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 until August 2008. During this time the reactor was operational 4690h so that the gratings received a total thermal/fast neutron fluence ~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. After more than eight years exposure the amplitude and the shape of the gratings spectra remained unchanged for fibers devoid of hydrogen loading, while FBGs fabricated using hydrogen loading did exhibit moderate changes. The results show that fiber Bragg gratings indeed have potential for long-term temperature monitoring in nuclear installations.