The application of distributed optical fiber sensing technology in nuclear island safety monitoring is mainly studied in this paper. The anti-radiation ability of the system is an important index. The distributed optical fiber system in this paper is designed with a special anti-radiation optical fiber. The temperature of power supply cables and other facilities in the nuclear island containment can be distributed monitored in full period in real time. Radiation tests are carried out to validate the anti-radiation performance of the system. 60Co is used as a gamma ray radiation source to generate continuous pulses with an average energy of 1.25 MeV. The rate of radiation dose is 1800 Gy/h and the total radiation dose is 1950 Gy. The anti-radiation ability of anti-radiation optical fiber and common optical fiber is compared. It is proved by tests that common optical fiber sensor has great fiber loss in radiation environment, about 0.5dBm, and the loss is decreasing continuously. Anti-radiation optical fiber has little fiber loss in radiation environment, about 0.12 dBm, and the loss tends to be saturated. During the radiation test, the temperature measurement performance of the distributed optical fiber sensing system using anti-radiation optical fiber and the one using common optical fiber is tested in real time. The results show that the anti-radiation distributed optical fiber sensing system performs well in the whole process, which meets the requirements of temperature monitoring for 1339.2 Gy total radiation dose in the refueling cycle of nuclear island.
In this paper, we introduce an optimal bias voltage searching strategy and maintaining method in BOTDA system based on dual sideband modulation. The system utilizes both up shifted and down shifted continuous wave light to generate Brillouin scattering light, while using the amplified pulsed light to boost the signal by stimulated Brillouin scatting effect. In order to obtain a clean Brillouin shifted curve along the fiber under test, the probe light must be in good signal to noise ratio. In addition, the intensity of shifted light should be stable as well. Therefore, we propose a novel control method of frequency shift in sideband modulation of a BOTDA system, which keeps the first order frequency-shifted light at its maximum signal to noise ratio and minimum light intensity fluctuation. We also implement static experiments to verify our proposed scheme. Strain/temperature tests were taken out to evaluate the performance of the BOTDA. Results showed that the resolution of BOTDA system could reach 13με/0.65°C at 50 km fiber cable with spatial resolution of 0.5m.
An optical model to simulate the distributed fiber optical sensor based on spontaneous Brillouin spectrum is derived. The reliability of this model is validated with experimental measurements. Using this analytical expression, parametric studies are conducted to investigate impacts of key factors including fiber loss, signal to noise ratio, bandwidth and scanning step on the optical fiber sensor measurement error. The simulation results exhibit good agreement with previous published calculation results. Applying this novel model into the data interpretation, measurement error of distributed fiber optical sensor based on spontaneous Brillouin scattering can be better controlled.