Demonstration of long term optical fiber thermometry in the in-core region of a nuclear reactor

An experimental demonstration of fiber optic temperature sensing in the in-core region of Japan Materials Testing Reactor from 250 to 750 °C is described. Temperature data could be obtained for two full-power weeks with neutron fluxes of ̃10¹⁴ n/cm²/s and gamma dose rates of ̃5 × 10³ Gy/s. The measurements were based on thermally generated infrared light within the optical fiber itself. The fiber thus served as both signal generator and signal transmitter to the out-of-core region. The fibers utilized in the experiments where of high OH pure-silica-core type and showed good radiation resistance. In the infrared region the transmission of the fibers was only weakly affected by the incident radiation. Radiation induced luminescence and Cerenkov radiation in the optical fibers were found to have small influence on the signal in the infrared window. The high OH content of the fibers used in the present experiment precluded the use of the spectral regions at 945, 1245 and 1390 nm, due to the high intrinsic and radiation induced absorption at these wavelengths. The use of silica fibers limited the maximum temperature to <1000 °C. The present experiments show that optical sensors based on infrared emission can be used to monitor temperature in the in-core region of nuclear reactors for extended periods of time.