# Report on TESCA irradiation of optical sensors and tests of glasses

Guy Cheymol, Marion Agoyan, Ayoub Ladaci, Florence Martin, Christophe Destouches

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Guy Cheymol, Marion Agoyan, Ayoub Ladaci, Florence Martin, Christophe Destouches. Report on TESCA irradiation of optical sensors and tests of glasses. EPJ Web of Conferences, 2026, ANIMMA 2025 - Advancements in Nuclear Instrumentation Measurement Methods and their Applications, 338, pp.04023. 10.1051/epjconf/202533804023 . cea-05513813

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Submitted on 16 Feb 2026

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# Report on TESCA irradiation of optical sensors and tests of glasses

Guy Cheymol<fnref n="1" />, Marion Agoyan<fnref n="2" />, Ayoub Ladaci<fnref n="1" />, Florence Martin<fnref n="3" /> and Christophe Destouches<fnref n="3" />

**Abstract**—The TESCA irradiation campaign, conducted from November 16 to December 17, 2023 (one reactor operating cycle) in the SCK•CEN BR2 research reactor, enabled testing of optical sensors at different development stages, under intense radiation: fast neutron fluence (E > 1 MeV) of 1 to 2 x 10¹⁹ neutrons/cm<fnref n="2" /> and gamma dose of about 5 GGy. The temperature ranged between 100°C and 500°C. TESCA irradiation enabled on-line testing of Fabry-Perot extensometers, and miniature, non-contact, high-temperature optical pyrometry sensors. We also report an online measurement of radiation-induced attenuation on a sapphire sample. Because all sensors and measurements are temperature-sensitive, it was crucial to either maintain stable temperatures or accurately monitor them to assess the impact of radiation on sensor response stability at constant load or on glass parameters. Brief temperature jumps were also applied to test sensors accuracy. An overview of results from monitoring the various sensors and experiments throughout the irradiation cycle are provided. A well thermally stabilized Fabry-Perot extensometer maintained high stability, with a drift below 1 µm under constant load. It also ensured accurate thermal expansion measurements during temperature jumps. The pyrometric sensors operated effectively under intense irradiation at steady temperature: the collected blackbody emission spectrum was preserved, as expected with constant temperature. The unusable spectral range around the OH peak will be extended to [1250–1430 nm]. In contrast, the RIA setup showed susceptibility to temperature variations and mechanical vibrations, indicating the need for improved mechanical robustness.

**Keywords** —Fabry-Perot extensometer, Glasses under irradiation, Irradiation of optical sensors, Optical pyrometry sensors, Optical sensors in Material testing reactor.

## I. INTRODUCTION

NUCLEAR energy remains a key solution to meet future energy demands while limiting carbon dioxide emissions and other atmospheric pollutants. In this context, research and development efforts increasingly rely on highly instrumented experiments conducted in Material Testing Reactors (MTRs). These experiments are crucial for extending the lifetime of nuclear reactors, optimizing fuel management, and enhancing the safety of both current and next-generation nuclear power

plants.

Optical Fiber Sensors (OFS) offer several advantages for instrumentation within MTR irradiation devices. Their ability to enable remote measurements, multiplex multiple sensing points, and resist electromagnetic interference makes them particularly attractive. Moreover, their compact size is ideal for the constrained spaces typical of irradiation setups, minimizing perturbation of the thermal and radiation fields near the tested material samples. OFS can be used to monitor a wide range of physical parameters through various optical principles, even in harsh environments [1].

However, in order to qualify these sensors for nuclear applications, it is essential to assess their performance under irradiation and realistic operating conditions.

This article presents some results from the TESCA irradiation campaign, which enabled the testing of optical sensors—at different stages of development—under intense radiation and elevated temperatures (100 to 500 °C). In addition, several optical glasses intended for integration into future confocal chromatic sensors were evaluated. Both online Radiation-Induced Attenuation (RIA) measurements and optical path variation measurements were conducted, along with post-irradiation assessments. In this paper, we present an example of online RIA measurement results obtained for a sapphire sample.

The following sections describe the experimental setup, test conditions, materials and sensors used, followed by a presentation and discussion of key results. The article concludes with a summary of the findings and perspectives.

## II. DESCRIPTION OF EXPERIMENT

### A. Irradiation and Test Device

The irradiation was conducted in the BR2 reactor at the SCK•CEN site in Mol, Belgium. The experiments were placed in the reflector region, inside the Liberty test device, which consists of five elongated "needles" (Figure 1). Each needle has an outer diameter of 18 mm and a length slightly under 6 meters, with its lower section located in the reactor core. The lower part of each needle, aligned with the reactor's midplane, contains a heating core that enables temperature regulation over a vertical span of approximately 40 cm. This heating core limits the experimental volume to a cylindrical region with a diameter of 9 mm and a length of 40 cm. From each needle emerge a metallic tube containing the fibers that carry the probe beam

<footnote n="1">¹ Université Paris-Saclay, CEA, Service de Recherche en Matériaux et procédés Avancés, 91191, Gif-sur-Yvette, France</footnote>

<footnote n="2">² Univ. Lyon, UJM, CNRS, IOGS, Laboratoire Hubert Curien, UMR 5516, 18 rue Prof. B. Lauras, 42000 Saint-Etienne, France</footnote>

<footnote n="3">³ CEA, IRESNE, Cadarache, DER/SPESI, F-13108 Saint-Paul-lez-Durance, France</footnote>
and the optical signal, as well as the electrical wires of the heating core and five thermocouple (TC) wires. The experimental zone within each needle is maintained in a helium atmosphere.

<figure><img src="image_2.png" /><figcaption>Figure 1: Left: a needle and a flexible tube housing relay fibers, thermocouples and heating wires; right: picture taken during insertion of a part of the experimental load in the heating core.</figcaption></figure>

The irradiation period lasted 30 days. The fast neutron flux (E > 1 MeV) ranged from 5×10¹² to 1×10¹³ n/cm²/s, depending on the specific needle and vertical positioning, resulting in a total fluence between 1×10¹⁹ and 2×10¹⁹ n/cm². The associated gamma heating was approximately 2 kGy/s, leading to a total dose of 5 GGy.

## B. Sensors and Tests Conducted

We tested hardened Fabry-Perot (FP) extensometers (Figure 2, left), designed to operate at high temperatures (up to 400 °C), in MTRs. Previous work [2] described the sensor and its performance at high temperature without irradiation. Here the objective is to test the sensor response under high neutron and gamma irradiation, at high temperature, at constant stress and also to test the sensor response when change in stress is produced by deliberate temperature shifts generating thermal expansion of the support.

<figure><img src="image_3.png" /><figcaption>Figure 2: Left: FP sensor welded onto stainless steel support (gauge length: 15 mm); right: pyrometry sensor designed to collect IR emission at 90° to the fiber direction.</figcaption></figure>

Miniature, non-contact, high-temperature optical pyrometers (Figure 2, right), developed for Loss Of Coolant Accident (LOCA) simulation experiments [3,4], were also tested. Three such sensors faced highly oxidized surfaces held at controlled temperature during most of the irradiation cycle, with some intentional temperature jumps introduced. In multispectral pyrometry, once calibration has been completed, it is important to minimize differential spectral variations in the collected signal, other than those caused by changes in the surface temperature, because they can be interpreted as temperature

variations. The main objective here is to study the differential spectral variations of the received optical signal at constant temperature that may result from irradiation, particularly RIA in the optical fiber.

Additionally, tests were conducted as part of the development of a confocal chromatic displacement sensor, intended for in-core measurements such as fuel rod swelling [5]. To evaluate RIA in candidate optical glasses for this sensor, an assembly with seven miniature optical lines was constructed (Figure 3, top). Each line consisted of a radiation-hardened fiber whose end face was positioned at the center of curvature of a polished stainless steel concave mirror, with a small glass sample (1.4 × 1.4 × 3 mm³) placed in between. RIA was determined by the decrease in optical signal during irradiation, after subtracting attenuation in the fiber and losses from mirror reflection. These baseline losses were measured on a reference line without any glass sample.

<figure><img src="image_4.png" /><figcaption>Figure 3: Top: assembly with 7 independent lines; bottom: diagram of a single line of RIA glass sample measurement.</figcaption></figure>

## C. Temperature

Since all tested sensors exhibit temperature sensitivity, maintaining thermal stability was essential for evaluating radiation effects on sensor response. Given the variability of nuclear heating, supplementary electrical power was required to regulate the temperature. Heating wires embedded in the core were connected to the BR2 facility's temperature control system. Five thermocouples were installed per needle, with one selected as the reference for active regulation.

Figure 4 illustrates the temperature evolution in one needle: needle A dedicated to Fabry-Perot sensors. The control thermocouple (TC1) successfully maintained the setpoint temperature, demonstrating the effectiveness of the regulation system. However, other thermocouples located further from the control point showed significant deviations and fluctuations during the irradiation cycle, highlighting temperature inhomogeneities.
<figure><img src="image_5.png" /><figcaption>Figure 4: Temperatures at various points, gamma heating and reactor power on needle A dedicated to Fabry-Perot sensors. The heights of the TCs relative to the core median plane are respectively: +150 mm (TC1), +80 mm (TC2) and -130 mm (TC5).</figcaption></figure>

### III. RESULTS AND ANALYSIS

#### A. Fabry-Perot Extensometer

Figure 5 presents the evolution of temperature and optical cavity length (Lc) of Fabry-Perot sensor no. 1 during the irradiation cycle. At constant temperature, variations in Lc attributable to the experiment—particularly to irradiation— remain below 1 µm. The associated fast neutron fluence (>1 MeV) reaches approximately 1.5 × 10¹⁹ n/cm² at the end of the irradiation cycle.

<figure><img src="image_6.png" /><figcaption>Figure 5: Optical cavity length (Lc) of Fabry-Perot sensor no. 1, temperature, gamma heating, and reactor power during the irradiation cycle.</figcaption></figure>

The temperature sensitivity of the optical cavity length is due to the thermal expansion of both the sensor and the stainless-steel support [2]. After accounting for the sensor's own expansion, the device effectively measures the expansion of the support structure. It is therefore a validation of the capacity to measure small extensions that we aim for by making 3 temperature jumps of approximately -100 °C during irradiation. The measured cavity length changes are consistent with those recorded during post-manufacturing tests (+2.6 µm/100 °C), confirming the preservation of measurement accuracy.

Additionally, the signal level showed strong radiation resistance, with attenuation remaining below 3 dB throughout the cycle. The sensor was built with a single-mode (SM) radiation-hardened fiber made from a preform provided by Exail (formerly iXblue).

#### B. Pyrometric Sensor.

Figure 6 displays normalized spectra collected by a pyrometric sensor at several time points when the corresponding thermocouple indicated temperatures close to 500 °C (±2 °C). Spectral normalization facilitates comparison of the signal shape, in line with multispectral pyrometry principles. A variable peak at 1280 nm—attributable to oxygen fluorescence [6]—was observed. To reduce its impact on temperature measurements, the spectral exclusion window (1300–1430 nm, corresponding to the OH peak) [3,4] should be extended to include the 1280 nm region. Outside this range, differential attenuation appears negligible. The spectrum of the collected blackbody emission is preserved, as expected since the temperature is constant. This means that the measurement is not affected by the irradiation environment encountered in this experiment.

However, due to issues during assembly, the initial black-body-based calibration—which provided a temperature reference for the spectral data—was lost. Therefore, we are unable to associate specific temperatures with the recorded spectra, which complicates the precise and quantified assessment of the irradiation-induced error.

The same is true for temperature jumps of about 30°C; but we still observed variations in intensity, at a given wavelength, in agreement to within a degree of the evolution expected according to Planck's laws.

<figure><img src="image_7.png" /><figcaption>Figure 6: Normalized spectra from the pyrometric sensor at different time points. Fast neutron fluence ~10¹⁹ n/cm² for the last timepoint.</figcaption></figure>

#### C. RIA of glass samples

Two main challenges were encountered: loss of measurement channels due to fiber breakage along the transport lines, and unexpected signal fluctuations caused by slight sample displacements within their housings. These displacements likely come from mechanical vibrations and unintended temperature changes. A slight defocusing considered to counter this effect was difficult to implement. This reduces the confidence in the signal intensity and, by extension, the derived Radiation-Induced Attenuation (RIA) values. Nevertheless, Figure 7 presents RIA data for a double-pass sapphire crystal (6 mm total length), obtained by subtracting the reference signal measured on a channel without a sample.
<figure><img src="image_8.png" /><figcaption>Figure 7: RIA(i) at the beginning and midpoint of the cycle (fast neutron fluence ~0.8 10¹⁹ n/cm²) for the sapphire sample alone (double-pass, 6 mm length), derived by subtracting the RIA of the reference channel. Temperature during the period: between 110 and 270°C (average: ~150°C).</figcaption></figure>

Although the absolute values remain uncertain, the spectral profile can be considered reliable, as it indicates stronger attenuation in the lower part of the spectrum—consistent with post-irradiation measurements performed later (to be presented in a forthcoming article). Similar increases in RIA at wavelengths below 1 µm have been reported in [7] for sapphire optical fibers irradiated at comparable temperatures but under lower fluences. Reference [8] also presents post-irradiation, thermally-annealed absorption data for sapphire samples, again obtained at lower neutron fluences. These differences in irradiation conditions complicate any direct comparison of RIA levels.

## IV. CONCLUSIONS

We have presented the on-line testing of Fabry-Perot extensometers, miniature non-contact high-temperature optical pyrometric sensors, and radiation-induced transmission changes in optical glass samples during the TESCA irradiation experiment in the BR2 reactor at SCK•CEN. The experimental setup and test conditions were described. Among the various parameters, temperature control is often particularly critical for evaluating radiation effects on sensor responses. Achieving both homogeneous and stable thermal conditions in such environments remains a significant challenge.

Under fast neutron fluences (E > 1 MeV) of $1–2 \times 10^{19}$ n/cm² and total gamma doses of approximately 5 GGy, the RIA observed in radiation-hardened fibers - manufactured by Exail (formerly iXblue) - was sufficiently low to ensure satisfactory sensor performance. Specifically, an RIA of ~3 dB is well within acceptable limits for interferometric measurements using the Fabry-Perot sensor. Furthermore, the low differential RIA observed in 400 µm core multimode fibers at temperatures around 500 °C had minimal impact on multispectral pyrometry. The effective double-pass fiber length exposed to ~70% of the maximum flux is estimated at about 50 cm.

The Fabry-Perot extensometer demonstrated excellent stability under high radiation, showing a drift of less than 1 µm under constant load and providing reliable measurements of thermal expansion. Pyrometric measurements also remained functional under intense irradiation, with an extension of the unusable spectral range to [1250–1430 nm].

However, the RIA measurement setup exhibited sensitivity to temperature fluctuations and mechanical vibrations, indicating that its mechanical robustness should be improved.

Finally, we performed on-line optical path measurements on glass samples, the results of which will be detailed in a forthcoming publication. These on-line observations will be complemented by post-irradiation characterizations of additional glass samples irradiated in dedicated containers and retrieved approximately one year after the TESCA experiment.

The Fabry-Perot extensometer and the pyrometric sensor are suitable for use in MTR irradiation devices, given their demonstrated resistance to radiation under the tested conditions.

## ACKNOWLEDGMENT

This work was supported by the French Alternative Energies and Atomic Energy Commission (CEA), Division of Nuclear Energy. The authors thank the staff of SCK.CEN BR2 and in particular Andrei Goussarov for their technical assistance in the preparation and conduct of the irradiation test.

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