The evolution of transmission spectra of Bragg gratings written in an N-doped silica-core fiber in the course of H(2) loading at a pressure of 6 MPa is investigated. It is shown, that penetration of hydrogen molecules in the region of fiber core with written gratings causes irreversible spectrum changes, which do not disappear after subsequent H(2) outcome from the fiber. Bragg gratings' spectra monitoring in the process of H(2) loading is viewed from the angle of photosensitivity mechanisms responsible for formation in N-doped silica-core fibers photoinduced Bragg gratings, capable to operate at very high temperatures.