Fig. 8-3 Observed power and temperature by the measurement of radio-luminescence and thermal radiation in the optical fiber itself During heavy irradiation in the core region of a reactor, an optical fiber generates strong radio-luminescence in the wavelength range 400-1,400 nm with some peaks and thermal radiation in the infrared. The intensity of radio-luminescence was directly proportional to the reactor power. From the thermal radiation due to the gamma-ray heating dependent on reactor power, temperatures could be estimated by Planck's law. (a) Relation of the reactor power and 450 nm radio-luminescence peak in the optical fiber itself. (b) Estimated temperatures measured by thermal radiation due to the gamma-ray heating. Developed radiation resistant optical fibers kept their good transmission characteristics even in a reactor irradiation up to a neutron flux of 1018 n/m2 * s and a gamma-ray dose rate of 103 Gy/s. Also, radiation resistant optical fibers survived neutron irradiation up to 1024 n/m2 and a temperature of 1,100 K.

Fig. 8-4 A new reactor instrumentation system using optical fibers An optical instrumentation system using the quantum effects in the optical fiber itself such as radio-luminescence, thermal radiation and scattered optical signals, could measure radiation, temperature, pressure, strain of piping, and so on. An optical measuring method could reduce the number of sensors and be a simple and convenient system.

Until now, the optical transmission characteristics of optical fibers were thought to be vulnerable to radiation due to radiation-induced defects and the formation of color centers. It is known that radiation causes ionization and atomic displacement within the molecular bonding network of silica (SiO2) glass. However, recent research for the development of radiation resistant optical fibers, has found that the content of fluorine (F) to silica glass improves the radiation resistance of optical fibers. Fluorine content reduces the formation of color centers such as the E'-center and the non-bridging oxygen hole center in the silica core glass. Developed fluorine contained optical fibers were found to have good radiation resistance even after a reactor irradiation up to a neutron fluence of 1024 n/m2 and gamma-ray doses of larger than 109 Gy at a temperature of 1,100 K. A new in-core measuring method such as the power and temperature monitor using radiation resistant optical fibers was demonstrated in the core region of JMTR. Figure 8-3 shows an example of observed power and temperature by the measurement of radio-luminescence and thermal radiation in the optical fiber itself. Figure 8-4 shows a new optical reactor instrumentation system for advanced nuclear power reactors using quantum effects in the optical fiber itself such as radio-luminescence, thermal radiation, scattered optical signals and so on. An optical measuring method could reduce the number of sensors and be a simple and convenient system. Furthermore, an optical in-core imaging system using radiation resistant optical fibers is planned for the High Temperature Testing Reactor.

Reference T. Kakuta et al., Application of Optical Fibers to Instrumentation System in Advanced Nuclear Power Reactors, ICONE-7128: Proc. 7th Int. Conf. on Nuclear Engineering, Apr. 19-23, Tokyo, Japan (1999).

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