Melter Feed Reactions at T ≤ 700°C for Nuclear Waste Vitrification

Abstract

To understand feed‐to‐glass conversion for the vitrification of nuclear waste, we investigated batch reactions and phase transitions in a simulated nuclear waste glass melter feed heated at 5 K/min up to 700°C using optical microscopy, scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. To determine the content and composition of leachable phases, we performed leaching tests; the leachates were analyzed by inductively coupled plasma atomic emission spectroscopy. By 400°C, gibbsite and sodium borates lost water and converted to amorphous phase, whereas other metallic hydroxides dehydrated to oxides. Between 400°C and 700°C, carbonates decomposed before 500°C; amorphous aluminum oxide and calcium oxide reacted with the sodium borate and formed the more durable amorphous borate phase along with intermediate crystalline products; above 500°C, quartz began to dissolve, and hematite started to convert to trevorite.

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

“…EGA helps us to identify gases evolved during the batch‐to‐glass conversion, detect gases responsible for primary foaming, and understand how gas evolution responds to time‐temperature history that could occur during conversion in the cold cap. Although EGA alone cannot differentiate between the individual reactions that produce the same gas or exactly describe the reaction pathways, it is still an important tool when combined with other methods, such as x‐ray diffraction, x‐ray spectroscopy, leaching tests, or residual anion analysis …”

Conversion of Nuclear Waste to Molten Glass: Cold‐Cap Reactions in Crucible Tests

“…We used a melter feed for a simulated high‐alumina HLW glass (Table ) that has been characterized in previous studies . The feed was prepared as slurry, as described in a previous paper .…”

“…The reaction kinetics of feed‐to‐glass conversion was investigated using thermal analysis techniques including differential scanning calorimetry‐thermogravimetric analysis (DSC‐TGA) and gas chromatography‐mass spectrometry (GC‐MS) . Further information regarding feed reactions and phase transitions that cannot be identified with those techniques were investigated using laboratory‐scale melter tests and crucible tests . Detailed understanding of all stages of feed‐to‐glass conversion as they occur over the temperature range in the cold cap is needed for advanced melter feed formulation to enhance the glass production rate …”

Rhenium volatilization in waste glasses

“…These salts are miscible when molten, forming a single liquid phase, the primary melt, in which Tc is dissolved in the form of pertechnetate (or Re as perrhenate in nonradioactive simulants). As temperature increases, carbonates, nitrates, and nitrites react with the solid feed components, releasing gases (Table 4), while the cations (usually alkalis) enter the glass-forming phase [29,[34][35][36]. As a result, the salt-phase composition changes, turning from a mixture dominated by nitrate, nitrite, and carbonate to a mixture of sulfate, chromate, halides, and pertechnetate/perrhenate.…”

“…Substantial differences in glass-forming and glass-modifying feed materials used for HLW and LAW borosilicate glasses and in HLW and LAW waste compositions result in differences in the feed-to-glass conversion process. For example, the high surface area of amorphous aluminum oxide from the decomposition of aluminum hydroxide, a major component of the HLW glass (Table 1) [36,45], provides a large internal surface that may selectively immobilize the salt melt, whereas LAW feeds consist mainly of water-free minerals ( Table 3) that do not provide a comparable active surface area.…”