# Radiation vulnerability of optical fiber cables for underground nuclear waste monitoring

Jérémy Perrot, Adriana Morana, Emmanuel Marin, Youcef Ouerdane, Aziz Boukenter, Johan Bertrand, Stéphane Poirier, Hanaa Houjeij, Evzen Novak, Tomas Peltan, et al.

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## To cite this version:

Jérémy Perrot, Adriana Morana, Emmanuel Marin, Youcef Ouerdane, Aziz Boukenter, et al. Radiation vulnerability of optical fiber cables for underground nuclear waste monitoring. Optical Fiber Technology, 2025, 92, pp.104181. 10.1016/j.yofte.2025.104181. hal-05045923

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Submitted on 24 Apr 2025

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Radiation vulnerability of optical fiber cables for underground nuclear waste monitoring

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Jérémy Perrot<sup>a, *, †</sup>, Adriana Morana<sup>a</sup>, Emmanuel Marin<sup>a</sup>, Youcef Ouerdane<sup>a ‡</sup>, Aziz Boukenter<sup>a</sup>, Johan Bertrand<sup>b</sup>, Stéphane Poirier<sup>c</sup>, Hanaa Houjeij<sup>c</sup>, Evzen Novak<sup>d</sup>, Tomas Peltan<sup>d ‡</sup>, Sylvain Girard<sup>a, e</sup>

ARTICLE INFO

Keywords:

Optical fibre

Optical fibre cable

Rayleigh scattering

Brillouin scattering

Temperature monitoring

Strain monitoring

Radiation effects

Hydrogen

Gamma-rays

Neutrons

ABSTRACT

This work presents our evaluation of the radiation vulnerability of optical fiber cables candidate to monitor temperature and strain in nuclear waste repositories. For this, the cables have been both exposed to γ-1 MGy Total Ionizing Dose (TID) and to mixed-field neutron-γ-rays up to 150 Gy TID and neutron fluence of 2.8 × 10<sup>13</sup> n/cm<sup>2</sup>. The effect of hydrogen-rich atmospheres is also investigated as this constraint is associated with the targeted environments. The evolution of the properties of seven optical fiber cables, differing in their compositions and structures, were evaluated during these two irradiation campaigns. Radiation-Induced Attenuation (RIA), Brillouin Frequency Shift (BFS), and Rayleigh Frequency Shift (RFS) were measured online at room temperature. Additional post-irradiation assessments have been performed: thermo-mechanical properties of both Brillouin and Rayleigh scatterings were evaluated pre- and post-irradiation, along with carbon coating H<sub>2</sub>-blocking capabilities and radiation-induced degradation. Results demonstrated that radiation induces hydrogen diffusion from some of the cable structures, particularly from gel components. This, in addition to RIA, significantly increases the optical loss levels at infrared wavelengths under γ-rays (>1200 dB/km, at 1550 nm). The study explores the origins of these losses and the radiation-induced BFS and RFS levels. Post-mortem analyses, including spectral assessments, macroscopic cable degradation observations, and hydrogenation, provided further insights into cable behavior in such conditions. A PEEK-structured cable showed notable resilience, with minimal changes in its Brillouin/Rayleigh sensitivity, no visible degradation, and the lowest H<sub>2</sub>-blocking capability degradation under irradiation. This work offers a comprehensive qualification process for evaluating optical fiber cable performance for nuclear waste monitoring, and the findings exhibit broader implications for various nuclear industry applications.

1. Introduction

Geological disposal is considered the safest and most sustainable option for the final management of high-level waste (HLW) and spent fuel classified as waste [1]. In France, the Industrial Centre for Geological Storage (Cigéo) is designed to store HLW and intermediate-level long-lived waste (IL-LLW) from French nuclear facilities. This future reversible deep geological disposal facility, located at a depth of 500 m, is planned to operate for at least 100 years, starting around 2035,

in the Meuse/Haute-Marne region. The French National Radioactive Waste Management Agency (Andra), responsible for the Cigéo project, intends to monitor the site by equipping the disposal cells with instruments and sensors to track physical parameters such as temperature and deformation.

The environment around the nuclear waste cannisters imposes harsh conditions to the monitoring. Concerning HLW cells, the cumulative doses reached after 100 storage years has been evaluated up to 10 MGy (SiO<sub>2</sub>) and up to 0.8 MGy(SiO<sub>2</sub>) for IL-LLW cells from a mixed-field

<footnote><sup>a</sup> Université Jean Monnet Saint-Etienne, CNRS, Institut d'Optique Graduate School, Laboratoire Hubert Curien UMR 5516, F-42023 Saint-Etienne, France</footnote>

<footnote><sup>b</sup> National Radioactive Waste Management Agency (ANDRA), F-92298 Chatenay-Malabry, France</footnote>

<footnote><sup>c</sup> Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSN-RES/SCA/LECEV, F-91400 Saclay, France</footnote>

<footnote><sup>d</sup> Centrum Výzkumu Rež s.r.o., 250 68, Husinec- Rez 130, Czech Republic</footnote>

<footnote><sup>e</sup> Institut Universitaire de France (IUF), Ministère de l'Enseignement Supérieur et de la Recherche, 1 rue Descartes, F-75005 Paris, France</footnote>

<footnote>* Corresponding author.
E-mail address: jeremy.perrot@univ-st-etienne.fr (J. Perrot).</footnote>
neutron/γ exposure with a neutron flux simulated (for HLWs) at 0.8 × 10⁵ n.cm⁻².s⁻¹ (at the beginning of the operation) and 0.6 × 10⁴ n. cm⁻².s⁻¹ (after 170 years of operation), at the sensor location with a maximum for neutron energy of 500 keV. In addition, because of the high activity of the wastes a temperature increase is expected up to 90 °C (HLW cells) and 70 °C for IL-LLW cells from the initial temperature of 15 °C. Moreover, wastes can emit low amounts of radioactive gases (³H, ¹⁴C and ⁸⁵Kr) and generate hydrogen (H₂) originating from wastes due to radiolysis phenomena (mainly in IL-LLWs) but also from anoxic corrosion of metallic materials inside the canisters (mainly in HLW wastes) [1]. The tracking of physical measurands should withstand this harsh environment and its constraints. The major nuclear waste activity relies on γ-rays emission and few neutrons.

Using optical fibre systems offers many advantages compared to the use of point sensors to ensure this monitoring under such harsh conditions. It offers a deported, distributed [2] and real-time sensing capabilities as well as the monitoring of temperature [3], strain [4], gas concentration [5], water levels [3] and is suitable for dosimetry applications [6] in radiation environments. This technology is applicable in various environments such as those associated with high energy physics [7], nuclear reactors or nuclear waste repositories [8], with high spatial resolution and enabling the positioning of sensors in harsh environments. This emerging technology provides high performances in a variety of applications such as for structural health monitoring [9], aeronautics [10], space programs [11,12], geotechnical domain [12] and medicine [13].

Optical fibres can be implemented in a versatile manner, by tuning the composition and structure of the fibres to the requirements of the desired application. In addition, placing optical fibres around any geometrical form (such as nuclear waste canisters) enables a 3D-mapping [14] of parameters such as temperature and strain at the location of the optical fibre.

Continuously distributed sensors based on Rayleigh and Brillouin scattering are widely used to monitor the strain and temperature of various civil engineering structures. For the geological disposal monitoring application (such as Cigéo-Andra), radiations (γ-rays and neutrons) are likely to impact the performances of optical fibre sensors. First, at the fiber level, as it was shown that most of the silica-based optical fibers exhibit high vulnerability to radiation and can darken, through the Radiation Induced Attenuation (RIA) phenomenon, even after exposure to relatively low radiation doses. Standard fibers are then not suitable for high-radiation (MGy dose levels) environments. Radiation-hardened fibers (F-doped and pure silica) are then required for deployment in environments with high exposure (dose-rates and TID) [15]. The impact of radiation on fiber performance must be carefully characterized before deploying fibers in high-radiation environments. All cable materials must be tested under high radiation levels and demonstrate an operational lifespan of at least 100 years, ensuring accurate measurement of temperature and strain in the waste environment. The main radiation influence on the fibre properties is the RIA [16]. The fiber doping such as type of dopants and impurities, their concentration and radial distribution, as well as the manufacturing process parameters, have been shown to strongly influence the RIA [17].

To characterize our sample, we used a Stimulated Brillouin Scattering (SBS) interrogator known as Pump-Pre Pulse Brillouin Optical Time Domain Analyzer (PPP-BOTDA) working at 1550 nm. The counter-propagation of the two-pump pulse and continuous probe along the waveguide allows Brillouin backscattering, containing the Brillouin spectrum signature, when the two counter-propagative waves match the Brillouin frequency. In fact, through the electrostriction effect [18], the stimulation of acoustic waves in the fiber core and cladding allows Brillouin spectrum measurement along the fiber length. Compared to a classic BOTDA instrument, limited to the 1-m spatial resolution, the PPP-BOTDA overcomes this limitation by sending a pre-pulse stimulating the phonon prior the sensing pulse exhibiting a reduced width compared to BOTDA. This technique then allows reaching spatial

resolutions down to the centimeter [19].

Brillouin frequencies (ν<sub>B</sub>) of the optical fibers (their specific and intrinsic responses) depend on both temperature and strain. Typical sensitivity coefficients related to temperature (C<sub>TB</sub>) and strain (C<sub>εB</sub>) for the standard types of fibers are of 1 MHz/°C and 0.05 MHz/με [20], respectively, originating from the silica fiber core glass density and its refractive index profile, which are both temperature (T) and strain (ε) dependent. The central Brillouin Frequency Shift, Δν<sub>B</sub>, follows relation (1) [21]:

$$ \Delta\nu_B = C_{TB}\Delta T + C_{\epsilon B}\Delta\epsilon \quad (1) $$

The frequency shifts due to temperature (ΔT) and strain (Δε) variations according to the fiber sensitivities.

The Rayleigh scattering signature measurement is performed using a Tunable Wavelength Coherent Optical Time Domain Reflectometer (TW-COTDR). This technique relies on the tuning of the laser source that is swept in wavelength. This light is injected into the fiber and a heterodyne detection chain is used to interfere the reference source light with the backscattered signal from the optical fiber sample. A beat frequency between the reference arm and the backscattered light is associated with a location inside the fiber. In fact, the phase difference between the reference and the scattered light is proportional to the time delay of the backscattered signal in the fiber. The random inhomogeneities in glass density (that can be due to temperature, strain, irradiation...) within the fiber core lead to fluctuations in the refractive index, which are observed as a random distribution of the power spectrum in the Coherent OTDR (C-OTDR) trace. The frequency domain approach is referred to as Rayleigh Optical Frequency Domain Reflectometry (R-OFDR), while the time domain can be realized using the TW-COTDR method [22]. The tunable wavelength distributed feedback laser and frequency scanning are used to obtain power spectrum. The measurements are compared with the one in reference state, and via cross-correlation to calculate the frequency shift (Δν<sub>R</sub>). The Rayleigh Frequency Shift (RFS) induced by the strain and/or temperature changes are expressed by (2):

$$ \Delta\nu_R = C_{TR}\Delta T + C_{\epsilon R}\Delta\epsilon \quad (2) $$

While classic OTDR (Optical Time Domain Reflectometry) technique is limited to the 1-m spatial resolution, the TW-COTDR can reach the cm-order spatial resolution [22]. This shift can also be measured with the R-OFDR interrogator via the Rayleigh trace with sub-millimeter spatial resolution over tens of meters through the swept wavelength interferometry technique [23]. R-OFDR also allows measuring the RFS by cross-correlating the measured trace with a reference trace being able to monitor small environmental variations (strain, temperature, pressure, gas concentration, etc.) inducing localized optical fiber refractive index changes.

Both Brillouin and Rayleigh scattering-sensing are known to be radiation sensitive. Radiation effects on sensors exploiting scattering-based measurements are the Radiation-Induced Brillouin or Rayleigh Frequency Shifts (RI-BFS/RI-RFS) and RIA.

RIA in Ge-doped (radiation tolerant) and F-doped (radiation hardened) fibers has been extensively investigated under X-rays and γ-rays [15,24]. When irradiated, point defects are generated in the silica fiber core and cladding, leading to the creation of new spectral absorption bands in the fiber transmission window. Depending on the nature of the involved optically active defects, RIA usually grows with the Total Ionizing Dose (TID) and can decay after irradiation as some defects recover [24]. RIA also leads to a decrease in the Brillouin spectrum signal amplitude. Moreover, the Brillouin spectrum is also subjected to radiation-induced modification of its spectrum through RI-BFS [13]. RI-BFS leads to systematic temperature and strain errors as the BFS is used to monitor those parameters. RIA and Brillouin spectrum modifications under irradiation have been shown to depend on the fiber composition, on the operating wavelength and on environmental conditions such as
the irradiation temperature, TID and dose rate [2].

RIA in F-doped fibers exhibits lower RIA levels and RIA temperature dependence from –80 °C to +80 °C at 1550 nm [25] than Ge-doped fibers. Those last fibers exhibit a high RIA temperature dependence with levels of 900 dB/km, 100 dB/km and 30 dB/km at –80 °C, +20 °C and +80 °C, respectively. For comparison, F-doped fiber presented RIA levels of 30 dB/km and 5 dB/km at –80 °C and +20 °C/+80 °C, respectively. This attenuation also affects the Brillouin signature leading to a decrease of the Brillouin peak amplitude following the RIA kinetics and dependencies [25,26].

The γ-ray effect on Brillouin signature has been studied on both Ge-doped and F-doped fibers at room temperature up to 10 MGy dose, exhibiting positive RI-BFSs of +4 MHz and +2.3 MHz, respectively [27]. These frequency shifts can be interpreted as variations of +4 °C or +80 με for the Ge fiber, which correspond to radiation induced errors. RI-RFSs of –7 GHz and –3 GHz have been measured on Ge-doped and F-doped fibers respectively after 1 MGy γ-exposure [26].

Hydrogen presence in silica-based optical fibers also affects the waveguide transmission properties. In irradiated optical fibers, hydrogen can interact with radiation-induced point defects passivating some of them [28,29]. This leads to a decrease in the RIA, mostly in the visible domain. In the Near InfraRed (NIR) domain, the hydrogen diffusion into the fiber core (pristine or irradiated) could lead to the increase of losses through its presence or the generation of Si-OH groups, resulting in the growth of optical absorption band [30] at 1380 nm (absorption of the hydroxyl groups). A third hydrogen loading-related effect has been demonstrated and corresponds to a variation of the fiber refractive index [31]. The H₂ diffusion behavior has been theoretically and experimentally described showing that by increasing the temperature, the glass saturation is reached much more quickly but at a lower absolute H₂ gas concentration. Hydrogen diffusion inside the fiber changes also the refractive index leading to a Hydrogen-Induced Refractive Index Change (HI-RIC) that could be measured through Rayleigh-based sensors [5]. Brillouin scattering is also affected by hydrogen diffusion (Hydrogen Induced Brillouin Frequency Shift, Hi-BFS), its impact being explained by both a modification of the glass refractive index together with an increase of the acoustic velocity with the gas concentration [32]. Undesired H₂ diffusion in the optical fiber can then be misinterpreted as a variation of the environmental measurement. Some mitigation techniques have been proposed to prevent from H₂-diffusion such as using a thin layer of hermetic carbon coating (20–50 nm thickness) or aluminum coating deposited around the fiber cladding [33]. Such coatings prevent from both hydrogen penetration inside the fiber core and strength degradation from environmental moisture.

The combined effects of hydrogen and γ-ray exposure on RIA have been studied in the NIR domain for both Ge- and Pure Silica Core (PSC) F-doped cladding optical fibers. These studies reveal that radiation creates precursor sites that interact with hydrogen, leading to a significant increase in attenuation [34,35]. The extent of attenuation changes induced by the combined effects of radiation and hydrogen largely depends on the dopant type and concentration. F-doped fibers exhibit high tolerance to these changes, while Ge-doped fibers show pronounced sensitivity to Hydrogen-Induced Attenuation (HIA) after γ-ray exposure up to 32 kGy. An increase of 29.3 dB/km and 60.7 dB/km in attenuation was observed for Ge-doped fibers after hydrogenation compared to their solely irradiated counterparts, for fibers with ~2.5 mol%(GeO₂) and ~10 mol%(GeO₂)-doped cores, respectively. In contrast, the PSC F-doped cladding fiber sample, when both irradiated and hydrogenated, exhibited an attenuation increase of 2.3 dB/km compared to its solely irradiated counterpart.

When exposed to neutrons, an additional effect occurring in silica-based material is related to displacement damages inducing both defects and glass compaction. This leads also to a change in refractive index (compaction) originating from local density changes. In extreme conditions, bulk amorphous silica material exposed to a very large fast

neutron fluence (>10¹⁹ n/cm²) exhibits an increase of RI up to 3% [36]. A study on rad-hard F-doped graded-index optical fibers with acrylate coating (0.2 wt% at the core center to 5 wt% in the cladding) under a mixed neutron/γ-environment (from 0.02 MGy to 2 MGy TID) and neutron fluences of 10¹⁵ n/cm² to 10¹⁷ n/cm², showed that the processes at the origin of the defect generation remain dominated by ionization process for both kinds of irradiation (γ and neutrons) at such dose and fluences [37].

However, when inserting the silica-based optical fiber in cable structures its radiation, thermal and mechanical responses are susceptible to change due to an influence of the cable on the fiber properties. In fact, under irradiation, the materials and components surrounding the silica-based fibers (polymers [38], gels, and corroded metals [8]) can be degraded and even release H₂ molecules [39] through radiolysis or thermal desorption [38]. It is then necessary to perform qualification testing for optical fiber cables under such harsh conditions. The assessment involves analyzing RIA levels and radiation effects on both Brillouin and Rayleigh scattering-based sensing from online measurements of the seven tested cables during irradiation campaigns. Furthermore, pre- and post-mortem analysis have been conducted based on multiple criteria (e.g. RI-BFSs/RI-RFSs), RIA, temperature and strain sensitivities, robustness, longevity...). This research has involved laboratory testing of seven different Telecom or sensing grade silica-based optical fiber cables (mostly commercially available), to understand the radiation impact on their performances. Irradiations have been performed with a ⁶⁰Co γ-source to reach TID of ~MGy and in a core reactor (mixed field neutron/γ) reaching neutron fluences of 2.8 × 10¹³ n/cm² and TID of 150 Gy. This work proposes a comprehensive qualification process for evaluating optical fiber cable performances for nuclear industry applications.

## 2. Materials and methods

In this section, we present the tested cable samples, the irradiation campaign conditions and the methods used for online and post-irradiation characterization of those cables.

### 2.1. Investigated samples

This study was performed on seven different optical fiber cable types. Fiber used are of telecom-grade type, with radiation responses and characteristics similar to those of Corning's SMF28 Ultra fibers [40] at 1550 nm [41]. They strongly differ in their cable layers composition and geometry, as illustrated in Fig. 1. Among the seven tested optical fiber cables, the first five are commercially available, the two last are prototypes developed in collaboration between Neubrex (Japan) and Andra (France):

i) The BRUSens DTS-STL-PA model (labeled as CBT) is optimized for temperature sensing, isolating the fiber sensor from strain perturbations and allowing fast temperature transfer to the sensing fiber (acrylate primary coating). This cable is made of an inner steel tube filled with gel surrounding the optical fiber with an outer layer of metallic protection made of several tubes coiled around the steel tube. The outer polymer sheath is made of polyamide.

ii) The BRUSens DSS V9 model by Solifos (labeled as CBS) is optimized for strain sensing with a high strain transfer capacity from outer strain perturbation into the sensing fiber (polyimide primary coating). This second cable is made of an inner polyimide coating surrounding the sensing fiber, a steel tube surrounding the polyimide and an outer polyamide multi-layer buffer sheath.

iii) The BRUsteel cable model from Solifos (labeled as NE) is a stainless-steel strength wires toron armored cable surrounding an inner metallic tube filled with gel surrounding the sensing fiber (polyimide primary coating).
<figure><img src="image_4.png" /><figcaption>Fig. 1. Overview of the different investigated optical fiber cable samples: description of their content, composition and specificities for: CBT, NE, FN1, PEEK, FN2, CFB and CBS cables.</figcaption></figure>

iv) The Neubrex FN-SSL-3 is a flat and adhesive cable (labeled as FN1), manufactured for surface-tape temperature and strain sensing, made of a film protecting fiber (acrylate primary coating), an adhesive layer and a release liner.

v) The Neubrex FN-SILL-3 is a flat cable (labeled as FN2), made for accurate temperature and strain sensing, made of an embossed olefin elastomers-made sheath maintained by two reinforcing metallic bars strength steel rod member surrounding optical fibers (acrylate primary coating).

vi) The Loose cable tube (labeled as CFB) is made of a proprietary soft buffer surrounding the sensing fibers (acrylate primary coating) and a polyamide sheath.

vii) The last cable is developed by ANDRA and Neubrex consisting in a simple PolyEtherEtherKetone (PEEK) sheath surrounding the sensing fiber.

## 2.2. γ-rays irradiation campaign

In this section, the experimental details about the first γ-ray steady state irradiation (high dose) campaign, performed with a $^{60}$Co source at the IRMA facility (IRSN, France) are reported. The seven samples (FN1, FN2, PEEK, CFB, CBT, CBS and NE) are connected to the Neubrescope (NBX-7020F) via an optical switch (2 × 8 NBX-1000-2) both from Neubrex [42], allowing for sequential Brillouin-scattering and Rayleigh

<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Test plan of the γ-irradiation campaign at the $^{60}$Co source at the IRMA facility for all 7 tested cable samples (10 m each). (b) Parameters of the loop sequence running of the Neubrescope measurements connected to the optical switch.</figcaption></figure>
scattering-based measurements on each of its 7 ports. Brillouin-
scattering based distributed measurements are performed with the
PPP-BOTDA interrogator embedded inside the Neubrescope instrument
acquiring Brillouin gain spectra with a high spatial resolution (~ cm)
and over-time during both irradiation and recovery phases. The online
measurement provides the center frequency of the fundamental Bril-
louin peak in optical fiber cable samples sections. Rayleigh-based scat-
tering measurements are performed with the C-OTDR interrogator,
embedded in the Neubrescope instrument, acquiring Rayleigh back-
scattered traces over time with a high spatial resolution (~cm). TW-
COTDR is used for distributed Rayleigh Frequency shift signature mea-
surement inside optical fiber cable samples over time with a high spatial
resolution (~cm) also embedded in the Neubrescope instrument. All
three interrogator measurements are sequenced in a loop to consecu-
tively measure each distributed Brillouin and Rayleigh signatures, as
presented in Fig. 2.b.

A first measurement sequence is performed using the COTDR in-
strument to acquire traces from each line on the different ports of the
optical switch. The raw data and the measured losses are recorded for
each cable. Once these measurements are completed, the instrument
performs a PPP-BOTDA measurement followed by a TW-COTDR mea-
surement. From the PPP-BOTDA measurement, the raw data and the
central frequency of the Brillouin peak are recorded. From the TW-
COTDR measurement, the raw data and the Rayleigh frequency shift,
compared to the reference, are recorded. When these two measurements
are completed, the next line is probed. Once all seven lines have been
probed, the sequence starts again with COTDR measurements. This loop
is repeated throughout the irradiation and recovery phases of the irra-
diation campaign. It takes about 3 h to complete the entire loop.

From the Brillouin spectrum measurement, the central frequency of the fundamental Brillouin peak is extracted by applying a polynomial fit at the top of the peak to deduce the maximum value. The RI-BFS is obtained by: i) averaging the center frequency measured over the 10 m cable section; ii) subtracting the reference value of the peak (before irradiation) and; iii) subtracting the temperature contribution multi- plying the measured temperature profile with the measured Brillouin temperature sensitivity coefficient. From the RFS measurement (returned by the Neubrescope), the RI-RFS is obtained by: i) averaging the RFS measured over the 10 m cable section; ii) subtracting the reference value (before irradiation) and; iii) subtracting the temperature contribution multiplying the measured temperature by the thermo- couple with the measured Rayleigh temperature sensitivity coefficient.

The samples (10 m each) are placed around the source on a circular
support at a distance (23 cm radius) chosen to reach a TID of 1 MGy
during the run duration. In more details, a TID of 877 kGy was expected
at the bottom of the support and 1.09 MGy at its top after 550 h of
irradiation at 1.62 kGy/h and 1.95 kGy/h, respectively. As shown in

Fig. 3, radiation hardened transport fiber cables link the instrument to the samples in the irradiation chamber. Two type-K thermocouples were installed around the sample cylinder to monitor the temperature in real-time within the irradiation chamber and to allow for temperature impact corrections on the Brillouin and Rayleigh measurements. All tests are done at room temperature (~25 °C).

2.3. Neutron/γ-rays mixed field irradiation campaign

As for the γ-irradiation previously described, CFB, PEEK, FN2 and FN1 cable samples (10 m each) were connected to the Neubrescope (NBX-7020F) via an optical switch (2 × 8 NBX-1000-2) probing Brillouin and Rayleigh signatures during irradiation and recovery phases (Fig. 4). Irradiation tests were performed in the LR-0 research reactor core at the Czech Republic Research Centre Řež (CVŘ reactor) and allowed to reach a neutron fluence of 2.8 × 10¹³ n/cm² with a continuous nominal thermal neutron flux of about 10⁹ n/cm²/s and a fast (above 1 MeV) neutron flux of 2.10⁸ n/cm²/s [43]. The TID is about 150 Gy (at RT, ~20 °C), after all the performed runs, with an associated dose-rate of ~1.2Gy/h. The irradiation consisted in 5 runs during 5 days with irradiation phases of ~8 h/day at RT. In addition to the previous γ-irradiation campaign, an online dosimetry measurement has been performed in the middle of the cable coil to assess the TID deposited in the cable samples during all the runs. This measurement has been performed using a P-doped radiation sensitive phosphosilicate single mode optical fiber (100 m) associated with an optical loss measurement interrogator at 1530 nm. This P-doped fiber is known to exhibit a linear RIA/dose sensitivity of 4 dB.m⁻¹.Gy⁻¹ for doses below 500 Gy [44].

As for the previous irradiation, BFS, RFS and optical losses are
measured online in the four cables during all the 5 days of campaign
with the PPP-BOTDA and TW-COTDR respectively resulting in a mea-
surement every 25 min. Only four out of the previously tested seven
cables were chosen for this irradiation campaign. The CBT, CBS, and NE
cables were disqualified based on the results of the IRMA γ-irradiation
campaign. The sample coil consisted in a 23 cm diameter coil connected
to the instruments via radiation hardened optical fiber transport cables
as presented in Fig. 5. The samples were positioned in the dry central
channel at the center of the reactor core, where flux non-homogeneities
remain within ±4 % of the average flux value across all particle energy
levels along the radial axis. Along the channel depth, at the sample
location, the non-homogeneities have been calculated by the facility of
±5 % of the averaged flux value across all particle energy levels [45].
Four type-K thermocouples were installed inside the sample cylinder to
monitor the temperature in real-time in the reactor core to correct the
results from the temperature contribution.

<figure><img src="image_6.png" /><figcaption>Fig. 3. Illustrations of the instrument room and the irradiation chamber presenting the installed setup of the γ-irradiation campaign from the ⁶⁰Co source of optical cable samples at the IRMA facility for all 7 tested cable samples.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Irradiation campaign plan setup of the instrument room and the irradiation chamber presenting the installed setup of the neutron/γ-irradiation campaign in the LR-0 reactor core for CFB, PEEK, FN1 and FN2 cables.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. Illustrations of the instrumentation room and the irradiation chamber for the neutron/γ-irradiation campaign in the LR-0 reactor core.</figcaption></figure>

## 2.4. Post-irradiation characterizations

### 2.4.1. Spectral analysis

The fiber cable spectral response has been characterized before and after irradiation in the near-infrared (NIR, 800 nm – 1800 nm) domain. The 1 m long samples (non-irradiated and irradiated) were connected to a white light source (Ocean Insight-Deuterium-Halogen UV-Vis-NIR) from one end, whereas the other end was linked to a NIR spectrometer

(Ocean Optics-NIR512). Based on this measurement we estimated the OH-content inside the fiber, through the optical losses at 1380 nm. From the transmitted spectrum, we calculate the hydrogen-related attenuation levels by subtracting the –OH peak baseline from its amplitude. According to [46], the Si-OH-related extinction coefficient in silica has been measured to be ~60.6 dB·km⁻¹·ppm⁻¹ at 1380 nm, allowing to deduce the related OH-content.

Raman spectra were collected using the LabRAM Aramis (Horiba
Jobin Yvon group), an integrated confocal micro-Raman system. Excitation was provided by a HeCd laser emitting at 442 nm (2.81 eV). Laser power could be adjusted through software by selecting one of the available density filters. The Raman backscattered light passes through the confocal hole at the spectrograph's entrance. To eliminate the Rayleigh signal, an additional filter is used. The spectrograph then disperses the multichromatic Raman signal onto the CCD multichannel detector, capturing Raman scattering intensities over a range of 100 cm⁻¹ to 1500 cm⁻¹.

### 2.4.2. Hydrogen loading testing

To test the effectiveness of the cable coatings to serve as a protective barrier against hydrogen diffusion into the fiber core, we spliced approximately one meter of each cable samples (pristine and irradiated) to form a single line, which was placed in a pressure stainless steel vessel filled with high-purity H₂ gas at 100 bars at the temperature of 30 °C. The H₂-loading chamber is thermally controlled with a surrounding heater with an accuracy of ±0.5 °C. The outer part of the fiber is connected to the analysis instrument via an SMF28e + transport optical fiber. The distributed losses (1.1 mm spatial resolution) and the Rayleigh Frequency Shift (RFS) are measured (every minute) with a R-OFDR interrogator from Luna Technologies (OBR-4600) operating from 1529 nm to 1571 nm and centered at 1550 nm. The associated frequency shift is obtained by comparing the reference trace (before hydrogenation) along the optical sample coil (~13 m total length) to the measured Rayleigh trace during the H₂-loading process. The sample coil is not subjected to any additional strain variation and the measured RFS, obtained by averaging the RFS signature on the sample section, is corrected by subtracting the RFS value obtained for small temperature fluctuations on the non-irradiated sample. All RFS results are given with an uncertainty of ±0.05 GHz. From this process, the sole contribution to the RFS is attributed to the H₂-loading.

### 2.4.3. Temperature and strain Brillouin/Rayleigh sensitivities

To characterize the radiation effects on Brillouin and Rayleigh temperature ($C_{TB}$ and $C_{TR}$) and strain ($C_{eB}$ and $C_{eR}$) sensitivities of our studied samples, temperature cycling measurements were performed on both pristine and γ-ray irradiated (1 MGy TID) samples between 0 and +40 °C. For $C_T$ characterization, temperature steps were uniformly applied to the sample placed in a heating/cooling chamber (BINDER MK115). Multiple temperature cycles (increase and decrease) were applied at 2 °C/min for 10 min ramps followed by 1 h of stabilization before each measurement for 10 °C steps from 0 °C to +40 °C. The Brillouin spectrum was acquired every minute with both BOTDA (from OzOptics) and R-OFDR (OBR-4600) interrogators respectively to track the induced temperature BFSs and RFSs. Samples were loosely coiled with no constraint applied to avoid any strain contribution.

Both Brillouin and Rayleigh strain sensitivity coefficients ($C_{eB}$ and $C_{eR}$) measurement setup involved two mechanic holders placed at a distance of 2.75 m ($C_{eB}$ measurements) and 37 cm ($C_{eR}$ measurements) with corresponding longitudinal elongation steps of 0.5 mm and 0.25 mm, respectively. A force sensor was positioned at one of the holders to monitor any potential slippage of the cable during the drawing process. A sub-millimetric displacement of the pre-strained sample on the support enabled the monitoring of the strain-induced BFSs and RFSs up to 5000 με using both BOTDA and R-OFDR interrogators respectively. The temperature of the room was kept constant (within ±0.2 °C) to avoid any temperature parasitic effects on BFS and RFS signatures.

## 3. Results

In this section, we first present the MGy TID level γ-ray results from RIA, Brillouin and Rayleigh online measurements as well as post-irradiation characterizations. Then we present the mixed-field (neutron/γ) exposure results through online RIA, Brillouin and Rayleigh measurements.

### 3.1. High-dose γ-exposure qualification

#### 3.1.1. Temperature profile

The temperature profile was measured during the whole irradiation campaign (before, during and after irradiation). Fig. 6 shows the temperature profile calculated by averaging the temperatures given by the two thermocouples. The temperature oscillates between 30 °C and 23 °C and is impacted by the ⁶⁰Co source activity and local environment such as day/night and activities in the instrument room. When the source is removed from its shielding storage canister and positioned at the center of the coil, the temperature rises by 2.5 °C, then drops by 1.8 °C at the end of the irradiation. The highest temperature peak before the start of irradiation is related to the operational staff activity next to the samples. Day/night cycles are reported on the temperature profile counting close to 23 periodic patterns during the irradiation phase corresponding to the 23 days of irradiation. This temperature profile is essential for the temperature contribution correction to both BFS and RFS signatures in optical fiber cables.

#### 3.1.2. Online radiation induced attenuation measurements

The RIA levels and kinetics were obtained from the loss measurements using the Neubrescope (NBX-7020F) C-OTDR interrogator operating at 1550 nm. Via the measured trace, the RIA is determined by calculating the slope over the cable section (10 m) and subtracting the reference slope corresponding to the cable's intrinsic losses to obtain the sole contribution of radiation. Fig. 7 illustrates the increase of the optical losses in the irradiated cable section over time (cumulated dose) for CBS, CBT, NE and PEEK cables. The C-OTDR output signal is launched into the optical switch connected to radiation tolerant transport optical fiber cables linking the instrument room with the irradiation chamber. Each pigtailed (with 20 cm of SMF-28) cable installed around the ⁶⁰Co source, as presented in Fig. 3, are subjected to the increase of losses from irradiation exposure. This section (tested cable section), highlighted in green in Fig. 7 is used for RIA calculation over the cumulated dose and during the recovery phase. The different sections of interest of the optical path are delimited by splicing points between optical fiber transport radiation tolerant cables, pigtails and tested fiber cable samples.

The loss distribution along the optical path is shown in Fig. 7.a and 7.b for CBS and CBT, respectively, highlighting significant losses in the tested cable sections that increase with cumulated dose. Both cables exhibit uncommon behaviors that differ from the well-known behavior of Ge-doped fibers under similar radiation fields [15], as demonstrated

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Temperature profile measured, during the IRMA γ-irradiation campaign, around the optical fiber cable samples inside the irradiation chamber, before, during and after irradiation.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. Online C-OTDR traces measurement for RIA calculation on (a) CBS, (b) CBT, (c) NE and (d) PEEK cables during $^{60}$Co exposure up to 984 kGy at 1.79 kGy/h, at room temperature (~ 25 °C), measured with the NBX-7020F C-OTDR instrument at 1550 nm.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Online RIA results for FN1, FN2, CFB and PEEK cables during $^{60}$Co exposure up to 984 kGy at 1.79 kGy/h, at room temperature (~ 25 °C), measured with the NBX-7020F C-OTDR instrument at 1550 nm.</figcaption></figure>
by the loss distribution evolution of the PEEK cable in Fig. 7.d. The PEEK cable, representative of the typical behavior of standard Ge-doped fiber cables such as CFB, FN1, and FN2 (which exhibit similar patterns), shows a uniform loss increase along the cable section with the cumulated dose. In contrast, Fig. 7.a and 7.b illustrate a distinct behavior for CBS and CBT, characterized by a stagnation stage in losses up to 580 kGy and 100 kGy of cumulated dose, respectively. Beyond these thresholds, the losses increase drastically, leading to a shift in the loss behavior of the cable compared to its bare fiber. Fig. 7.c illustrates the loss distribution behavior along the optical path of the NE cable, showing a uniform loss increase within the cable section as the cumulated dose rises, ultimately resulting in losses exceeding those of the PEEK cable by more than 20 times. The radiation loss behavior is quantified for all cables and reported in Fig. 8.

Fig. 8.a presents the results for the four first cables: FN1, FN2, CFB, PEEK. Those results show RIA levels and kinetics typical of those measured for germanosilicate (Ge-doped core) fibers [15]: 188 dB/km, 198 dB/km, 140 dB/km and 197 dB/km at 984 kGy TID. On these four cables, no significant effect of the cable design on RIA levels and kinetics is observed.

However, not all tested cables exhibited RIA levels and kinetics associated with the Ge-doped bare fibers. Specifically, CBT and CBS cables showed RIA levels similar to those of standard germanium-doped fibers up to 100 h and 300 h, respectively as presented in Fig. 8.b. Afterward, the RIA levels increase significantly, reaching 2670 dB/km and 1220 dB/km for CBT and CBS cables respectively after an accumulated dose of 984 kGy. The increased losses observed in those cables are attributed to the generation of hydrogen in the fiber environment, most probably hydrogen generated by the radiolysis of cable materials, such as gel and polymer. A similar behavior was observed in [39] when comparing a gel-filled loose optical cable to a dry counterpart. The study reported that the inner-cable gel contributed to a 17 dB/km increase in losses during the recovery phase after a γ-cumulated dose of 17.4 kGy (0.60 kGy/s) at room temperature. This increase was particularly evident at 1380 nm (Si-OH related absorption peak) during the recovery phase, where significant loss surges were experienced.

These cables contain gel within their inner metallic layers, which aids fiber insertion during manufacturing. Radiation effects on these gels is likely the source of the significant increase in losses due to hydrogen aging, as hydrogen diffuses from the outer coatings of the cable into the fiber core. After irradiation, traces of gel were found inside these cables, while gel-free cables (CFB, FN1, FN2, and PEEK) showed no increase in hydrogen-related losses. Therefore, we attribute the substantial RIA increase primarily to hydrogen released from the cable's gel composition.

When exposed to ionizing radiation, the gel releases H₂ gas through radiolysis as gamma rays deposit energy into the material. This process leads to the diffusion of the gas into the fiber core, introducing OH-related optical defects. During the hydrogenation, Si-OH and Ge-OH hydroxyl groups are formed as hydrogen interacts with either drawing or radiation-induced defects, such as NBOHCs (Non-Bridging Oxygen Hole Center), ≡Si-O• and ≡Ge-O• respectively [2,3]. In addition, as radiation modifies the silica matrix inducing radiation-related defects such as Si-NBOHCs and Ge-NBOHCs [49], presenting absorption band peaking at 1380 nm and affecting the 1550 nm measurement window leading to significant increase in RIA levels at 1550 nm.

The diffusion times differ between the CBT and CBS cables (100 h and 300 h, respectively), probably due to differences in the compositions and structures of the external coatings of these two cables.

The NE cable also reaches high RIA levels up to 2250 dB/km but the RIA kinetics does not suggest the same H₂-diffusion behavior where losses drastically increase as the ⁶⁰Co source is placed in the center of the cable spool. NE cable does not present the characteristic H₂-diffusion kinetics without any diffusion time as the losses rapidly increase with dose presenting a higher sensitivity than the expected telecom Ge-doped fiber levels [15]. Without any characteristic H₂-diffusion kinetics and

such high sensitivity, we suggest a difference in the fiber composition. In fact, such radiation sensitivity can be attributed to P-doping in the fiber core [15] (radiation sensitive grade fibers) or recently showed that Ultra Low Loss (ULL) PSC fibers can exhibit such sensitivity when exposed to irradiation [50] with levels of ~1600 dB/km after 100 kGy X-ray exposure exhibiting kinetics such as: i) a fast RIA increase at the beginning of the irradiation; ii) high RIA levels; iii) high recovery levels after irradiation (almost full-recovery of the losses). However, this requires a deeper investigation to understand the origin of the NE cable behavior and no such recovery kinetics (of ULL PSC fibers) has been measured on the NE cable.

### 3.1.3. Brillouin frequency shift

Online RI-BFSs measurements were acquired using the PPP-BOTDA interrogator. Fig. 9 presents the RI-BFS evolution, after temperature contribution correction, as a function of TID up to 984 kGy. The temperature correction has been performed using experimentally-measured Brillouin temperature sensitivities for pristine samples ($C_{TB}$) and shown in Table 1.

It should be noted that due to high loss levels caused by H₂-diffusion inside the fiber core, NE and CBT cables have quickly reached and overcome the optical budget of the Brillouin interrogator.

The presented results have to be considered with a ± 0.5 MHz uncertainty margin resulting from the temperature variations inside the irradiation chamber that cannot be perfectly corrected. The sole contribution of the γ-irradiation resulted in RI-BFS levels around +6 MHz, +5 MHz, +3 MHz, +6 MHz, +3 MHz for CFB, FN1, PEEK, CBS and FN2 cables, respectively. An equivalent radiation-induced temperature monitoring error is evaluated to +3 °C, +2 °C, +1 °C, +2 °C and +2 °C respectively after 1 MGy TID. In another study, the RI-BFS of Ge-doped (5.2 wt% in the core) optical fibers have been tested in close conditions at +4 MHz [26]. The highest RI-BFS level is measured for the CBS cable for which a drastic loss increase is attributed to H₂-loading of the sample. The loss excess tendency cannot be retrieved in the RI-BFS kinetics meaning that both result from a distinct mechanism. However, the Brillouin signature is affected by both the RIC and the acoustic velocity change. It has been shown that the acoustic velocity (related to a change in the density of the silica) plays a preponderant role in the RI-BFS over the RIC [26]. The H₂-loaded cable exhibits a higher RI-BFS level than non-hydrogenated ones but no distinct H₂-loading kinetics can be found on the RI-BFS. It is known that hydrogen loading process induces positive BFSs [32] and is responsible for contributing to the higher level of the RI-BFS in Fig. 9.d, an estimation of the hydrogen contribution to the CBS BFS profile is proposed at the end of the section II.4.

Besides the CBS cable, no significant effect of the cable composition or structure has impacted the fiber response to γ-irradiation up to 1 MGy. The Brillouin radiation response of the cable is equivalent to the fiber without cable coating structure for CFB, FN1, PEEK and FN2 cables.

### 3.1.4. Rayleigh frequency shift

RI-RFS was acquired using the TW-COTDR interrogator as described in section II.2. Fig. 10 presents the RI-RFS evolution, after temperature contribution correction, as a function of TID for CFB, FN1, PEEK, and FN2 cables. The correction has been performed using the Rayleigh temperature sensitivities measured for pristine samples ($C_{TR}$) and shown in Table 2.

RI-RFS results presented in Fig. 10 exhibit levels of -9 GHz, -4 GHz, -7 GHz, -5 GHz for non-hydrogenated cables leading to corresponding temperature monitoring errors around +2 °C, +2 °C, +1 °C and +2 °C, for CFB, FN1, PEEK and FN2, respectively, after 984 kGy TID. Where in [26], an order of -6 GHz of RI-RFS has been found after such TID on a Ge-doped fiber (5.2 wt% in the core). RI-RFS results have to be considered with a ± 0.5 GHz uncertainty margin resulting from the temperature variations inside the irradiation chamber that cannot be perfectly corrected.
<figure><img src="image_12.png" /><figcaption>Fig. 9. Online RI-BFS results for: (a) CFB, (b) FN1, (c) PEEK, (d) CBS and (e) FN2 optical fiber cable samples during ⁶⁰Co exposure up to 1 MGy, at room temperature (~ 25 °C), measured with the NBX-7020F PPP-BOTDA interrogator at 1550 nm.</figcaption></figure>

<table><caption>**Table 1**
Pristine optical fiber cable brillouin temperature sensitivity coefficients.</caption><thead><tr><th>Fiber cable samples</th><th>CFB</th><th>FN1</th><th>PEEK</th><th>CBS</th><th>FN2</th><th>NE</th><th>CBT</th></tr></thead><tbody><tr><td>Brillouin temperature sensitivity<br>C<sub>TB</sub>(MHz/°C)</td><td>+2.35</td><td>+2.05</td><td>+2.44</td><td>+2.53</td><td>+1.64</td><td>+1.28</td><td>+0.93</td></tr></tbody></table>

For all the four cables presented in Fig. 10, no significant influence of the cable structure and composition exhibit an effect on their Rayleigh frequency signature response under irradiation with levels and kinetics corresponding to Ge-doped optical fibers.

On the other hand, due to high loss levels NE and CBT cables have quickly reached and overcome the optical budget of the interrogator, allowing to measure RI-RFS only during the first 250 h for CBT and 125 h for NE cables. RI-RFS results are presented in Fig. 11 for cables exhibiting high RIA levels: CBS, CBT and NE samples.

However, for the CBS cable sample (Fig. 11.a), an increase of the RI-RFS occurs around 300 h after the irradiation start corresponding to the excess loss profile when hydrogen is diffusing in the fiber core from the cable composition. RI-RFS levels are subjected to a decrease of around –14 GHz due to H₂-loading up to 984 kGy TID as shown in Fig. 11.a. Prior this decrease, the CBS cable exhibited a behavior similar to RIA and to non-loaded cables with a RI-RFS level stabilizing at –4 GHz. A similar behavior has been measured for CBT cable exhibiting a Ge-doped fiber expected RI-RFS level and kinetics up to 100 h of irradiation reaching a level stabilizing around –3 GHz RI-RFS. Then, hydrogen diffusion induces an increase of RI-RFS level around –9 GHz. NE cable quickly reaches a stabilized RI-RFS level around –5 GHz but does not present H₂-diffusion related kinetics as for the RIA kinetics presented in Fig. 8.b. The RI-RFS signatures of all cables during irradiation exhibit

the same RIA kinetics behavior. In the CBS and CBT cables, however, H₂-diffusion kinetics are evident in both their RIA and RI-RFS signatures.

As discussed in section III.3, our results confirm that RI-BFS mostly depends on the acoustic velocity change under irradiation rather than the RIC. In fact, RI-RFS measurement is only affected by RIC during irradiation (after temperature correction) and exhibits a high sensitivity to H₂-loading where RI-BFS exhibits no H₂-loading kinetics. For the CBS cable, the Hydrogen contribution to the RI-RFS (–14 GHz) level leads to a RIC of +10⁻⁴ and an acoustic velocity change of ~ 2 m/s estimated from [51]. Leading to an estimated linear increase of +4 MHz of the hydrogen contribution to the BFS during the diffusion process, in accordance with our measured RI-BFS profile in Fig. 9.d.

In addition, NE cable RI-RFS level stabilizes while RIA kinetics keep increasing during the irradiation phase. For both CBS and CBT cables, the hydrogen diffusion led to additional equivalent temperature monitoring errors around +3 °C and +6 °C, respectively.

### 3.1.5. Post-irradiation diagnostics

#### 3.1.5.1. Post-mortem brillouin and rayleigh temperature and strain sensitivities.
We evaluated the effect of the irradiation on the two Brillouin and Rayleigh temperature ($C_{TB}$ and $C_{TR}$) and strain ($C_{εB}$ and $C_{εR}$) sensitivity coefficients using setups described in section II.4.3 of FN2
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Online RI-RFS results for: (a) CFB, (b) FN1, (c) PEEK, (d) FN2 optical fiber cable samples during ⁶⁰Co exposure up to 1 MGy, at room temperature (~ 25 °C), measured with the NBX-7020F TW-COTDR interrogator centered at 1550 nm.</figcaption></figure>

<table><caption>**Table 2**
Pristine Optical fiber cable Rayleigh temperature sensitivity coefficients.</caption><thead><tr><th>Fiber cable samples</th><th>CFB</th><th>FN1</th><th>PEEK</th><th>CBS</th><th>FN2</th><th>NE</th><th>CBT</th></tr></thead><tbody><tr><td>Rayleigh temperature sensitivity C<sub>TR</sub>(GHz/°C)</td><td>-5.01</td><td>-2.75</td><td>-5.07</td><td>-5.44</td><td>-3.10</td><td>-1.36</td><td>-1.61</td></tr></tbody></table>

and PEEK cables, which exhibited the least sheath degradations after irradiation. Sensitivity coefficient results are compiled in the Table 3. Fig. 12.a shows the $C_{ER}$ measurement result for the PEEK cable and the related RFS due to strain on the pooled cable section between 0 µε and 4794 µε. Fig. 12.b shows the result of the RFS level averages on the pooled section versus the applied strain. $C_{ER}$ is then calculated from a linear fit for both increasing and decreasing applied strains. Same measurements are performed for the FN2 cable, both pristine and irradiated samples.

No hysteresis has been detected on both cables exhibiting a mean sensitivity (from strain increase and decrease measurements) of -0.107 GHz/µε and -0.102 GHz/µε for PEEK and FN2 pristine cables respectively. Irradiated samples exhibit lower sensitivities of -0.099 GHz/µε and -0.060 GHz/µε for PEEK and FN2 resulting in 7.5 % and 41 % decreases respectively due to the 1 MGy TID γ-irradiation. The same

measurement has been performed for $C_{EB}$ Brillouin sensitivities presented in Fig. 13 with an example of the PEEK pristine cable. Fig. 13.a present the tracking and Lorentzian fitting of the Brillouin central frequency peak versus the applied strain. The resulting linear sensitivity coefficient calculation is shown in Fig. 13.b. This measurement has been repeated for pristine and irradiated samples (PEEK and FN2). No hysteresis has been reported on both cables exhibiting a mean sensitivity (from strain increase and decrease measurements) of +0.052 MHz/µε and +0.032 MHz/µε for PEEK and FN2 pristine cables respectively. Irradiated samples exhibit lower sensitivities of +0.046 MHz/µε and +0.026 MHz/µε for PEEK and FN2 resulting in 11.5 % and 18.8 % decreases respectively. Uncertainties have been evaluated to ± 5·10⁻⁴ MHz/µε and ± 3·10⁻⁴ GHz/µε for Brillouin and Rayleigh strain sensitivities respectively.

Both cables showed a notable decrease in strain sensitivity after irradiation, whereas bare optical fibers are known to exhibit negligible irradiation induced $C_{EB}$ changes after 10 MGy TID γ-irradiation [27]. The cable sensitivity decrease is attributed to an alteration of the PEEK coating and the FN2 olefin sheath that become slightly more friable and entail separation between the cable coating and the sensing fiber.

The temperature sensitivity measurements are performed with respect to the setup described in section II.4.3, with an example for irradiated samples presented in Fig. 14. The results presented in Table 3 showing that both Brillouin and Rayleigh temperature coefficients
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Online RI-RFS results for: (a) CBS, (b) CBT, (c) NE optical fiber cable samples during ⁶⁰Co exposure up to 1 MGy, at room temperature (~25 °C), measured with the NBX-7020F TW-COTDR interrogator centered at 1550 nm.</figcaption></figure>

<table><caption>**Table 3**

Irradiated Optical fiber cable Rayleigh and Brillouin temperature and strain sensitivity coefficients.</caption><thead><tr><th rowspan="2">Fiber cable samples</th><th rowspan="2"></th><th colspan="2">Rayleigh sensitivities</th><th colspan="2">Brillouin sensitivities</th></tr><tr><th>C<sub>TR</sub>(GHz/°C)</th><th>C<sub>ER</sub>(GHz/με)</th><th>C<sub>TB</sub>(MHz/°C)</th><th>C<sub>EB</sub>(MHz/με)</th></tr></thead><tbody><tr><td rowspan="2">PEEK</td><td>pristine</td><td>-5.07</td><td>-0.107</td><td>+2.44</td><td>+0.052</td></tr><tr><td>irradiated</td><td>-5.37</td><td>-0.099</td><td>+2.45</td><td>+0.046</td></tr><tr><td rowspan="2">FN2</td><td>pristine</td><td>-3.10</td><td>-0.102</td><td>+1.64</td><td>+0.032</td></tr><tr><td>irradiated</td><td>-3.18</td><td>-0.060</td><td>+1.74</td><td>+0.026</td></tr></tbody></table>

exhibit a slight increase after irradiation where bare fibers are known to face a 6% decrease after 1–10 MGy TID γ-irradiation [27,52]. However, no significant hysteresis has been detected after the first temperature cycle on all tested samples.

The measurement has been performed on both pristine and irradiated cable samples and all the results have been reported in Table 3. The Brillouin and Rayleigh temperature sensitivity uncertainties are evaluated at ± 0.05 MHz/°C and ± 0.04 GHz/°C respectively. Some small but significant Rayleigh temperature sensitivity increases of 6% and 2.5% have been measured for PEEK and FN2 cables, respectively. Brillouin temperature sensitivities have not faced any changed considering the evaluated uncertainties.

**3.1.5.2. Spectroscopy.** Some pre-existing centers [47–48], as well as radiation-induced defects [49], can interact with the diffused hydrogen

resulting for example in Si-OH and Ge-OH groups formed as hydrogen interacts with Non-Bridging Oxygen Hole (NBOH, ≡Si-O• and ≡Ge-O•). This interaction leads to an increase in absorption losses in the IR domain, mainly around 1380 nm. NIR spectral measurements have then been performed on irradiated and pristine samples to evaluate the OH content induced during the irradiation. Transmitted NIR spectra (described in section II.5) of both pristine and irradiated (after 1 MGy γ-exposure) CBS and FN1 cables, are presented in Fig. 15.a and 15.b, respectively, showing that some hydrogen enters the CBS cable, while it is not the case for FN1 cable.

Fig. 15.a shows that the post-irradiation CBS sample transmitted spectrum exhibits a drastic increase of absorption around 1380 nm, confirming the role of hydrogen diffusion in the loss increase at 1550 nm and in the increase of the measured RI-RFS. Fig. 15.b shows that no significant increase of OH-related content in the FN1 cable due to radiation. We can notice the presence of a small OH peak in this fiber cable originating from the optical fiber manufacturing process [47].

The losses at 1383 nm were measured using an OTDR (EXFO FTB-500) on both pristine and irradiated samples to determine the RIA at 1383 nm one-year post-irradiation. Table 4 presents the data for the seven tested cables.

One year after irradiation (conserved at room temperature), irradiated cables exhibit high RIA levels at 1380 nm suggesting a lasting contribution of the OH absorption peak for the H₂-loaded cables from the irradiation (CBS and CBT). Although the OH content in these two cables has been evaluated as low, with a maximum of 59 ppm for CBT and 53 ppm for CBS cables. The corresponding equivalent [OH] content

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Example of a strain Rayleigh sensitivity coefficient measurement on PEEK cable before irradiation. (a) The RFS trace over the cable length evolution with strain measured with a R-OFDR interrogator scanning range centered at 1550 nm, at room temperature (~ 25 °C). (b) The resulting strain sensitivity calculation for both strains increase and decrease.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 13. Example of a strain Brillouin sensitivity coefficient measurement on PEEK cable before irradiation. (a) The Brillouin Gain Spectrum trace evolution with strain measured with a BOTDA interrogator at 1550 nm, at room temperature (~ 25 °C) with the corresponding Lorentzian fits. (b) The resulting strain sensitivity calculation for both strains increase and decrease.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 14. Example of a temperature Rayleigh sensitivity coefficient on PEEK and FN2 cables after irradiation. (a) The RFS evolution with temperature measured with a R-OFDR interrogator centered at 1550 nm from 0 °C to 40 °C. (b) The resulting temperature sensitivity calculation for all temperature increases and decreases.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 15. (a) CBS and (b) FN1, cable pre and post-irradiation (after 1 MGy $^{60}$Co γ-exposure) characterization of NIR transmission spectra. Normalized to their maximum amplitude.</figcaption></figure>
<table><caption>**Table 4**
Attenuation in pristine and Irradiated fiber cable samples measured at 1383 nm.</caption><thead><tr><th>Cable Samples</th><th></th><th>CBS</th><th>CBT</th><th>CFB</th><th>FN1</th><th>FN2</th><th>NE</th><th>PEEK</th></tr></thead><tbody><tr><td>Attenuation</td><td>pristine</td><td>0.96</td><td>2.00</td><td>2.66</td><td>4.20</td><td>6.83</td><td>8.75</td><td>0.29</td></tr><tr><td>@ 1383 nm (dB/km)</td><td>Irradiated</td><td>3189</td><td>3587</td><td>99</td><td>149</td><td>87</td><td>1437</td><td>44</td></tr><tr><td></td><td>RIA</td><td>3188</td><td>3585</td><td>96</td><td>145</td><td>80</td><td>1428</td><td>44</td></tr></tbody></table>

was calculated based on the OH-related extinction coefficient in silica of 60.6 dB/km/ppm, as referenced in [46]. The [OH] impact on the sensing length and the long-term reliability of the monitoring system remains critical. However, additional radiation induced mechanisms can influence the RIA level at 1380 nm from other tails of broad absorption bands also contributing to the RIA such as GeY (peaking at 1.38 eV with a FWHM of 0.71 eV) and Ge-STH (peaking at 0.54 eV with a FWHM of 0.35 eV) [15]. The calculated equivalent [OH] content is obtained by considering only the sole contribution of the OH-related peak, whereas other neighboring spectral contributions may also be present. Similar measurement with the same OTDR have been performed on CBS, CBT, NE and PEEK cables at 1310 nm, 1550 nm and 1625 nm. Results are reported in Table 5.

Comparing CBS and CBT losses at 1310 nm, 1550 nm, and 1625 nm to their counterparts at 1383 nm suggests a clear contribution from diffusing hydrogen, with losses peaking at the [OH] spectral signature, reaching RIA levels of 3188 dB/km and 3585 dB/km, respectively. At 1310 nm, CBS and CBT exhibit RIA values of 211 dB/km and 647 dB/km, respectively, while at 1550 nm, the RIA values increase up to 605 dB/km and 1219 dB/km for CBS and CBT. Conversely, the NE cable does not show a significant increase in losses at 1383 nm (1428 dB/km) compared to the RIA levels measured at 1310 nm (1460 dB/km) and 1550 nm (1399 dB/km). This confirms that the high RIA levels observed in the NE cable is not attributed to significant hydrogen diffusion, further supported by the absence of hydrogen diffusion kinetics in Fig. 8. b. An extensive study has been performed on optical fiber and their cable counterparts for corium monitoring in case of severe accident in nuclear power plant [35], highlighting the crucial role of the filling gel. The latter was found to induce significant RIA levels at 1380 nm, with an estimated OH content of 20 ppm in the fiber after a cumulative γ-dose of 1.5 MGy (5 kGy/h) on a radiation-hardened fiber (PSC and F-doped cladding) cable at 22 °C. In contrast, no OH content was detected in the extracted fiber irradiated under the same conditions.

The composition of the NE fiber is at stake when explaining the measured RIA kinetics of this cable. We performed Raman spectroscopy on the extracted NE fiber for the pristine sample. We also compare its Raman spectral response (in the core and cladding) to the one of a P-doped fiber (9 wt% phosphorus in the core). Results are presented in Fig. 16 where identified glass structure are reported from [53–55] for SiO₂ and P-doped silica fibers. Setup details are reported in section II.4.1.

From both measured pristine and cladding Raman spectra, no measurable P-doping signature can be found from the extracted NE optical fiber. In addition, from [42], no GeO₂-signature can be found on

the tested NE fibers. The conclusion suggests that no Germanium or Phosphorus signatures were detected in the NE cable fiber, which exhibits high radiation sensitivity; however, its exact origin remains unknown to date.

### 3.1.5.3. Macroscopic degradations.
Radiation affects not only the optical properties of the cable but also its mechanical properties, which must be considered for its installation and use in nuclear waste monitoring or generally in the nuclear industry. The general condition of each cable has been inspected before and after irradiation. Photographs of CBT, CBS, CFB, PEEK and FN1 cables are shown in Fig. 17. No visible degradation has been noticed on both NE cable types. The stainless-steel strength wires toron armored layers of the NE cables are not visibly affected by radiation. However, the irradiated FN2 cable demonstrated easier removal of the outer embossed olefin elastomer sheath, with a slightly more detachable capacity, while remaining one of the most resilient cables. The PEEK cable only exhibited a color change to a yellow nuance after irradiation but no significant change in manipulation has been testified and remains the most resilient cable type along with the NE cable.

The other cables have been degraded by the irradiation, as illustrated in Fig. 17. Besides its outer color change, the CBT cable gel contained inside liquified and leaked out of the cable in the irradiation chamber leading to a lubrication of the cable, due to irradiation (no major temperature increase). The CBS cable color changed and the friability of the external sheath increased. The CFB cable outer loose tube turned friable and cracks from the outer sheath can be easily seen from Fig. 17 leading to the fall of some outer sheath pieces when constrained or manipulated. The FN1 cable protection films have been highly degraded during irradiation. It became highly friable and falls apart not ensuring its protection purpose. The adhesive liner lost some adhesive properties and colors changed drastically.

A majority of the cables are not suitable for applications in high dose radiative environment regarding the radiation induced degradation of protective layers. Among these cables, CBT, CFB, and FN1 pose a risk of breakage, leakage, and failure if installed for monitoring in such environments. On the other hand, metallic structures and the PEEK coating appears unaffected by the irradiation up to 1 MGy at ~ 25 °C.

### 3.1.5.4. Post-mortem hydrogen-exposure qualification.
Previous CBT and CBS cables exhibited inner hydrogen diffusion from inside the cable to the fiber core from the elements the cables are composed of. We should also consider that the targeted environment is also subjected to ambient hydrogen either from nuclear wastes that can emit low amounts of radioactive gases (³H, ¹⁴C and ⁸⁵Kr) and generate hydrogen (H₂) due to radiolysis phenomena (mainly in IL-LLW) but also from anoxic corrosion of metallic materials inside the canisters (mainly in HLW wastes) [2]. Consequently, this section explores the effects of a pre-irradiation on the cable response when exposed to a high-pressure, H₂-rich environment. For this, optical fiber Rayleigh signature was probed during the H₂-loading process at 100 bars, at 30 °C, for pristine and irradiated samples (FN1, FN2, PEEK) to compare their behaviors. The H₂-induced RFS (HRFS) was measured up to ~ 120 h with the R-OFDR interrogator (OBR-4600) on a one-meter long sample providing the data shown in Fig. 17.

Sample behavior during the H₂-loading is described by the diffusion equation in cylindrical geometry [31,32] and depends on the coating type, temperature, and vessel pressure. Afterward, the diffusion of hydrogen increases in the optical fiber and follows an Arrhenius-type

<table><caption>**Table 5**
Attenuation in pristine and Irradiated fiber cable samples measured at 1310 nm, 1550 nm and 1625 nm.</caption><thead><tr><th>Samples</th><th></th><th>Attenuation @ 1310 nm (dB/km)</th><th>Attenuation @ 1550 nm (dB/km)</th><th>Attenuation @ 1625 nm (dB/km)</th></tr></thead><tbody><tr><td>PEEK</td><td>pristine</td><td>0.34</td><td>0.19</td><td>0.20</td></tr><tr><td></td><td>Irradiated</td><td>59</td><td>72</td><td>68</td></tr><tr><td>CBT</td><td>pristine</td><td>3.74</td><td>0.90</td><td>2.65</td></tr><tr><td></td><td>Irradiated</td><td>650</td><td>1220</td><td>1179</td></tr><tr><td>CBS</td><td>pristine</td><td>1.14</td><td>1.80</td><td>4.18</td></tr><tr><td></td><td>Irradiated</td><td>212</td><td>607</td><td>655</td></tr><tr><td>NE</td><td>pristine</td><td>4.71</td><td>1.62</td><td>5.21</td></tr><tr><td></td><td>Irradiated</td><td>1465</td><td>1401</td><td>1507</td></tr></tbody></table>
<figure><img src="image_19.png" /><figcaption>Fig. 16. Comparison of pristine Raman spectra optical fibre extracted from the NE cable (measured in the core and cladding) and a P-doped fiber.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Fig. 17. Comparison photographs of pristine and irradiated (after 1 MGy $^{60}$Co γ-exposure) optical fibre cables: CBT, CBS, CFB, PEEK and FN1 samples.</figcaption></figure>

temperature-dependent behavior until the H₂-saturation is reached. In Fig. 18, the results for FN1, FN2 and PEEK cables agree with literature [5,31] with gas penetration in standard Ge-doped fiber inducing RFSs of -95 GHz, -94 GHz and -90 GHz, respectively after 116 h at 30 °C. From Fig. 18.a, one can notice that PEEK cable is slightly less impermeable to H₂-loading considering an uncertainty of ±0.05 GHz on the HI-RFS measurement. However, CFB cable exhibits very low HI-RFS levels after 116 h of hydrogenation measured at -0.7 GHz for the pristine sample showing that this cable outer layers and coatings present significant H₂-blocking capabilities in these conditions.

By subtracting the pristine curve to the respective irradiated cable HI-RFS response, we obtain the sole contribution of the γ-irradiation on the HI-RFS shown in Fig. 18.b. After 116 h of hydrogenation, the γ-induced HI-RFS is measured of -9.7 GHz, -6.7 GHz, -16.8 GHz and -24.3 GHz for CFB, PEEK, FN2 and FN1 cables respectively, showing that the cable structure physical barrier has been significantly degraded.

Such radiation induced HI-RFS level are equivalent to calculated temperature monitoring errors of +1.94 °C, +1.32 °C, +7.37 °C and +8.84 °C when using data from Table 2, for CFB, PEEK, FN2 and FN1 samples, respectively. This latter cable is the most degraded and presents a high hydrogen diffusion permeability whereas the PEEK cable presents the highest radiation tolerance in terms of H₂-blocking capability. It is important to note that the pressure vessel is set at 100 bars of pure H₂-gas and that no such high-pressure conditions are expected during the monitoring in nuclear wastes facilities during 100 years. This measurement is an accelerated aging process testing of the radiation induced H₂-blocking capability degradations. In addition, it is known that increasing temperature (from 30 °C to 80 °C) increases both the time to reach H₂-loading saturation but also that lower saturation levels are reached [56] confirming that a qualification at lower temperature corresponds to the worst case scenario.

Following the same procedure, we investigate the responses of both
<figure><img src="image_21.png" /><figcaption>**Fig. 18.** FN1, FN2, PEEK and CFB cable H₂-loading (at 30 °C and 100 bars) pre- and post-irradiation (after 1 MGy ⁶⁰Co γ-exposure) where Rayleigh Frequency Shifts are measured with a R-OFDR scanning range centered at 1550 nm. (a) Hydrogen Induced RFS during hydrogenation for all pristine and irradiated cables; (b) Radiation Induced HI-RFS of irradiated cables during H₂-loading where the pristine fiber HI-RFS contribution has been subtracted to the irradiated HI-RFS.</figcaption></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>**Fig. 19.** Extracted optical fibers from NE and CBT cables H₂-loading (at 30 °C and 100 bars) pre- and post-irradiation (after 1 MGy ⁶⁰Co γ-exposure) where Rayleigh Frequency Shifts are measured with a R-OFDR scanning range centered at 1550 nm. The figure presents hydrogen Induced RFS during hydrogenation for pristine and irradiated fibers.</figcaption></figure>

not-irradiated and pre-irradiated samples of NE, CBT cables. Their HI-RFS results are presented in Fig. 19.

Both CBT fibers exhibit standard Ge-doped HI-RFS level and kinetics before and after irradiation (–97 GHz after 116 h) and a very small γ-exposure induced HI-RFS of –0.4 GHz after 116 h. This change can be both attributed to the acrylate primary coating degradation of the fiber after 750 kGy TID [57]. However, we notice that standard Ge-doped extracted (from cables) optical fibers exhibit low radiation induced HI-RFSs compared to cables. We confirm that the degradation shown in Fig. 18.b preponderantly results from the degradation of the layer structure constituting the cable with a small contribution from the fiber primary coating.

The fiber extracted from the NE cable exhibits a smaller HI-RFS level (–84 GHz after 116 h) than standard Ge-doped fibers and exhibit a different contribution of irradiation on the Fig. 19. We suggest that this effect may be related to differences in fiber composition; however, a defect-focused study, which falls outside the scope of this work, would be required to confirm this.

## 3.2. γ-irradiation campaign: conclusion

In section III.1, we have combined online and post-irradiation measurement to characterize the behaviors of seven optical fiber cables via RIA, RI-BFS, RI-RFS, spectroscopy and hydrogenation when exposed to harsh environment. We have discriminated cables over their radiation and hydrogen responses showing that CBS an CBT cables exhibited high RIA levels due to the hydrogen diffusion inside their core originating from their cable composition. Other qualified cables have been tested in an aging hydrogen diffusion process to assess the radiation degradation caused on their H₂-blocking capability. CFB, FN1, FN2 and PEEK cables exhibited RIA, RI-BFS and RI-RFS levels and kinetics of their sensing Ge-doped fibers, showing that no significant effect of the cable structure or composition has been measured. However, CFB exhibited the highest blocking capability with low HI-RFS levels but high sheath degradation preventing from any installation in such harsh environment. PEEK cable exhibited the lowest radiation-induced HI-RFS levels and no significant protective layer degradations. FN1 exhibited large cable outer layers degradations of the while FN2 outer olefin sheath was not significantly impacted by the γ-ray exposure in addition with lower radiation induced HI-RFS levels than FN1 cable. Finally, Brillouin and Rayleigh strain/temperature sensitivities have been measured for pristine and irradiated PEEK and FN2 samples demonstrating low decrease of strain coefficients and low increase of temperature coefficients after 1 MGy TID.

This first γ-irradiation campaign results in a special interest in the PEEK cable for ensuring monitoring in such harsh environment.

## 3.3. Mixed-field neutron/γ qualification

The qualification not only concerns hydrogen and γ-irradiation effects but cables are also subjected to neutrons from nuclear wastes. In this section, we performed a second irradiation campaign on PEEK, FN1, FN2 and CFB cables exposed to a mixed field neutron/γ from the LR-0 nuclear reactor core at the Czech Republic Research Centre Rež as described in section II.3. Below, irradiation conditions are described such as temperature and dosimetry profiles followed by online RIA, Brillouin and Rayleigh signature measurements during the irradiation campaign composed of 5 runs allowing us to reach a neutron fluence of $2.8 \times 10^{13}$ n/cm² with a continuous nominal thermal neutron flux of about $10^9$ n/cm²/s and a fast (above 1 MeV) neutron flux of $2.10^8$ n/cm²/s. The TID is about 150 Gy with an associated dose-rate of ~1.2Gy/h.

### 3.3.1. Irradiation conditions: temperature and dosimetry profiles

Temperature profile has been measured, resulting from the mean of
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /><figcaption>**Fig. 20.** Measured profiles of the mixed-field neutron/γ irradiation conditions during the entire campaign of: (a) the temperature inside the reactor core, at the sample location; (b) the measured Radiation Induced Losses profile performed with a P-doped fiber at 1530 nm.</figcaption></figure>

the four type-K thermocouples installed as presented in section II.3, and reported in Fig. 20.a. This temperature profile exhibits an oscillating response between 19 °C and 20 °C and is used for Brillouin and Rayleigh signatures temperature contribution corrections. In addition, the optical losses in this calibrated P-doped optical fiber are known to present a linear loss/dose dependence of 4 dB/km/Gy [44]. Radiation induced losses have been measured and results are presented in Fig. 20.b during irradiation runs and recovery phases in between runs.

The 100-m radiation sensitive fiber optical loss levels are reported in Table 6, where related run loss radiation induced levels between each phase (run and recovery) are analyzed to deduce the RIA and the deposited dose for each run using the fiber RIA to dose sensitivity dependence. The RIA evolution after each irradiation runs is also reported in Table 6 exhibiting no recovery effect but a slight RIA increase. This specificity of the P-doped fibers can be explained by the conversion of the radiation induced metastable Phosphorus Oxygen Hole Center (POHC) into P1 centers [58]. As P1 defects are stable at RT, this results in an increase of their concentration after irradiation [59]. The maximum increase of the RIA during the recovery phase has been evaluated at +6.7 % after the first run.

The measured deposited dose reduces for the last two runs due to reduced irradiation times for the last two days. The total deposited dose has been measured at 152.23 Gy.

### 3.3.2. Online measurements

#### 3.3.2.1. RIA.
Throughout the irradiation runs, losses in the optical fiber cables (FN1, FN2, CFB and PEEK) has been tracked with the TW-COTDR interrogator at 1550 nm. Subtracting the reference loss level prior irradiation exposure, we deduce the RIA profile of each cable during the campaign. RIA results are presented in Fig. 21.

Results exhibited no significant RIA increase during all the irradiation campaign and levels measured are included within the uncertainty

<figure><img src="image_26.png" /><figcaption>**Fig. 21.** Online RIA results for FN1, FN2, CFB and PEEK cables during neutron/γ mixed-field exposure with a neutron fluence of 2.8·10¹³ n/cm² and a TID of 150 Gy, at room temperature (~ 25 °C), measured with the NBX-7020F TW-COTDR instrument at 1550 nm.</figcaption></figure>

window leading to the conclusion that no significant increase of RIA has been measured on all four cable samples when exposed to a neutron fluence of 2.8 × 10¹³ n/cm² and an associated TID of 150 Gy.

#### 3.3.2.2. Brillouin and Rayleigh frequency Shifts.
As for the γ-irradiation, both BFS and RFS signatures have been measured during the campaign (run and recovery phases) with PPP-BOTDA and TW-COTDR

**Table 6**

<table><caption>Dosimetry results measured with a 100-meter-long P-doped fiber (Radiation Induced losses (RI-Losses), RIA and doses) at 1530 nm.</caption><thead><tr><th rowspan="2">Irradiation Runs</th><th colspan="2">1</th><th colspan="2">2</th><th colspan="2">3</th><th colspan="2">4</th><th colspan="2">5</th></tr><tr><th>Irradiation</th><th>Recovery</th><th>Irradiation</th><th>Recovery</th><th>Irradiation</th><th>Recovery</th><th>Irradiation</th><th>Recovery</th><th>Irradiation</th><th>Recovery</th></tr></thead><tbody><tr><td>Initial RI-Losses level @ 1530 nm (dB)</td><td>0</td><td>14.66</td><td>15.53</td><td>29.83</td><td>30.84</td><td>46.02</td><td>47.00</td><td>58.84</td><td>59.78</td><td>64.69</td></tr><tr><td>Final RI-Losses level @ 1530 nm (dB)</td><td>14.66</td><td>15.53</td><td>29.83</td><td>30.84</td><td>46.02</td><td>47.00</td><td>58.84</td><td>59.78</td><td>64.69</td><td>64.62</td></tr><tr><td>RIA @ 1530 nm (dB/m)</td><td>0.15</td><td>0.01</td><td>0.14</td><td>0.01</td><td>0.15</td><td>0.01</td><td>0.12</td><td>0.01</td><td>0.05</td><td>0.00</td></tr><tr><td>Dose (Gy)</td><td>36.65</td><td>-</td><td>35.75</td><td>-</td><td>37.95</td><td>-</td><td>29.60</td><td>-</td><td>12.28</td><td>-</td></tr></tbody></table>
<figure><img src="image_27.png" /><figcaption>Fig. 22. (a) Online RI-BFS and (b) RI-RFS results for FN1, FN2, CFB and PEEK cables during neutron/γ mixed-field exposure with a neutron fluence of 2.8·10¹³ n/cm² and a TID of 150 Gy, at room temperature (~ 25 °C), measured with the NBX-7020F PPP-BOTDA and TW-COTDR instruments at 1550 nm, respectively.</figcaption></figure>

respectively. Results are presented in Fig. 22 for FN1, FN2, CFB and PEEK cable samples.

RI-BFS and RI-RFS in Fig. 21.a and 21.b respectively, do not exhibit significant variation levels during the irradiation runs. The observed RI-BFS and RI-RFS variations are attributed to temperature correction procedure residuals and not to radiation effects.

## 4. Discussion

This qualification campaign helps to establish a selection criteria framework for candidates to ensure temperature and strain monitoring in harsh environment such as the Cigéo nuclear waste storage program. Table 7 provides a simplified summary of the outcomes of this study. We discriminate the cable monitoring capability via their radiation tolerance, cable (layers) effects on the monitoring, hydrogen diffusion tolerance, thermo-mechanical changes in the cable integrity and their macroscopic changes useful for practical installation and longevity in such harsh environment. The green-colored boxes indicate a suitable characteristic for ensuring monitoring, the red boxes denote a disqualifying characteristic, and the orange boxes represent a drawback that is not disqualifying for monitoring.

From this qualification campaign, the PEEK cable presents promising features for radiation (neutron/γ mixed-fields) and hydrogen-rich environments. In addition, with high temperature and strain sensitivities exhibiting the lowest radiation-induced changes, PEEK cable also presents no significant macroscopic degradation facilitating its implementation in the targeted severe environment. It also exhibited the smallest degradation with radiation of its H₂-blocking capability.

It is possible to improve its performances knowing previous studies on radiation and hydrogen-hardened fibers. In fact, cables are now composed of telecom-grade optical fibers known to be radiation-tolerant but not as radiation-hardened fibers as F-doped fibers exhibiting low RIA, RI-RFS and RI-BFS levels even at high doses [25–27]. The behavior of the fibers (Ge- or F-doped) are well known under irradiation to this day [15]. In addition, it has been proven that adding a thin (~ 20 nm) carbon coating layer between the cladding and the primary coating of the fiber prevents from H₂-diffusion inside the sensing fiber core acting as a physical barrier [33]. It has also been shown that at high γ-doses (MGy) and high neutron fluences (10¹⁷ n/cm²) the carbon coating can ensure a physical barrier to H₂-diffusion entailing low level radiation induced HI-RFSs of ~0.2 GHz at 80 °C [56].

Leveraging the composition and structural characteristics of the

<table><caption>**Table 7**

Cable performances and discrimination over the irradiation qualification process summary.</caption><thead><tr><th>Criteria</th><th>Cables</th><th>FN1</th><th>FN2</th><th>CFB</th><th>PEEK</th><th>CBS</th><th>CBT</th><th>NE</th></tr></thead><tbody><tr><td rowspan="2">Temperature sensitive</td><td>C<sub>T<sub>R</sub></sub> (GHz/°C)</td><td>-2.75</td><td>-3.10</td><td>-5.01</td><td>-5.07</td><td>-5.44</td><td>-1.61</td><td>-1.36</td></tr><tr><td>C<sub>T<sub>B</sub></sub> (MHz/°C)</td><td>+2.05</td><td>+1.64</td><td>+2.35</td><td>+2.44</td><td>+2.53</td><td>+0.93</td><td>+1.28</td></tr><tr><td rowspan="2">Strain sensitive</td><td>C<sub>ε<sub>R</sub></sub> (GHz/°C)</td><td>-0.122</td><td>-0.062</td><td>-0.119</td><td>-0.107</td><td>-0.077</td><td>-0.005</td><td>-0.013</td></tr><tr><td>C<sub>ε<sub>B</sub></sub> (MHz/°C)</td><td>+0.036</td><td>+0.032</td><td>+0.033</td><td>+0.052</td><td>+0.021</td><td>+0.004</td><td>+0.004</td></tr><tr><td colspan="2">RIA tolerance</td><td colspan="5">Fiber-grade dependent (~200 dB/km)</td><td colspan="2">Hydrogen loading<br>From the cable structure<br>(RIA >1200 dB/km)</td></tr><tr><td colspan="2">RI-RFS tolerance</td><td colspan="5">Fiber-grade dependent</td><td colspan="2">Fiber-grade dependent<br>(RIA >2000 dB/km)</td></tr><tr><td colspan="2">RI-BFS tolerance</td><td colspan="5"></td><td colspan="2"></td></tr><tr><td colspan="2">RI-C<sub>T<sub>R</sub></sub> and RI-C<sub>T<sub>B</sub></sub> changes</td><td>x</td><td>+2.5%/+6%</td><td></td><td>+6%/+2%</td><td></td><td></td><td></td></tr><tr><td colspan="2">RI-C<sub>ε<sub>R</sub></sub> and RI-C<sub>ε<sub>B</sub></sub> changes</td><td></td><td>-18.8%</td><td>x</td><td>-11.5%</td><td></td><td>x</td><td></td></tr><tr><td colspan="2">Macroscopic degradations</td><td>Brittle</td><td>Brittle</td><td>Cracks</td><td></td><td>Brittle</td><td>Gel leakage</td><td></td></tr><tr><td colspan="2">RI-Hydrogen blocking capability (RI-HI-RFS)</td><td>-24.3 GHz</td><td>-16.8 GHz</td><td>-9.7 GHz</td><td>-6.7 GHz</td><td></td><td></td><td>x</td></tr><tr><td colspan="2">Robustness</td><td>Fragile</td><td></td><td>Thin</td><td></td><td></td><td></td><td></td></tr></tbody></table>
PEEK cable, along with radiation-hardened fiber and additional H₂-hermetic coatings like carbon, could meet all the requirements for temperature and strain sensing in such harsh environments.

## 5. Conclusion

This study examines how cable structure and composition affect optical fiber sensor performances under irradiation. We developed and implemented a testing methodology to evaluate temperature and strain sensing performances before and after irradiation, comparing seven cable types based on their ability to monitor in environments with high γ-doses, mixed neutron/γ radiation, and hydrogen-rich conditions. The cables were tested for online RIA, RI-BFS, RI-RFS, radiation-induced HI-RFS levels, physical macroscopic degradations, and changes in radiation-induced Brillouin/Rayleigh thermo-mechanical sensitivities. One candidate cable, featuring a PEEK outer sheath, demonstrated no adverse effects from the cable structure on RIA levels, fiber-grade related losses, or RI-BFS and RI-RFS levels. It exhibited the lowest changes in Brillouin and Rayleigh temperature and strain sensitivities, the smallest radiation-induced HI-RFS levels, and no macroscopic degradation. These characteristics, coupled with the future use of already-existing radiation-hardened fibers and an H₂-hermetic carbon coating, are suitable for temperature and strain monitoring in radiation and hydrogen-rich environments. This enhancement not only aids in nuclear waste monitoring for the Cigéo program but is also beneficial for a variety of applications within the nuclear industry.

## CRediT authorship contribution statement

**Jérémy Perrot:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Adriana Morana:** Writing – review & editing, Visualization, Validation, Supervision.
**Emmanuel Marin:** Writing – review & editing, Supervision.
**Youcef Ouerdane:** Writing – review & editing, Visualization, Supervision.
**Aziz Boukenter:** Project administration, Funding acquisition.
**Johan Bertrand:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.
**Stéphane Poirier:** Resources, Investigation.
**Hanaa Houjeij:** Resources, Investigation.
**Evzen Novak:** Resources, Investigation.
**Tomas Peltan:** Resources, Investigation.
**Sylvain Girard:** Writing – review & editing, Visualization, Validation, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

## Funding

This work is supported by the European Joint Program on Radioactive Waste Management (EURAD). EURAD has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement N°847593.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

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