Diffusion experiments under stagnant conditions in a constant aperture and a variable aperture slot were made to obtain data for simulation of simultaneous flow and diffusion in fractures. This approach was necessitated by the need to avoid buoyancy-induced flow caused by density differences generated by the presence of a tracer. For this purpose, to avoid flow but negligibly influence diffusion the slots were filled with agar, which generates a 99% porous matrix, which negligibly affects diffusion but essentially stops flow. A simple photographic technique was used to follow diffusion and to determine the aperture distribution on the variable aperture slot. With the obtained data, numerical simulations were performed to illustrate how a solute diffuses from a source into the water seeping past. The results support the simple analytical solution that has been used to determine the escape of radionuclides from a damaged canister containing spent nuclear fuel in a geologic repository in fractured rock.

Uranine is a dye commonly used in tracer experiments; it is chosen for its high visibility even at low concentrations. Uranine solutions are slightly denser than water at the same temperature. However, in laboratory experiments uranine solutions have been known to occasionally show unpredictable flow behaviors. This paper investigates the possible effect of light-induced density change to explain some of these behaviors. Uranine has a wide light absorption spectrum for visible light, which can heat the dye solution and lower its density to below that of the surrounding water, which induces buoyancy-driven flow. Simulations are made in both one dimension and two dimensions to determine the extent of the effect. The results are then compared to different experiments with unanticipated flow patterns.

In geologic repositories for nuclear waste located in crystalline rocks, the waste is surrounded by a bentonite buffer that in practice is not permeable to water flow. The nuclides must escape by molecular diffusion to enter the seeping water in the fractures of the rock. At high water-seepage rates, the nuclides can be carried away rapidly. The seepage rate of the water can be driven by the regional hydraulic gradient as well as by buoyancy-driven flow. The latter is induced by thermal circulation of the water by the heat produced by radionuclide decay. The circulation may also be induced by salt exchange between buffer and water in the fractures. The main aim of this paper is to explore how salt exchange between the backfill and mobile water in fractures, by buoyancy effects, can increase the escape rate of radionuclides from a repository.A simple analytical model has been developed to describe the mass transfer rate induced by buoyancy. Numerical simulations support the simple solution. A comparison is made with the regional gradient-driven flow model. It is shown that buoyancy-driven flow can noticeably increase the release rate.

Models for gas−solid reactions in porous particles typically consist of a set of mass and energy balances in the form of conservation equations. For spherical or close to spherical particles, these equations are formulated in 1D spherical coordinates. In the case where accumulation of gas inside the particle is significant, the balance equations contain convective terms. The present work presents a simple numerical scheme based on flux limited finite volume methods for discretizing conservation equations for convective and diffusive transport of mass and energy in radial direction in a porous sphere. The velocity is governed by Darcy's law coupled to an equation of state. The proposed scheme is applied to a series of test problems that admit full or partial analytical solutions. For the cases where only partial analytical solutions are available, a Comsol model is adopted for comparison. It is found that the scheme is able to resolve step gradients without generating oscillations and that it properly handles changes in the sign of the convective velocity. Applying the scheme for solving a common model for biomass pyrolysis reveals the importance of convective gas transport in the pyrolysis of thermally thick biomass particles.

An experiment with a vertical slot with horizontally seeping water with a dye diffusing from below was performed to help validate and visualize the Q-equivalent model, which describes the mass transfer rate from a source into flowing water, such as that in a repository for nuclear waste. The Q-equivalent model is used for quantifying mass transport in geological repositories. However, the tracer propagated much slower and to a lesser extent than predicted by the model. It was found that the tracer gave rise to a small density gradient that induced buoyancy-driven flow, overwhelming that driven by the horizontal hydraulic gradient. This dramatically changed the mass transfer from the dye source into the water in the slot. For the release of contaminants, this can have detrimental as well as beneficial effects, depending on whether positive or negative buoyancy is induced. These observations led to an analysis of when and how density differences in a repository can influence the release and further fate of escaping radionuclides in waste repositories. This and other experiments also showed that laboratory experiments aimed at visualizing flow and mass transfer processes in fractures could be very sensitive to the heating of the dye tracers by the lighting in the laboratory.