In-core high temperature measurement using fiber-Bragg gratings for nuclear reactors

The Pebble Bed Modular Reactor Pty. Ltd. (PBMR) has called for research into the possibility of distributed in-core temperature measurement. In this paper, several methods for distributed temperature measurement in high-pressure, -radiation and -temperature environments have been investigated by means of a literature study. The literature study revealed fiber-Bragg grating (FBG) temperature sensors to be the most feasible solution to the temperature measurement challenge.Various parameters affecting the propagation of light in optical fibers and consequently the FBG reflection profile was investigated. The differential equations describing FBG structures were solved and implemented in Matlab in order to simulate wavelength division multiplexing (WDM) of a distributed FBG sensing system. Distributed sensing with apodized FBGs written into the sapphire optical fibers is considered.Temperature measurement using wavelength division multiplexing of apodized FBGs written into silica optical fibers were also demonstrated in a test platform. The measured results corresponded with the theory. It was found that when there is a strong temperature gradient across the FBG, spectral widening of the reflection profile occurs. This fact should be taken into account when allocating bandwidth to a certain FBG and choosing a demodulation algorithm. Sapphire FBGs were also acquired and the optical properties investigated. Furthermore, high temperature stable FBGs written with femtosecond laser radiation in silica Sumitomo Z-Fiber have been evaluated and shown to be a good option for temperature measurement below 1000 °C.Finally, the implementation of FBGs in a high temperature nuclear reactor is discussed and recommendations are made for future work.

► High temperature (>1800 °C) distributed measurement using sapphire fibers has been developed.
► Fiber-Bragg gratings inscribed at regular intervals along an optical fiber.
► Signals analysed with optical spectrometer.
► Calibration of high temperature fiber-optic sensors.