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This paper addresses the particular need to develop suitable fabrication techniques of glasses to improve the radiation-resistance and extend the lifetime of optical fibres in high radiation background. Two radiation hardening techniques based on hydrogen and fluorine doping of glass have been applied on step index pure silica core optical fibres. The radiation-induced optical absorption and luminescence have been measured in gamma and fission reactor irradiation conditions at SCKCEN, Mol Belgium. The experimental results show that the lowest optical absorption is achieved in the hydrogen-treated fibre sample. However, the luminescence effect remains significant in any type of fibre and cannot be reduced by the investigated radiation hardening techniques.

Fused silica core optical fibers are expected to play crucial roles especially in the size-reduced International Thermonuclear Experimental Reactor (ITER-FEAT). Several radiation resistant optical fibers have been developed in Japan and the Russian Federation. The task force on radiation effects in diagnostic components in the ITER-EDA (engineering and design activity) promoted international round-robin irradiation experiments on the developed optical fibers. Ten different optical fibers were tested in a cobalt-60 gamma-ray irradiation facility and in the Japan Materials Testing Reactor. The paper reports results obtained on five different optical fibers, which include purified, hydrogen loaded, and fluorine doped ones. Results show that the developed optical fibers could be deployed in remote handling and out-of-vessel applications. But, for the in-vessel diagnostics in the visible range optical spectroscopy, further improvement of the radiation resistance of optical fibers will be needed.

For applying to optical diagnostics for fusion reactor, two types of radiation-resistant optical fibers have been fabricated and their optical phenomena were examined in the JMTR. The optical absorption due to radiation-induced defects and formation of color centers, is concentrated in the wavelengths less than 700 nm. By contrast, the absorption was quite low in the wavelength range from 700 to 1500 nm. The optical fibers appeared to have good radiation resistance and survived neutron irradiation up to 2 × 10 24 n/m 2 . During JMTR irradiation, strong optical emission from 400 to 1400 nm with a sharp peak intensity at 1270 nm was observed. The peak value at 1270 nm is directly proportional to the reactor power. The results of absorption and fluorescence phenomena suggest the possibility of multi-parameter sensing for fusion reactor diagnostics.

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.

The effects of irradiation on the performance of optical fibers with silica core were examined during irradiation in a fission reactor, JMTR. The fibers performed well in terms of their radiation characteristics up to the fast neutron fluence of 1.06 × 10 20 n/cm 2 and the gamma ray dose rate of 4.3 × 10 9 Gy at 460 K. Three kinds of optical radiation induced by the reactor irradiation were observed during irradiation. Origins of these optical radiations were studied. Broad optical radiations centered at 0.45 and 0.73 μm are thought to be Cherenkov radiation. The sharp optical radiation at 1.27 μm was thought to be generated in the silica core by the gamma-ray irradiations in the reactor.

Two kinds of optical fibers were irradiated in a fission reactor, JMTR (Japan Materials Testing Reactor), up to a 1.55 × 10 19 n/cm 2 fast neutron fluence and a 3.3 × 10 9 Gy ionizing dose at 370 K. Optical transmission spectra were measured in the wavelength range of 450–1750 nm, in situ. Growth of strong optical absorption bands and a peak were observed in the range of wavelength shorter than 750 nm. In the meantime, the fibers showed good radiation-resistance in the range of wavelength larger than 750 nm. Some optical radiation from the fibers was observed. The main cause of the optical radiation is thought to be so-called Cherenkov radiation.