Development of a Fiber-Optic Oxygen Sensor for Long-Term Corrosion Monitoring of Radioactive-Waste Repository

In order to monitor corrosion risk of radioactive-waste repository, a fiber-optic oxygen sensor for long-term use was proposed and its durability was evaluated. The sensing mechanism is based on quenching of luminescence by oxygen. Ruthenium complex whose counter-anions are dodecyl sulfate was utilized as oxygen-sensitive dye. This complex was dispersed into silicone resin. Excitation is achieved with a blue LED (470 nm) and the emitted light at around 610 nm is collected by optical fiber. The sensing membrane showed high sensitivity and quick response to gaseous and dissolved oxygen. Dependence of sensitivity on temperature was evaluated. It was found that the sensitivity decreased with ambient temperature increase. Furthermore, accelerated thermal degradation of the sensing membrane was observed when the sensor was exposed to a high-temperature environment. In order to clarify the degradation and long-term stability of the sensor, degradation tests were conducted in air condition at high temperature from 150 to 250 °C. The complicated degradation behavior was observed. The half decay period of the sensitivity strongly depended on the test temperature. From the simple lifetime estimation, it is clarified that sensing membrane would have good performance and long-term stability in practical environment. Furthermore, degradation tests in hot water at 80 °C were also conducted. In contrast to the degradation tests in air, the sensitivity gradually increased with immersion time.