Tests and foreseen developments of fibered-OSLD gamma heating measurements in low-power reactors

International audienceIn this paper are presented test measurements of a fibered-OSLD system performed during a dedicated experimental phase in EOLE zero-power reactor. The measurement setup consists of an OSLD crystal connected onto the extremity of an optical fiber and a laser stimulation system, manufactured by the CEA/LIST in Saclay. The OSL sensor is remotely stimulated via an optical fiber using a diode-pumped solid-state laser. The OSL light is collected and guided back along the same fiber to a photomultiplier tube. Results obtained using this system are compared to usual gamma heating measurement protocol using OSLD pellets. The presence of induced radio-luminescence in the OSLD during the irradiation was also observed and could be used to monitor the gamma flux.The feasibility of remote measurements is achieved, whereas further developments could be conducted to improve this technique since the readout procedure still requires to withdraw the OSLD off the gamma flux (hence from the core) on account of the dose rate (around a few Gy.h-1), and the readout time remains quite long for on-line applications. Several improvements are foreseen, and will be tested in the forthcoming years

Fiber Optic remote gamma dosimeters based on Optically Stimulated Luminescence: State-of-the-art at CEA

International audienceOptically Stimulated Luminescence (OSL) used in conjunction with optical fibres allows remote measurements of both dose and dose rate for the purpose of radiation protection in nuclear industry and medecine (radiology, radiotherapy), monitoring of installations (dismantling) as well as in process controls. Since 1995, two types of OSL systems have been developed by the Optical Measurement Laboratory (CEA) for gamma detection. The first system relies on rare-earth-doped Alkaline-Earth Sulphides and operates in the red/near-infrared part of the spectrum. It does not provide a tissue-equivalent dose measurement and is dedicated for dismantling operations and process control where a great fiber length is required (100 meters). Several systems of this kind are routinely used by COGEMA and CEA for assistance in their dismantling operations as they provide real-time in-situ operation in areas out of reach for usual dosimeters. The second system relies on alumina crystals and operates in the blue/green part of the visible spectrum. It has been developed for radiation protection purposes and may involve a patented compensation technique (both for angular incidence and photon energy)

Optical fiber sensors to improve the safety of nuclear power plants

Conference of 4th Asia-Pacific Optical Sensors Conference 2013, APOS 2013 ; Conference Date: 15 October 2013 Through 18 October 2013; Conference Code:101244International audienceSafety must always prevail in Nuclear Power Plants (NPPs), as shown at Fukushima-Daiichi. So, innovations are clearly needed to strengthen instrumentations, which went inoperative during this nuclear accident as a consequence of power supply losses. Possible improvements concern materials and structures, which may be remotely monitored thanks to Optical Fiber Sensors (OFS). We detail topics involving OFS helpful for monitoring, in nominal conditions as well as during a severe accident. They include distributed sensing (Rayleigh, Raman, Brillouin) for both temperature sensing and structure monitoring as well as H2 concentration and ionizing radiation monitoring. For future plants, Fiber Bragg Grating (FBG) sensors are considered up to high temperature for sodium-cooled fast reactor monitoring. These applications can benefit from fiber advantages: sensor multiplexing, multi-km range, no risk-to-people, no common failure mode with other technologies, remote sensing, and the ability to operate in case of power supply lost in the NPP

Packaged sapphire fiber Bragg gratings ability to withstand temperature up to 1500°C

International audienceA fiber Bragg grating has been inscribed in a 100-µm diameter sapphire optical fiber with the phase mask technique and a fs-laser emitting at 800 nm. The grating was placed inside a sealed alumina capillary to protect the fiber from the environment. Then the fiber was set inside an oven and cycled up 7 times to a maximum temperature of 1500°C during 2 h. We observed that after two cycles, the grating is stabilized and no more hysteresis on the Bragg wavelength is observed. However, the temperature uncertainty is as high as 15°C and is principally due to modal interference. Then the grating is submitted to a 3-day annealing and two annealing successive 4-day annealing – for a total of eleven days – at a temperature of 1500°C. During this treatment, the grating amplitude remained constant and the Bragg wavelength showed no significant drift. As a conclusion, the packaged grating did not exhibit any erasure during these annealing experiments and perform reliable temperature measurement up to 1500°C

Détection répartie de la corrosion par capteur à fibre optique et OFDR sur les armatures de renforcement en acier des structures en béton armé

International audienceCorrosion is a major pathology for Civil Engineering structures, affecting long-term reliability. Huge direct costs (maintenance, rehabilitation) and indirect costs (lost productivity, litigation, outages, delays, downtimes) justify strategies to minimize the impact of corrosion. Regarding civil engineering (CE) infrastructures constructed of reinforced concrete, steel reinforcing bars (rebars) are naturally corrosion-protected when embedded into concrete (pH ~ 13). Concrete carbonation (“generalized” corrosion) and chloride ion penetration (“pitting” corrosion) both accelerate their corrosion rate. Corrosion products grow in volume and the increase in pressure at the steel-concrete interface leads to cracks of the concrete layer and acceleration of degradation. They may escape as well through cracks, thus leading to a reduction in rebar diameter and global structure weakening.Until now, inspections are carried out periodically and involve indirect measurement techniques (e.g. chemical-, impedance-, potential-based) that are time-consuming, costly and probabilistic in nature. Imaging techniques (e.g. ultrasonics) provide information about internal damage or voids within concrete but are of limited range of investigation (limited to accessible surfaces). Since it is impossible to predict where corrosion would start in large infrastructures, distributed monitoring techniques are desirable in order to early detect its onset, particularly in hidden or inaccessible areas.We investigated the OFDR technique in the perspective of a Condition-Based Maintenance (CBM), identified as a future challenge in order to provide safe operating conditions and cost savings. The use of telecom-grade fibers as sensors is motivated by their cost-effectiveness, electrochemical passivity, electromagnetic immunity and networking/multiplexing capability. We report on an original fiber-based corrosion sensor design employing usual steel rebars in order to avoid galvanic corrosion to occur. Since the sensor is a rebar, it also behaves as an extensometer and a dedicated design is proposed to discriminate between global thermomechanical loadings and local corrosion mechanical effects. Changes in OFDR signals with respect to reference signals provide localization, identification and direct measurement of corrosion. The sensing device was successfully tested under accelerated pitting corrosion as a proof-of-concept

State-of-strain evaluation with fiber Bragg grating rosettes: application to discrimination between strain and temperature effects in fiber sensors

International audienceAn optical rosette that incorporates fiber Bragg gratings as strain gauges has been designed, fabricated, and tested. We investigated it by measuring the state of strain of a thin plate as the test structure submitted to an increasing load in a four-point bending configuration and for various angular orienta-tions. This device has also been successfully investigated as a self-temperature-compensated in situ uniaxial strain sensor without any angular dependence and with high accuracy in recovery analysis, leading us to expect many industrial applications. Printed circuit processes or integrated optics on polymers would provide a means for accuracy, reproducibility, and integration in a mass-produced process

Measurement of index modulation along Fiber Bragg Gratings by Side Scattering and Local Heating methods

International audienceA method for measurement of index modulation profile along fiber Bragg gratings is described which uses a hot finger that selectively heats some fringes. Results obtained from this local heating technique and side scattering technique are reviewed and compared

Fibre Remote Optoelectronic Gamma Dosimetry Based On Optically Stimulated Luminescence Of Al2_2o3_3:C

International audienceOptically stimulated luminescence dosimetry (OSL-D) used in conjunction with fibre optics enables a remote measurement of dose, for the purpose of radioprotection in the nuclear industry and in medicine (radiology, radiotherapy). Alumina OSL crystals are used because of their low Z, low fading and optical transparency, which improves the sensitivity. An optoelectronic portable dosemeter has been designed and tested that shows a dose detection of 50 $\mu$Gy with a 20 metre-long fibre. Following irradiation, all trapped electrons are released under light stimulation while the OSL is integrated to provide dose-equivalent measurements. A compensation technique is designed with the help of the MCNP4b code, so that both angular and photon energy characteristics comply with international standards (CEI 61066) for photon dose equivalent H$_p$(10). Two sensors are described that allow measurements over a wide solid angle (95% of 4$\pi$Sr), for photon energies ranging from 15 keV to 3 MeV

Fiber-Coupled, Time-Gated Al2_2O3_3:C Radioluminescence Dosimetry Technique and Algorithm for Radiation Therapy With LINACs

International audienceAn original algorithm for real-time In Vivo Dosimetry (IVD) based on Radioluminescence (RL) of dosimetric-grade Al$_2$O$_3$:C crystals is described and demonstrated in reference conditions with 12-MV photon beams from a Saturne 43 linear accelerator (LINAC), simulating External Beam Radiation Therapy (EBRT) treatments. During the course of irradiation, a portion of electrons is trapped within the Al$_2$O$_3$:C crystal while another portion recombines and generates RL, recorded on-line using an optical fiber. The RL sensitivity is dose-dependent and increases in accordance with the concentration of trapped electrons. Once irradiation is completed, the Al$_2$O$_3$:C crystal is reset by laser light (reusable) and the resultant OSL (Optically Stimulated Luminescence) is also collected back by the remote RL-OSL reader and finally integrated to yield the absorbed dose. During irradiation, scintillation and Cerenkov lights generated within the optical fiber (“stem effect”) are removed by a time-discrimination method involving a discriminating unit and a fiber-coupled BGO scintillator placed in the irradiation room, next to the LINAC. The RL signals were then calibrated with respect to reference dose and dose rate data using an ionization chamber (IC). The algorithm relies upon the integral of the RL and provides the accumulated dose (useful to the medical physicist) at any time during irradiation, the dose rate being derived afterwards. It is tested with both step and arbitrary dose rate profiles, manually operated from the LINAC control desk. The doses measured by RL and OSL are both compared to reference doses and deviations are about ±2% and ±1% respectively, thus demonstrating the reliability of the algorithm for arbitrary profiles and wide range of dose rates. Although the calculation was done off-line, it is amenable to real-time processing during irradiatio

Optical Fiber Sensor for corrosion detection and evolution monitoring in reinforced concrete structures

Conference of 9th International Workshop on Structural Health Monitoring: A Roadmap to Intelligent Structures, IWSHM 2013 ; Conference Date: 10 September 2013 Through 12 September 2013; Conference Code:114374International audienceCorrosion of reinforced bars (rebars) in concrete structures is a major issue in civil engineering structures, being its detection and evolution measurement a challenge for applied research. In this project we present a new methodology to detect corrosion and its evolution monitoring, based on the direct rebar-concrete interface changes measurement by the use of Optical Fiber Sensor (OFS) either a Fiber Bragg Grating (FBG) network or Optical Frequency Domain Reflectometry (OFDR) interrogation technique providing distributed strain measurement. A built-in pre-strain profile makes possible the detection of general and pitting corrosion with the same sensor. Accelerated corrosion tests were performed in an electrolytic solution under both pitting and general corrosion. A clear correlation is observed between the evolution of the spectral shift and the corrosion progress thus demonstrating the feasibility of this technique