(327d) Performance-Based Simulants for Hanford Radioactive Waste Treatment Process Testing

Radioactive wastes from nuclear fuel processing are stored in large underground storage tanks at the Hanford Nuclear Reservation near Richland, Washington. Treatment and remediation requires that waste feed from the storage tanks be delivered to the Hanford Tank Waste Treatment and Immobilization Plant (WTP) in a manner that ensures continuous WTP operations over the life cycle of the treatment mission. The complex physical and chemical processes required for this mission, the significant scale, and the hazardous nature of the waste necessitate the use of simulants in lab- and pilot-scale process testing.

As pointed out in the National Research Council (NRC) report Advice on the Department of Energy’s Cleanup Technology Roadmap: Gaps and Bridges, testing with a complete range of actual radioactive wastes to cover all process limits related to chemical, physical, and rheological properties would require large amounts of actual waste that is difficult to retrieve and handle, increases radiological safety hazards to laboratory staff, and in most cases is too time-consuming and costly. The benefits of developing accurate and reliable waste slurry simulants are discussed in the NRC report:

“The absence of adequate understanding of the behavior of process streams can necessitate overly conservative and costly process designs to minimize the risk of a process failure or the risk of unrecognized safety issues, which as a worst case can render a facility inoperable with the actual radioactive waste it was intended to process.”

Hanford simulants were initially developed by attempting to mimic the chemical composition of the waste. This was typically done by characterizing the chemical analytes of a tank waste sample, then matching the chemical composition either with precipitated metal hydroxides or by adding metal oxides. However, this approach can have significant drawbacks wherein the simulants developed that do not reproduce properties of the tank waste significant to treatment process performance, such as particle size or rheological properties. Because of these limitations, a new class of simulants, “performance-based simulants,” was developed to match physical properties salient to waste processing step performance. This approach targets the performance of the simulant relative to actual waste performance data, and has been developed into ASTM standard C1750-1 for simulant development. Development and testing results of three specific Hanford waste performance-based simulants addressing the waste feed delivery system and leaching and cross-flow filtration performance in the WTP are presented. These simulants developed using specific Hanford waste performance characteristics and have been used to successfully demonstrate and understand significant aspects of process performance.