Development of in-core monitoring system using radiation resistant optical fibers

With the aim to develop new in-core monitoring system using optical techniques, studies have been started on heavy neutron irradiation effects on optical fibers. Two types of highly radiation resistant optical fibers. Pure silica (SiO₂) with OH content core and SiO₂ with fluorine doped core fibers were irradiated in the core region of JMTR. The fibers were irradiated under fast neutron flux (E>1.0 MeV) of about 1×10¹⁷ n/m² s and gamma-ray dose rate of 2×10³ Gy/s. The optical absorption and the light emission spectrum change were measured in-situ along the irradiation. As a result, the following effects have become clear. Large absorption loss at wavelength of shorter than 700 nm and loss peak of 630 nm were appeared. However, the increase in absorption was quite low in the wavelength from 700 to 1700 nm. The SiO ₂ fibers were survived up to 3×10²³ n/m². During irradiation, the induced light emission in the wavelength from 400 to 1700 nm and light peak at about 1200 nm were observed. Induced light emission was directly proportional to the neutron yield