Foaming during nuclear waste melter feeds conversion to glass: Application of evolved gas analysis

Abstract

During the final stages of batch‐to‐glass conversion in a waste‐glass melter, gases evolving in the cold cap produce primary foam, the formation and collapse of which control the glass production rate via its effect on heat transfer to the reacting batch. We performed quantitative evolved gas analysis (EGA) for several HLW melter feeds with temperatures ranging from 100 to 1150°C, the whole temperature span in a cold cap. EGA results were supplemented with visual observation of batch‐to‐glass transition using the feed expansion tests. Upon heating, most of the gases—mainly H2O, CO2, NO, NO2, N2, and O2—evolve at temperatures below 700°C and escape directly to the atmosphere through open porosity. However, as open porosity closes when enough glass‐forming melt appears at ~720°C, the residual gas evolution leads to the formation of primary foam. We found that primary foaming is mostly caused by the decomposition of residual carbonates, though oxygen evolution from iron‐redox reaction can also play a role. We also show that the gas evolution shifts to a higher temperature when the heating rate increases. The implications for the mathematical modeling of foam layer in the cold cap are presented.

Effect of sucrose on foaming and melting behavior of a low‐activity waste melter feed

“…Even after salts melted (melting points of NaNO 2 and NaNO 3 are 284°C and 309°C, respectively, and the eutectic point is 226°C), the presence of other components separated the reactants, preventing chemical uniformity and even allowing NaNO 2 and NaNO 3 to react with other feed components, such as amorphous alumina and hematite, releasing NO. For the baseline feed (no sucrose added, C/N = 0.05), more CO 2 was produced than would have been available from as‐batched chemicals (see Figure B), probably because of the reaction of NaOH in the feed with CO 2 from air . The discrepancy between theoretical yield and measured CO 2 for C/N ratios above 1.02 in Figure B is most probably an artifact caused by the rapid release of gases at ~250°C (see Figres E and F) that were missed by the EGA, as described above.…”

“…Reactions of nitrates and nitrites with sucrose and other organics produce NO x , N 2 , and O 2 together with CO 2 and CO at temperatures lower than those at which the glass‐forming melt becomes connected, so the evolved gases produce less foam. Interestingly, although sucrose addition reduced foaming, did not significantly affect the glass production rate .…”

Heat transfer from glass melt to cold cap: Gas evolution and foaming

“…For the experimental conditions applied, = 50 mL min −1 and Φ = 10 K min −1 (Section 2); thus, = 2.23 × 10 −4 mol K −1 . The gases detected at temperatures above T FM were O 2 , CO 2 , and SO 2 ; H 2 O also evolved, but was not monitored. Figure shows releases of CO 2 , O 2 , and SO 2 at T > T FM , ie, T 0 = T FM in Equation ().…”

Glass production rate in electric furnaces for radioactive waste vitrification

“…This amount of gases (CO 2 , NO, and O 2 ) was released to the atmosphere when primary foam was collapsing, starting at T FM . The HWI‐Al‐19 feed is a representative of feeds exhibiting regular foaming with a single primary foam peak and no or little gas released to the atmosphere during the temperature interval between T FO and T FM . Feeds HWI‐Al‐28 and HLW‐E‐Al‐27 (see Figure and the foaming curves in Appendix ) belong to this class.…”

“…The T FM was used to estimate the T B (see Section 5.1) for feeds that released little or no gas within the temperature interval from T FO to T FM . This was checked by performing evolved gas analysis (EGA) …”