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/cm2 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.

Over the past decade, additive manufacturing has made tremendous progress. In particular, Laser Powder Bed Fusion (L-PBF) applications have increased dramatically due to its ability to produce complex shapes as lattice structures. In parallel, optical fiber sensors have demonstrated their ability to measure physical parameters such as temperature or stress inline. Recent studies suggest the possibility of incorporating optical fiber sensors into L-PBF parts. This allows the sensor to be placed at the desired location within the complex structure during manufacturing. After a brief introduction of the study and the position of optical fiber sensors, the paper presents the proposed approach for inserting the sensors during L-PBF fabrication and the associated experiments. The results show the quality of the integration and the impact on the mechanical properties of the parts. An example of application, the monitoring of lattice structure manufactured by L-PBF for shock absorbers of radioactive waste storage, is finally given. (c) 2024 The Authors. Published by Elsevier B.V.

The advent of additive manufacturing as a cutting-edge method of contemporary manufacturing has changed how metallic parts are made and used. In fact, the design flexibility provided makes it possible to incorporate sensors throughout the production process to meet the expanding demand for structural health monitoring. In this work, strain and temperature were measured using a Fabry-Perot optical sensor while a metallic object was being created using a laser powder bed fusion technique. The objective was to measure the metal part's internal strain and temperature during an application. In order to prevent sensor damage or deterioration and preserve the parts' good mechanical qualities at the same time, an improved laser printing approach was devised.

Over the past decade, additive manufacturing has made a tremendous progress; This technology gets a great interest to the development of mechanical parts with complex geometries and compositions. The freedom of conception allows using additive manufacturing process in order to integrate optical sensors during the printing process and opens the way to the production of instrumented components for SHM (Structural Health Monitoring). The latter required a particular process with adapted printing strategy and an interruption of the procedure in order to implement the sensor. The impact of this insertion needs to be investigated in order to assess its influence on the mechanical parts behaviors.

To measure fuel rod swelling in a nuclear research reactor, our research has focused on the development of an optical confocal chromatic sensor, allowing contactless measurements of radial changes in fuel rods. This sensor must be able to operate in the harsh and hot environment of a reactor, especially under high neutron fluences up to $10^{19}~{n}_{\mathrm{ fast}}/\text {cm}^{2}$ and gamma doses of a few GGy. When exposed to such radiation levels, the properties of bulk optical glasses are known to be affected by two main phenomena: radiation-induced attenuation (RIA) and radiation-induced refractive index change (RIRIC). With the objective of testing glasses as candidates for a future confocal chromatic sensor, we created a dedicated setup for the online monitoring of the glasses’ RIRIC in the core reactor with a measurement principle relying on interferometry. Since not only the refractive index (RI) but also the length of the samples changes under neutron radiation through compaction, we measured the variation of the optical path (OP) ( $nL$ ). To assess the performance of our setup, preliminary in-lab tests were performed to evaluate the feasibility of measuring the small RI changes caused by external temperature changes. Due to this study, we were able to obtain data that were not yet available in the literature regarding the temperature-induced RI variation of several bulk glasses, including radiation-hardened glasses, up to 350 °C. This information is crucial for the targeted application, as the confocal chromatic sensor will have to operate at such elevated temperatures.

Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments.

The optical fiber (OF) silica glass compaction is the least studied effect induced by radiation, since it takes place at very high fluences, mainly in mixed environments combining gamma-rays and neutrons. Although the radiationinduced structural reorganization has already been investigated in bulk materials, OFs are a more complex case study, due to its inhomogeneous nature and internal stress distribution. We hereby investigate the structural changes induced in a pure-silica core OF by fast neutron fluence of up to 5 x 1019 n/cm2 and total gamma-dose of -4 GGy(SiO2), through micro-Raman spectroscopy allowing us to study the radial distribution of several signatures. The structural changes induced by neutrons on this OF are weaker than those observed on bulk glass at similar fluences, in agreement with the lower observed compaction. The main cause is the higher irradiation temperature, even if a key role is played by the internal stress present in the fibers.

Confocal chromatic microscopy is an optical technique allowing measuring displacement, thickness, and roughness with a sub-micrometric precision. Its operation principle is based on a wavelength encoding of the object position. Historically, the company STIL based in the south of France has first developed this class of sensors in the 90’s. Of course, this sensor can only operate in a sufficiently transparent medium in the used spectral domain. It presents the advantage of being contactless, which is a crucial advantage for some applications such as the fuel rod displacement measurement in a nuclear research reactor core and in particular for cladding-swelling measurements. The extreme environmental conditions encountered in such experiments i.e. high temperature, high pressure, high radiations flux, strong vibrations, surrounding turbulent flow can affect the performances of this optical system. We then need to implement mitigation techniques to optimize the sensor performance in this specific environment. Another constraint concerns the small volume available in the irradiation rig next to the rod to monitor, implying the challenge to conceive a miniaturized sensor able to operate under these constraints.

In [1], the address of author Pierre Ferdinand is incorrect. The correct address should read:

We have developed a Fabry-Perot extensometer that is able to operate at high temperature (up to 400 degrees C), for in pile experiments in material testing reactor, under high neutron and gamma flux. For that purpose, we have revised the basic scheme of an extrinsic Fabry-Perot interferometer (EFPI) to limit the effects of radiation and especially of the fast neutron fluence and to adapt the sensor to high temperature. After a description of the sensor and its operation, we present, in this article, two types of tests that were carried out to check the accuracy of the sensor at high temperature. First, four sensors are fixed either on stainless steel plates or on tantalum plates and the temperature is raised from room temperature to 400 degrees C. Knowing the thermal expansion of both the supports and the sensor parts, it is possible to predict the expected variation of the sensor output. The comparison between the predicted and experimental outputs provides the information about the accuracy of the sensor. Second, we performed high-temperature tensile testing at 300 degrees C and 350 degrees C. The Fabry-Perot measurements are compared, in the elastic domain, with the expected strain given by Hooke's law, and on a larger strain domain, with the measurements of a high-temperature axial extensometer. The experiments and the results are presented. The error appears to be limited to about 1 mu m when the strain is low (0.1%) and can be of few micrometers on a larger range (up to 2%). Strain measurements up to more than 5% are also reported.

Fabry-Perot (FP) sensors like other Fiber Optic (FO) sensors may be of particular interest for in pile experiments in MTR with little room available thanks to their compact size. Light weight also reduces gamma heating hence limiting the thermal effect. Different physical parameters such as temperature, strain, displacement, vibration, pressure, or refractive index may be sensed through the measurement of the optical path length difference in the cavity. We have developed a Fabry-Perot extensometer able to operate at high temperature (up to 400°C), under a high level of radiation (neutron and gamma flux). The measurement based on interferometry is largely insensitive to radiation induced attenuation (RIA) thanks to the wavelength encoding of the useful signal, but for such high fluence as encountered in a reactor core, a special rad-hard fiber is needed. Operating in the wavelength domain around 1ím remains preferable to minimize the impact of irradiation. Moreover, fast neutron radiation is expected to change the structure of the fiber and possibly others materials in the transducer. Then, we revised the basic scheme of Extrinsic Fabry-Perot Interferometer (EFPI) so that the effects of compaction remain limited. Tests under mixed neutron and gamma irradiation permitted to verify the general behavior and particularly the low drift with radiation induced compaction (RIC). Also, two types of tests have been conducted to verify the accuracy at high temperature. The first ones are “measurements” of thermal dilatation of materials: the sensor is fixed on a sample and knowing its thermal expansion, it is possible to predict the measurement expected from the optical sensor when the temperature is increased from low to high temperature. The comparison between the predicted and experimental outputs informs on how the sensor is accurate. The second types are tests on a tensile test bench operating at high temperature. The Fabry-Perot measurements are compared, in the elastic domain, with the expected strain given by the Young modulus of the material, and also on a larger strain domain, with the measurements of a high temperature axial extensometer. Both types of tests are presented and commented.

The Fukushima-Daiichi nuclear accident of March 2011, and the subsequent loss of internal power supplies after the NPP (Nuclear Power Plant) water flooding caused by the tsunami, leaving the operator TEPCO with almost no information from the reactor pits, demonstrates that safety must always prevail. Accordingly, the French public authorities initiated the RSNR research program, to stimulate and fund new R&D projects to improve the safety of nuclear reactors in service and those of future NPPs. The DISCOMS project (Distributed Sensing for Corium Monitoring and Safety) aimed at developing and testing innovative and passive sensors dedicated to Nuclear Safety, namely an instrumented pole equipped with long length SPNDs (Self-Powered Neutron Detectors)-Thermocouple poles, and Distributed Optical Fiber Sensors, to be installed ex-core in both the reactor pit and concrete floor. The sensors, remotely operated from a safe place, will not only provide additional information during the Severe Accident, but also in post-accidental situation, even in case of loss of all power supplies. The modelling of a 60 year normal operation followed by a Severe Accident for two generations of reactors (Gen II, Gen III) permitted to demonstrate that ex-core long length SPNDs can identify different scenarios: reactor shut down, Normal Operation, Severe Accident without corium relocation, and Severe Accident with corium pouring on the concrete floor. Long length SPNDs were designed and manufactured, along with their electronics, to measure low currents ranging from 1 pA to 100 nA collected under radiations, and qualified in a research reactor with fluxes compliant with modelled scenarios. Optical Fibers Sensor cables are devoted to monitor the Molten Core – Concrete Interaction (MCCI): temperature and strain profiles can be provided in the concrete depth by embedded cables, as a result of using the Raman DTS, Brillouin and Rayleigh OFDR reflectometry techniques, based on the analysis of the backscattered light in single-mode optical fibers, for distributed measurements potentially up to 1000°C with Brillouin instrumentations. Additionally, such sensor cables can be used as fuses with telecom or photon counting OTDRs to detect corium vicinity. Sensor cables and radiation resistant optical fibers have been selected and tested to comply with the radiation conditions in the reactor pits as depicted by the modelling. A final MCCI experiment with prototypical corium, performed at the VULCANO CEA facility, involving also two instrumented SPNDs-Thermocouple poles, has demonstrated the ability of both kinds of sensors and corresponding instrumentations to deliver useful information about the corium status and its progression through the concrete.

This project addresses the development of a remote monitoring solution, able to improve the Nuclear Power Plants safety, and strengthen the third containment barrier, in case of severe accident with reactor breakthrough and corium casting. To overcome any consequence of electrical power supplies loss, distributed sensing technologies are considered. Carefully characterized and selected, rad hard single mode optical fibers have been inserted into cables, with the aim to be embedded in the concrete floor below the reactor vessel, at depths determined by modelling, to resist high temperatures and ionizing radiations. Complementary, Self Powered Nuclear Detectors have been developed to identify the reactor core fusion when deployed in the vicinity of the reactor vessel. Optical sensors are interrogated by Raman, Brillouin and/or Rayleigh-based instrumentations, along with SPND by an innovative multichannel ultra-low current threshold detection system. In case of serious accident, such dual monitoring system will be able to detect the reactor breakthrough, the corium casting, the erosion phase of the concrete floor and the corium cooling.

Experimental Investigation) device will be integrated into the future experimental Jules Horowitz Reactor [1] of the CEA to provide a feedback on potential nuclear accidents, for example the Loss-Of-Coolant Accident (LOCA) [2]. This device will allow investigating the burst conditions of nuclear fuel claddings when the environment of the cladding, as liquid pressurized water during “normal” nuclear activity, becomes a steam atmosphere. In case of LOCA, the heat produced by the residual power is not evacuated and the cladding surface temperature increases drastically. In the LORELEI device, the thermal contribution of the surrounding rods will be reproduced by using a heating Inconel tube placed around the cladding under test. To study in detail the temperature profile of the cladding before the burst, the surface temperature of the cladding has to be monitored by a non-intrusive technique avoiding a modification of the thermal environment and, consequently, of the burst conditions. The accuracy of the measurement has to be close to ten degrees. In preliminary studies, we designed a device and a data treatment procedure able to measure the surface temperature of Zircaloy-4 (Zy-4) claddings, without any contact, under air, up to about 850°C [3, 4]. Two pyrometry methods were investigated: the bichromatic calculation and the Multispectral Radiation Thermometry (MRT) estimation [5]. This latter measurement method revealed an uncertainty < 10°C in the temperature determination in the experimental conditions of the laboratory bench. We selected the spectral ranges 1.0–1.3 μm and 1.45–1.6 μm for the pyrometry measurements according to the future LORELEI conditions [1]. Thermocouples were used to provide a reference temperature and allowed us to validate the constant emissivity profile hypothesis in the spectral ranges of calculation. Emissivity values ranging between 0.85 and about 0.90 were determined, in agreement with the literature [6]. In high temperature surroundings, it appears obvious that a contribution from the surroundings will increase the energy

The light-water one-rod equipment for loss-of-coolant accident (LOCA) experimental investigation (LORELEI) device, one of the experiments of the future experimental nuclear reactor Jules Horowitz of Commissariat a l'Energie Atomique et aux Energies Alternatives, will be dedicated to the study of the LOCA. For a better understanding of the temperature conditions during a LOCA, the surface temperature of the fuel cladding has to be monitored. For this purpose, we are designing a noncontact temperature measurement device based on multispectral pyrometry, adapted to the extreme conditions of LORELEI. In this paper, we present a radiative transfer model, taking into account the direct flux from the measured surface and contributions from the surroundings, to simulate the impact of high-temperature surroundings on the temperature estimated using the multispectral radiation thermometry (MRT) method. The hypothesis of a constant emissivity profile in the spectral ranges of calculation is used to estimate the surface temperature. Experimental pyrometry measurements are performed on Zircaloy-4 and Inconel surfaces in high-temperature surroundings to validate the model. As in LORELEI, an optical fiber is used to carry the infrared thermal flux from the cladding to the spectrometer. Thermocouples are used to give a reference temperature and to provide a complete temperature monitoring of the device. A temperature difference is observed between the "true" cladding temperature and the MRT temperature estimation, when the temperature of the surroundings is between 50 degrees C and 150 degrees C below the cladding temperature, and this difference increases with the cladding temperature. This provides a correction value to apply on the temperature estimation to approach the real cladding temperature.

Fiber Bragg grating (FBG) sensors are expected to provide valuable data in extreme radiation environments associated with nuclear research reactors. However, when the fast neutron fluence reaches 10(18)-10(19) n/cm(2), the radiation-induced changes in the material density and refractive index may drastically bias the measurements. This paper evaluates the radiation effect on the FBG performances by comparing their properties before and after their exposure to fast neutron fluences exceeding 10(19) n/cm(2) (E > 1 MeV). We studied the responses of FBGs manufactured by three different laboratories in the same single-mode optical fiber but using different inscription conditions. The Bragg wavelength and the reflectivity were measured before and after irradiation thanks to a dedicated mounting. For nearly all FBGs, the Bragg peak remains visible after the irradiation while the radiation-induced Bragg wavelength shifts (RI-BWSs) vary from a few picometers (equivalent temperature error <1 degrees C) to nearly 1 nm (similar to 100 degrees C error) depending of the FBG inscription conditions. Such high RI-BWSs can be explained by the huge refractiNe-index variation and compaction observed for hare fiber samples through other experimental techniques. Our results show that by using specific hardening techniques, the FBG-based temperature measurements in a nuclear research reactor experiment may become feasible.

Optical fibre sensors (OFS) are worthy of interest for measurements in nuclear reactor thanks to their unique features, particularly compact size and remote multi-point sensing for some of them. But besides non negligible constraints associated with the high temperature environment of the experiments of interest, it is well known that the performances of OFS can be severely affected by high level of radiations. The Radiation Induced Attenuation (RIA) in the fibre is probably most known effect, which can be to some extent circumvented by using rad hard fibres to limit the dynamic loss. However, when the fast neutron fluence reaches 1018 to 1019 n/cm2, the density and index variations associated to structural changes may deteriorate drastically the performances of OFS even if they are based on rad hard fibres, by causing direct errors in the measurements of temperature and/or strain changes. The aim of the present study is to access the effect of nuclear radiations on the Fabry Perot (FP) and of Fibre Bragg Grating (FBG) sensors through the comparison of measurements made on these OFS - or part of them - before and after irradiation [1]. In the context of development of OFS for high irradiation environment and especially for Material Testing Reactors (MTRs), Sake 2 experiment consists in an irradiation campaign at high level of gamma and neutron fluxes conducted on samples of fibre optics – bare or functionalised with FBG. The irradiation was performed at two levels of fast neutron fluence: 1 and 3.1019 n/cm2 (E>1MeV), at 250°± 25°C, in the SCK•CEN BR2 reactor (Mol Belgium). An irradiation capsule was designed to allow irradiation at the specified temperature without active control. The neutron fluence was measured with activation dosimeters and the results were compared with MCPN computations. Investigation of bare samples gives information on the density changes, while for the FBGs both density and refractive index perturbation are involved. Some results for bare fibres were reported recently. In this paper, we will focus on the measurements made on FBGs that have been manufactured by different laboratories on SMF 28 fibers: CEA, University of St-Etienne and University of Mons. Tested gratings have been written using various conditions (type of fibre, of laser, writing wavelength, power density, post writing thermal annealing,…), leading to various behaviours after Sake 2 irradiation. Bragg wavelength and reflectivity have been measured before and after irradiation thanks to a special mounting at the same temperature. It appears that a change in the shape after irradiation of the Bragg peak disturb the retrieval of the Bragg wavelength. The measurements show that for nearly all gratings the Bragg peak remains visible after the irradiation, and that Radiation Induced Bragg Wavelength Shifts (RI-BWSs) vary from few pm (equivalent to an error of less than 1°C for a temperature sensor) to nearly 1 nm (equivalent to 100°C) depending of the FBG types. High RI-BWSs could indeed be expected when considering the huge refractive index variation and compaction of the bare fibre samples that have been measured by other techniques. Post writing thermal annealing is confirmed as a key parameter in order to obtain a more radiation tolerant FBG. Our results show that specific annealing regimes allow making FGBs suitable to perform temperature measurements in a MTR experiment.

•Good agreement between pyrometry and thermocouple temperatures.•Good quality of the residuals from minimization.•No strong variation of emissivity for samples with oxide layer thickness from 8 to 16µm.

In the framework of the development by CEA and SCK·CEN of a Fabry Perot Sensor (FPS) able to measure dimensional changes in Material Testing Reactor (MTR), the first goal of the SAKE 1 (Smirnof extention - Additional Keytests on Elongation of glass fibres) irradiation was to measure the linear compaction of single mode fibres under high fast neutron fluence. Indeed, the compaction of the fibre which forms one side of the Fabry Perot cavity, may in particular cause a noticeable measurement error. An accurate quantification of this effect is then required to predict the radiation-induced drift and optimize the sensor design. To achieve this, an innovative approach was used. Approximately seventy uncoated fibre tips (length: 30 to 50 mm) have been prepared from several different fibre samples and were installed in the SCK·CEN BR2 reactor (Mol Belgium). After 22 days of irradiation a total fast (E > 1 MeV) fluence of 3 to 5×10 19 n fast /cm 2 , depending on the sample location, was accumulated. The temperature during irradiation was 291°C, which is not far from the condition of the intended FPS use. A precise measurement of each fibre tip length was made before the irradiation and compared to the post irradiation measurement highlighting a decrease of the fibres' length corresponding to about 0.25% of linear compaction. The amplitude of the changes is independent of the capsule, which could mean that the compaction effect saturates even at the lowest considered fluence. In the prospect of performing distributed temperature measurement in MTR, several fibre Bragg gratings written using a femtosecond laser have been also irradiated. All the gratings were written in radiation hardened fibres, and underwent an additional treatment with a procedure enhancing their resistance to ionizing radiations. A special mounting made it possible to test the reflection and the transmission of the gratings on fibre samples cut down to 30 to 50 mm. The comparison of measurements made before and after the irradiation, at the same temperature, allowed us to measure the loss in reflectivity as well as the Bragg wavelength drift. The results are quite promising for some of the investigated gratings.