Soil Decontamination by Subcritical Carbon Dioxide
Development and Demonstration of Technology for Decontamination of Solid Surfaces and Soils by Subcritical Carbon Dioxide
Tech Area / Field
- ENV-RED/Remediation and Decontamination/Environment
8 Project completed
Senior Project Manager
Ryzhova T B
Khlopin Radium Institute, Russia, St Petersburg
- NPO Mayak, Russia, Chelyabinsk reg., Oziorsk\nMining and Chemical Complex, Russia, Krasnoyarsk reg., Zheleznogorsk
- Idaho National Engineering and Environmental Laboratory, USA, ID, Idaho Falls\nCEA / DCC / CEN Valrho, France, Pierrelatte\nUniversité Louis Pasteur, France, Strasbourg\nBritish Nuclear Fuels Ltd (BNFL), UK, Chesire, Risley Warrington
Project summaryProblem concerned with the removal of radionuclides from solid surfaces is one of the fundamental problems on decommissioning of nuclear power facilities and rehabilitation of contaminated territories. Along with the widely known decontamination technologies for contaminated surfaces and soils, in the last 10 years a method of supercritical fluid decontamination (SFD) has been actively developed, that is, the decontamination by gases at supercritical temperatures and pressures.
Compared to the traditional methods of decontamination (washing of surfaces by acidic and alkaline solutions of oxidants and complexones), the SFD method enables to reduce the amount of secondary liquid radioactive waste and to do away with the use of toxic, corrosive and explosion-fire-hazardous reagents. Carbon dioxide (CO2) is generally used in SFD as a "solvent", since it possesses relatively low critical parameters (Tcr. = 31°C and Pcr. = 73 atm), it is non-toxic, fire-explosion-proof, easily available and inexpensive. Reduction in pressure and/or increase of temperature causes SC-CO2 to become a gas, providing the solvent recycle and isolating the extracted elements in dry and compact form.
During the last 10 years work is underway on development and improvement of commercial equipment for supercritical fluid extraction of metals and decontamination of surfaces, as well as in the field of decontamination technologies. CEF (France), BNFL (Great Britain), DOE (USA), JAERI (Japan) and RPA "Radium Institute" (Russia) have been deeply involved in this research. Studies on supercritical extraction of metals are predominantly aimed at the development of methods for metal removal from solid surfaces by SC-CO2 to decontaminate soils, fabrics, asbestos, rubber etc and to simplify sampling procedures in environmental analysis. The first dry-cleaning workshop with the use of liquid CO2 was opened in the USA in 1998. CEA of France in collaboration with VTT company elaborates technology and a mobile facility (4 contactors, 1 m3 each) for SC-CO2 decontamination of soil. In Russia studies are pursued for demonstrating the potentialities of supercritical fluid decontamination of really contaminated surfaces and for determining the solubility of radionuclide complexes in SC-CO2, including the study under ISTC Project # 1160.
However, the supercritical fluid decontamination envisages conducting the process at P ~ 300 atm and T ~ 70 °C. Similar conditions are rather dangerous and process equipment is expensive. As compared to supercritical decontamination, the efficiency of subcritical decontamination is somewhat lower (by a factor of 2-4), but the equipment cost for subcritical process (<70 atm) is 15-30 times lower than that of SFD (300 atm). Besides, it is more reliable to conduct processes at 70 atm than under a pressure of ~ 300 atm. The existing pilot facilities suitable for work with radionuclides can operate at ~ 70 atm and can be used only for subcritical decontamination.
The experience gained in supercritical decontamination of solid surfaces and soils, as well as the data on solubility of radionuclide complexes in sub- and supercritical CO2 make it possible to start the development of soil subcritical decontamination technology.
Hence, the main objective of this project is to develop decontamination technology for soils and other solids with the use of subcritical CO2 and to test this technology on an enlarged scale, reasoning from the data on supercritical decontamination and solubility of radionuclide complexes.
The project execution should lead to the following main results:
– A wealth of evidence will be obtained and processed on extraction of radionuclides (U, Pu, Np, Am, Cs and Sr) by subcritical CO2 at 50-70 atm and at 20-40 °C in the presence of commercial complexing agents such as TBP, D2EHPA, polyethylene glycols, fluorineted carboxylic acids and b-diketones.
– Technology for decontamination of solids (protective clothes, inpidual protection means) and soils with the use of subcritical carbon dioxide will be developed.
– Pilot facilities for subcritical extraction at RI and MCC will be modernized as applied to decontamination problems.
– Technology for subcritical decontamination will be tested on enlarged scale with the use of really contaminated samples, including soils.
Thus, the execution of studies under the project should permit to elaborate the technology of subcritical fluid decontamination, to improve the pilot facilities for subcritical fluid decontamination and to obtain the data needed for feasibility study concerned with development of full-scale subcritical decontamination facility and for preparation of Statement of Work on its development.
The obtained could be used for development of low-waste decontamination facilities as applied to the following objects:
– small metallic and plastic complex-shaped articles, including those with inner cavities;
– protective overalls and shoes;
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