No abstract available

[en] The airborne release of ¹⁴C from various nuclear facilities has been identified as a potential biohazard due to the long half-life of ¹⁴C (5730 years) and the ease with which it may be assimilated into the biosphere. At ORNL, technology has been developed for the removal and immobilization of this radionuclide. Prior studies have indicated that ¹⁴C will likely exist in the oxidized form as CO₂ and will contribute slightly to the bulk CO₂ concentration of the gas stream, which is air-like in nature (approx.300 ppM/sub v/ CO₂). The technology that has been developed utilizes the CO₂-Ba(OH)₂.8H₂O gas-solid reaction with the mode of gas-solid contacting being a fixed bed. The product, BaCO₃, possesses excellent thermal and chemical stability, prerequisites for the long-term disposal of nuclear wastes. For optimal process operation, studies have indicated that an operating window of adequate size does exist. When operating within the window, high CO₂ removal efficiency (effluent concentrations <100 ppB/sub v/), high reactant utilization (>99%), and an acceptable pressure drop across the bed (3 kPa/m at a superficial velocity of 13 cm/s) are possible. Three areas of experimental investigation are reported: (1) microscale studies on 150-mg samples to provide information concerning surface properties, kinetics, and equilibrium vapor pressures; (2) macroscale studies on large fixed beds (4.2 kg of reactant) to determine the effects of humidity, temperature, and gas flow rate upon bed pressure drop and CO₂ breakthrough; and (3) design, construction, and operation of a pilot unit capable of continuously processing a 34-m³/h (20-ft³/min) air-based gas stream

[en] Many of the processing steps in the nuclear fuel cycle generate aqueous effluent streams bearing contaminants that can, because of their chemical or radiological properties, pose an environmental hazard. Concentration of such contaminants must be reduced to acceptable levels before the streams can be discharged to the environment. Two classes of contaminants, nitrates and heavy metals, are addressed in this study. Specific techniques aimed at the removal of nitrates and radioactive heavy metals by biological processes are being developed, tested, and demonstrated. Although cost comparisons between biological processes and current treatment methods will be presented, these comparisons may be misleading because biological processes yield environmentally better end results which are difficult to price. The fluidized-bed biological denitrification process is an environmentally acceptable and economically sound method for the disposal of nonreusable sources of nitrate effluents. A very high denitrification rate can be obtained in a FBR as the result of a high concentration of denitrification bacteria in the bioreactor and the stagewise operation resulting from plug flow in the reactor. The overall denitrification rate in an FBR ranges from 20- to 100-fold greater than that observed for an STR bioreactor. It has been shown that the system can be operated using Ca²⁺, Na⁺, or NH₄⁺ cations at nitrate concentrations up to 1 g/liter without inhibition. Biological sorption of uranium and other radionuclides (particularly the actinides) from dilute aqueous waste streams shows considerable promise as a means of recovering these valuable resources and reducing the environmental impact, however, further development efforts are required

[en] Many of the processing steps in the nuclear fuel cycle generate aqueous effluent streams bearing contaminants that can, because of their chemical or radiological properties, pose an environmental hazard. Concentration of such contaminants must be reduced to acceptable levels before the streams can be discharged to the environment. Two classes of contaminants, nitrates and heavy metals, are addressed in this study. Specific techniques aimed at the removal of nitrates and radioactive heavy metals by biological processes are being developed, tested, and demonstrated. Although cost comparisons between biological processes and current treatment methods are presented, these comparisons may be misleading because biological processes yield environmentally better end results which are difficult to price. However, a strong case is made for the use of biological processes for removing nitrates and heavy metals fron nuclear fuel cycle effluents. The estimated costs for these methods are as low as, or lower than, those for alternate processes. In addition, the resulting disposal products - nitrogen gas, CO₂, and heavy metals incorporated into microorganisms - are much more ecologically desirable than the end products of other waste treatment methods

[en] Chlorination of cooling waters may be a source of environmentally significant pollutants. Many water-soluble chlorine-containing organic compounds of low volatility were found in a sample of cooling water chlorinated to a 2-mg/l chlorine concentration in the laboratory. The compounds were separated and detected using a coupled ³⁶Cl-tracer--high-resolution liquid chromatographic technique developed at the Oak Ridge National Laboratory for determination of chlorinated organics in process effluents. For a chlorination contact time of 75 min at 25⁰C, the yield of chlorine in the form of chloro-organics amounted to 0.78% of the chlorine dosage. It is estimated that the yield is about 0.5% under typical reaction conditions in the electric power plant cooling system chosen for study. Because chlorine is commonly used to remove slime films from the cooling systems of electric power plants, as a means of maintaining high operational efficiency, it is estimated that several hundred tons of chlorinated organics are produced annually in the nation by this antifoulant process. The chromatographic elution positions of some of the separated constituents correspond to those of compounds separated and partially identified from chlorinated sewage treatment plant effluents. The results of this study indicate the formation of chloro-organics during the chlorination of cooling waters should be thoroughly examined, particularly with respect to their identification and determination of possible toxicological properties

[en] It is widely known, but often forgotten, that the rate of release of a contaminant from a waste form is proportional to the available surface area. In addition, typical leach tests to determine rate of release are of relatively short duration and show disappointingly high release rates. By taking a novel approach to the modeling of leaching and using some available long-term leach data, we have been able to demonstrate a potential for obtaining the long-term leach rates from short-term data along with known or measurable properties of the waste form. (author)

[en] Disposal in geological media is being considered for the long-term isolation of high-level radioactive waste. In this study, a full-equilibrium model was developed to describe the transport and fate of the radionuclides in the fracture. Sorption onto the rock matrix, fracture surface and sorption onto mobile and immobile colloids are included. The effect of colloidal particle size was also considered. Mass partition mechanisms between the colloids and solid matrix and between colloid and contaminant are represented by local equilibrium. In the three-phase system, the three parameters, the retardation coefficient, the hydrodynamic dispersion coefficient and fracture width, are modified to include the equilibrium distribution coefficient of a contaminant with a carrier. In the three-phase model, much smaller retardation and hydrodynamic dispersion coefficients are obtained and the effect of the fracture width is larger. Much faster transport of contaminant has resulted mainly because the radionuclides attached to colloidal particles are not subject to retardation by diffusion into the rock matrix. With the additional consideration of colloidal particle sizes, these effects become ever larger. Numerical solutions for the model were obtained using a fully implicit finite difference scheme. A significant sensitivity to model parameters was discovered, and, in particular, the equilibrium distribution coefficients between a contaminant and the carrier were found to be the most important factors