Reducing The Volume Of Radioactive Waste: Catalyst Enhanced Molten Salt Oxidation Is The Answer

Abstract:

The nuclear energy industry is facing the challenges of new build and the need for reducing the amounts and volume of radioactive waste, amongst many other technical challenges. The general public particularly support the reduction of Low and Intermediate Level Wastes (LLW and ILW) and the number of such. The industry generally prefers to develop existing technologies with which they are familiar and have experience. This contribution describes a well researched novel technique that can reduce the volume of LLW and ILW currently being idly held in large warehouses on nuclear sites, specifically contaminated paper, cotton wipes and lab coats.Molten Salt Oxidation, MSO, is a technique for the flameless oxidation of waste and originally employed oxygen (from air) and molten sodium carbonate at 900-1100°C. Oxidation was slow and often incomplete, particularly for paper, cotton and plastics, and abandoned twenty years ago. However, its efficiency for other wastes was particularly high. We have now established that the early workers had misunderstood the mechanism of the oxidation process. Consequently, upon reassessment, we have improved the speed and efficiency of the process. Using a ternary carbonate melt at 550°C our process, Catalyst Enhanced Molten Salt Oxidation, completely oxidised paper, cotton and plastics. Thus CEMSO is a particularly useful technique for reducing the quantities of both slightly and heavily contaminated radioactive waste.CEMSO oxidises the organic component of waste into CO2 and steam while retaining all the inorganic and radioactive constituents in the molten carbonate, from which they can be readily removed, including as insoluble phosphates, now suitable for vitrification (and the carbonate recycled). Previously molten carbonate was considered to act as a catalyst but our studies found that oxidation proceeds because oxygen dissolves chemically, not physically, in molten carbonate, yielding peroxide and superoxide ions, the active species. Their concentration is further increased when nitrate is added as catalyst, since nitrate reacts with peroxide to form superoxide and nitrite and the nitrite is now regenerated into nitrate by reaction with oxygen. This has been confirmed by Density Functional Theory: the formation constants of the adduct intermediates all show negative entropy.Molten Salt Oxidation destroys completely ion exchange resins and hence is a suitable treatment for exhausted radioactive resins.We have shown that tributyl phosphate and kerosene are completely oxidised, with phosphorus remaining in the melt as phosphate, thus revealing that the quantities of exhausted TBP and kerosene on nuclear sites can be very considerably reduced in volume to phosphates plus phosphates of the radionuclides, again suitable for vitrification.We have also shown that UO2, including ceramic UO2, and uranium compounds are converted into diuranates, the solubilities of which are < 500 ppm.Plutonium contaminated waste yielded >99.9% plutonium in the melt and the element was only detected in the first pre-filter of the off-gas.Sr, Eu and Cs compounds, to simulate fission products, have been added to molten carbonate sparged with oxygen, and all remained completely in the melt.Since aqueous cyanide has been reported as >99.99% destroyed we consider that the volume of raffinate might be lowered by CEMSO.Finally, other hazardous wastes have been oxidised by>99.9999% efficiency, including sarin and other chemical warfare agents, PCBs, and both liquid and solid chlorinated hydrocarbons.We shall describe how CEMSO can be implemented to reduce and render radioactive waste into smaller volumes and suitable for immobilisation.

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