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

Abstract

Reductants, such as sucrose (C12H22O11), are added to nuclear waste melter feeds containing high fractions of nitrates and nitrites to reduce excessive foaming during feed‐to‐glass conversion, decrease sulfate segregation, and increase technetium retention. The effect of sucrose on foaming and melting reactions during the conversion was examined using the feed volume expansion test, thermogravimetric analysis, evolved gas analysis, x‐ray diffraction, and scanning electron microscopy with energy dispersive x‐ray spectrometry. Different amounts of sucrose were added to vary the carbon to nitrogen (C/N) ratio in the melter feed. As the C/N ratio increased, the extent of foaming decreased, and the N2/NO ratio increased in the evolved gas. Significant foam suppression, rapid gas release at approximately 250°C, and reduction in transition metal oxides were observed at C/N > 1.1.

Melting rate correlation with batch properties and melter operating conditions during conversion of nuclear waste melter feeds to glasses

“…In LAW feeds, on the other hand, the high concentration of NO + O 2 in foam bubbles keeps Na 2 SO 4 and Fe 2 O 3 oxidized. When nearly all nitrates were decomposed, the partial pressure of oxygen in the glass‐forming melt dropped and the redox reactions were allowed to reach equilibrium by releasing oxygen 23,45,48,49 . Thus, the outburst of secondary gases, O 2 and SO 2 , indicates the end of primary foaming under the cold cap, and the minimum of O 2 observed in EGA curves identifies the cold‐cap bottom temperature, T B , as marked by the dotted vertical lines in Figure 3.…”

“…For LAW melts, in which solid inclusions are not expected to be present at T B (this was confirmed for LAWE4H 23 ), we used the reported viscosity-temperature relationships [35][36][37][38][39][40][41] to estimate the melt viscosity at T B . The results showed a similar range for T B , about 10-100 Pa s; see the solid circles in the gray shaded area in Figure 5.…”

“…Whereas HLW feeds and commercial batches contain relatively modest fractions of Na 2 O (~5–10 wt% for HLW and ~15–20 wt% for LAW) and produce mostly CO 2 , LAWs typically contain high fractions of Na 2 O and produce large volumes of NO x 19‐22 . Together with other fluxes, Na 2 O triggers early formation of glass‐forming melt while NO x from nitrites and nitrates continues to evolve at temperatures at which the glass‐forming melt is connected into a continuous phase 23 . This overlap of melt formation and gas evolution causes vigorous foaming, resulting in tumultuous, boiling‐like behavior over an extended temperature interval (~600°C–950°C).…”

“…[19][20][21][22] Together with other fluxes, Na 2 O triggers early formation of glass-forming melt while NO x from nitrites and nitrates continues to evolve at temperatures at which the glass-forming melt is connected into a continuous phase. 23 This overlap of melt formation and gas evolution causes vigorous foaming, resulting in tumultuous, boiling-like behavior over an extended temperature interval (~600°C-950°C). This is in contrast to HLW and commercial batches, which typically produce moderate foam; only some exhibit extensive foam, and very few vigorous, boiling-like foam.…”

“…Maximum porosity of vigorous foams can reach value of 0.96. 23 When observing a small laboratory sample, the volume of foams with LAW feeds does not grow to a single distinct maximum, but fluctuates around a value determined by the gas release rate rather than the melt viscosity. As the viscosity decreases with increasing temperature, the foam gradually dwindles.…”