Thermodynamic Coupling of Heat and Matter Flows in Near-Field Regions of Nuclear Waste Repositories

Abstract

In near-field regions of nuclear waste repositories, thermodynamically coupled flows of heat and matter can occur in addition to the independent flows in the presence of gradients of temperature, hydraulic potential, and composition. The following coupled effects can occur: thermal osmosis, thermal diffusion, chemical osmosis, thermal filtration, diffusion thermal effect, ultrafiltration, and coupled diffusion. Flows of heat and matter associated with these effects can modify the flows predictable from the direct effects, which are expressed by Fourier's law, Darcy's law, and Fick's law. The coupled effects can be treated quantitatively together with the direct effects by the methods of the thermodynamics of irreversible processes. The extent of departure of fully coupled flows from predictions based only on consideration of direct effects depends on the strengths of the gradients driving flows, and may be significant at early times in backfills and in near-field geologic environments of repositories. Approximate calculations using data from the literature and reasonable assumptions of repository conditions indicate that thermal-osmotic and chemical-osmotic flows of water in semipermeable backfills may exceed Darcian flows by two to three orders of magnitude, while flows of solutes may be reduced greatly by ultrafiltration and chemical osmosis, relative to the flows predicted by advection and diffusion alone. In permeable materials, thermal diffusion may contribute to solute flows to a smaller, but still significant, extent.

Formulation of electro-chemico-osmotic processes in soils

“…Assuming that the solid's density, Ps is constant, the equation of solid mass conservation for a nonreactive solid matrix can be expressed by [13] d(1 -n) PsV'(l-n)Vs+Ps dt -0 (8) from which we obtain 1 dsn (9) V'Vs-l_n dt Assuming vertical compressibility only, and no change in total stress, ~, i.e., d~ --0, we can write the following expression by employing Terzaghi's definition of effective stress, ~' = ~ -Ip, where p is the pore pressure, and I is the unit matrix dse dsa' dsp V 9 V~ _= -d~-= a ---d~-= a -~-,…”

“…Such a system was studied by Elrick et al [7]. Thermodynamically coupled flows in near-field regions of nuclear waste repositories have been studied by Carnahan [8]. Electrokinetic salt rejection in hyperfiltration has been investigation by Jacazio et al [9].…”

Thermal impact of waste emplacement and surface cooling associated with geologic disposal of high-level nuclear waste

“…For the low-permeability, small-deformation effects expected for the waste impact, the simple approach is usually justifiable. However, it should also be realized that studies employing coupling are needed for better understanding of the physical and chemical processes involved, especially in the near-field (Tsang 1980;Carnahan 1983;LBL 1984;de Harsily 1985;Cook 1985). The coupling processes of primary concern are…”

Simulation of Chemically Reactive Solute Transport Under Conditions of Changing Temperature

“…thermal osmosis. chemical osmosis, and ultrafiltration) arising from thermodynamic coupling between flows of heat and matter (Carnahan, 1984(Carnahan, , 1985 are absent. These assumptions allow the calculation of heat transport to be done independently of the calculations of solute transport and chemical equilibria.…”