The optical fiber (OF) silica glass compaction is the least studied effect induced by radiation, since it takes place at very high fluences, mainly in mixed environments combining γ-rays and neutrons. Although the radiation induced structural reorganization has already been investigated in bulk materials, OFs are a more complex case study, due to its in homogeneous nature and internal stress distribution. We hereby investigate the structural changes induced in a pure-silica core OF by fast neutron fluence of up to 5 × 1019 n/cm2 and total γ-dose of ~4 GGy(SiO2), through micro-Raman spectroscopy allowing us to study the radial distribution of several signatures. The structural changes induced by neutrons on this OF are weaker than those observed on bulk glass at similar fluences, in agreement with the lower observed compaction. The main cause is the higher irradiation temperature, even if a key role is played by the internal stress present in the fibers.