Off-gas Treatment System Research Articles

Application of Silver Impregnated Iodine Adsorbent to Nuclear Facilities

Radioactive iodine is one of the most important nuclides to be prevented for release from nuclear facilities and many facilities have off-gas treatment systems to minimize the volatile nuclides dispersion to the environment. Silver impregnated inorganic adsorbents were known as inflammable and stable fixing materials for iodine and the authors started to develop 25 years ago a kind of inorganic adsorbent that has better capability compared with conventional ones. Aluminum oxide (Alumina) was selected as a carrier material and silver nitrate as an impregnated one. Pore diameters were optimized to avoid the influence of impurities such as humidity in the off-gas stream at lower temperatures. Experiments and improvements were alternately conducted for the new adsorbent. The tests were carried out in various conditions to confirm the performance of the developed adsorbent, which clarified its good ability to remove iodine. Silver nitrate impregnated alumina adsorbent (AgA) has about twice the capacity for iodine adsorption and higher iodine removal efficiency at relatively high humidity than conventional ones. The AgA chemically and stably fixes radioactive iodine and fits the storage and disposal of used adsorbent. AgA is now and will be applied to nuclear power plants, reprocessing plants, and research facilities.

Remediation of Elemental Mercury Using in Situ Thermal Desorption (ISTD)

In situ thermal desorption (ISTD) is a soil heating method that simultaneously applies heat and vacuum to the subsurface at temperatures up to 600 degrees C. As the soil is heated, the vapor pressure of the contaminant increases allowing mass transfer to the gas phase and extraction from the soil using vacuum wells. The overall goal of this research is to assess the feasibility of using ISTD to remove elemental mercury from soils. The initial phase of research included design of a laboratory soil column apparatus and preliminary soil column experiments with surrogate nonaqueous phase liquids (perfluorocarbons) to test the apparatus and investigate the effects of air flow rate and temperature on the ISTD process. Following the preliminary experiments, a mercury off-gas treatment system was added and mercury experiments were conducted. Experiments performed using elemental mercury showed greater than 99.8% removal of the mercury from Ottawa sand. These results show that ISTD can remove mercury from soil at temperatures well below its boiling point and that perfluorodecalin can be used as a surrogate for elemental mercury in laboratory experiments. A flow and transport simulator was used to model the results from both the perfluorocarbon and the mercury experiments.

Demonstration plasma gasification/vitrification system for effective hazardous waste treatment

Plasma gasification/vitrification is a technologically advanced and environmentally friendly method of disposing of waste, converting it to commercially usable by-products. This process is a drastic non-incineration thermal process, which uses extremely high temperatures in an oxygen-starved environment to completely decompose input waste material into very simple molecules. The intense and versatile heat generation capabilities of plasma technology enable a plasma gasification/vitrification facility to treat a large number of waste streams in a safe and reliable manner. The by-products of the process are a combustible gas and an inert slag. Plasma gasification consistently exhibits much lower environmental levels for both air emissions and slag leachate toxicity than other thermal technologies. In the framework of a LIFE-Environment project, financed by Directorate General Environment and Viotia Prefecture in Greece, a pilot plasma gasification/vitrification system was designed, constructed and installed in Viotia Region in order to examine the efficiency of this innovative technology in treating industrial hazardous waste. The pilot plant, which was designed to treat up to 50 kg waste/h, has two main sections: (i) the furnace and its related equipment and (ii) the off-gas treatment system, including the secondary combustion chamber, quench and scrubber.

温度スイング吸着(TSA)プロセスを導入した物質循環型オフガス処理システムの開発 (3) TSAプロセスの設計と装置規模の評価

温度スイング吸着(TSA)プロセスを導入した物質循環型オフガス処理システムの開発 (3) TSAプロセスの設計と装置規模の評価

Extraction Behavior of TRU Elements in the Nuclear Fuel Reprocessing

The extraction behaviors of uranium, plutonium and other nuclides in the fuel reprocessing have been investigated to study the capability of confining the radioactivity in the PUREX process. Reprocessing tests using spent fuel of 44GWd/t have been carried out in alpha-gamma cell equipped with reprocessing equipment in NUCEF(Nuclear Fuel Cycle Safety Engineering Research Facility). The reprocessing equipment consists of a dissolver, six mixer-settlers, off-gas treatment and aqueous waste treatment systems. The mixer-settlers were assigned to diluent washing, co-decontamination, FP scrub, U recovery and U/Pu partitioning steps. The concentration profiles of nitric acid, U, Pu, neptunium and americium were obtained under normal operation of PUREX flow sheet. The distribution fractions of the nuclides were calculated by the concentrations and the flow rates of organic and aqueous solutions. It was found that U and Pu were recovered from the spent fuel dissolver solution and were well separated from each other. About 10% of Np was distributed into the raffinate solution in the co-decontamination step. Almost ail of the Am migrated to the raffinate solution. The experimental results were used for the evaluation of the simulation code ESSCAR(Extraction process simulation code) developed for the safety analysis of reprocessing.

A Simple Model Predicting Iodine Profile in a Packed Bed of Silica-Gel Impregnated with Silver Nitrate

Based on a simple adsorption theory, a mathematical model was proposed to predict axial iodine profiles of the column of silica gel impregnated with silver nitrate (hereinafter referred to as Ag-S) in an off-gas treatment system for spent fuel dissolution. The unknown parameters of the model: the effective diffusion coefficient Dea and the Langmuir coefficient K were determined by curve fitting of iodine profile experimentally obtained. At 423 K, Dea and K were found to be 5.60×10-7 m2.s-1 and 1.0×105 m3.kg-1, respectively. Using the parameter values, the model could well predict the iodine profiles obtained at 423 K in the previous works under different experimental conditions. Furthermore, the effect of silver contents on the iodine profiles was reasonably predicted. It was suggested that the proposed model is simple and would be useful to predict the iodine profiles in Ag-S adsorbent columns.

Behavior of Hazardous and Radioactive Metals in a Laboratory Furnace and a Demonstration-Scale Incinerator

The behaviors of hazardous and radioactive metals in analytical and real combustion conditions were observed to understand their vaporization and to predict their release into the atmosphere at a real incineration plant. In the laboratory furnace experiment, vaporization fluxes were obtained based on the measurement of weight losses with temperatures. Vaporization fluxes were compared to maximum theoretical fluxes obtained from the Herz-Knudsen's kinetic theory of gas. The vaporization coefficient, α, for volatile metal chloride appears to be a characteristic value of the evaporating substance, not varying with increasing temperatures. With some theoretical considerations on interfacial equilibrium, mass transfer coefficient of vaporization, KG, was determined as a function of absolute temperature. To evaluate the behavior of metals and radionuclides in an incinerator and off-gas treatment system, test burns of simulated waste containing several toxic metals and radioisotopes were performed at the demonstration-scale incineration plant. The previously analyzed characteristics of volatilization of hazardous metals and radioactive nuclides were used to explain their distributions in the demonstration plant. The decontamination factor for low-volatility species was above 105, which was equivalent to the removal efficiency of particulate matter in the plant. The factors for semivolatile components were in the range between 102 and 104, showing the strong dependency on volatility. Key words: Metal; radionuclide; volatility; incineration; removal efficiency

Removal of Dissolved VOCs from Water with an Air Stripper/Membrane Vapor Separation System

Treatment of water contaminated with volatile organic compounds (VOCs) is a major problem for the United States chemical industry. Currently, VOCs are removed from moderately contaminated wastewater streams by processes such as steam stripping and from dilute wastewaters by air stripping combined with a carbon adsorption off-gas treatment system. This paper describes the development and performance of a hybrid process that combines air stripping with membrane organic-vapor separation to recover VOCs from the stripper off-gas. A number of prototype systems have been constructed and evaluated. The optimum system appears to be a tray stripper fitted with a high-pressure compression-condensation membrane separation unit. Such a system can remove 95 to 99% of the VOCs present in contaminated water; the removed VOCs are recovered as a liquid condensate. The economics of the technology are competitive with alternative processes, particularly for streams containing more than 500 ppm VOC and having flow rates less than 10 to 30 gal/min.

Stripping of TCE from water at low air/water ratios

AbstractTrichloroethylene was removed from water by air stripping in a packed column using low air/water ratios, and the measured removal efficiencies were used to correlate the height of a transfer unit using a model which takes into account both liquid‐ and gas‐phase resistance to mass transfer. The resulting correlation was used to optimize the design of a combined air stripper and offgas treatment system. Estimated overall cost savings of up to 26% compared to conventional air strippers can be realized by use of the optimum air/water ratio.

Separation Technology for Radioactive Iodine from Off-Gas Streams of Nuclear Facilities

Iodine separation technology using an inorganic adsorption material has been investigated in order to apply the technology to the off-gas treatment systems of nuclear facilities. Iodine removal efficiencies were checked by laboratory experiments using simulated off-gas streams of various conditions and the developed adsorbent, silver impregnated alumina (AgA). Laboratory test results demonstrated effective iodine removal with high decontamination factors (DF's) at relatively high temperatures (≥100°C). Then the removal efficiency were confirmed using actual off-gas streams sampled from the dissolver off-gas treatment system of the Karlsruhe reprocessing plant. The DF's were over 103 with the AgA bed depth of 10 cm and showed little change during the adsorption period, which indicated applicability of the iodine removal technology with AgA to nuclear fuel reprocessing plants. Iodine adsorption capacity and its release property were also investigated using simulated off-gas streams. The former had a value of ~0.22g/g-AgA and this value could well predict the breakthrough property. The adsorbed iodine was judged stable during the storage of AgA saturated with iodine in air at temperatures below 500°C and in water at ~20°C after changing the adsorbed iodine form from AgIO3 to Agl. Thus, the separation technology provided effective and stable iodine separation from the off-gas of nuclear facilities.

Constraints on mass balance of soil moisture during in situ vitrification

In situ vitrification (ISV) is an environmental remediation technology used to melt contaminated soil sites into more stable configurations. The behavior of water and other volatile constituents in the soil-melt system is important to the overall performance of the ISV technology. Mass and volume balance constraints are used to derive a method to indirectly estimate the volume of: (1) soil that dehydrates and releases water vapor to the off-gas, and (2) outside air pulled into the off-gas treatment system. These constraints allow us to speculate on whether some water may remain in the soil rather than being completely transported into the off-gas system. The method is tested with data from a field-scale test. Results suggest that, contrary to previous conceptual models, not all water that is vaporized reaches the surface and captured by the off-gas treatment system. It is probable that some percentage remains within the soil beneath and around the molten ISV mass.

Separation Technology for Radioactive Iodine from Off-Gas Streams of Nuclear Facilities.

Iodine separation technology using an inorganic adsorption material has been investigated in order to apply the technology to the off-gas treatment systems of nuclear facilities. Iodine removal efficiencies were checked by laboratory experiments using simulated off-gas streams of various conditions and the developed adsorbent, silver impregnated alumina (AgA). Laboratory test results demonstrated effective iodine removal with high decontamination factors (DF's) at relatively high temperatures (≥100°C). Then the removal efficiency were confirmed using actual off-gas streams sampled from the dissolver off-gas treatment system of the Karlsruhe reprocessing plant. The DF's were over 103 with the AgA bed depth of 10 cm and showed little change during the adsorption period, which indicated applicability of the iodine removal technology with AgA to nuclear fuel reprocessing plants. Iodine adsorption capacity and its release property were also investigated using simulated off-gas streams. The former had a value o...

On-line, real-time measurements of decontamination factors for a low-level waste incinerator

A method is described to monitor the distribution of radioactive isotopes through the off-gas treatment system of an incinerator. Simulated commercial waste spiked with known amounts of five fission products, 131I, 106Ru, 137Cs, 59Fe, 60Co, were incinerated. High resolution photon detectors were installed on the off-gas handling system of the controlled air incinerator at the Los Alamos National Laboratory to determine relative decontamination factors, which ranged from a low of 40 for 137Cs to a high of 5000 for 131I. Background measurements were made to determine the residual activity in the incinerator. Due to the constant purging of the system to maintain a negative pressure with respect to the sorrounding environment, the residual activity decays more rapidly than dictated by its half-life.

Selected conversion of NOx by catalytic reduction with ammonia. An effective off‐gas treatment system is an environmental must in the reprocessing of nuclear fuels. An up‐to‐date progress report

The General Atomic Company off-gas treatment system is designed to process simulated radioactive or other noxious volatile and gaseous constituents in dissolver off-gas (DOG) streams indigenous to several nuclear fuel cycles, e.g., light-water reactors (LWRs). A series of NO/sub x/ converter tests was carried out with the GA engineering-scale system. The major reactions and optimum conditions were characterized for the selective catalytic reduction of pure NO, pure NO/sub 2/, and NO/sub x/ with NH/sub 3/ over synthetic hydrogen mordenite (Zeolon 900 H). Steady-state conversions in the range 99 to 99.9% were obtained for the specified conditions without observing any breakthrough of NH/sub 3/ in the effluent. The NO/sub x/ reduction capacity for a single-stage adiabatic converter is approximately 1% NO/sub x/ in the feed gas and is restricted by catalyst temperature limitations. 7 references, 9 figures. (JMT)