Nuclear plants, decontamination

Staehle, R. W. Presented at the Conference on Decontamination of Nuclear Plants Columbus, OH, 1975. 

The cobalt deposition rate on new, replacement, or decontaminated recirculation piping surface has been reduced by pretreating the piping using an atmosphere of oxygenated wet steam to form an oxide film (25). Studies have been conducted for both PWRs and BWRs to reduce the cobalt content of materials used in the nuclear parts of the plants, particularly in hardened and wear surfaces where cobalt-base alloys ( 50% Co) are used (26). Some low cobalt materials have been developed however, the use of the materials is limited to replacement parts or new plants. 

The solvent extraction process that uses TBP solutions to recover plutonium and uranium from irradiated nuclear fuels is called Purex (plutonium uranium extraction). The Purex process provides recovery of more than 99% of both uranium and plutonium with excellent decontamination of both elements from fission products. The Purex process is used worldwide to reprocess spent reactor fuel. During the last several decades, many variations of the Purex process have been developed and demonstrated on a plant scale. 

Children, like adults, can be exposed to nuclear agents through an attack on a nuclear power plant they can be exposed to radiologic agents through the release of a dirty bomb or an unintentional release. The decontamination and treatment of patients exposed to radioactive contaminants are discussed elsewhere in this book (refer to chapter 20). This section outlines pediatric-specific recommendations for care. 

Radioactive waste treatment applications have been reported [3-9] for the laundry wastes from nuclear power plants and mixed laboratory wastes. Another interesting application of reverse osmosis process is in decontamination of boric acid wastes from pressurized heavy water reactors (PHWRs), which allows for the recovery of boric acid, by using the fact that the latter is relatively undissociated and hence wdl pass with water through the membrane while most of the radioactivity is retained [10]. Reverse osmosis was evaluated for treating fuel storage pool water, and for low-level liquid effluents from reprocessing plants. 

Laboratory and pilot plant experiments carried out at INCT showed that reverse osmosis is very useful for the treatment of liquid low-level radioactive wastes from Polish nuclear laboratories. However, to reach high decontamination the process should be arranged as a multistage operation with microfiltration or ultrafiltration pretreatment [32,33]. 

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. 

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 the nuclear research centers, the wastes are obtained from research and experimental reactors, hot cells, pilot plants, and decontamination services. These wastes have varying physical, chemical, and radioactive nature due to the great diversity of isotopes used and the wide range of processes in which they are applied. 

Ultrasound has been successfully employed for the removal of radioactive cruds by Japanese workers [250], Radioactive crud deposited on the various apparatus of a nuclear power plant was removed by electropolishing followed by ultrasonic cleaning. Their method exhibited excellent removal efficiency, and was found to be especially useful for removing deposits on the inside wall of coolant conduits. Similar results were obtained by Gauchon et al. [251] who studied the decontamination of German BWR nuclear reactors by chemical, electrochemical, and water-jet methods employing ultrasound. Their results showed that simultaneous electrolysis and ultrasound gave the most efficient method for decontamination. 

In Donnan dialysis for decontamination of radioactive wastes from nuclear power plants. 

Use of these Purex equilibrium charts will be illustrated by calculating the number of equilibrium extracting and scrubbing stages needed in the uranium decontamination unit 2D of the Barnwell Nuclear Fuel Plant, data for which were given in Fig. 10.11 and Tables 10.7 and... 

Table 10.10 Adjusted material balance for calculation of number of theoretical stages in uranium decontamination unit 2D of Barnwell Nuclear Fuel Plant"... 

Decontamination of potable water of the Dnieper river by using a combination of dust-like clinoptilolite and aluminum sulfate followed by filtration through clinoptilolite layers led to a drastic decrease of radioactivity [66, 67]. In addition, filters of clinoptilolite tuffs were suggested to extract radionucleides from the drainage water of the encapsulated Chernobyl nuclear power plant. Filtration reduced Cs by 95% and Sr by 50-60%. After one year the filters carrying a radioactivity of lO Ci/kg were exchanged and buried [66]. 

EPRI-NP-6433, "Source Book for Chemical Decontamination of Nuclear Power Plants," August 1989. 

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