In Situ Gamma Radiation‐Induced Attenuation in Sapphire Optical Fibers Heated to 1000°C

Abstract

The purpose of this work was to determine the suitability of using instrumentation utilizing sapphire optical fibers in a high‐temperature gamma radiation environment. In this work, the broadband (500–2200 nm, or 0.56–2.48 eV) optical attenuation of commercially available sapphire optical fibers was monitored in situ during continuous gamma irradiation from room temperature up to 1000°C. The gamma dose rate of the irradiation facility was measured to be 370 rad/h (dose in sapphire). Results show rapid growth of an absorption band centered below 500 nm (the minimum detectable wavelength in this work) and extending as far as ~1000 nm that reached a dynamic equilibrium after approximately 4 h of irradiation at room temperature. Increasing temperature generally reduced the added attenuation, although the added attenuation came to a nonzero equilibrium at temperatures of 1000°C and below. No peaks in the added attenuation were observed above 1000 nm, and the added attenuation at 1300 and 1550 nm remained less than 0.7 dB throughout the entire experiment. Fitting the attenuation spectra to Gaussian functions at each temperature revealed a large absorption band near 3 eV that monotonically decreased both in magnitude and width with increasing temperature. A second smaller band near 1.73 eV monotonically increased in both magnitude and width with increasing temperature, suggesting that some interconversion between defects may be occurring at higher temperatures.

Embedded metallized optical fibers for high temperature applications

“…Fiberbased sensors for measuring parameters such as temperature, pressure, and strain are commercially available, and certain sensing technologies offer the possibility of spatially distributed measurements. The high-temperature capabilities, chemical inertness, and radiation resistance of fiber optics has led to an increased interest in using fiber optic-based sensors for harsh environment applications, including (1) environments with temperatures up to 1000 °C or more [1][2][3][4], (2) down-hole applications in the oil and gas industry [5,6], and (3) high radiation dose applications [7][8][9][10][11][12][13]. Fiber optic-based sensors were considered for active strain monitoring of components and structures to identify early indications of failure and to allow for preventative maintenance before failure occurs [14,15].…”

Demonstrate embedding of sensors in a relevant microreactor component

“…On the other hand, spatially distributed strain can be measured by directly embedding metal coated fibers in the component and compensating for temperature effects using loosely-coupled fiber-optic temperature sensors or TCs, similar to previous efforts [2,3]. Fiber-optic sensors have been demonstrated at temperatures up to 1,000°C [16,17,21] and in high radiation environments [22][23][24][25][26][27][28][29][30]. For extremely high-temperature applications beyond 1,000°C, singlecrystal sapphire optical fibers can be used [30][31][32].…”

Embedment of sensors in ceramic structures

“…Type K and Type N thermocouples have a long history of use in high-temperature reactors, so they are the primary candidates for embedded fuel temperature monitoring. Although not as welldemonstrated for nuclear applications, fiber optic temperature sensors have the potential for spatially distributed temperature measurements [7], and they have been shown to survive temperatures up to 1,000°C and moderate neutron fluence [8][9][10][11][12][13]. Therefore, fiber optic temperature sensors are also being considered for embedding in ceramic structures.…”