Distributed measurements made with fiber optic instrumentation have the potential to revolutionize data collection for facility monitoring and process control in industrial environments. Dozens of sensors etched into a single optical fiber can be used to instrument equipment and structures so that dozens of spatially distributed temperature measurements, for example, can be made quickly using one optical fiber. Optically based sensors are commercially available to measure temperature, strain, and other physical quantities that can be related to strain, such as pressure and acceleration. Other commercially available technology eliminates the need to etch discrete sensors into an optical fiber and allows temperature measurements to be made along the length of an ordinary silica fiber. Distributed sensing with optical instrumentation is commonly used in the petroleum industry to measure the temperature and pressure profiles in down hole applications.
The U.S. Department of Energy is interested in extending the distributed sensing capabilities of optical instrumentation to high temperature reactor radiation environments. For this technology extension to be possible, the survivability of silica optical fibers needed to be determined in this environment. In this work the optical attenuation added to silica optical fiber exposed simultaneously to reactor radiation and temperatures to 1000°C was experimentally determined. Optical transmission measurements were made in-situ from 400nm-2300nm. For easy visualization, all of the results generated in this work were processed into movies that are available publicly [1].
In this investigation, silica optical fibers were shown to survive optically and mechanically in a reactor radiation environment to 1000°C. For the combined high temperature reactor irradiation experiments completed in this investigation, the maximum attenuation increase in the low-OH optical fibers was around 0.5db/m at 1550nm and 0.6dB/m at 1300nm. The radiation induced optical attenuation primarily affected wavelengths less than 1000nm and this attenuation cannot be avoided in silica. Thermal effects dominated the increase in attenuation at wavelengths above 1000nm and it may be possible to mitigate these effects. Fortuitously, commercial optical instrumentation typically utilizes wavelengths centered around 1300nm and 1550nm where the radiation induced attenuation was minimal. The maximum continuous use temperature of silica optical fiber may be limited to 900°C with intermittent use to 1000°C. The silica optical fibers tested in this project are inexpensive and commercially available. Optical sensors were not tested in this project and development and testing of radiation hard optical sensors is recommended as future work.