Fiber Bragg gratings are all-fiber wavelength-selective refractive index structures manufactured through UV exposure of optical fibers. Their applications range from WDM all-fiber filters, dispersion compensators and fiber laser resonators for optical telecommunication applications to different types of point or distributed optical fiber sensors for a large variety of applications. The nuclear industry is considering the use of photonic technology for data communication in the next generation of nuclear power plants. In addition, fiber Bragg grating sensors are being evaluated by the nuclear industry for structural integrity and temperature monitoring. This work aims to study, in harsh radiation environments, a new type of FBG referred to as chemical composition grating. These gratings differ from other types of FBG in that their refractive index structure is attributed to a change in the chemical composition. Chemical composition gratings have shown to be extremely temperature stable surviving temperatures in excess of 1000 °C. We have experimentally studied the effect of very harsh gamma-neutron radiation on the properties of chemical composition gratings fabricated in a Ge-F doped silica optical fiber.