Drift-Scale Heater Test at the Exploratory Studies Facility at Yucca Mountain, Nevada, provide a basis to evaluate conceptual and numerical models used to simulate thermal-hydrological coupled processes expected to occur at the proposed repository. A three-dimensional numerical model was built to perform the analyses. All model simulations were predicated on a dual (fracture and matrix) continuum conceptualization. A 20-percent reduction in the canister heat load to account for conduction and radiation heat loss through the bulkhead, a constant pressure boundary condition at the drift wall, and inclusion of the active fracture model to account for a reduction in the number of fractures that were hydraulically active provided the best agreement between model results and observed temperatures. The views expressed herein are preliminary and do not constitute a final judgment of the matter addressed or of the acceptability of its use in a license application... 

The active fracture model for unsaturated flow through fractured rocks proposed by Liu et al. (1998) was used to modify flow through fractures. This model is based on the hypothesis that only a portion of connected fractures is active in conducting water. The hypothesis stipulates that (i) all connected fractures are active if the system is fully saturated, (ii) all fractures are inactive if the system is at residual saturation, and (iii) the fraction of fractures that is active is related to water flux through the fractures. Liu et al. (1998) proposed that the fraction of active fractures be a power function of effective water saturation in connected fractures. 

Matrix temperature, matrix saturation, and fracture saturation are calculated after four years of heating using the 3D model with dual continua, an active fracture model, and an open boundary at the drift wall (Figures 6-8). This model conceptualization provided the best agreement with temperatures measured during the DST at boreholes 158, 160, andl62. Temperatures calculated for the fracture continuum are the same as those calculated for the... 

Liu, H.H., C. Doughty, and G.S. Bodvarsson. An active fracture model for unsaturated flow and transport in fractured rock. Water Resources Research 34(10) 2,633-2,646. 1998. 

The rate of flow between fluids in fractures and in the rock matrix is a crucial factor for transport and reaction in fractures. For consistency with the formulation for flow between fractures and matrix used in the Yucca Mountain Project, the reactive surface area for minerals in unsaturated fractures has been related to the fracture-matrix interaction area based on a modified form of the Active Fracture Model (Liu et al., 1998 Sonnenthal el al., 2003). In this way, the wetted surface area for mineral-water reactions is consistent with that for flow and diffusion. 

Because the chemical changes are so closely tied to the TH processes and fracture-matrix interaction, modeling several chemical components provides an independent method for validating and refining TH models (e.g., the Active Fracture Model for fracture-matrix interaction and the dualpermeability method). The methodology developed for the THC model of the DST can then be applied with more certainty to long-term predictions of nuclear waste repository performance. 

Conceptual models for the DST were evaluated by comparing simulated temperatures with temperatures measured at three boreholes. The models with the best agreement between calculated and measured temperatures included dual continua (matrix and fracture) and active fracture, to reflect actual heat loads. The canister heat load was decreased by 20 percent to account for radiative and conductive heat loss through the thermal bulkhead. 

The present discussion of continuum modeling of dynamic fracture is not an exhaustive review. Rather, it points out the variety of approaches which have been, and are still being, pursued to provide methods for calculating dynamic fracture phenomena. Such work is still quite active and considerable effort... 

In 1936, de Boer formulated his theory of a stressed bond which, despite its simplicity, still constitutes the basis for most models of chemical reactivity under stress [92], In order to fracture an unstressed bond which, in the absence of any vibration, is approximated by the Morse potential of Fig. 18, an energy D must be supplied. If, however, the bond is under tension due to a constant force feitt pulling on either end, the bond rupture activation energy will be decreased by an amount equivalent to the work performed by the mechanical force over the stretching distance from the equilibrium position. The bond potential energy in the presence of stress is given by ... 

A number of models for catalytic active sites have been proposed to explain the isospecific polymerisation of propylene in the presence of TiC -based catalysts [68]. It was Natta who proposed [366], for the first time, that the steric control in isospecific propylene polymerisation is caused by the structure of the active species located on the borders of crystal layers of violet titanium trichloride. Arlman and Cossee [277] suggested that the isospecific active sites are located on lateral crystal surfaces, which in violet titanium trichloride such as oc-TiCh correspond to (110) planes. Titanium atoms present on the above-defined fracture surface have a vacant octahedral site bonded to five chlorine atoms (Figure 3.26) [1,68]. 

Based on these considerations about the possible nature of the effects observed, let us construct a phenomenological model of the process, not going deeply for the time being into the reason for the high chemical activity of the surfaces formed during the fracture. 

Burger, K. N., Nieva, J. L., Alonso, A., and Verkleij, A. J. (1991), Phospholipase C activity-induced fusion of pure lipid model membranes. A freeze fracture study, Biochim. Biophys. Acta, 1068,249-253. 

A molecular transition model involving the -relaxation in PMMA has been first put forward by Johnson and Radon They explained the transition in crack speed behavior based on a correlation between the temperature dependence of a time to failure inferred from fracture experiments and the temperature variation of the reciprocal frequency of the P-relaxation peak. They thus assumed that the crack transition is caused when the P-process is fully active. Also the fracture mechanics parameter K, governing the transition from slow to fast crack growth, shows a time and temperature dependence equal to that of the p-transition... 

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