This paper presents the state of the art distributed sensing systems, based on optical fibres, developed and qualified for the French Cigéo project, the underground repository for high level and intermediate level long-lived radioactive wastes. Four main parameters, namely strain, temperature, radiation and hydrogen concentration are currently investigated by optical fibre sensors, as well as the tolerances of selected technologies to the unique constraints of the Cigéo’s severe environment. Using fluorine-doped silica optical fibre surrounded by a carbon layer and polyimide coating, it is possible to exploit its Raman, Brillouin and Rayleigh scattering signatures to achieve the distributed sensing of the temperature and the strain inside the repository cells of radioactive wastes. Regarding the dose measurement, promising solutions are proposed based on Radiation Induced Attenuation (RIA) responses of sensitive fibres such as the P-doped ones. While for hydrogen measurements, the potential of specialty optical fibres with Pd particles embedded in their silica matrix is currently studied for this gas monitoring through its impact on the fibre Brillouin signature evolution.

Distributed optical-fiber sensors (OFSs) based on scattering phenomena are nowadays used for several applications such as temperature or strain monitoring. Whereas very promising results have already been reported for the applications of Rayleighand Raman-based OFSs in harsh environments, only few studies concern the Brillouin-based ones. Two phenomena influence the Brillouin-based OFS when the sensing fiber is exposed to radiations: the radiation-induced attenuation (RIA), which reduces the sensing range by decreasing the signal-tonoise ratio, and a frequency shift of the Brillouin peak, which entails an error on the temperature or strain measurements. We report here an in situ study on the effects of γ-rays on Brillouin optical time-domain analysis (BOTDA) OFS up to the accumulated dose of 10 MGy(SiO ₂ ) and those of X-rays on Brillouin optical time-domain reflectometer sensor up to 1 MGy. We compare the vulnerability levels for fiber composition, fiber packaging, as well as the influence of a preirradiation on the sensing performances. Whereas the Ge-doped fibers present a too high RIA that limits drastically the sensing range at high doses, especially with the BOTDA technique; the F-doped ones appear suitable for integration in high dose levels environments. Indeed the error induced by the Brillouin frequency shift on temperature measurements is smaller than 5 °C after a 10-MGy dose.

In this paper, we demonstrate and highlight a proof of concept for the feasibility of an innovative technique to regenerate on-site irradiated optical fiber links in nuclear facilities. Using Hole-Assisted optical fibers (HAOF), a longitudinal gas-loading is easy to perform thanks to the fibers' dedicated holes located in the outer part of the cladding. All along the fiber length, gas ( H2 or D2) diffuses from the holes into the silica matrix, interacts with radiation induced point defects and passivates them, reducing the Radiation Induced Attenuation (RIA) levels. The validity of our approach is demonstrated considering the changes occurring at infrared wavelengths during the H2 treatment of a MGy irradiated single mode Ge-doped HAOF. Within just a few hours, a reduction of about 50% is observed for the RIA at 1550 nm of the 10 MGy irradiated HAOF, acting only from one of its two ends. An additional study is done on a set of fibers with various core dopants (F, Ge, P) and without holes to give an overview of the pertinence of developing HAOF fibers with these dopants for various applications. Using HAOF and this recovery technique appears very promising for samples based on pure-silica, Ge or F-doped cores and operating in the ultraviolet-visible spectral domains such as plasma diagnostics. This approach exhibits another interesting feature which may be extension to higher dose ranges and lifetime of P-doped distributed dosimeters used in high energy physics facilities or nuclear power plants.

Raman-based Distributed Temperature Sensors (RDTS) allow performing spatially resolved (1 m) reliable temperature measurements over several km long Optical Fibers (OFs). These systems are based on the temperature dependence of the intensities of both the Stokes and anti-Stokes components of the Raman back-scattered signal. One of the specific issues associated with RDTS technology in radiation environments is the differential Radiation Induced Attenuation (RIA) between the two components that induces huge errors in the temperature evaluation. Such problem is particularly evident for commercially available single-ended DTS using one laser source. Double-ended configuration could be used to correct for the differential attenuation but are limited by RIA in terms of sensing range. In the present work, we show how a Radiation-Hardened-by-Design DTS (RHD-DTS) overcomes the observed radiation issues keeping the single-ended interrogation scheme. In the tested RHD-DTS two infrared excitation laser sources (~1550 nm and ~1650 nm) are employed: the wavelength of the Stokes component due to the first excitation source coincides with the wavelength of the second excitation; vice versa, the wavelength of the anti-Stokes component due to the second excitation source coincides with the wavelength of the first excitation. The overall result is that the two signal intensities are automatically corrected for the differential RIA all along the OF sensor length and the temperature measurements becomes robust against radiation effects. This study demonstrates the potential of such a sensor by reporting preliminary experimental results obtained with a prototype developed by Viavi Solutions exploiting radiation-sensitive or radiation-hardened optical fibers.

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.