Remediation of Spent Oxalic Acid Nuclear Decontamination Solutions Using Ozone

Abstract

The Savannah River Site (SRS) has forty-three remaining very large underground tanks containing significant quantities of nuclear waste generated primarily from cold-war radiochemical separations. All of the tanks eventually must be closed. As part of decommissioning/closing the nuclear waste tanks, even residual quantities of the waste must be removed. Although most sludge can be removed mechanically, chemically cleaning (i.e. decontamination) with oxalic acid is used to aid in the removal of residual quantities. Although oxalic acid works for cleaning the tanks, its downstream impacts are considered detrimental. To better understand and quantify the impacts, detailed models were developed to account for different potential processing strategies for handling the spent oxalic acid nuclear decontamination slurries. Although the results vary, the models show that regardless of the oxalate handling strategies: 1) significant washing to decrease sodium concentration/solids concentration in vitrification feed will be required, and 2) the creation of copious future additions of feed for salt processing will be unavoidable. Using a Theory of Inventive Problem Solving (TRIZ) approach, a modified form of the Chemical Oxidation Reduction Decontamination (CORD) ultraviolet (UV) light was identified as being used with an analogous but already resolved problem that could be adapted to SRS HLW tank cleaning. A novel preliminary process called Enhanced Chemical Cleaning (ECC) was envisioned. Many people have aided me in this research and made this thesis possible. Naming all key people would likely be impossible; therefore, I will probably leave someone out. In advance, please accept my apologies. From my time at High-Level Waste Engineering, I would like to thank Renee Spires, Tommy Caldwell, Mark Mahoney, and Neil Davis. They were the first people to honestly trust my novel TRIZ determination that using oxalic acid to clean the High-Level Waste tanks, tied closely to decomposing the resultant spent acid solution represented the "ideal solution." At Savannah River National Laboratory, Chris Martino and Bill King, who spent considerable time helping me understand much of the oxidation chemistry. At AREVA NP (Lynchburg) Dennis Jones, John Remark, Ray Beatty, and Sarah Evans, all who helped me gain valuable insights. My supervisor Professor Colin Boxall who encouraged me throughout the process. And finally, to my many friends and colleagues that supported me during this research. viii

Catalytic reactions of oxalic acid degradation with Pt/SiO2 as a catalyst in nitric acid solutions

“…In nuclear fuel reprocessing plants, oxalic acid is commonly used as a precipitant or complexing agent to separate actinide ions from other elements through the formation of undissolved actinide oxalic acids in aqueous solution. [1][2][3][4][5][6][7] For example, plutonium is recycled through plutonium oxalic acid precipitation from solution during a uranium-plutonium co-decontamination separation process that dates back to the 1940s. 3 However, aer the precipitation step, small amounts of plutonium oxalic acid solids remain in the radioactive ltrate, and small amounts of plutonium must be extracted from the ltrate.…”

“…Therefore, oxalic acid must be removed from ltrates as much as possible to reach the limiting oxalic acid concentration (10 −4 to 10 −5 M). 6,7 At present, this method for oxalic acid in ltrates oxidized by KMnO 4 is normally used in nuclear fuel reprocessing plants. This method has the advantages of simple operation and short reaction time, but it results in continuous accumulation of K + and Mn 2+ compared to alternative methods, [6][7][8][9] and it does not comply with the principle of nuclear waste minimization.…”

“…6,7 At present, this method for oxalic acid in ltrates oxidized by KMnO 4 is normally used in nuclear fuel reprocessing plants. This method has the advantages of simple operation and short reaction time, but it results in continuous accumulation of K + and Mn 2+ compared to alternative methods, [6][7][8][9] and it does not comply with the principle of nuclear waste minimization. Therefore, alternative methods to replace this method are needed.…”

“…Various methods of oxalic acid destruction, such as the addition of oxidizing agents, 10,11 electrolytic oxidation 12,13 and using a catalyst in nitrate solution, [6][7][8][14][15][16][17][18][19][20] have been developed to replace the method for oxalic acid oxidized by KMnO 4 . H 2 O 2 is the most popular reagent for the addition of oxidizing agents in this approach and has the advantage of not importing additional impurity ions into the reaction system, but its drawback is that oxalic acid degradation to limiting oxalic acid concentration is excessively prolonged due to the low reaction rate.…”

“…Oxalic acid degradation using catalysts in nitric acid solutions is a research hotspot, and depends on catalysts being soluble in water, and these are classied into heterogeneous and homogeneous catalysts. In homogeneous catalysis, metal ions, such as vanadium ions 14 and manganese ions, [6][7][8] are used as catalysts or combined with ozone and ultraviolet irradiation, with manganese ions being the most mature and widely studied.…”