Thermal Gradient Effects on Redox Evolution and Volatility-Driven Fractionation in Ternary U/Ce/Cs Condensates.

Area of Science:

• Materials Science
• Chemical Engineering
• Nuclear Engineering

Background:

• High-temperature condensation in complex materials like nuclear debris requires understanding thermal history's effect on redox evolution and chemical fractionation.
• Ternary U/Ce/Cs systems are relevant for modeling nuclear debris formation.

Purpose of the Study:

• To test how distinct thermal regimes in a plasma flow reactor influence redox pathways and elemental partitioning in U/Ce/Cs systems.
• To quantitatively link thermal gradients to redox evolution and volatility-driven fractionation.

Main Methods:

• Utilized a modified plasma flow reactor with controlled thermal gradients (ambient cooling vs. furnace-assisted hold-up at ~1400 K).
• Characterized condensed phases using transmission electron microscopy (TEM) for morphology and nanoscale element distribution.
• Quantified bulk elemental ratios using inductively coupled plasma-mass spectrometry (ICP-MS).

Main Results:

• Uranium and cerium condensed as UO2 and CeO2 across both thermal regimes.
• Extended ambient cooling led to partial uranium oxidation to α-UO3.
• Furnace-assisted hold-up preserved UO2, partially reduced cerium to Ce2O3, and increased cesium incorporation into Cs2O and Cs-uranate phases.
• ICP-MS confirmed stable U/Ce ratios and delayed cesium enrichment.

Conclusions:

• Thermal history significantly influences redox states and elemental partitioning in U/Ce/Cs systems.
• Plasma flow reactor experiments can differentiate between equilibrium-governed and kinetically-controlled condensation processes.
• Results provide a quantitative method for analyzing condensation behavior in multicomponent systems under varying thermal conditions.