Radiation effects in ytterbium-doped silica optical fibers are still poorly studied despite their importance in space-based applications as optical inter-satellite links. We investigate such effects by coupling thermally stimulated luminescence and optical absorption measurements. Connections between traps populated by X ray irradiations and the radiation-induced optical absorption (RIOA) are established. The color centers responsible for RIOA are proved to correspond to reduced forms of defects related to dopants. Holes released upon reduction processes are shown to be trapped on defects of the silica matrix.