Isotope separation by selective migration8 March 2017
Nuclear wastewater is typically quite low in radioactive activity, but almost all of the activity must be removed before disposal. All current processes to remove the activity require large, expensive equipment, which also must be properly disposed of once it has served its purpose.
Aprocedure referred to as Selective Migration has been proposed to reduce the activities of selected radioactive species in water, without the use of conventional ion exchange, membrane or filtration techniques, thereby eliminating the secondary waste volumes associated with such methods and reducing the subsequent requirement to fund and manage extra waste disposal.
The technique migrates the species onto layer or film, sustained by a supportive substrate.
The proposed technique was tested in research at the University of South Florida (USF). The project incorporated a phase to scale the technology to an industrial application. A project in the Netherlands was chosen to test the technology with a successful reduction of activity in a commercial setting.
Significant reduction of iron-55 was achieved in initial trials of this technology. The results also show a significant reduction in tritium.
Overview of the technique
This proposed technology, developed by CBT of Florida, USA, involves a proprietary method of sweeping the pH of a volume of active water across a range, to effect a change in redox potential, sequestering radioactive species into a smaller, isolable volume.
This not only decontaminates the water but also obviates the need to bury large volumes of purification media and equipment, leaving a comparatively small amount of material consisting of reduced mass and volume of a disposable film.
CBT believes that its technology has the potential to be automated and inexpensively produced. Another benefit of this technology would be that it can remove nearly all radioactivity material from a solution, depending on the matrix. The technique can be used in parallel with, or to augment, conventional techniques, especially in high salt content matrices.
Description of method
In the laboratory-scale at USF, the activity reduction process took place within a primary vessel, with a recirculation pump. The vessel was filled with test solution. Into the vessel are inserted catalysts in the form of proprietary metallic rods. The rods serve as a substrate onto which a film is slowly deposited over time. The system is operated at ambient temperature and pressure.
The pH of the test solution is important, as it provides the interaction between the matrix and the catalyst material inserted into the vessel. As the recirculation progresses, periodically samples can be collected and counted.
During the test the vessel was sealed at the top with para-film. The initial level of fluid was marked on the side of the vessel to monitor excessive evaporation or fluid level changes, indicating a leak from tubing.
In practice, when using the selective migration technique, more than one vessel containing catalyst can be installed in series or parallel. In studies carried out outside the USF project, typically three vessels were used: a primary, a secondary to contain the catalyst, and a third for dedicated monitoring. Disturbance of the secondary waste substrate can agitate the collected film and isotopes can be released back into the test solution. This accounts for the increase and decrease readily observed in the data graph for iron-55.
System scale and volume
There were two reasons to carry out the study at USF. The first, of course, was to validate the technique. The next reason was to show that the technique could be scaled up to larger industrial scale volumes.
The technique was studied initially using low-energy iron-55, which although low in energy, is representative of typical transition metals, especially cobalt-60, that are among the active species that must be removed from waste. Once the procedure was established in removing iron-55, the technique was tested on tritium, which is much more difficult to remove.
Conclusions from USF study
The results obtained in the studies at USF and elsewhere show that significant reduction of iron-55 was achieved using the selective migration technology (Figure 1). The results also show a significant reduction in tritium levels (Figure 2). Note that in both cases samples were counted on parallel instruments for validation, although results from only one instrument are shown for simplicity. Tritium values were verified by quench evaluation and standard addition.
The initial activity levels of the test matrix were equivalent to those measured in the waters of some commercial reactor facilities, for both iron-55 and tritium.
After the technique was employed, counts performed on secondary waste were close to background for both tritium and iron-55.
For both isotopes, the results indicated that the volume of secondary waste could be minimised without the use of ion exchange resins, membranes, mechanical filters or chelation agents.
Additional studies could be conducted, relating to a detailed documentation of the mechanism of reduction, removal and disposition into a secondary waste medium. These studies could aid in the optimization and development of specifications for designs for full-scale industrial use.
The first-phase project was at laboratory scale, using a 1-2 litre vessel. Using resources and information gained from the project, researchers from the University of South Florida designed a larger scale system. The larger-scale version of the technology was sized at around 200 litres and it was successfully utilised through JB NPP of Sweden and UniTech Services, Europe, in a commercial nuclear uniform laundering facility within weeks after the USF project was completed. Discharge activity was reduced by a factor of 10. This verified that the technique is scalable. It also showed that it could be used in a variety of environments: the test matrix used at the laundry had particulates and was of high salt content. It also included detergents.
Additional field-testing outside of the USF and laundry studies was conducted successfully at different volumes – 5, 10, 60 and 130 litres. In addition, around 20 smaller studies were conducted (in 1 litre Marinelli beakers for convenience).
Tests on reactor water were conducted with tritium levels as high as 108 Bq/kg. The mass of the secondary waste (film or oxide) indicates a secondary waste reduction of several magnitudes when used independently or in conjunction with conventional methods.
Additional field-testing outside of the USF project has demonstrated the technique is useful for selective removal of isotopes in low or high salt content water, including chemical waste tanks, floor drain water, the fuel pool, reactor water and detergent matrices. The technique was successful in reducing the activity of active water containing principal gamma emitters (including cobalt-60), transition metals, carbon-14, tritium, actinides and several others.
Considering that the half-life of tritium is approximately 12 years, the equivalent of around 24 years of radioactive decay based on half- life was removed within two months utilising this technology (Figure 2). Note the initial increase shown is due to mixing equilibration.
CBT believes that the technique could be used to augment current efforts to manage the effects of the tsunami at Japan’s Fukushima plant. Selective migration could be of benefit if it is used along with current tritium cleanup efforts at the disaster site.
More information contact [email protected] The USF study “Isotope separation by selective migration” by Dr Bhethanabotla can be found at: http://www.eng.usf.edu/~bhethana/Projects2.html