The formation of nanosized particles in pure silica glass irradiated in water in the reactor core with fast neutrons up to 1019 cm-2 and gamma radiation has been studied by optical spectroscopy and X-ray diffraction. Irradiation with neutrons leads to the destruction of the initial mesostructure of a - and β - quartz with dimensions of 1.7 and 1.2 nm and the formation of cristobalite and tridymite nanocrystallites with dimensions of 16 and 8 nm in displacement thermal peaks.