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The DIstributed Sensing for COrium Monitoring and Safety (DISCOMS) project considers the use of optical fiber sensing cables embedded into the concrete floor under the reactor vessel for remote monitoring of a severe nuclear accident. This article focuses on the selection and testing of single-mode optical fibers with limited radiation-induced attenuation (RIA). To simulate the normal operation period of the reactor, followed by a severe accident, several gamma and mixed (neutron–gamma) irradiations were performed, thanks to the Atomic Energy Commission (CEA) Saclay facilities, POSÉÏDON irradiator and ISIS reactor, up to a gamma cumulative dose of about 2 MGy and fast neutrons fluence (E > 1 MeV) of 6 × 1015 n/cm2. In comparison with the first gamma test on standalone fibers, a significant increase of the RIA at 1550 nm appeared during the second test on fibers encapsulated in sensing cables. Molecular hydrogen generated by the cable compound radiolysis induced an increase of the hydroxyl (OH) attenuation broad spectral peak centered at 1380 nm. The radiation-induced OH growth mechanism is confirmed by the comparison of the extended absorption spectral responses of the standalone and encapsulated fibers. A third gamma irradiation run permitted to measure the radiolytic hydrogen production yield of some compounds of a dedicated temperature cable sample. The efficiency of a carbon-coating layer over the silica cladding, acting as a barrier against hydrogen diffusion into the optical fiber core, was successfully tested under gamma irradiation as well as in a final test under neutron irradiation. [ABSTRACT FROM AUTHOR]

We have developed a Fabry–Perot extensometer that is able to operate at high temperature (up to 400 °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 °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 °C and 350 °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~μm 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. [ABSTRACT FROM AUTHOR]

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 à 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 °C and 150 °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. [ABSTRACT FROM AUTHOR]

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 1018–1019 n/cm2, 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 1019 n/cm2 ($E > 1$ 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 °C) to nearly 1 nm (~100 °C error) depending of the FBG inscription conditions. Such high RI-BWSs can be explained by the huge refractive-index variation and compaction observed for bare 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. [ABSTRACT FROM AUTHOR]

Optical fibre sensors (OFS) can bring substantial advantages over conventional sensing approaches for in- situ measurement in fission Material Testing Reactors (MTRs) and other nuclear research or industrial facilities: easy remote sensing, possibilities of multiplexing, passive operation, low sensitivity to electromagnetic interference, compact size, high resolution and accuracy even under high radiation dose and high temperature, once necessary adaptations have been achieved. In this paper firstly we remind the three undesired effects of high level of irradiation on OFS: radiation induced attenuation, radio luminescence and compaction due to fast neutrons. Then we present two types of sensor that we develop: elongation Fabry Perot sensor - jointly with SCK\!\! \cdot \!\! CEN - and Michelson type displacement sensor; they both rely on white light interferometry to retrieve the desired measurement. We report the results of irradiation of Fabry-Perot sensors in the core of BR2 material testing reactor in Mol (Belgium), under intense neutron-gamma flux and at high temperature. Six Fabry Perot fibre optics sensors are mounted on a support insensitive to radiation. The objective is to test the survival and the drift of the sensors. The temperature is maintained steady at 200^\circ C during 22 days then the temperature is increased up to 390^\circ C. Among five sensors built according to nominal scheme, four are still alive at the end of the cycle and three show a low drift of 1 to 4~\mu\m. These results show a clear progress compared to the previous irradiation and make it possible to consider the use of these sensors in real tests of material in MTR reactor. However, improvement in the robustness of the interfaces is still necessary. The next part is devoted to the measurement of displacements perpendicular to the direction of the lead-in fibre, of interest for small room environments where the fibre cannot make a 90^\circ turn. The optical scheme is based on the Michelson interferometer. The fibre and signal conditioner are the same as used with the Fabry Perot extensometer. The sensor is expected to be applied for the measurement of variations in the diameter of the fuel cladding during exposure in a research reactor. Three sensors arranged on a circle, within 120^\circ from each other, must measure displacements of the external surface of the cladding with an error lower than 10~\mu\m, over a 0.5 mm range. The irradiation device considered is in development for the Jules Horowitz Reactor (RJH) under construction in Cadarache (France). We have developed and tested up to 400^\circ C the optical module which will be inserted in the mechanical part comprising the mechanical probe. With a bit larger room available, we also develop a device capable of measuring displacement, under high nuclear flux in the vicinity of the core of Sodium Fast Reactor. The objective could be to sense more or less directly the deformation of the core support structure and core movement. The uncertainty expected is near 1~\mu\m with a range of more than 10 mm. [ABSTRACT FROM PUBLISHER]

In the framework of the Joint Instrumentation Laboratory (LCI), gathering resources from SCK-CEN (Belgium) and CEA (France), we are developing an optical sensor in order to accurately measure radiation-induced elongation of material placed in the core of a Material Testing Reactor (MTR). This extensometer displays common advantages of Fibre Optic (FO) sensors: high resolution, easy remote sensing and multiplexing, and also compact size which is of particular interest for in pile experiments with little room available. In addition, light weight reduces gamma heating hence limiting the thermal effect. In accordance with the specifications, the sensor has preferably two fixing points defining a gauge length of 10 to 15 mm. The diameter is less than 2 mm. Intense gamma and neutron irradiation as well as high temperatures are the most difficult environment conditions to withstand. Reactor radiation produces huge losses in common optical fibre. The losses can be limited by selecting the fibres, the wavelength range (800–1200 nm), and a measurement based on interferometry (largely insensitive to losses in the fibre thanks to the wavelength encoding of the useful signal). Heavy neutron—mainly—and gamma flux such as in MTR, also produce compaction of silica, resulting in a significant drift and preventing the use of commercial FO sensors in such environment. Knowing this issue we revised the basic scheme of Extrinsic Fabry Perot Interferometer (EFPI) in order to limit the effects of compaction. A first sensor prototype fixed on a stainless steel support is tested with the Smirnof test facility in the BR2 MTR in Mol (Belgium). The support is subject to a constant mechanical and thermal stress, and then his dimensions are not supposed to vary. This test shows a very low drift of the revisited EPFI design under high irradiation field in comparison with a commercial EFPI. This result has to be confirmed with second generation sensors with an increased robustness. The other difficulty to face is high temperature. Fibre optics with metal coating allow safe operation under temperatures up to 400^\circC and even higher. But differential dilatation between silica and typical metallic material produces differential elongation in the range of 0.7 10^-2 i.e 5000 \mu\varepsilon for an increase in temperature of 400^\circC. Such large elongation has to be considered carefully in the sensor design and its fixing on the sample. We are currently implementing metal-coated fibre and we are preparing the next in pile irradiation on the BR2 reactor scheduled for end of 2011. Other applications of the sensor can be considered. [ABSTRACT FROM AUTHOR]

In-situ measurement is a key issue for advanced irradiation programs in nuclear research reactors, especially in Material Testing Reactors (MTR). In that prospect, Optical Fiber Sensors (OFS) can feature unique intrinsic properties that bring substantial advantages over conventional sensing approaches. One of the objectives of the Joint Instrumentation Laboratory (JIL), gathering SCK\bulletCEN (Belgium) and CEA (France) resources and know-how, is to develop Optical Fiber Sensors (OFS) for metrology under high radiation. A preliminary condition to the development of OFS is to ensure that the Radiation Induced Absorption (RIA) of the light in the fiber does not exceed a pre-defined threshold. We report the results of a long lasting irradiation experiment, at higher dose than previously reported: up to a fast neutron fluence of 10^20 n/cm^2 and an ionising dose of 16 GGy, carried out on various fibers, including hollow core photonic band gap (PBG) fibers, singlemode (SM) and multimode (MM) fibers. If we consider that RIA measurements losses < 10 dB are acceptable for the development of OFS in harsh environments such as MTRs, it can be stated that multimode and single mode fibers exist that fulfill this criteria, as far as the most favourable spectral region, in the 800 to 1100/1200 nm wavelength range is considered. Possible applications of OFS sensors in the vicinity of research reactors core are presented. We focus then on the project of dimensional measurement, based mainly on low coherence interferometry with an Extrinsic Fabry Perot. We report some first irradiation experiments on commercial sensors, then we present the adapted schemes designed to avoid the consequences of the radiation induced silica compaction. On that basis, we have set the configuration, built some first prototypes in metallic capillaries and we have worked on the anchoring of the sensor on a sample. We present the prototypes developed and the first results of testing accuracy. The irradiation of some sensors anchored on a stainless steel plate is planed in 2009 in the BR2 reactor facility of SCK\bulletCEN in Mol/Belgium using a dedicated irradiation rig. The purpose of this experiment will be to assess, in a first step, the survivability and the level of signal drift under high radiation when a stable strain is applied on the sensor. [ABSTRACT FROM PUBLISHER]

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