Overview

In this chapter, the competing and synergistic catalytic effects of highly loaded metal ion mixtures on the rates of decomposition of oxalic acid by ozonation in acidic oxalate-rich slurries are studied. Although research on general organic and, specifically, oxalate decomposition has been widely performed using single transition metal catalysts, the decomposition of oxalate using a mixture of transition metals and metal oxalates, with each potentially competing or aiding the decomposition, has not been well studied.

However, this study is not merely driven by scientific curiosity. As described in Chapter 1, the SRS near Aiken, South Carolina, USA remains home to forty-three very large, underground, carbon steel tanks, storing a total of approximately 1.2×108 litres of liquid radioactive HLW. To inhibit corrosion of tank fabric, the liquid waste is first rendered heavily alkaline, with a hydroxide concentration typically > 1 M. However, these conditions result in metal ion precipitation from the HLW liquid, forming a sludge that is predominately comprised of the oxides and hydroxides of iron, aluminium, and manganese and mainly compacted into a solid mass. Each of these forty-three tanks must eventually be emptied, cleaned, and closed.

Although the bulk of the sludge in each tank can be removed using a hydraulic slurrying technique, use of chemically aided techniques to partially digest remaining sludge, thus rendering it more amenable to suspension with subsequent removal via slurrying, have been deployed. Given the high metal hydro(oxide) concentration of the sludge, digestion using technologies adapted from other uses in the nuclear industry, particularly decontamination methods, have been explored.

As discussed in Section 2.3, the new process identified to clean the HLW tanks was termed Enhanced Chemical Cleaning (ECC), wherein the process, dry oxalic acid and make-up water are combined to make a dilute oxalic acid solution. The oxalic acid solution is added to the tank being cleaned, lowering the pH and digesting the sludge. After the sludge solids are digested, they are suspended through use of slurring or mixer pumps. The suspended solids are then transferred out of the HLW tank being cleaned, as part of a spent acid slurry. The spent acid slurry is then treated with ozone and UV, where the oxalate decomposes into CO2 and then is off-gassed. The oxalate decomposition process increases the pH causing sludge oxide solids to reform as a result of hydroxide-promoted precipitation. The thick slurry containing sludge oxide solids are transferred out of the oxalate decomposition process. Using an existing Evaporator, a significant fraction of the liquid will be separated/removed from the thick slurry. The resultant (mostly) dewatered solids are transferred to the deposition tank (where the solids are combined with other HLW sludge to become eventual feed to vitrification), while the evaporator condensate is transferred back towards the tank being cleaned, as fluid to be recycled/refreshed/restored. As part of regenerating the acid, anhydrous oxalic acid is added to the recycled-fluid immediately before the fluid is returned to the HLW tank being cleaned. The regenerated/refreshed cleaning fluid is then added back into the HLW Tank Being Cleaned for further digestion of the sludge.

The slurries created from oxalate-assisted digestion of HLW sludges would have the potential to be highly radioactive. Therefore, use of an immersed UV lamp (i.e. a quartz encased UV lamp as detailed in Section 4.5.1) is considered problematic in both providing the regulatory required pedigree of primary containment, as well as from a maintenance/cleaning perspective. Thus, an alternative to the conceptual design ECC Process for post-decontamination oxalate degradation was sought.

Given both the high oxalate and dissolved transition metal loadings in the recovered slurries, the focus of the search changed to determine whether the metal catalysts already present within the slurries can catalyse the oxalate decomposition to an endpoint of < 100 ppm (1.1×10-3 M) within an acceptable period of time.

In addition to addressing SRS’s objective to close its radioactive liquid HLW tanks in a timely manner, such a study allows for a number of generic knowledge gaps to be confronted. With concepts for ozonation chiefly originating from well-characterised minimal-constituent dilute water-type systems (e.g., water treatment), there is a fundamental lack in understanding of the crucial factors affecting the decomposition rate of spent organic acids in highly loaded metal slurries (e.g. spent decontamination solutions, metal etch sludge waste, etc.). Additionally, although research on general organic and oxalate decomposition has been primarily performed using single transition metals, the decomposition of oxalate making use of a mixture of transition metals, with each potentially competing or aiding the decomposition also is a system not studied to date.

Because of the unique concerns associated with radioactive liquid waste, caustic additions, i.e. NaOH, known to optimise the hydroxyl radical yield (Zepp et al., 1992), before any treatment with ozone, would only add additional waste and further complicate downstream processing. Thus, the preference is to work under the as received low pH slurry conditions (i.e. a starting pH ~2), so affording an opportunity to investigate ozone-initiated oxalate decomposition under less studied acidic conditions.

Since testing using real HLW26 (detailed in Chapter 5) necessitated its performance in a laboratory shielded hot cell – and hence, had significant safety controls and size limitations imposed on it – the process tests performed to understand the oxalate decomposition mechanisms were performed with simulant and are discussed in Chapter 427,28, while the results of Chapter 5 are provided to show confirmation of applicability.

Section 4.2 provides an overview of the purpose/goals associated with each of the simulant based oxalate decomposition tests. Section 4.3 provides a synopsis on making the sludge simulants used in making the simulant decomposition test slurries, as well as the associated nomenclature used for identifying the slurries. Section 4.4 provides an overview of the

Simulant Decomposition Test Apparatus design, including the UV Lamp Apparatus design, as well as general procedures associated with performing the simulant based oxalate decomposition (with Appendix 3, containing a detailed equipment list (including model numbers) and additional design performance details. Section 4.5 determines if UV light is vital to decompose the oxalate in an industrially relevant time frame (i.e. less than 24 hours required to decompose the oxalate concentration to 1.1×10-3 M in each approximate 60 litre batch of simulant based slurry). Section 4.6 determines the impact of the three competing

26 _{Real HLW is commonly also referred to as actual HLW, with the two terms commonly usedf}

interchangeably .

27 _{Similar to Chapters 1, 2, and 3 where components, functions, and streams associated with the process}

being presented by the respective chapter are italicised, the components, functions, and streams associated with the Simplified Simulant Decomposition Test Apparatus, shown by Figure 9, are italized within this chapter.

28 _{For or completeness, the full set of simulant based decomposition test data are included in Table 41}

through Table 43, which are found in Appendix 5, while the full set of real HLW based decomposition test data are contained in Table 44 through 47, also contained in Appendix 5.

transition metals, Fe, Ni, and Mn on the decomposition process. Section 4.7 demonstrates that pH can be used as a field measure to confirm when oxalate decomposition is complete (as stated above, taken to be corresponding to < 100 ppm / 1.1×10-3 M oxalate in solution). Section 4.8 summarizes the results of a study, which strongly suggests that the observed oxalate decomposition, especially under the initial acid conditions, is not the result of a hydroxyl radical-driven process.