Development of a Temperature Distributed Monitoring System Based ...

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 6, DECEMBER 2014

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Development of a Temperature Distributed Monitoring System Based On Raman Scattering in Harsh Environment C. Cangialosi, Student Member, IEEE, Y. Ouerdane, S. Girard, Senior Member, IEEE, A. Boukenter, S. Delepine-Lesoille, J. Bertrand, C. Marcandella, P. Paillet, Senior Member, IEEE, and M. Cannas Abstract—Raman Distributed Temperature Sensors (RDTSs) offer exceptional advantages to monitor the envisioned French deep geological repository for nuclear wastes, called Cigéo. Both -ray and hydrogen release from nuclear wastes can strongly affect the temperature measurements made with RDTS. We present experimental studies on how the performances of RDTS evolve in harsh environments like those associated with -rays or combined radiations and release. The response of two standard and one radiation tolerant multimode fibers (MMFs) are investigated. In all fibers the differential induced attenuation between Stokes and anti-Stokes signal, causes a temperature errors, up to with standard multimode fibers (100 m) irradiated at 10 MGy dose. This degradation mechanism that is more detrimental than the radiation induced attenuation (RIA) limiting only the sensing range. The attenuation in the [800-1600 nm] spectral range at room temperature is explored for the three fibers -irradiated and/or hydrogen loaded to understand the origin of the differential RIA. We show that by adapting the characteristics of the used fiber for the sensing, we could limit its degradation but that additional hardening by system procedure is necessary to correct the T error in view of the integration of our RDTS technology in Cigéo. The current version of our correction technique allows today to limit the temperature error to for 10 MGy irradiated samples. Index Terms—Distributed temperature, gamma radiation, hydrogen release, optical fiber sensing, radiation effects, Raman scattering.

I. INTRODUCTION

D

ISTRIBUTED optical fiber temperature sensors based on the temperature dependence of Raman scattering in silica have been intensively investigated during the last years [1], [2]. Manuscript received July 11, 2014; revised October 07, 2014; accepted October 31, 2014. Date of publication December 04, 2014; date of current version December 11, 2014. C. Cangialosi is with Lab. Hubert Curien, Université de Saint-Etienne, F-42000 Saint-Etienne, France, and also with Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, 90100 Palermo, Italy (e-mail: [email protected]). Y. Ouerdane, S. Girard, and A. Boukenter are with Lab. Hubert Curien, Université de Saint-Etienne, F-42000 Saint-Etienne, France (e-mail: [email protected]; [email protected]; [email protected]). S. Delepine-Lesoille and J. Bertrand are with the French National Radioactive Waste Management Agency (Andra), 92298 Chatenay-Malabry, France (e-mail: [email protected]; [email protected]). C. Marcandella and P. Paillet are with CEA DAM DIF, F91297 Arpajon, France (e-mail: [email protected]; [email protected]). M. Cannas is with Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, 90100 Palermo, Italy (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2014.2368787

They are now employed in various industrial application ranges such as the fields of (i) public safety like fire detection inside buildings and tunnel or (ii) the industrial process monitoring. This technology has also recently shown considerable promise for the temperature monitoring of nuclear facility [3]–[6]. Raman Distributed Temperature Sensors (RDTS) are intrinsic optical fiber sensors, in which the optical fiber is not only a transmission medium but also the sensing material. RDTS offer exceptional advantages over traditional electronic sensors for temperature monitoring of the envisioned French deep geological repository for long-lived high-level (LL/HL) and intermediate-level (LL/IL) nuclear wastes, called Cigéo [7]. In particular, they provide temperature changes along the fiber over distances extending up to several kilometers with one meter spatial resolution and thus overcome limitations of traditional sensors, whose information is restricted to local effects. The study of the vulnerability of RDTS technology in Cigéo radiation environment requires evaluating the influence of different constraints such as -rays and hydrogen effects on their performances. Inside the storage cell, the temperature varies between and . The gamma radiation dose rate varies between 1Gy/h and 10Gy/h depending on the considered wastes. These radiation levels lead to a significant degradation of Telecom-grade optical fibers over the facility lifetime (100 years) through the radiation induced attenuation (RIA) phenomenon. Moreover, small hydrogen releases ( mmol/hour for few IL nuclear wastes) originating from nuclear waste release and anoxic corrosion of materials are expected. These small concentrations could slowly and regularly increase when ventilation stops with cell closure. Its maximum levels could approach 100% hydrogen content in the atmosphere [8]. So, these mobile species can diffuse into the fiber and affect its transmission properties. We recently investigated the degradation mechanisms of RDTS for some types of aggressions, such as those associated with or -rays [9], [10]. We tested diverse classes of standard fibers, with different dopant species, cladding and coating composition. We observed that the Raman response is strongly influenced by the dose and by the hydrogen presence into the fiber core. The RIA influences the relative intensities of Stokes and anti-Stokes components used by the RDTS system to monitor temperature, leading to both large errors in the temperature measurements and limiting measurable range of length. Indeed, we observed the impossibility to use P-doped fibers as RDTS

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sensing elements due to their high RIA values preventing the detection of the Raman signal. Moreover, through the same effect, Hydrogen Induced Attenuation (HIA) will even more affect the RDTS performances, leading to wrong temperature evaluation ( error). In the case of hydrogen-loaded fibers, the amplitude of the errors is related to the hydrogen concentration into the fiber. Nevertheless, we showed that by adapting the characteristics of the used fiber for the sensing, it is possible to limit this -related degradation. Using carbon coated fibers avoid the presence of hydrogen inside the core. Some methods to correct this error due to the RIA have been suggested for RDTS system. Two procedures envisaged the use of an open-ended arrangement: the first method requires two thermocouples for the calibration and assumes the uniformity of the RIA along the fiber. The second technique exploits a loop arrangement of the sensing fiber to calibrate the ionizing field distribution effects [4], [6]. Another procedure, tested until 300kGy, using double ended arrangement is proposed. In this configuration the sensor is probed from each end of the fiber and the increasing temperature error along the sensing fiber due to RIA is eliminated [5]. The drawback of double ended configuration for our application is i) due to the impossibility to perform temperature measurement in case of the fiber breakage during an accident; ii) to multiply the fiber length of the sensor by , increasing the negative impact of RIA on the sensor dynamic. This is a crucial feature to assure the safety of Cigéo facility and explains our first choice of a single-ended system. II. PRINCIPLE

OF

RAMAN DISTRIBUTED TEMPERATURE SENSORS

RDTS acts as a transducer converting temperature variation into optical power intensity change. Its temperature sensitivity is achieved through the modulation of the backscattered intensity, owing to the phenomenon of spontaneous Raman scattering. Indeed, when a high power probe laser launches a pulsed optical signal into optical fiber at the probe frequency , most of the scattered light originates from Rayleigh scattering at the same frequency . Only a small fraction of probing power is transferred to frequency downshifted (Stokes) or upshifted (anti-Stokes) and is Raman-backscattered to the fiber input [11]. The Raman component of the backscattered light is caused by thermally driven molecular vibrations and is used to obtain information about the temperature distribution along the fiber length [12]. Since the intensity of the anti-Stokes component is more sensitive to the temperature than the intensity of the Stokes line [13], RDTSs derive the temperature from the anti-Stokes to Stokes optical power ratio, R, according to the equation:

(1)

where THz for silica, h and k are the Planck’s and Boltzmann’s constants respectively, T is the temperature of the optical fiber (in ). These systems derive the location along the

TABLE I CHARACTERISTICS OF TESTED FIBERS

multimode fiber from the time of flight of the probe pulse using optical time domain reflectometry techniques (OTDR) [14]. III. EXPERIMENTAL PROCEDURE A. Tested Fibers Three commercial multimode fibers, described in Table I, are investigated. The set includes two types of Step Index Fibers (SI-MMF) and one Radiation Resistant Fiber (RR-MMF). We tested 6 different samples of Fibers I and II: a pristine sample (it means without any treatment, used as a reference), three not pre-treated -irradiated samples at doses of 3 MGy, 6 MGy and 10 MGy and two samples -loaded at different times before the -irradiation at 6 MGy. Then, one sample was -irradiated when there was still a large amount of hydrogen inside it, whereas for the other one the -loading was performed two months before -irradiation so that no mobile species remains in the sample. Only the pristine and -ray irradiated samples (3 MGy, 6 MGy, 10 MGy) of Fiber III were tested. B. Irradiation Condition All fibers were -irradiated using the Brigitte facility of SCK CEN to accumulated doses of 3, 6 and 10 MGy. Post mortem (one month after irradiation) temperature measurements were performed simultaneously with the RDTS system on pristine and on the three irradiated samples (100 m length) of all fiber types to highlight the radiation effects on the sensor performances. During the period, between the irradiations campaign and the post mortem measurements, all the samples were kept at room temperature, so this study highlights the stable part of the RIA involved. C. Design of the Experiment Raman set up All measurements were performed using a commercial openended Sentinel DTS-SR Raman distributed temperature sensor from Sensornet [15] provided with an 8 channels multiplexer. The best instrument claimed performances are: probe wavelength of 1064 nm, of accuracy (for distances km), spatial resolution of 1 m and longest range (continuous monitoring for up to 30 km). All the measurements are made at normal pressure and controlled room temperature conditions, under the same calibration parameters, with a spatial averaging length of 1.02 m. Pre-Hydrogen Loading Condition The samples were placed into hydrogen specific vessels at pressure of 202 bars and during 62 hours to achieve the

CANGIALOSI et al.: DEVELOPMENT OF A TEMPERATURE DISTRIBUTED MONITORING SYSTEM

Fig. 1. Distributed Temperature measurement performed on a standard MMF, Fiber I, at room temperature. Results of pristine and -irradiated samples at 3, 6, 10 MGy are shown.

saturation. Then one of the samples was left at ambient conditions and hydrogen desorbed naturally within few days [16]. The other one was preserved at low temperature to prevent the outgassing before the irradiation campaign. It is important to note that the concentration of present into the fibers strongly decreases during the irradiation run of few weeks but we assumed that the level was above the threshold needed to observe the bleaching of radiation-induced defects. OSA Measurement An Optical Spectrum Analyzer AQ6370C (from Yokogawa), was used to investigate the [600-1700 nm] wavelength range with the maximum resolution of 0.02 nm. All measurements were done at room temperature one month after the end of the irradiation. IV. EXPERIMENTAL RESULTS A.

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Fig. 2. Distributed Temperature measurement performed on a standard MMF, Fiber II, at room temperature. Results of pristine and -irradiated samples at 3, 6, 10 MGy are shown.

Fig. 3. Distributed Temperature measurement performed on the radiation resistant MMF, Fiber III, at room temperature. Results of pristine and -irradiated samples at 3, 6, 10 MGy are shown.

- Radiation Effects

Fig. 1 and Fig. 2 illustrate the -ray effect on the Raman temperature measurements performed at room temperature ( ) on Fibers I and II, respectively. Gamma radiations lead the device to evaluate wrong temperature (real temperature is given by the pristine sample). Moreover, we observe for -irradiated samples of both fibers a significant linear increase of temperature evaluated by the RDTS with the fiber distance: a temperature of is detected over a length of 100 m even if the whole sample is kept at the temperature of . This RDTS temperature error increases both with the distance and with the dose. As discussed in [10], this negative impact is caused by the RIA. However for such sensors, the RIA level is not the only parameter to be considered, the other one, very impacting on RTDS response is its spectral dependence in the domain of the Stokes and anti-Stokes signals. Whereas RIA levels will limit the possible sensing distance, the RIA spectral dependence will cause the temperature measurement errors. The radiation response of RDTS using the radiation resistant Fiber III is shown in Fig. 3. We note a limited increase of temperature error (with respect to the pristine sample) measured along 100 m of the fiber up to . The temperature error increases more

weakly with the distance than with other tested fibers and the error tends to saturated in the investigated dose range. Fig. 4, 5 and 6 report the attenuation spectra of pristine and -irradiated (at 3 MGy, 6 MGy, 10 MGy) samples of Fibers I, II and III, respectively. For all fibers, the irradiated samples present different values of attenuation at the anti-Stokes and Stokes wavelengths at the three doses. Moreover, as the degradation of the Raman-based sensor efficiency is mainly caused by the differential RIA between the two signals, , it is obvious from our measurements that radiations differently affect the two signals. Fibers I and II show similar spectral behavior as function of the dose. In the spectral region of interest (between AS and S lines at 1064 nm), the RIA is of the same order of magnitude ( dB/m) for both fibers. By contrast, the radiation resistant fiber (Fig. 6) shows lower values of RIA and less differences between the two Stokes and anti-Stokes signals. This leads to a better RDTS response, as observed in Fig. 3. B. Combined - Radiation and

Effects

Fig. 7 and 8 highlight the impact of -rays on pretreated samples of Fibers I and II, respectively. Both figures compare dif-

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Fig. 4. Attenuation spectra of Fiber I. Results of pristine and -irradiated samples at 3, 6, 10 MGy are shown. Vertical lines mark the wavelength position of Raman intensity anti-Stokes, IAS, and Stokes, IS (at 1014 nm and 1114 nm, respectively).

Fig. 5. Attenuation spectra of Fiber II. Results of pristine and -irradiated samples at 3, 6, 10 MGy are shown. Vertical lines mark the wavelength position of Raman intensity anti-Stokes, IAS, and Stokes, IS (at 1014 nm and 1114 nm, respectively).

Fig. 6. Attenuation spectra of the radiation resistant MMF, Fiber III. Results of pristine and -irradiated samples at 3, 6, 10 MGy are shown. Vertical lines , and Stokes, mark the wavelength position of Raman intensity anti-Stokes, (at 1014 nm and 1114 nm, respectively).

ferent samples: pristine (the reference), -irradiated at 6 MGy, -irradiated at 6 MGy with present during irradiation and -irradiated at 6 MGy two months after -loading and out-diffusion. In both fibers we observe a meaningful negative effect


Fig. 7. Distributed Temperature measurement performed on Fiber I, at room temperature. Results of pristine (light blue line), -irradiated at 6 MGy (red inside (green line) and -irradiated at line), -irradiated at 6 MGy with -loading (blue line) samples are shown. 6 MGy two months after

Fig. 8. Distributed Temperature measurement performed on Fiber II, at room temperature. Results of pristine (light blue line), -irradiated at 6 MGy (red inside (green line) and -irradiated at line), -irradiated at 6 MGy with -loading (blue line) samples are shown. 6 MGy two months after

of -rays on the sample -loaded two months before irradiation campaign and left to desorb naturally at ambient conditions. Indeed, the response of the sensor is worse than without the pre-treatment. We observe a temperature error exceeding the one of the 10 MGy irradiated sample (not reported in Fig. 7 and Fig. 8). However, our measurements show that, at the opposite, the sample irradiated in presence of during the irradiation is slightly affected by this combined treatment one month after the end of the irradiation. The spectral measurements performed on all the samples of Fibers I and II, are presented in Fig. 9 and Fig. 10, respectively. In both fibers, high RIA values are observed for the sample -loaded and left to desorb naturally at ambient conditions for two months before irradiation at 6 MGy with respect to the pristine one. In both -loaded samples, we note a main peak at nm due to the OH presence inside the fiber, but only in the sample irradiated when there was still inside are observed two further OH overtones (at m and nm) [17]. Between and (1014 nm and 1114 nm, respectively), the RIA is , whereas it is negligible in both


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fiber-composition dependent but all fibers, even the radiation tolerant ones, seem too much affected by these effects. A. Origin of the Infrared RIA

Fig. 9. Attenuation spectra of Fiber I. Results of pristine, -irradiated at 6 MGy, -irradiated at 6 MGy with inside and -irradiated at 6 MGy two months -loading samples are shown. Vertical lines mark the wavelength poafter , and Stokes, (at 1014 nm and sition of Raman intensity anti-Stokes, 1114 nm, respectively). All measures were done at room temperature.

The exposure of the material to radiation results in the generation of defects; actually, this often occurs by transformation, or conversion, of other defects preexisting in the as-grown material before irradiation, and known as precursors. For Fibers I and II, the sample -loaded two months before the irradiation campaign shows the worse Raman response and the higher RIA level among all tested samples of these two fibers. We interpret this finding assuming that the -preloading in the fiber creates functional groups, such as SiOH and SiH, precursors for -rays induced generation of NBOHC and E’ center that are responsible for absorption bands in the visible and UV range thus resulting in an increase of RIA [18], [19]. We note also that the RIA increases after irradiation in the IR spectral domain above 1100 nm, however the nature of the defects absorbing in this region is unknown yet. Otherwise, we observe a correct RDTS response with the hydrogen loaded samples during the irradiation. We assumed that the defects generated by -rays have been bleached by reactions with molecular hydrogen present in the fiber, improving the -irradiation hardness of the sensor. B. Differential RIA Between S and AS Wavelengths

Fig. 10. Attenuation spectra of Fiber II. Results of pristine, -irradiated at inside and -irradiated at 6 MGy two 6 MGy, -irradiated at 6 MGy with -loading samples are shown. Vertical lines mark the wavelength months after , and Stokes, (at 1014 nm and position of Raman intensity anti-Stokes, 1114 nm, respectively). All measures were done at room temperature.

the pristine and loaded samples. This proves that the RDTS response is related to the spectral dependence of the RIA around 1064 nm. Also in this case we note a correlation between Raman response and differential RIA. Indeed the loaded-samples two months before -irradiation show: i) a remarkable error in the temperature measures, reported in Fig. 7 and Fig. 8. ii) a not negligible difference ( ), reported in Fig. 9 and Fig. 10. V. DISCUSSION Our study shows that the response of Raman device (instrument fiber) is strongly affected by both -rays and loading and that combined effects will have to be considered for Cigéo applications. Radiation induced attenuation degrades the fiber transmission and sensing capacities. With dose, the RIA increase limits the fiber length that can be used to monitor temperature within a facility whereas the differential RIA values at Stokes and anti-Stokes wavelengths strongly perturb the T measurements. The amplitude of these effects is shown to be

The presence of ionizing radiation can lead to optical fiber deterioration and a non-uniform increment of its attenuation in the IR spectral domain [20]–[22]. Moreover, the backscattered light also undergoes an attenuation and , for the anti-Stokes and the Stokes signals respectively, creating the differential RIA, ( ), causing the error in the measured temperature distribution. Thanks to the spectral measurements, it is clear that the Raman response is mainly related to the spectral dependence of the RIA that impacts differently the Stokes and anti-Stokes wavelengths. Indeed, in irradiated samples we note important changes of the differential attenuation compared to the pristine sample whose results are negligible, as observed in Fig. 4, 5 and 6. Ergo, if the differential RIA is not taken into account, the measurement error increases with the distance along the fiber limiting the sensing length and hence the sensor performances. The assumption that is commonly used in single-ended RDTS systems that is constant with time and/or uniform along the length of a fiber is not suitable if the fiber is subjected to environment where its transmission is likely to be degraded (like in presence of ionizing radiation and hydrogen release). Therefore, to obtain correct values of temperature, the measurements must be compensated for the differential attenuation . If we take into account attenuation in the formula (1), we obtain the anti-Stokes to Stokes optical power ratio, R(T,z), as a function of the position of the scattering element du [5]

(2) where

is the distance from the fiber input.

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Fig. 11. Natural logarithm of anti-Stokes over Stokes ratio as function of the fiber length calculated for the 10 MGy irradiated Fiber I (black line). The red . line indicates linear fit used to obtain the differential attenuation

C. New Correction Procedure We developed a new procedure to correct the systematic error of Raman temperature measurements due to radiation effects. The application method of this correction will be illustrated only for the Fiber I in this paper. We found that the response of our device depends on various factors such as the type of fiber, differential attenuation for the Raman Stokes and anti-Stokes lines due to -ray and presence and also instrumental parameters. Taking into account these issues we have advanced an innovative correction technique to improve the performance of our instrument in a harsh environment that affects uniformly the whole fiber length. We found that the temperature can be evaluated as in (3): (3)

, is a temperature offset parameter and where is the differential attenuation, anti-Stokes minus Stokes. In the equation (3) the two factors, and , are unknown and have to be determined. In addition, we can notice that complementary micro-Raman measurements (not reported in this paper) performed on irradiated samples with doses up to 10 MGy, show no significant structural change of the silica matrix leading to no significant change on the Stokes and anti-Stokes lines wavelengths. To obtain the differential attenuation, The following method is used to find the value of the differential attenuation for each sample. When the temperature is constant along the fiber from the equation (2) or (3) we obtain:

(4) Doing a linear fit of the natural logarithm of the anti-Stokes over Stokes ratio as function of the distance, as in (4), we can directly obtain for each sample. The linear fit done on 10 MGy irradiated sample of the Fiber I is shown in Fig. 11.

Fig. 12. Differential attenuation, , evolution obtained by the linear fit, as function of the irradiation dose observed in the Fiber I. The black points indicate of the samples irradiated at different doses. Instead, the red the differential obtained from the samples submitted to both radiation ones designate the effects. and

The differential attenuation value obtained through the linear regression is dB/m, close to the one estimated through OSA spectral measurement (Fig. 4) equal to dB/m. Fig. 12 reports the various differential RIAs, acquired by fitting the AS/S ratio along Fiber I, as function of the dose. We observe an increase of the attenuation followed by saturation at larger doses. Moreover, the sample irradiated at 6 MGy when there was still inside shows the lowest value . Moreover, this value is comparable with the obtained for the pristine one. Instead, the samples irradiated at the same dose but two months after the -loading exhibits the highest value among all samples. These behaviors are consistent with the experimental evidences shown beforehand. Therefore, the differential RIA affects significantly the response of the distributed temperature sensor based on Raman scattering. Table II reviews for all samples of the three investigated fibers, the values extracted from both the linear fit and the OSA spectral measurements. To acquire the instrumental parameter, Considering the offset factor is mandatory to extract the correct value of the temperature. To determinate this parameter we have extracted, through the formula (3), the value for each sample of the Fiber I, imposing in the first point of the fiber ( m) the temperature value detected by external thermocouples. Then, it is possible to correct the RDTS measurements using Eq. (3) and reconstruct the temperature distribution along the fiber. For the worst case (10 MGy), we then obtain a variation temperature of only after correction. Fig. 13 reports, for the Fiber I -irradiated at 10 MGy, the temperature error calculated with respect to a reference temperature detected in the initial part of the sensing fiber, where the effect of attenuation is smallest. In the graphic this temperature error is compared to the error we obtained after our procedure correction on the same data set. It’s clear that this new technique leads to remarkable improvements on the temperature measurements performed in harsh environment. Moreover, for the Fiber I we found a linear behavior plotting the obtained offset parameters in function of the dose (black point of Fig. 15).


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TABLE II DIFFERENTIAL RIA OF TESTED FIBERS

Fig. 13. Comparison between the error on the temperature obtained by the instrument measurements and the T error evaluated after our correction of radiation effects for Fiber I - irradiated at 10 MGy.

Fig. 15. The offset parameter, , calculated in function of the -radiation dose for Fiber I. Black points were obtained by single samples measurement. Red points are the values acquired by measurement with configuration: 3 MGy Pristine.

observe different results. It indicates that the offset parameter depends on the power of the injected light in the fiber. A study of this dependence will permit us to know the right value of for each measurement and hence to obtain an automatic correction function for RDTS system. VI. CONCLUSION

Fig. 14. Linear fits (red and blue lines) realized on the natural logarithm of ratio anti-Stokes over Stokes intensities plotted in function of the fiber length calculated by measurement performed in this configuration: 3 MGy Pristine for Fiber I.

To verify that the depends on the power of injected light, an additional test was done on Fiber I. Raman measurements were performed with following line of measures: 3 MGy pristine sample. Also in this case we calculated with a linear fit the offset factor for both samples and we plotted the obtained parameters as a function of the dose, as in Fig. 15. Comparing the value of acquired for the pristine sample, through the serial measurement (3 MGy pristine configuration) and that one drew from single measurement (only pristine sample), we

To ensure the efficiency of RDTSs inside deep geological repository for long-lived high-level and intermediate-level nuclear wastes, their vulnerability of the system was characterized under harsh conditions representative of Cigéo. The vulnerabilities of three classes of sensing optical fibers were characterized under -rays and in presence of various amounts of hydrogen during the exposure (from zero to nearly fully-loaded). Experimental results have shown that the differential RIA strongly affects the response of Raman sensor. It leads to an increase of the temperature error with the distance, up to in 100 m length of standard fibers. Even if the amplitude of these errors is fiber-composition dependent and reduced by using a radiation resistant fiber, the error is still larger than after a short 100 m length. The situation is even more complex by considering combined radiations and hydrogen effects. We observed a good RDTS response on the hydrogen loaded samples during the irradiation, in which the defects generated by -rays have been recovered by molecular hydrogen presents in the fiber, improving the -irradiation hardness of the sensor in terms of

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permanent damages. However, in case of online measurements, hydrogen induced attenuation (HIA), more deeply investigated in [10] will strongly limit the interest of such loading. Moreover, the -presence before the irradiation seems to negatively impact the sensor performance by enhancing the RIA differences between Stokes and anti-Stokes wavelengths. Hydrogen issues can be mitigated with specific hermetic coatings around the fibers, but RIA and differential RIA will be nearly impossible to fix by acting at the component level. However, at the system level, we have developed a new procedure to correct the systematic error of Raman temperature measurements due to radiation effects. It authorizes today to limit, after correction of the data, the errors to in case of homogeneous irradiation along the fiber. As perspectives, this procedure needs to be tested - online - under radiation during continuous exposure of the fiber. More complex radiation hardening techniques will also be developed to consider the use of RDTS systems in more complex scenarios involving non homogeneous dose and dose rate kinetics over the fiber sensor. REFERENCES [1] B. Culshaw and A. Kersey, “Fiber-Optic Sensing: A historical perspective,” J. Lightw. Technol., vol. 26, pp. 1064–1078, 2008. [2] J. P. Dakin and D. J. Pratt, “Distributed optical fibre Raman temperature sensor using a semiconductor light source and detector,” Electron. Lett, vol. 21, no. 13, pp. 569–570, 1985. [3] F. Jensen, E. Takada, M. Nakazawa, T. Kakuta, and S. Yamamoto, “Distributed Raman temperature measurement system for monitoring of nuclear power plant coolant loops,” Proc. SPIE, vol. 2895, pp. 132–144, 1996. [4] F. Jensen, E. Takada, M. Nakazawa, T. Kakuta, and S. Yamamoto, “Consequences of radiation effects on pure-silica-scattering-based temperature measurements,” IEEE Trans. Nucl. Sci., vol. 45, no. 1, pp. 50–58, Feb. 1998. [5] A. Fernandez Fernandez, P. Rodeghiero, B. Brichard, F. Berghmans, A. H. Hartog, P. Hughes, K. Williams, and A. P. Leach, “Radiation-tolerant Raman distributed temperature monitoring system for large nuclear infrastructures,” IEEE Trans. Nucl. Sci., vol. 52, no. 6, pp. 50–58, Dec. 2005. [6] A. Kimura, E. Takada, K. Fujita, M. Nakazawa, H. Takahashi, and S. Ichige, “Application of a Raman distributed temperature sensor to the experimental fast reactor JOYO with correction techniques,” Meas. Sci. Technol., vol. 12, pp. 966–973, 2001.

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