Long-term exposure of fiber Bragg gratings in the BR1 low-flux nuclear reactor

We report the results of a long-term exposure of fiber Bragg gratings in the BR1 low-flux nuclear reactor at SCK-CEN in Mol, Belgium. Gratings fabricated in the photosensitive and the standard fibers were installed in February 2000 in a research channel of the reactor and remained there till August 2008. During this time the reactor was operational 4690h so that the gratings received a total thermal/fast neutron fluence up to 16.9/1.47×10¹⁷ n/cm², and a gamma-dose ~10 MGy. The temperature cycled from 10 to 80°C. All the gratings were easily detectable at the end of the experiment. At that time most grating showed a slight amplitude degradation. However, for three gratings the amplitude remained unchanged within the measurement error. The Bragg wavelength tracing allows perfect match of the temperature data obtained with standard the thermocouples placed near the gratings. The results show that fiber Bragg gratings can be used for long-term temperature monitoring in nuclear installations.