Alkaline high-level wastes are produced in nuclear fuel reprocessing plants as a result of the neutralization of initially acidic solutions with sodium hydroxide. The typical neutralized alkaline wastes are tank supernatants in the United States at DOE sites (Hanford, Savannah River, Oak Ridge) having the actual NaOH concentration of 0.75 to 7 M.
Radioactive alkaline solutions are also generated when decommissioning fast neutron reactors, which utilize liquid sodium or a sodium-potassium blend as a coolant of a primary circuit. Large quantities of spent metallic sodium are treated with water resulting in 10-30 M sodium hydroxide solutions. The largest contribution to the sodium hydroxide waste activity is provided by 137Cs and, to a lesser but remarkable extent, 90Sr.
One of the management strategies of radioactive sodium hydroxide effluents is aimed at minimizing its activity before the final processing by selective removal of radionuclides by means of solid/liquid extraction based on an ion-exchange process. Decontaminated NaOH solution of a low-level activity, cesium-loaded solids (organic and inorganic ion-exchangers), and/or stripped Cs matter are produced as secondary radioactive wastes, which are subject to conversion into final chemically stable solid forms by cementation, ceramization, or vitrification processes. The technical problem for 137Cs and 90Sr removal from NaOH solutions is the high sodium concentration. Dilution and/or neutralization of sodium hydroxide solutions are often required to provide the efficient extraction of the radionuclides by ion exchangers.
In our recent study entitled “One-step immobilization of cesium and strontium from alkaline solutions via a facile hydrothermal route” published in the Journal of Nuclear Materials, we suggested an effective low-temperature process to immobilize 137Cs and 90Sr from alkaline radioactive waste in pollucite or analcime based solid matrices. The processing of 137Cs and 90Sr bearing sodium hydroxide solution is carried out in an autoclave at 150 °C in the presence of hollow aluminosilicate microspheres from coal fly ash, known as “cenospheres.”
Cenospheres are the microspherical glassy component of coal fly ash characterized by a low bulk density (0.2-0.8 g/cm3) and can be easily separated in aqueous and air media by gravitational methods. The content of cenospheres in fly ash results from the combustion of different types of coals and varies over a rather wide range — from 0.01 to near 5 wt. %. Due to the sphere-shaped morphology, low density, high hydrostatic strength, thermal stability, and liquid-like fluidity, cenospheres are mainly used as fillers of composite and heat-insulating materials, refractory coatings, components of backfills and proppants, etc.
The high-tech application of cenospheres (as microreactors, catalysts, adsorbents, membranes, etc.) is based on the possibility to stabilize their composition and morphology by classification of a cenosphere concentrate at the expense of an inequality of physical parameters (size, density, magnetic properties) of different cenosphere globules. One more high-tech approach to use cenospheres of appropriate silicon and aluminum contents is a fabrication of microsphere sorbents for trapping 137Cs and 90Sr from aqueous radioactive waste and their accommodation in insoluble pollucite and Sr-anortite based ceramics via the subsequent heating of the 137Cs/90Sr loaded cenosphere-based sorbent.
How Cenospheres Interact with 137Cs- and 90Sr-Bearing Sodium Hydroxide Solutions
Cenosphere properties have been found to be suitable for their use as a template core and a Si and Al source in the zeolite synthesis. The interaction of the cenosphere glassy material under hydrothermal conditions with sodium hydroxide, in dependence on reaction parameters, resulted in zeolitic phases of different structural types, such as NaX, NaA, NaP1 (GIS), chabazite (CHA), and analcime (ANA).
For now, it was established that the hydrothermal treatment of a reaction mixture composed of cenospheres and cesium- or strontium-bearing sodium hydroxide solutions at 150 °C resulted in scavenging the radionuclides 137Cs and 90Sr from solutions and their accommodation in insoluble mineral-like forms, such as microsphere mono- and polyphase crystalline materials. Cesium is immobilized in a crystal lattice of a pollucite phase, CsAlSi2O6, or pollucite-analcime solid solutions and strontium are fixed as a strontium silicate compound, providing the degree of Cs and Sr recovery from alkaline solutions of 90-99 %.
This approach makes it possible to minimize both the alkaline solution activity and amounts of the secondary radioactive wastes as well as to treat other Cs waste, e. g. Cs traps or Cs raffinates, in the mixture with the sodium hydroxide effluents. The limited quantities of radioactive alkaline solutions can be treated as a batch process using an autoclave reactor.
These findings are described in the article entitled One-step immobilization of cesium and strontium from alkaline solutions via a facile hydrothermal route, recently published in the Journal of Nuclear Materials. This work was conducted by a team including Tatiana A. Vereshchagina, Ekaterina A. Kutikhina, Yana Yu. Chernykh, Leonid A. Solovyov, Anatoly M. Zhizhaev, Sergei N. Vereshchagin from the Institute of Chemistry and Chemical Technology of the Siberian Branch of the Russian Academy of Sciences (ICCT SB RAS), Federal Research Center “Krasnoyarsk Science Center SB RAS”, Russia, and Alexander G. Anshits from ICCT SB RAS and the Siberian Federal University, Russia.