Thermal Processing of Chloride-Contaminated Plutonium Dioxide.

Area of Science:

• Materials Science
• Nuclear Chemistry
• Radiochemistry

Background:

• Chloride contamination in plutonium dioxide (PuO2) samples from degraded poly(vinyl chloride) (PVC) packaging poses a challenge.
• Historical PuO2 samples from 1974 and 1980 exhibit significant chloride levels (2000-5000 ppm).
• Understanding chloride behavior during thermal treatment is crucial for safe storage and processing.

Purpose of the Study:

• To investigate the effects of heat treatment on chloride-contaminated PuO2.
• To determine the optimal conditions for thermal stabilization and chloride removal.
• To characterize the different forms of chloride present in the PuO2 samples.

Main Methods:

• Performed over 80 heat treatment experiments on chloride-contaminated PuO2 samples.
• Analyzed chloride content using caustic leaching and ion chromatography.
• Quantified mass loss on heating (LOH) and volatilized chloride.
• Utilized electron microscopy to observe morphological changes.

Main Results:

• Mass loss on heating primarily results from adsorbed water and gases, not chloride.
• Chloride volatilization begins above 400°C, with over 100% of leachable chloride removed above 700°C.
• A non-leachable form of chloride was identified, indicating incomplete removal by simple heating.
• Heat treatment at 800°C in flowing air, followed by cooling and packaging under argon, is proposed as optimal.

Conclusions:

• Heat treatment effectively reduces leachable chloride, but a non-leachable fraction persists.
• Optimal thermal stabilization requires specific conditions: 800°C in air, with careful cooling and packaging.
• Morphological changes were observed, with platelet particles degrading more than trapezoidal ones.