Optical fibers provide a variety of options for instrumentation in reactor environments. Optical fibers can be used to measure multiple physical phenomena including, temperature, strain, pressure, and fluid level. In addition to the various sensing applications, optical fibers are immune to electromagnetic interference, have a small footprint (~100 ?m), and a fast response. The Department of Energy and Idaho National Laboratory have considered optical fibers for use as in-pile instrumentation in the Transient Reactor Test Facility (TREAT). TREAT was designed to test reactor fuels under accident conditions by replicating accident conditions for a variety of reactor transients, such as those associated with a loss of coolant accident (LOCA). This work investigates silica fiber optic temperature sensors with inscribed type-II fiber Bragg gratings (FBGs) under conditions similar to those that would be experienced in a TREAT transient. Separate effects testing was used to test the sensors under high-temperature step transients and under irradiation up to a total fluence similar to that of TREAT. Specifically, this work investigates distributed temperature measurements, using the Optical Frequency Domain Reflectometry (OFDR) sensing technique, using a Luna Innovations Optical Backscatter Reflectometer (OBR) 4600, with silica optical fibers inscribed with type-II fiber Bragg gratings (FBGs). In conclusion, separate effects testing of type-II FBGs in silica optical fiber, to high temperature and to neutron fluences that are an order of magnitude larger than fluences that are anticipated for TREAT tests, demonstrate that type-II FBGs in silica optical fiber hold great promise for high-temperature reactor instrumentation in TREAT.
