MODELING CHEMICAL AND PHYSICAL PROCESSES IN LEACHING SOLIDIFIED WASTES zq # n ?
Solidification is an important treatment process in hazardous waste management and will continue to be so until waste minimization and waste recycle processes are perfected for all hazardous wastes. It is generally recognized that immobilization of contaminants in solidified wastes occurs through both physical and chemical mechanisms. Standard techniques for analyzing contaminant leaching measure only an observed diffusivity that does not separate chemical and physical factors. The suitability of the standard data analysis procedure is reviewed and alternative models developed that describe the separate effects of chemical and physical processes. These models describe physical transport through a solidified waste matrix according to Fick's law. Chemical reactions of sorption/desorption and precipitation/ dissolution are described. The observed diffusivity that would be calculated by ignoring chemical processes is shown to depend on the true effective diffusivity and coefficients that describe the chemical phenomena.
Assessment of the loss of radioactive isotopes from waste solids to the environment. Part I. Background and theory
A report is given of several theoretical expressions based on mass transport phenomena that relate the radioactivity escaping from such solids to diffusion, dissolution processes, surface conditions, and radioactive decay. Representative available data for radioactive waste solids incorporated in cement, asphalt, ceramic, and glass media are analyzed using the theoretical expressions presented. These analyses show that an expression taking into account diffusion and concentration-dependent dissolution gives good agreement with the data for most of the products considered. In the main, these products can be categorized as waste solids of low solubility incorporated in inert matrices. The effective diffusivities obtained are in the range of mid 10$sup - 17$ to mid 10$sup -12$ cm$sup 2$/sec, the dissolution rate constants are in the range of high 10$sup -9$ to low 10$sup -7$ sec$sup -1$, and the surface transfer constants are in the range of low 10$sup -4$ to low 10$sup -3$ sec$sup -1$ for the products analyzed. Once determined, such parameters can be used to compare various waste products and to estimate releases from these products, in particular, long-term releases. (auth)
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.