Wastes, radioactive evaporation

Low Level Waste Treatment. Methods of treatment for radioactive wastes produced in a nuclear power plant include (/) evaporation (qv) of cooling water to yield radioactive sludges, (2) filtration (qv) using ion-exchange (qv) resins, (J) incineration with the release of combustion gases through filters while retaining the radioactively contaminated ashes (see Incinerators), (4) compaction by presses, and (5) solidification in cement (qv) or asphalt (qv) within metal containers. 

Oxalate Destruction in Waste Stream. Although the reactions are more rapid at 8M HNO3, manganous ion catalyzes the oxidation of oxalate at lower acidities. Approximately 1M HNO3 seems to be required. As the major solids producing reagent in the waste stream is oxalic acid, it is much cheaper to oxidize the oxalic acid to CO2 gas than to store it as radioactive waste. Waste streams are therefore acidified with nitric acid, Mn(N03>2 added and the solutions evaporated. During the evaporation, the oxalate ion is oxidized to CO2 gas. 

Chemical treatment is a weU-proven technique and is less expensive. Generally, specific chemical precipitation is carried out in situ in big tanks holding the radioactive waste stream to remove radionuclides while permitting nonradioactive ions to be released in the effluent from the process. The radionuchdes present in the waste are precipitated, co-precipitated, carrier precipitated, or adsorbed by insoluble compounds. A clariflocculator is used for flocculation and clarification. This technique is used as a batch or a semibatch process for large volumes of effluents containing only low concentrations of activity. It provides lower DF values (10-100) than evaporation. 

In radioactive waste treatment, significant operational aspects include the following. Since the operation requires the use of high pressures, there is a need to ensure control of the activity release from possible leaks. As with evaporation, pretreatment of the feed may be necessary to prevent scaling, and where dirty waters are to be fed directly it would be advisable to consider the use of equipment with larger membrane flow channels, which would permit periodic foam ball cleaning of the membrane surface. 

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. 

Liquid radioactive waste was directed from the waste storage tank to the 8 m feed reservoir. After pretreatment with PP depth filters and injection of antisealant, the wastes were directed to the first stage of RO. The retentate from this stage was concentrated in the third RO unit. The concentrated solution could be directly solidified if the concentration of the total solute was appropriate (<250 g/dm ). The salt concentration is limited by the conditions of concrete solidification. If the concentration was not sufficient, the further concentration took place in the evaporator. Permeate from the first and third stages was directed to the permeate reservoir before the second RO unit. The product from the membrane installation (permeate from the second stage) was of required radiochemical purity and after the control of specific activity and salinity was discharged to the communal sewage. 

Evaporation is a widely used method for radioactive waste processing. One of disadvantages of the process is radionuclides carry over with small droplets. The contaminated condensate needs additional polishing with ion-exchange resins. The installation of MD module for final cleaning of the condensate can avoid the use of ion exchange. The unit that plays the role of demister can be driven with waste heat from nuclear power plant. 

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. 

Historically, a classic example of an evaporation process is the production of table salt. Maple syrup has traditionally been produced by evaporation of sap. Concentration of black liquor from pulp and paper processing constitutes a large-volume present application. Evaporators are also employed in such disparate uses as desalination of seawater, nuclear fuel reprocessing, radioactive waste treatment,preparation of boiler feed waters, and production of sodium hydroxide. They are used to concentrate stillage waste in fermentation processes, waste brines, inorganic salts in fertilizer production, and rinse liquids used in metal finishing, as well as in the production of sugar, vitamin C, caustic soda, dyes, and juice concentrates, and for solvent recovery in pharmaceutical processes. 

Anthony, D. Improved evaporators for radioactive waste. Chem. Eng. Progress 1983, 58-63. 

Radiochemical and Radioanalytical Letters Figure 5. IR evaporating system for the treatment of a-radioactive waste (21)... 

One method of radioactive waste minimization is to compact the waste into a smaller, more densely packed volume. A second method is by incineration, specifically of exposed organic materials. A third method of waste volume reduction is by either dewatering (filtration) and/or evaporation for water removal and recovery. 

Evaporation. Evaporation is a process whereby a solution or a slurry is concentrated by vaporizing the solvent, normally water. Then a residue with a high solids content, usually a sludge, will be formed that is handled as the radioactive waste concentrate. 

Wl. Warner, B. F., et al. Operational Experience in the Evaporation and Storage of Highly-Active Fission-Product Waste at Windscale, Proceedings of the Management of Radioactive Wastes from Fuel Reprocessing, Paris, 1972, Report CONF-721107, Mar. 1973, p. 339. 

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.)... 

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. 

FIGURE 25.21 Hybrid processes for radioactive waste treatment, (a) Combination of evaporator with MD module, (b) Combination of RO unit with two modules of MD. (c) MD module combined with enhanced UF with regeneration of complexing agent. 

There are many methods used for liquid radioactive waste (LRW) treatment, including chemical precipitation, sedimentation, ion exchange, evaporation, biological methods [7,8], and membrane permeation. The relative costs of all separation processes are an important criterion in the selection of a treatment process. For example, Figure 26.1 shows the relative costs of some desalination technologies as a... 

Forty years of nuclear materials production at the Savannah River Site (SRS) has generated over 300 million liters of aqueous radioactive waste to date. This waste has been primarily generated from the two fuel reprocessing facilities as fission products from reactor irradiations. The current volume of waste is less than half the above due to processing through evaporators to remove excess non-radioactive water. The total activity stored in the tanks is over 600 million Ci (2.2E19 Bq). 

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. 

Aqueous solutions containing uranyl or thorium compounds can be evaporated to dryness and the residues disposed of as radioactive waste. Because of their toxicity, solidification may be necessary prior to burial at a low-level radioactive waste site. 

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. 

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