Design of a mobile dissolved air flotation system with high rate for the treatment of liquid radioactive waste

https://doi.org/10.1016/j.psep.2020.07.016Get rights and content

Abstract

The global trend towards sustainable development with low carbon emissions has encouraged the expansion of nuclear energy. The nuclear reactors, under normal operating conditions or radiological emergency, generate different types of radioactive waste, mainly liquids and solids that require efficient and mobile treatment systems. This work presents the design proposal of a mobile dissolved air flotation system (mDAF) with a high rate for the treatment of liquid radioactive waste generated in the attention and response to radiological post-emergencies of nuclear power plants or environmental emergency responses associated with oil spills. The evidence obtained showed that the design used allows to reach, under the conditions of the experiment tested, removal levels higher than 95 % in the concentrations of oil waste, SO4−2, NO3-, PO3-3, Ni and Cr. Removal levels of approximately 94 % and 75 % for the total Co and 60Co, respectively, were achieved. It was possible to demonstrate the feasibility of designing a mDAF of low cost with enough versatility to treat radioactive liquid waste generated in a radiological post-emergency.

Introduction

There is a global trend towards the sustainable economic development of low carbon emissions, aimed at contributing to climate change mitigation efforts caused by global warming (Dedinec et al., 2016). Different strategies and public policies to mitigate climate change have been proposed in recent years, examples of these are the decrease in fossil fuel consumption by the transport sector (Andersson et al., 2020), promote sustainable land use and regulate land changes (Ellison et al., 2014; Kissinger et al., 2019), among others. In the energy sector, the incorporation and massification of renewable energies and the expansion of nuclear energy are contemplated the latter as alternatives for reducing CO2 emissions (Goh and Ang, 2018; Peng et al., 2019; Hassan et al., 2020).

There are different arguments against the expansion of nuclear energy, the main ones being a) the risks of radiological emergencies such as those that occurred in Fukushima (International Atomic Energy Agency (IAEA, 2015; Hassan et al., 2020; Kosai and Unesaki, 2020), and b) the generation of radioactive waste (Knapp and Pevec, 2018; Mahmood et al., 2020). This debate is not new and will surely continue for several more years.

Concerning the production of radioactive waste, these are mainly generated from the operation and decommissioning of nuclear power plants, and fuel cycle facilities (Watanabe et al., 2019). The nuclear power plants, under normal operating conditions, generate gaseous, liquid and solid waste, with diverse chemical and radiological characteristics (Kerlin and Upadhyaya, 2019). Gaseous wastes are mainly non-condensable gases, fission product gases (131m, 133m, 133, 135, 135m,137, 138Xe, 85, 85m, 87, 88Kr, 131,133I, mainly), water activation gases and radiolytic gases (Chae et al., 2019). Liquid radioactive wastes are produced in greater volume and are characterized by low radioactive activity (137Cs, 134Cs, 60Co, 58Co, 131I, etc.), and small concentrations of other chemicals (surfactants, oily residues, sulphates, oxides, silicates, etc.) (Ortiz-Oliveros and Flores-Espinosa, 2019). As a result of the treatment of these liquids, wet, solid radioactive waste (resins, filters, evaporator concentrates and sludges) is produced (International Atomic Energy Agency (IAEA, 2017). Concerning radioactive solid waste, these include compactable waste as well as personnel and other materials (paper, plastics, etc.) produced in the different activities carried out at the power nuclear plant, and metal materials, concrete materials and other inert waste that are non-compactable (Ojovan et al., 2019). The most important radioactive solid waste, from a radiological safety perspective, are spent fuels; these are characterized by containing fission products of 235U, 239Pu, 241Pu and their subsequent radioactive decays; minority actinides generated by neutron capture of 238U and its transmutations (Np, Pu, Am and Cm, mainly) and neutron activation products of zircaloy bars, structural steels and UO2 impurities, etc. (International Atomic Energy Agency (IAEA, 2020).

In general, radioactive waste can be classified into 6 large classes according to their final disposal: exempt waste, very short-lived waste, very low-level waste, low-level waste, intermediate-level waste, and high-level waste (International Atomic Energy Agency (IAEA, 2009).

In a nuclear power plant, during and after a radiological emergency, large amounts of contaminated solid and liquid materials are generated (with 134Cs and 137Cs, mainly), as well as radioactive waste (Tateda et al., 2020). The radiological characteristics of these depend on the nature of the emergency, for example, at the Fukushima Daiichi nuclear power plant (as of November 2014) around 131,900 m3 of debris, 79,700 m3 of trees and thousands of cubic meters of water, contaminated with various radionuclides, had been generated (International Atomic Energy Agency (IAEA, 2015; Tanaka et al., 2018). Therefore, treatment and storage facilities had to be built according to the large volumes of materials and waste generated.

In general, radioactive waste treatment systems are usually installed in buildings within or near nuclear power plants, and in more recent cases in centralized facilities dedicated to the treatment of radioactive waste managed by independent management entities (Ojovan et al., 2019; Watanabe et al., 2019). In both cases, the design and construction of the treatment systems are built-in, fixed and robust installations, often underutilized (International Atomic Energy Agency (IAEA, 2014a). In recent years, the IAEA has recommended the development and use of mobile treatment systems because they offer greater flexibility compared to those built-in permanent facilities (International Atomic Energy Agency (IAEA, 2014b). Further, its performance, cost and flexibility can make mobile systems attractive for use in nuclear power plants or a radiological post-emergency. These novel systems offer multiple applications in radioactive waste management, being able to adapt to small and large scales, as in the facilities of the fuel cycle (International Atomic Energy Agency (IAEA, 2014a,2014b).

In general, liquid radioactive wastes are generated in a higher volume, both in nuclear power plants and during a radiological emergency. Regardless of the source of the radioactive liquid waste, these are traditionally treated by evaporation, ion exchange, chemical precipitation, filtration, membrane technology (nano-filtration, ultrafiltration and reverse osmosis) and centrifugal separation (Dobre et al., 2018; Kim et al., 2019; Ojovan et al., 2019; Zdorovets et al., 2020). Organic liquid wastes are treated, mainly, by incineration (Purkait et al., 2020). In recent years, unconventional treatment techniques in the nuclear industry, such as dissolved air flotation (DAF) has been explored as alternative technologies for the treatment of these wastes, demonstrating the removal of ionic and colloidal radionuclides from liquid radioactive waste (Ortiz-Oliveros and Flores-Espinosa, 2019).

DAF systems are a separation technique based on the dissolution of air and the generation of microbubbles that accelerate the separation of contaminants, hydrocarbons, minerals, algae, radionuclides, and thickening of activated sludge, among others (Ortiz-Oliveros et al., 2012; Oh et al., 2019; Zhang et al., 2019; Cagnetta et al., 2019; Mohammadi et al., 2020; Tang et al., 2020). The separation of ionic and colloidal materials is mainly due to the physical-chemistry of surface phenomena (surface interaction between the particle and microbubbles) and the hydrodynamics of the system (Zhang et al., 2020; Li et al., 2020). Dissolved air flotation systems are formed by a coagulation/flocculation stage, a pressurization tank, an air injection system and a flotation cell. These systems compared to sedimentation, filtration, and membrane technology is considered to have moderate operating costs and is designated a clean technology because it uses minimal amounts of air and coagulants/flocculant, low retention times, high hydraulic load surfaces, high separation efficiencies, low capital costs and high operational versatility (An et al., 2017; Yu et al., 2017; Ortiz-Oliveros and Flores-Espinosa, 2019). These characteristics show it as a competitive treatment system.

In this context, we present, within this work, the design proposal of a mobile dissolved air flotation system (mDAF) with a high rate for the treatment of liquid radioactive waste generated in the attention and response to radiological post-emergencies of nuclear power plants, as well as in environmental emergencies such as those occurring in oil spills or the recovery of oily waste. Likewise, the experimental results of the flotation tests of the proposed system are presented.

Section snippets

Methods

As shown in Fig. 1, the design of a mDAF with a high rate for the treatment of liquid radioactive waste was carried out considering three basic equipment: (1) coagulation–flocculation reactor, where the destabilization of the colloidal materials occurs; (2) pressurized vessel, packed with rashing rings, causing the gas to dissolve in the radioactive liquid waste where the bubbles are generated; and (3) flotation cell where the collision of the bubbles and flocs is carried out. In this, it is

Design of flocculator

From the geometrical relationships-standardized stirred reactors, identified in Table 2 as well as in Eqs (1) and (2), it was estimated that the reactor has a volume of 0.150 m3. In Table 3, the geometrical characteristics of the flocculator are presented, considering a safety factor of 20 %. This included a hopper in a truncated cone shape for the discharge of the liquid (the height was 0.60 m, with an inclination of 60°, see Fig. 2), which allowed for maintaining the geometric arrangement

Discussion

As shown in Table 7, the proposed system allows removal levels above 95 % in turbidity, as well as in the oil, SO4−2, NO3, PO3−3, Ni, Cr and Pb concentrations. Levels of 85 %–88 % are obtained for the concentration of total solids and Fe. In the case of Co, the removal levels of total cobalt were obtained from the 94 % equivalent to a concentration of 0.32 mg/L and 75 % for the 60Co equivalent to a specific activity of 0.02 kBq/mL.

Under the pH conditions of the experimental test, Co and 60Co

Conclusions

The experimental results obtained with the mDAF, clearly showed that the system design methodology is adequate for the treatment of radioactive liquid waste of complex composition. This allowed high levels of removal of radioactive and non-radioactive contaminants to be achieved.

Comparison of removal levels achieved with the mDAF versus environmental and nuclear standards evidenced compliance with both regulations. Likewise, evidence was found that allows us to infer that the mechanism of

Funding

This research received no external funding.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements

This work is part of project DR-001 of the Radioactive Waste Department, National Institute of Nuclear Research (ININ). The technicians of ININ are also acknowledged, especially to C. Duarte-Alanis for the drawings made

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