Steady γ-Ray Effects on the Performance of PPP-BOTDA and TW-COTDR Fiber Sensing

Abstract

We investigated the evolution of the performances of Pulse Pre Pump-Brillouin Time Domain Analysis (PPP-BOTDA) and Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR) fiber-based temperature and strain sensors when the sensing optical fiber is exposed to two γ-ray irradiation conditions: (i) at room temperature and a dose rate of 370 Gy(SiO2)/h up to a total ionizing dose (TID) of 56 kGy; (ii) at room temperature and a dose rate of 25 kGy(SiO2)/h up to a TID of 10 MGy. Two main different classes of single-mode optical fibers have been tested in situ, radiation-tolerant ones: fluorine-doped or nitrogen-doped core fibers, as well as Telecom-grade germanosilicate ones. Brillouin and Rayleigh Sensitivities of N-Doped fibers were not reported yet, and these characterizations pave the way for a novel and alternative sensing scheme. Moreover, in these harsh conditions, our results showed that the main parameter affecting the sensor sensitivity remains the Radiation Induced Attenuation (RIA) at its operation wavelength of 1550 nm. RIA limits the maximal sensing range but does not influence the measurement uncertainty. F-doped fiber is the most tolerant against RIA with induced losses below 8 dB/km after a 56 kGy accumulated dose whereas the excess losses of other fibers exceed 22 dB/km. Both Rayleigh and Brillouin signatures that are exploited by the PPP-BOTDA and the TW-COTDR remain unchanged (within our experimental uncertainties). The strain and temperature coefficients of the various fibers under test are not modified by radiations, at these dose/dose rate levels. Consequently, this enables the design of a robust strain and temperature sensing architecture for the monitoring of radioactive waste disposals.

Extreme Radiation Sensitivity of Ultra-Low Loss Pure-Silica-Core Optical Fibers at Low Dose Levels and Infrared Wavelengths

“…These two F-doped fibers have pristine attenuation (before irradiation) at 1550 nm, respectively, of 0.83 dB/km and less than 0.5 dB/km. Moreover, both of them have already been demonstrated to be resistant to steady state irradiations [ 12 , 13 ] and able to be used for Brillouin, Rayleigh, or Raman based sensing up to MGy dose levels [ 14 , 15 , 16 , 17 ].…”

France’s State of the Art Distributed Optical Fibre Sensors Qualified for the Monitoring of the French Underground Repository for High Level and Intermediate Level Long Lived Radioactive Wastes

“…Based on promising results from [72], N-doped fibre has also been tested but it revealed less adapted to Cigéo than the qualified F-doped fiber [21]. …”

“…Andra and LabHC use two different techniques to evaluate strain sensitivities of optical fibres. The first design is based on pulleys [ 21 ]. Several meter-long fibre is supported by several pulleys, fixed at the two extremities; and strained over a length section L by applying a stepwise displacement ΔL using a micrometer screw gauge ( Figure 6 , right).…”

“…Another solution is to coil the Fibre Under Test (FUT) around a borosilicate [ 21 ] or a quartz [ 22 ] tube, by applying different controlled strains along the sample length. An illustration is reported in Figure 7 with a step strain every 15 m. Borosilica has a thermal expansion coefficient (TEC) identical to that of silica.…”

Combined Radiation and Temperature Effects on Brillouin Scattering based Sensing with Ge-Doped Optical Fibers

“…In addition to post irradiation analysis, steady-state measurements were performed to follow the transient mechanisms occurring during the irradiation. Online -ray tests revealed no transient degradation of the Brillouin response during a low dose/rate exposure (1 kGy/h for ~ 6 days) up to cumulated doses of 160 kGy, in various types of fibers [3] [4]. Meanwhile, it has been shown that the radiation influence may significantly differ as a function of the fiber surrounding temperature, both on Rayleigh [5] and Raman scattering processes [6].…”