Fiber-Optic Sensors for Structural Health Monitoring of Nuclear Power Plants: Challenges and Opportunities

Fiber-optic sensors are gaining traction in the nuclear industry due to their high accuracy, compact size, and ability to perform distributed measurements. To-date, most research has focused on the use of fiber-optic sensors for monitoring and mapping of in-core temperatures. Radiation-induced attenuation is a concern for any in-core application, but recent works have shown this to be manageable even at extremely high dose (>10²⁰ n/cm²). Radiation-induced drift is arguably a more daunting challenge for in-core temperature sensing. For advanced reactor applications, fiber-optic sensors could offer more value as a solution for structural health monitoring if they can survive for extended durations at temperatures beyond the capabilities of conventional technologies. These sensors could measure both static and dynamic (e.g., acoustic vibrations or acoustic emissions) strain, with the latter capable of filtering out low-frequency effects such as drift that occurs over longer timescales. Utilizing fiber-optic sensors for structural health monitoring may not require exposure to in-core radiation dose levels but instead presents a different set of challenges. Robustly attaching these sensors to various reactor components such as pressure vessels, pumps, heat exchangers, and primary system piping is crucial. The primary difficulty lies in managing the significant mismatch in thermal expansion between the fused-silica optical fibers and metal components. This mismatch can induce substantial stress on the optical fiber during operation, potentially leading to failure. Researchers are actively exploring a wide range of bonding techniques to address this issue. These techniques include adhesives, electroplating, welding, brazing, and advanced manufacturing methods like additive manufacturing and electric-field assisted sintering. Each method offers unique advantages and disadvantages regarding residual stress, maximum operating temperature, and suitability for large-scale components. This paper summarizes ongoing research to identify and optimize the most effective bonding techniques to ensure the long-term reliability and performance of fiber-optic sensors in nuclear power plants.