In‐Situ Reactor Radiation‐Induced Attenuation in Sapphire Optical Fibers

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The purpose of this work was to determine the suitability of using instrumentation utilizing sapphire optical fibers in a nuclear reactor environment. In this work, the broadband (500–2200 nm, or 0.56–2.48 eV) optical transmission in commercially available sapphire optical fibers was monitored in‐situ prior to and during reactor irradiation. The sapphire fibers were irradiated at a neutron flux of 1.5 × 1012 n/(cm2·s) and a gamma dose rate of 75 kGy/h (dose in sapphire) to a total neutron fluence of 1.1 × 1017 n/cm2 and total gamma dose of on the order of 1.5 MGy. Consistent with previous gamma irradiation experiments, an absorption band centered below 500 nm (the minimum measurable wavelength using the measurement system described in this work) and extending as far as ~1000 nm reached saturation during irradiation in the gamma shut‐down field (the gamma‐ray radiation field that is present in the reactor post‐operation, as a consequence of the decay of radioisotopes that were produced during reactor operation) of the reactor prior to reactor irradiation. Beginning reactor irradiation and increasing the reactor power caused rapid increases in attenuation, followed by a linear increase with irradiation time at constant reactor power. Shutting down the reactor caused a decrease in the added attenuation; however, restarting the reactor caused the added attenuation to rapidly return to values almost identical to those observed at the end of the previous irradiation. The decrease in attenuation that was observed after the reactor was shut down shows the importance of the in‐situ nature of the measurements made in this work (previous ex‐situ attenuation measurements could not have captured this effect). A model is proposed for the experimentally obtained values of the radiation‐induced attenuation that involves three previously identified color centers including a composite V‐center, a center, and a center. The model accounts for gamma radiation‐induced ionization of pre‐existing defects, generation of new defects via displacement damage, and conversion between defect centers via ionization and charge recombination.