[en] Full text: For study of influence of preliminary structure defects and type of cladding material on additional defects accumulation kinetics we investigated the absorption spectra of optical fibers (OF) marked FVP-400 (quartz core and quartz clad) and FSHA-800 (quartz core and polymeric clad) preliminary irradiated by fast neutrons fluencies 10¹², 10¹³, 10¹⁴, 10¹⁵ cm^-2 before and after additional irradiation by γ-rays of ⁶⁰Co source. Preliminary irradiation of samples was conducted in cadmium-plated channel of the reactor. Dose rate of accompanying γ-radiation is determined as 1250 R/s using KI quartz glass based dosimeter. The spectra of the induced absorption (losses) were calculated by an expression A(λ)=(10/L)x lg[T(λ)/T₀(λ)], where T₀(λ) and (λ) are transmissions of samples before and after irradiation, L-sample length [m], A(λ)- optical losses [dB/m]. It's established that the transformation processes of previously existing structure damages arising during fiber drawing into other ones and creations of additional defects under influence of neutrons depends on hydroxyl content and type of fibers cladding material. It's shown that under influence of γ-rays at doses 10⁵, 5·10⁵, 10⁶, 5.10⁶, 10⁷, 5·10⁷ and 10⁸ R the two-stage accumulation of non-bridging oxygen hole centers (NBOHC) is observed in preliminary neutron-irradiated OF. The first stage is caused by appearance of potential NBOHC arising during fiber drawing and irradiation of OF by neutrons. The dose value of γ-rays at the beginning of the second stage that is connected to creation of additional NBOHC under γ-rays action decreases with growth of preliminary irradiation fluence. We suppose that under neutron irradiation of OF along with transformation of previously existing damages and creation of additional point defects there is formation of areas with the higher density than in quartz glass which concentration increases with growth of irradiation fluence. Hence, the volume of such areas where ≡Si-O-Si≡ bonds are strained increases too. In condensed quartz glasses the rupture of strained ≡Si-O-Si≡ bonds under γ-irradiation arising during condensation is dominant during NBOHC creation processes. Thus, we assume that the first stage of increasing the number of NBOHC is caused by creation of previously existing NBOHC and transformation of potential centers into NBOHC under γ-rays action. After the certain values of γ -irradiation doses the creation of additional NBOHC in OF begins at the expense of strained ≡Si-O-Si≡ bonds rupture under γ-rays action. The probability of formation of additional NBOHC near and in volume of germs is higher than in other structure damage types. The increase of concentration and type of structure defects with growth of neutron fluence results in decreasing the value of γ -irradiation dose at which the second stage of NBOHC increase is observed. (author)