In situ reactor radiation-induced attenuation in sapphire optical fibers heated up 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 nuclear reactor environment. For this, the broadband (500–2200nm, or 0.56–2.48eV) optical transmission in commercially available sapphire optical fibers was monitored in situ prior to, during, and after reactor irradiation. Portions of the sapphire fibers were heated to temperatures up to 1000°C during irradiation. The sapphire fibers were irradiated, mostly at a neutron flux of 5.0×1011n/cm2/s and a gamma dose rate of 28kGy/h (dose in sapphire), to a total neutron fluence of 6.4×1016n/cm2 and total gamma dose on the order of 1MGy. Results were generally consistent with the results of previous in situ measurements of the transmission in unheated sapphire fibers during reactor irradiation. Added attenuation at 850, 1300, and 1550nm, appears to be limited by the growth of radiation-induced defect centers that are located in the ultra violet to the visible range and is therefore less at 1300 and 1550nm than at 850nm. A linear increase in attenuation, due to displacement damage effects, was observed with increased irradiation time at constant reactor power. However, the rate of increase of the added attenuation during constant power reactor irradiation monotonically decreased with increasing temperature up to 1000°C, with the most significant decrease occurring between 300 and 600°C. Additional calculations predicted that the majority of (if not all of) the observed increases in attenuation during irradiation at 600 and 1000°C were due to effects in the unheated sections of the irradiated sapphire fibers. These results suggest that, for a reactor radiation environment similar to that tested in this work, heating sapphire fibers to temperatures of 600°C or greater during irradiation would significantly reduce (or possibly eliminate entirely) the rate of growth of the added attenuation in the sapphire fibers.