High-Temperature Measurements With a Fabry–Perot Extensometer

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 (as a minimum). 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 (mainly with high purity silica core), the wavelength range (800–1200nm), 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 was tested in the Smirnof test facility in the BR2 MTR in Mol (Belgium). The support was subject to a constant mechanical and thermal stress, and his dimensions were not supposed to vary. This test showed 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 allows safe operation under temperatures up to 400°C and even higher. But differential dilatation between silica and typical metallic material produces differential elongation in the range of 0.5 10–2 i.e 5000μe for an increase in temperature of 300–400°C. Such large elongation has to be considered carefully in the sensor design and its fixing on the sample. Laboratory tests with a second generation of adapted sensors demonstrated their good accuracy: less than 1% error over a displacement of 100μm (< 1 μm error), at room temperature. The displacement measured can reach 250μm (20000μe with a gauge length of 12.5 mm) or more. We are currently implementing metal-coated fibre and we are preparing the next in pile irradiation on the BR2 reactor scheduled for mid-2011. Other applications of the sensor can be considered.

To perform a high temperature strain measurements, the sapphire extrinsic Fabry-Perot interferometer (EFPI) sensor head was attached to a silicon carbide rod and placed in a RF induction heater mounted on an MTS load frame. The high-temperature ceramic adhesive is designed to withstand temperatures up to 4000° F. As a basis for comparison, a gold-coated silica EFPI sensor was attached along side the sapphire EFPI. Initially, strain data was recorded at room temperature as the silicon carbide rod was slowly loaded to 4500 lbs. When the temperature was stabilized at a maximum of 2000° F, the SiC rod was again loaded to determine the effects of temperature on strain. Although the sapphire sensor was capable of measuring strain at higher temperatures, die induction heater, being poorly insulated, was not able to further heat the test material. The purpose of the second experiment was to determine the survivability and repeatability of tiie sapphire EFPI due to high-cycle fatigue loading. The long-term survivability of the sensor, in both static and fatigue loading, has been demonstrated. Although the load frame was shutoff after 200,000 cycles, one would expect to be able to fatigue load tiie sensor for millions of cycles since the components of the EFPI sensing head are free to move inside the alignment tube. This is consistent widi data obtained for similar silica opdcal EFPI sensors diat were fadgue loaded to over 10 million cycles as compared to resistive strain gages that typically can be cycled only to 60,000 at 2000°F.

Optical fiber Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibers. The gap between the fiber end- faces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fiber and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.

Optimizing the life cycle of nuclear systems under safety constraints requires high-performance experimental programs to reduce uncertainties on margins and limits. In addition to improvement in modeling and simulation, innovation in instrumentation is crucial for analytical and integral experiments conducted in research reactors. The quality of nuclear research programs relies obviously on an excellent knowledge of their experimental environment which constantly calls for better online determination of neutron and gamma flux. But the combination of continuously increasing scientific requirements and new experimental domains -brought for example by Generation IV programs- necessitates also major innovations for in-pile measurements of temperature, dimensions, pressure or chemical analysis in innovative mediums. At the same time, the recent arising of a European platform around the building of the Jules Horowitz Reactor offers new opportunities for research institutes and organizations to pool their resources in order to face these technical challenges. In this situation, CEA (French Nuclear Energy Commission) and SCK'CEN (Belgian Nuclear Research Centre) have combined their efforts and now share common developments through a Joint Instrumentation Laboratory. Significant progresses have thus been obtained recently in the field of in-pile measurements, on one hand by improvement of existing measurement methods, and on the other hand by introduction in research reactors of original measurement techniques. This paper highlights the state-of-the-art and the main requirements regarding in-pile measurements, particularly for the needs of current and future irradiation programs performed in material testing reactors. Some of the main on-going developments performed in the framework of the Joint Instrumentation Laboratory are also described, such as: a unique fast neutron flux measurement system using fission chambers with <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">242</sup> Pu deposit and a specific online data processing, an optical system designed to perform in-pile dimensional measurements of material samples under irradiation, an acoustical instrumentation allowing the online characterization of fission gas release in Pressurized Water Reactor fuel rods. For each example, the obtained results, expected impacts and development status are detailed.

Radioisotopes are widely used in industry, agriculture, healthcare, and other fields, they are important basic materials for nuclear science and technology. The research reactor scheme designed by our institute is characterized by high neutron flux, flexible rector core layout, strong irradiation capability, and convenient operation. It has important applications in nuclear fuel and material irradiation test, radioisotope production, neutron science research, etc. By arranging irradiation targets in the thermal irradiation channels of this reactor to produce radioisotopes, it is expected to obtain satisfactory radioisotope yield. This paper analyzes the characteristics of radioisotope production with the high flux reactor, and the productions of rare isotopes, medical isotopes and industrial isotopes are considered. With the relatively high thermal neutron flux, the target irradiation period could be shortened and the specific activity would be improved. Based on the characteristics of radioisotope conversion, a reasonable arrangement scheme for irradiation targets will help to improve the neutron utilization and isotope production.

Under irradiation the post-transition corrosion rates of Zircaloy are enhanced relative to out-of-flux rates. In an effort to better understand the effects of different mixed-field radiation environments and the variables that may contribute to this enhancement a series of in situ electrochemical impedance spectroscopy (EIS) tests were conducted in the Halden Test Reactor under PWR type conditions. The EIS tests were conducted using five identically fabricated electrode cells, which employed Zircaloy-4 tipped working electrodes. Approximately 30 μm thick oxides were prefilmed on the electrode tips prior to test insertion. The electrode test train was designed in a manner that enabled a wide range of gamma and neutron radiation flux conditions to be evaluated. Gamma fluxes (E ≥1 MeV) ranged from 1.7×1013 to 1.3×1014 cm−2 s−1 while the fast neutron fluxes (E ≥1 MeV) ranged from 2.9×1010 to 3.2×1013 cm−2 s−1. In addition, the EIS tests were conducted over temperatures ranging from 250 to 335°C. Electrode sets were situated into two basic radiation environments, which were differentiated by the magnitude and relative proportion of the mixed radiation field. In one environment, where the fast neutron flux was low, the relative gamma radiation component was high (i.e., high gamma-to-neutron ratio environment). In the other environment, where the fast neutron flux was high, the relative gamma radiation component was low (i.e., low gamma-to-neutron ratio environment). Based on the low frequency limits, the impedance moduli and inferred corrosion rates were found to be a complex function of temperature, and the relative proportion of gamma and neutron radiation. At low temperatures of 250°C it was found that the electrode sets which were exposed under high gamma-to-neutron ratios exhibited low frequency impedances that were almost 2.5 times lower than the case for the electrode sets which were placed in low gamma-to-neutron ratio. This occurred even though the absolute magnitude of the gamma and neutron flux fields decreased. This would imply that the corrosion rate is higher under high gamma-to-neutron ratios than under low gamma-to-neutron ratios. Conversely, at high temperatures, 335°C, only very small differences in impedance were noted as a function of radiation environment. Post-test destructive examination of the Zircaloy tipped electrodes indicates that the growth of the oxide formed during the in-pile phase of testing is consistent with the in situ EIS measurements obtained in-pile. The porous nature of the post-transition oxide coupled with the apparent correlation with gamma radiation suggests that heterogeneous radiolysis within the corrosion product film plays an important role in enhancing post-transition corrosion kinetics.

High temperature response of a single crystal CVD diamond detector operated in current mode

For treatment of deep-seated tumors, high epithermal neutron beam flux with minimal contamination from gamma rays as well as thermal and fast neutrons is desired for boron neutron capture therapy (BNCT). For this purpose, a beam shaping assembly (BSA) was designed to provide an epithermal neutron beam with little contamination. The Monte Carlo N-Particle code was employed for a design rendering epithermal neutron flux of [Formula: see text] n cm−2 s−1. Different mesh sizes were implemented to improve the accuracy of the computed dose map and in-phantom parameter in the head phantom and tumor. In the present work, the material test reactor (MTR) with consideration of the general specifications of the Tehran Research Reactor (TRR) was considered as a neutron source. The MAKXSF auxiliary processing code was used to calculate temperature of every part of the research reactor core.

In the framework of the development by CEA and SCKrCEN 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∗1019 nfast/cm2, 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.

An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for measurement of strain at high ambient temperatures. The sensor is fabricated using a femto-second (fs) laser. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber and is then self-enclosed by fusion splicing another piece of singlemode fiber. The fs-laser based fabrication makes the sensor thermally stable to sustain temperatures as high as 800 &deg;C. The sensor is relatively insensitive towards the temperature as compared to its response towards the applied strain. The sensor can be embedded in Carbon fiber/Bismaleimide (BMI) composite laminates for strain monitoring at high ambient temperatures.

SiC/SiC composites are considered for fusion applications due to their neutron irradiation stability, low activation, and good mechanical properties at high temperatures. The projected magnetic fusion power plant first wall and the divertor will operate with surface heat flux ranges of 0.5 to 1 and 4 to 6 MW/m{sup 2}, respectively. To maintain high thermal performance at operating temperatures the first wall and divertor coolant channels must have transverse thermal conductivity values of 5 to 10 and 20 to 30 W/mK, respectively. For these components exposed to a high energy neutron flux and temperatures perhaps exceeding 1000{degrees}C, SiC/SiC composites potentially can meet these demanding requirements. The lack of high-purity SiC fiber and a low through-the-thickness (transverse) thermal conductivity are two key technical problems with currently available SiC/SiC. Such composites, for example produced from Nicalon{trademark} fiber with a chemical vapor infiltrated (CVI) matrix, typically exhibit a transverse conductivity value of less than 8 W/mK (unirradiated) and less than 3 W/mK after neutron irradiation at 800{degrees}C. A new SiC/SiC composite fabrication process has been developed at MER Corp. This paper describes this process, and the thermal and mechanical properties which are observed in this new composite material.

The neutron field of various irradiation positions of the TRIGA Mark II reactor of the Jožef Stefan Institute has been thoroughly characterized by neutron activation dosimetry and miniature fission chambers techniques. In order to have a fully validated calculation scheme to analyze and plan experiments, the gamma field also has to be experimentally validated. The 10-year long collaboration between CEA and JSI is a perfect framework to carry out such a study, and measurements of the gamma field started in late 2016. Several measurement techniques were investigated in in-core and ex-core positions. On-line measurements were carried out using miniature ionization chambers manufactured by the CEA and PTW Farmer ionization chambers. Positional dependence was studied, showing a decrease in the delayed gamma contribution to the total gamma flux with increasing distance from the reactor core center. To characterize the gamma dose in the core, as well as in the periphery, thermo- and optically stimulated luminescent detectors were tested. These detectors are commonly used at CEA to measure the gamma dose in a given material in order to study the nuclear heating in various core elements (control rod, baffle, structural material). Different filters were used in order to assess an integrated dose ranging from a few Gy up to several kGy. The feasibility of such measurements demonstrates the complementarity between measurements with dosimetry and ionization chambers from low to very high gamma-dose environment, such as in material testing reactors.

As an important member of optical fiber sensor, fiber optic Extrinsic Fabry-Perot Interferometric (EFPI) sensor has prospects for a wide range of industrial applications due to its small size, compact configuration, good reliability and flexibility. In a white light based fiber optic EFPI sensor system, which has the advantages of large dynamic range and high resolution, spectral analysis equipment is the most important part which takes the major cost of the system. Therefore, multiplexing spectral analysis equipment to measure multiple sensors simultaneously is an effective and necessary way that can save the cost of the whole sensor system. In this paper, a scheme based on the frequency division multiplexing of EFPI optical fiber sensors is proposed. Two EFPI sensors with different gap lengths are parallely connected through the optical fiber coupler. The overlapped interference spectrum of the two EFPI sensors is measured by using an optical spectrum analyzer, which is separated into two individual interference spectrums with a Finite Impuse Response (FIR) band-pass filter. Their envelope components are extracted with Hilbert transform, and then the interference spectrums are normalized by the envelope components. The respective gap lengths of the two EFPI sensors are achieved with the cross correlation calculation. Using this frequency division spectral analysis method, a two-channel multiplexing EFPI sensor system is implemented in the laboratory. The deployment of this frequency division multiplexing technology can greatly reduce the cost of the whole system which has extensive potential applications for distributed fiber optic EFPI sensor system.

An innovative diamond detector layout is presented that is designed to operate at high temperature under intense neutron and gamma fluxes. It is made of a 500 μm “electronic grade” diamond film with 100 nm thick Ag metal contacts deposited onto each surface of the film by means of thermal evaporation. A thick layer of 6LiF has been deposited on top of one of the two Ag contacts to make the detector sensitive to thermal neutrons. The device was tested at the ISIS spallation neutron source (Rutherford Appleton Laboratory, UK) using the INES beam line. The detector was continuously irradiated for 100 hours in vacuum , exposed to a neutron flux of about 106 n cm−2 s−1 at a temperature . The aim of this experiment was to study the time dependence of the diamond detector performance while operating at high temperature under irradiation, providing a first experimental proof of reliable continuous operation for 100 hours at high temperature in a harsh environment.

Status and outlook of the neutron time-of-flight facility n_TOF at CERN