Growth of optical transmission loss at 850 nm in silica core optical fibers during fission reactor irradiation

Pure, OH-doped and F-doped silica core optical fibers were irradiated in a fission reactor at 400 +/- 10 K using an electric heater at a reactor power greater than 10 MW (20% of the full power). The temperature was not controlled well at the early stage of the reactor startup, when the temperature was about 320-340 K. The optical fibers were irradiated with a fast neutron (E > 1 MeV) flux of 3.2 x 10(17) n/cm(2)s and a gamma dose rate of 3 x 10(3) Gy/s for 527 h, Optical transmission loss at 850 nm was measured in situ during irradiation. A prompt increase in optical transmission loss was observed as irradiation started, which was probably due to dynamic irradiation effects caused by short-lived and transient defects and is probably recoverable when irradiation ceases. After the prompt increase in optical transmission loss, a so-called radiation hardening was observed in fibers containing OH. Radiation hardening was also observed in 900 ppm OH-doped fiber at the second startup. The optical transmission loss increased linearly with irradiation dose, denoted as the accumulated loss, which we believe is due to irradiation-induced long-lived defects. Accumulated loss dominates radiation-induced optical transmission loss in a fission reactor irradiation. (C) 1998 Elsevier Science B.V.