Distillation, radioactive waste

Leaching with Soxhlet Apparatus. A Soxhlet leaching apparatus has been used extensively( ) in radioactive waste measurements. The sample is contacted with continuously replenished distilled water. Leaching rates have been shown to be inhibited by ions previously leached from the solid material( ). This technique eliminates interference from ions in solution. 

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. 

Since membrane distillation exhibits high abihty of concentration of aqueous solutions with high retention of acids, salts, and other low-volatile compounds, it can be used for concentration of different radioactive waste streams with high volume reduction and high retention factors [97-99]. 

The process conducted in batch-type counter-flow apparatus (Figure 30.17) equipped with capillary PP Acccurel membranes showed good effectiveness of membrane distillation for purification of radioactive waste. Permeate obtained was pure water. All solutes together with radioactive compounds were rejected by the hydrophobic membrane. At tenfold volume reduction of the initial portion of waste, approximately tenfold concentration of radioactivity in the retentate stream was reached, while radioactivity of permeate retained on the level of namral background (Figure 30.18). As was observed in experiments small sorption in the system took place. However, permeate was free of radioactive substances and other dissolved compounds, the concentration and radioactivity factors sometimes slightly differed from volume reduction factors. 

Membrane distillation for concentration of radioactive waste was also tested in pilot plant experiments. The facihty used in the tests consisted of spiral-wound module, equipped with PTFE membrane, effective surface area of 4 m. The installation enabled the recovery of the part of heat by two installed heat exchangers (Figure 30.19). 

FIGURE 30.19 A scheme of the pilot plant for radioactive waste concentration. (1) MD module, (2) distillate reservoir, (3) feed tank, (4,5) pumps, (6) heater, (7,8) heat exchangers, (9,10) prefilters. (Reprinted from Zakrzewska-Trznadel, G., Harasimowicz, M., and Chmielewski, A.G., J. Membr. Sci., 163, 257, 1999. Copyright [1999] with permission from Elsevier.)... 

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. 

Zakrzewska-Trznadel, G., Harasimowicz, M., and Chmielewski, A.G., Concentration of radioactive components in Uquid low-level radioactive waste by membrane distillation, J. Membr. Sci, 163, 257, 1999. 

Zakrzewska-Trznadel, G., Membrane distillation for radioactive waste treatment. Membrane Technology, An International Newsletter, 103, 9, 1998. 

Membrane technologies have a great potential in the treatment of radioactive liquid wastes, as it has been proved throughout this chapter. In this sense, it is expected a growing use of the membrane processes in the radioactive field, with different possibilities alone, combined between them (microfiltration or ultrafiltration and reverse osmosis) or combined with other conventional processes like evaporation or ion exchange. Furthermore, some special membrane processes, like membrane distillation or liquid membranes, could be applied for the specific treatment of radioactive wastes. 

Separate the condensate by low-temperature distillation into (a) an Ar-Kr fraction to be bottled for permanent storage as radioactive waste and (b) an Xe fraction. 

These distillation units, with capacities of typically up to 50 m /d, have played a major role in the treatment of radioactive waste since their steady and reliable operation over many years has ensured that the high-level hquid waste from the nuclear fuel cycle routinely has been reduced in volume for safe storage prior to further treatment to condition it for disposal and long-term isolation from the biosphere. 

FIGURE 25.18 Concentration of radioactive waste in batch-type MD apparatus. (Reproduced from Chmielewski, A.G. et al.. Purification of radioactive wastes by low temperature evaporation (membrane distillation), Sep. Set TechnoL, 32(1-4), 709, 1997. With permission from Taylor Francis Group.)... 

The question of purity of the circulating water is not at all critical. For example, the boron tolerance for a loss in k of 0.0001 is roughly 10 parts per million. This is easily attained by distillation, and if it were not for the possibility of oil contamination, power boiler condensate would be entirely satisfactory. However, it probably will prove desirable to have a water evaporation imit available for concentrating radioactive wastes anyhow, and this same unit may well be used for producing distilled water for the pile circulating system. 

MD is an effective process for desalination, the concentration of salts and acidic solntions, and distilled water prodnction it can also be applied to water and waste-water treatment (Hogan et al. 1991 Gryta 2002 Couffin et al. 1998 Khayet 2011). The use of this process for low-level radioactive waste treatment leads to a concentration of radioactive species in a small volnme, appropriate for fossilization and the production of clean water streams and for discharge (Zakrzewska-Trznadel 1998 Zakrzewska-Trznadel et al. 1999). MD nnits can be employed in the front end of liquid radioactive waste processing to improve the economy of the treatment by an initial concentration before evaporation or in the back end to obtain better separation of the radionuclides and produce clean efflnents. 

Direct contact membrane distillation (DCMD) is an effective process applied for the concentration of dissolved matter in water solutions. The process can be considered a feasible method for liquid radioactive waste treatment and, in some cases, it can effectively serve in reducing the hazard connected with its toxicity. 

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