Operation experience of a high-temperature fluoride salt test facility (FLUSTFA): Issues identified and paths forward

Fluoride-salt-cooled High-temperature Reactors (FHRs) are promising Generation IV nuclear reactors, with a passive decay heat removal system serving as one of their key design features. A high-temperature FLUoride Salt Test FAcility (FLUSTFA) was designed and constructed to perform both integral-effect tests, such as validating the design of a scaled-down passive decay heat removal system, and separate-effect tests, including evaluating the thermal–hydraulic performance of compact heat exchangers. FLUSTFA utilizes FLiNaK (LiF-NaF-KF: 46.5–11.5–42 mol%) as the working fluid and operates up to 700 °C near the atmospheric pressure. It is comprised of a reservoir tank for melting and storing the salt, a primary molten salt loop that simulates the reactor primary coolant system, a secondary molten salt loop that represents the passive decay heat removal system, an air loop, and a chilled water loop, all of which are thermally coupled via heat exchangers. Several shakedown tests were carried out using high-temperature nitrogen and FLiNaK salt as the working fluid. A few issues were identified during initial operation of FLUSTFA at 550–600 °C, such as hydrogen fluoride generation and leakage, localized hot spots where excessive heat loss occurs, molten salt pump malfunctions, abnormal readings from ultrasonic flow meters, and blockage of salt charging and recycling lines. In addressing all these issues, paths forward have been successfully identified and implemented. The lessons learned are valuable in improving future design and construction of high-temperature molten salt systems for molten salt reactors, fusion reactors, and next-generation concentrated solar power plants.