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Transmission and Mechanical Characteristics of an Image Fiber under Gamma-ray Irradiation at High Temperature

In the nuclear industries, a visual inspection is required to confirm the integrity of the facilities and components. Recently, a silica-based image fiber has been used for visual inspection in the nuclear facilities. As the emphasis on visual inspection increases, an image fiber which can function in harsh environments, such as under high dose rate, narrow spaces and high temperature is required. It is thus necessary to study the transmission and mechanical characteristics of a silica-based image fiber exposed to γ-ray irradiation and elevated temperatures. We investigated the transmission loss characteristics and mechanical strength under γ-ray irradiation at high temperature with the objective of identifying the causes of deterioration of the image fiber. The induced loss of the transmission in the visible wavelength region was caused by È center and NBOHC (non-bridging oxygen hole center), and their annealing effect by thermal energy was confirmed. We believe that a high radiation resistant image fiber can be realized by improving its manufacturing condition, since it was identified that there was close relation between the induced loss of transmission and the thermal history of the glass material during melt-fused process of image fiber manufacturing. The mechanical characteristics were also investigated. The mechanical strength decreased by the deterioration of the primary coating. A secondary coating resistant to heat and radiation contributes to maintaining mechanical strength of the image fiber.