EVALUATION OF THE CURRENT STATE OF KNOWLEDGE FOR THERMOLYSIS OF ORGANICS WITHIN SRS WASTE FORMING VOLATILE ORGANIC COMPOUNDS (VOCS)
Revision 1 vi like that documented by Hanford studies cited in this report, could occur with SRS waste containing organics. The report on thermolysis testing of an actual SRS waste sample reported in Investigation of Thermolytic Hydrogen Generation Rate of Tank Farm Simulated and Actual Waste (SRNL-STI-2017-00611 Rev. 0) identified three unknown chromatograph peaks. Subsequent (and ongoing) analysis identified one peak as methane with the other two suspected as anomalous features. Preliminary
... estimates place the relative concentration of methane to hydrogen at ~30-35% for Tank 38 waste at boiling although the data is too sparse and too close to detection limits to provide a fully reliable value. The unknown chromatograph peak from simulant testing was also subsequently identified as methane. The simulant used conservative concentrations of multiple organics not likely to be present concurrently in a single waste feed. Preliminary estimates indicate equivalent concentrations of methane and hydrogen in the simulated waste tests that used high concentrations of the organics found in SRS waste. Presence of organo-mercury compounds may provide a route to methane formation, but confirmatory evidence is lacking for the waste matrix. Currently, the authors have not ascertained conclusive evidence of methane formation at SRS waste storage and processing conditions. The authors use recent data on stability of dimethyl mercury, which has been analyzed to be ~ 1 mg/L in some SRS CSTF tanks to provide an order of magnitude estimate of the potential generation rate for methane (e.g., methane generation rates of 9.1E-09 to 2.0E-06 ft 3 /gal . hr in the temperature range of 39 to 170 °C in a High Level Waste (HLW) simulant) for comparison purposes. Unfortunately, the authors have not yet located sufficient data on methyl mercury, present at levels up to ~ 200 mg/L in SRS CSTF tanks, to allow estimating the methane generation rate for that compound in alkaline solution. However, an estimated methane generation in reagent water is provided (e.g., methane generation rates of 2.2E-07 to 4.5E-02 ft 3 /gal . hr in the temperature range of 26 to 170 °C). Methane formation in the Bayer process from thermolysis tends to show a hydrogen-to-methane molar ratio of ≥75 (albeit at temperatures significantly higher than SRS waste processing conditions). Published studies of processing liquids obtained from the Bayer process cited in this report (from 2011 to 2016), albeit at more extreme conditions (175 °C to 275 °C), with a wide range of compounds, provide insight into the relative stability of classes of compounds toward hydrogen formation. By extrapolation and comparison to organics in SRS waste, antifoam is expected to show higher propensity for hydrogen (or flammable gas) formation. Ion exchange resins and solvents, in decreasing order, will likely yield lower quantities per unit mass of starting organic. Combined thermolysis and radiolysis studies for the cesium removal solvent suggest a potential methane-to-hydrogen ratio near ~0.24 with lesser amounts of other VOCs after long irradiation times. This magnitude agrees reasonably well with radiolysis data for PUREX solvent. Prior testing on off-gas production from thermolysis or radiolysis of antifoam does not provide insight into whether lighter VOCs formed. Preliminary findings from current SRNL HGR testing indicates that methane is produced at ~10X the rate as hydrogen from 100 °C thermolysis testing of the antifoam degradation product trimethylsilanol in a Tank 38 simulant. Decomposition of tributyl phosphates (TBP) and the degradation products in alkaline media is well studied. The authors found no literature evidence of methane generation from the TBP reaction system.