Frank Garisto

Solvation energy of ions in dipolar solvents

The solvation energy of hard spherical ions immersed in dipolar hard sphere solvents is investigated as a function of ion diameter. We apply both the mean spherical and linearized hypernetted‐chain approximations and show that for ions of physically realistic size both theories give qualitatively similar results. The ion solvation energy is obtained as the sum of two competing terms: UID, the direct ion–solvent interaction energy, which is negative, and UDD, the change in the solvent–solvent interaction energy per ion at infinite dilution. UDD is found to be positive and to make an important contribution to the solvation energy for all ion diameters.

The energy spectrum of α-particles emitted from used CANDU™ fuel

Abstract α-radiolysis could affect the dissolution behaviour of used UO2 fuel. To model this effect, one must first determine the α-radiation dose absorbed by the water near the fuel surface, e.g. in fuel cracks. This is the purpose of our paper. First, using the actinide inventories of used UO2 fuel, we calculate the number and energies of the α-particles created in various used fuels. However, the α-particles created in the fuel deposit a large fraction of their energy in the fuel matrix before reaching the fuel surface. In this paper, we show that the self-shielding by the surface layer of the fuel reduces the α-particle dose rate at the fuel surface by about a factor of seven.

The dissolution of UO2: a thermodynamic approach

The oxidative dissolution of UO/sub 2/ has been stimulated using thermodynamic reaction path calculations. The extent of dissolution (as a function of the reaction coordinate) and the solid oxidation sequences are reported for various pH values, ligand (carbonate and phosphate) concentrations and temperatures. There is qualitative agreement between the theoretical predictions and experiments.

Solid dissolution: effect of mass transport-precipitation coupling

Abstract Solubility-limited dissolution models are often used to calculate solid dissolution rates. In these models, the finite solid solubility limits the rate of solid dissolution. However, inhomogeneities such as temperature gradients can produce solubility gradients and, hence, can cause precipitation of solid downstream from the dissolving solid surface. Solubility-limited dissolution models, based on a one-dimensional diffusion-convection equation, are used to investigate the effect of precipitation on solid dissolution rates. Precipitation is found to increase solid dissolution rates. This effect is found to be particularly important at long times, especially if transport of dissolved solid is purely diffusion controlled, i.e. the convective velocity is zero.

Transport of uranium through a cracked clay barrier

Bentonite-based clays are proposed as a barrier to radionuclide migration in the geological disposal of used UO2 fuel. There is some evidence, however, that this buffer may be susceptible to cracking. Therefore, we have investigated the effect of cracking on the transport of dissolved uranium through the buffer using numerical models. The results indicate that, for swept-away boundary conditions at the rock/buffer interface, the total uranium flux through a cracked buffer is, as expected, greater than through an uncracked buffer. The effect of the cracks is strongly dependent on the ratio DDB, where D and DB are the pore-water and the buffer diffusion coefficients, respectively. In contrast, for semi-impermeable boundary conditions at the rock/buffer interface, the effect of the cracks on the total uranium flux is relatively small.

The effect of model dimensionality on source terms for the assessment of used fuel disposal

Abstract 1-D mass-transport models are often used to determine source terms for the safety assessment of used nuclear fuel disposal in geological vaults. In this paper, we investigate the deendence of solubility-controlled source terms on the dimensionality of the diffusion equation used to describe the mass transport of U solution species. Precipitation of U-containing solids downstream from the used-fuel matrix has been included in the model. Such precipitation enhances the dissolution rate of used fuel, and hence, increases the source term. The dimensionality of the diffusion equation is found to have a relatively small effect on radionuclide release for the assumed disposal vault geometries and for time periods less than 10 4 yr.

Reaction path calculations of mineral alteration products: Application to nuclear fuel waste management☆

The dissolution of microcline by water is simulated using reaction path calculations. Alteration products of the microcline dissolution are reported for various temperature and pH conditions. The theoretical predictions of the microcline alteration products agree with experimental results and geological field observations. The implications of rock alteration processes to the safety assessment of a deep underground vault for nuclear fuel waste disposal are discussed.

On the theoretical calculation of the dielectric constant of dilute electrolyte solutions

Abstract In dilute ionic solutions, the solute-dependent dielectric constant varies as a linear function of concentration. In this note we consider alternative theoretical routes to the limiting slope, and compare numerical results for ions in dipolar hard-sphere and water-like solvents.

The Effect of Cracks on Diffusive Mass Transport through a Clay Barrier

Clay-based buffers are often proposed as engineered barriers for underground disposal vaults of used nuclear fuel. Thus, in the Canadian conceptual vault design, each used-fuel container is emplaced in a vertical borehole in rock and surrounded by a compacted buffer material, made up of a mixture of sodium bentonite and sand. There is some evidence, however, that the buffer may be susceptible to cracking due to, for example, cementation or moisture depletion. In this paper we estimate numerically the consequences of cracking on the diffusion of uranium through a finite buffer layer. In particular, we study how the effects of cracks depend on the rock conditions at the edge of the vault. For swept-away boundary conditions at the rock/buffer interface (e.g., representing a major fracture zone in granite), the total flux through the cracked buffer system is greater than through the corresponding uncracked buffer, particularly for buffers with a very small effective diffusion coefficient. On the other hand, for the case in which the rock at the rock/buffer interface is intact and, thus, the mass transport of material from the buffer into the rock is small, the effect of the cracks on the total release flux is negligible.