Recent advancements in fiber optic manufacturing, sensor design, and fiber optic interrogators have provided significant opportunities towards the development of cross-cutting fiber optic sensing solutions across the nuclear industry. The addressable harsh nuclear environment markets include Gen II, II+ and IV nuclear reactors, fusion reactors, and accelerator systems. In this work the authors present a series of developments towards the implementation of singlefiber, multipoint, temperature and pressure sensors, test results in high-temperature and high-radiation environments, cryogenic environments, material compatibility studies for sensor packaging, and future development needs to address technical challenges towards sensor commercialization.
The authors have developed a single-mode sapphire sensor for distributed temperature and flow measurement to address the extreme environments encountered in energy applications. The sensor is designed to also detect and localize fouling and deposits that accumulate on its surface over time. Optical frequency-domain reflectometry (OFDR) and spectral back scatter analysis are employed with the single mode sapphire fiber to yield these distributed measurements. Temperature accuracy was on the order of 5°C for measurements ranging from room temperature to over 1000°C. Spatial resolution of 11 mm was attained and enabled visualization of the temperature gradients along a fiber passing through a furnace. The effects of cooling flow were characterized for steady operation, with the intent to leverage this data in future work to infer velocity profiles of high temperature flows. Dynamic cooling flow tests showed that the presence of simulated deposits on the outside of the sensor resulted in slower time response in the vicinity of the deposit. This technique could be used to determine the presence and location of deposits along the length of the sensor assembly. The newly developed sensor will be applicable to fossil energy production, nuclear energy production, and gas turbine engines or generators.
In the nuclear industry, there is a need for sensors that are resistant to both high temperatures and radiation. Fiber Bragg gratings inscribed into radiation resistant fibers are a potential solution to this as the femtosecond-infrared laser can inscribe Bragg gratings into fibers without a photosensitive core. In order for these gratings to be used for sensing, they need to be characterized to determine their temperature and radiation response and sensitivity. This paper characterized three commercially available fibers for use in high heat and high radiation environments. There are six fiber variants examined in this study. Three basic fiber designs are investigated: a germanium doped core, a germanium doped core with fluorine cladding, and a fluorine doped core and cladding. For each fiber design, normal and pre-irradiated versions are investigated. The fibers were tested for thermal response by heating them to 1000C and holding for 24 hours, and for radiation resistance by irradiating with gamma radiation. The germanium core doped fibers were more resistant to thermal effects but still had a wavelength shift during the 24-hour soak. The fluorine-doped fibers either had the gratings partially or completely erased during the thermal hold at 1000°C, but showed suitability for short term excursions to this temperature. The radiation data showed significant shifts in some cases, but there was not enough data to form a definitive conclusion. It appears that radiation introduces variability in the response of the Fiber Bragg Gratings (FBG).
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