A schematic diagram of the Shaft Seal Test Facility. (Image: NERS)
For 2,300 hours, the molten salt pump Shaft Seal Test Facility (SSTF) operated at the University of Michigan’s Thermal Hydraulics Laboratory, according to an article from UM. The large-scale experiment was designed to evaluate shaft seal performance in high-temperature pump systems. Fewer than 10 facilities worldwide have successfully operated fluoride or chloride salts for more than 100 hours using over 10 kilograms of material.
The project was led by UM’s Nuclear Engineering & Radiological Sciences (NERS) assistant research scientist Adam Burak. NERS graduate student Shuai Che designed and constructed the experiment and performed its daily operation. The work was aided by NERS professor Xiaodong Sun, whose research focuses on large-scale thermal hydraulic systems relevant to advanced reactors.
The experiment: The work was conducted to investigate shaft seal performance for molten salt pumps under typical conditions. The working fluid was 32 kilograms of FLiNaK, a common high-temperature salt mixture. During the experiment, the salt mixture was used at three different operating temperatures to evaluate seal behavior across varying thermal conditions: 500°C, 525°C, and 550°C
The goal of the experiment was to assess whether this type of mechanical seal could function reliably while exposed to a molten salt vapor. The SSTF’s ability to measure gas consumption, which is the amount of inert “cover gas” required to maintain a moisture- and oxygen-free environment around the seal, was also tracked.
In an operating reactor, cover gas must be absent of air and water vapor but can contain fission products that may migrate into the gas space. A higher gas consumption rate means more exhaust to treat, and gas treatment can become a significant operational expense. The SSTF experiment helped quantify this key parameter under long-duration conditions, offering important data for future reactor designers, the article noted.
Key accomplishments: According to the article, the data on the performance of a shaft seal for molten salt pumps is publicly available for the first time in the U.S. The data will help developers evaluate the sealing technologies that are best suited to specific designs.
Another accomplishment was safely performing maintenance and troubleshooting on a high-temperature system. Procedures were developed to intervene while the system was at temperature, a skill set that is critical for future reactor operations.
