Application of optical fibers for optical diagnostics in high temperature gas cooled reactor

Visibility of a core region of a high temperature gas cooled reactor (HTGR) is very poor in general with its solid graphite moderator. Realization of optical diagnostics will improve safety and maintenance of the HTGR considerably. The applicability of fused silica core optical fibers for optical diagnostics in a core of the High Temperature Testing Reactor (HTTR) of the Japan Atomic Energy Research Institute (JAERI) has been studied in the present research. Optical diagnostics are also expected to play crucial roles in advanced research planned in the HTTR. Optical transmission of the optical fibers was found not to degrade for several hundred hours at 1070K in air and helium environments in the wavelength range of 350-1800nm. In general. the optical fibers were found to be heat-resistant. To study radiation effects, the optical fibers were irradiated in Japan Materials Testing Reactor (JMTR). where the fast neutron(E>1MeV) flux was up to 1.5x1018n/m2s and the gamma-ray dose rate was up to about 5W/g for iron. The estimated fast neutron flux and the gamma-ray dose rate would be in the order of 1016n/m2 and about 0.1W/g for iron, respectively in the HTTR. In general, optical transmission loss increased substantially with a small irradiation dose in the visible wave length range, although some developed fibers showed better radiation resistance. Good optical transmissivity was kept in the infrared region with absorption rate of less than a few dB/m. Radioluminescence and thermoluminescence from sapphire and silica could be observed with optical fibers under irradiation. Cherenkov radiation was observed in the wavelength range of 600-1800nm, whose intensity was temperature-independent. Black-body radiation was dominant in the wavelength longer than 1200nm at elevated temperatures. The results showed that the silica core optical fibers could be used as an image guide as well as monitors for radiation dosimetry and for monitoring core environments in the HTTR. 13 refs