Methods for Monitoring a Chloride Volatility Process for Treating Nuclear Materials

More efficient fuel cycle options are sought for processing nuclear materials, such as spent nuclear fuel, with decreased losses and increased separation efficiencies. Reliable and accurate methods of monitoring and safeguarding materials used in these processes must also be developed. A three-step hydriding-chlorination-volatilization process is reported that was designed to separate impurities from metals while keeping the target bulk material in the solid phase. The bulk material chosen for this preliminary testing was cerium because it is commonly used as a surrogate for actinides and other rare earths. The impurities studied were aluminum, iron, gallium, tantalum, and uranium. The first step uses hydrogen gas between 0 and 300 °C to reduce the particle size of the metal and promote the chemical reaction with anhydrous chlorine gas. It was found that the particle size of the metal was quickly reduced by hydrogen gas from several cm to between 0 and 2,000 µm by forming CeH3 at 100 °C after 12 minutes. The second step results in the formation of porous chloride particles. This step proceeds quickly under flowing Cl2 by heating first to 250 °C and then to 500 °C with 100% Cl2. The final step utilizes the different vapor pressures of the chlorides to separate the impurities using chemical vapor transport. In early testing of this process, complete chlorination was prevented by contamination of the metal with dissolved oxygen. When steps were taken to reduce oxygen contamination, chlorination conversion increased up to 97%. The methods used to analyze the materials used in these experiments can also be used to monitor and safeguard materials in an actual process. For example, mass changes in the solid have been shown to indicate the fractional conversion of the metal to chloride based on correlation with conventional chemical analysis methods. Process monitoring signatures were identified that could be identified in real time based on changes in gas pressure, gas temperature, gas flow rate, and sample temperature.