Radioactive wastes membrane methods

This section aims to explain the unique features of membrane separation methods, their superior performance in contaminant removal, and their operational sensitivities and limitations. We focus particularly on the factors that need to be carefully assessed when the membrane technology to be used in the treatment of liquid radioactive waste is being considered. These include membrane configuration and arrangement, process application, operational experience, data related to key performance parameters, and plant and organizational impacts. 

Radiation resistance of the material used as well as commercial availability of the membrane units and auxiliary equipment, process competitiveness in comparison with conventional methods, its economics and feasibdity are important criteria for application of the membranes for radioactive waste processing. 

The RO process was implemented at the Institute of Atomic Energy, Swierk. The wastes collected there, from all users of nuclear materials in Poland, have to be processed before safe disposal. Until 1990 the wastes were treated by chemical methods that sometimes did not ensure sufficient decontamination. To reach the discharge standards the system of radioactive waste treatment was modernized. A new evaporator integrated with membrane installation replaced old technology based on chemical precipitation with sorption on inorganic sorbents. Two installations, EV and 3RO, can operate simultaneously or separately. The membrane plant is applied for initial concentration of the waste before the evaporator. It may be also used for final cleaning of the distillate, depending on actual needs. The need for additional distillate purification is necessitated due to entrainment of radionuclides with droplets or with the volatile radioactive compounds, which are carried over. 

One of the current researches devoted to membrane treatment of radioactive waste is directed toward seeded ultrafiltration and all methods, which combined with ultrafiltration, give considerable enhancement of separation (Table 30.6). 

The experiments have proved that membrane distillation can be applied for radioactive wastewater treatment. In one-stage installation the membrane retained all radionuclides and decontamination factors were higher than those obtained by other membrane methods. The distillate obtained in the process was pure water, which could be recycled or safely discharged into the environment. It seems the process can overcome various problems of evaporation such as corrosion, scaling, or foaming. There is no entrainment of droplets, which cause the contamination of condensate from thin-film evaporator. Operation at low evaporation temperature can decrease the volatility of some volatile nuclides present in the waste, such as tritium or some forms of iodine and ruthenium. The process is especially economic for the plants, which can utilize waste heat, e.g., plants operating in power and nuclear industry. 

In Table 30.15, decontamination factors for different processes for low- and medium-level radioactive waste treatment are shown. Membrane distillation with its high decontamination factors is a competitive method in this field. However, it has to be mentioned that these high-decontamination factors are achieved from low-volatile solute after adequate pretreatment. 

Membrane distillation modules are at present expensive in comparison with RO elements and their costs influence significantly the capital costs of the MD installations. The market of MD systems is limited in spite of the many advantages MD method is not widely accepted by the industry. Moderate interest of users influences the production capacities and in consequence reduces the wide implementation of the MD method in different branches of industry. It was proved that advantages of MD decreases with increase of the installation capacity big installations, of productivity comparable with RO, need large number of modules. Anyway, the comparison of two processes RO and MD proved technical and economic reasonability of the letter in some cases, such as radioactive waste concentration. The advantages of MD come from... 

Despite of some technical and process limitations, membrane techniques are very useful methods for the treatment of different types of effluents. They can be applied in nuclear centers processing low- and intermediate-level liquid radioactive wastes or in fuel reprocessing plants. All the methods reported in the chapter have many advantages and can be easily adapted for actual, specific needs. Some of them are good pretreatment methods the other can be used separately as final cleaning steps, or can be integrated with other processes. Membrane methods can supplement or replace techniques of distillation, extraction, adsorption, ion exchange, etc. Evaluation of membrane processes employed for liquid radioactive waste treatment is presented in Table 30.17. 

Advancement in Membrane Methods for Liquid Radioactive Waste Processing... 

Membrane Methods for the Treatment of Liquid Radioactive Waste.666... 

MEMBRANE METHODS FOR THE TREATMENT OF LIQUID RADIOACTIVE WASTE... 

Membrane processes are versatile and flexible they can be combined with other methods in hybrid processes. Adapted to actual needs, they can treat various process streams of different compositions and concentrations. MD coupled with EV or RO may improve the purification efficiency and increase DFs. The flow chart of such hybrid processes is presented in Figure 25.21. In Figure 25.21a, the combination of MD unit with evaporator is shown. EV is a widely used method for radioactive waste processing. One of the disadvantages of the process is radionuclides carry over with small droplets. The contaminated condensate needs additional polishing with IX resins. The installation of MD module for the final cleaning of the condensate can avoid the use of IX. The unit that plays the role of demister can be driven with waste heat from nuclear power plant. 

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