Fiber Optic Sensors for Monitoring Elongation, Swelling, and Temperature in a Simulated Nuclear Fuel Cladding

After the Fukushima-Daiichi accident, the development and use of accident tolerant fuel cladding materials in operational nuclear reactors necessitate extensive experimental research to evaluate the safety and mechanical properties of these new resilient fuel designs. Research includes in-situ diagnostic measurements of nuclear fuel rod temperature, fuel swelling and pressure, and cladding elongation during irradiation in test reactors. While some of these measurements were historically taken with single point sensors and averaged along the fuel pin through the Halden Reactor Project, obtaining more detailed and highly resolved measurements can significantly enhance understanding of these variables. This improved understanding can enhance reactor safety. One type of sensor that can provide high-resolution data on temperature and strain, including swelling and elongation, is the fiber optic sensor. However, a challenge exists in separating temperature and strain data at elevated temperature environments, including transients. In this study, temperature and strain fiber optic sensors were developed, constructed, and used to monitor wall temperature, diameter changes due to internal pressure increases up to 10.34 MPa (1500 psi), and elongation in both uniform and non-uniform steady-state temperature fields reaching 350°C of a simulated fuel pin cladding. The fiber sensors for strain measurements were helically wrapped and rigidly embedded in the cladding wall, enabling the monitoring of axial and circumferential components of strain. Additionally, a temperature fiber optic sensor was installed parallel to the strain fiber, and a methodology was developed to separate the effects of thermal expansion from mechanical strain for the strain sensor under steady-state conditions.