AbstractA periscope-type viewing system has been chosen as a reference of the ITER in-vessel viewing system due to its wide viewing capability. According to the ITER research and development program, development of a radiation hard periscope has been conducted by the Japan Home Team in collaboration with the JCT. As an intermediate stage, a sub-scaled radiation hard periscope with a length of 6 m was fabricated and irradiated at a dose rate about 10 kGy h–1. In this periscope, three types of radiation hard lenses made of alkaline barium glass, lead glass containing CeO2 and OH doped synthetic quartz were adopted on the basis of the irradiation experiments on various glasses. The irradiation test results of the sub-scaled periscope show no degradation of the viewing performance up to the accumulated dose of 50 MGy, while a standard-type (non-radiation hardness) periscope becomes invisible after 2 h irradiation. Based on this, a full-scale radiation hard periscope with a length of 15 m was fabricated and tested. As a result, it has been verified that the developed 15-m-long periscope has sufficient viewing capability and the focal adjusting mechanisms are capable of accommodating thermal expansion due to high temperature operation up to 250°C.