L. Oriol

Cea Leti, Grenoble, Rhône-Alpes, France

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Publications (16)7.46 Total impact

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    ABSTRACT: For the on-line monitoring of high fast neutron fluxes in the presence of a strong thermal neutron component, SCK·CEN and CEA are jointly developing a Fast Neutron Detector System, based on <sup>242</sup>Pu fission chambers as sensors and including dedicated electronics and data processing systems. Irradiation tests in the BR2 reactor of <sup>242</sup>Pu fission chambers operating in current mode showed that in typical MTR conditions the fission chamber currents are dominated by the gamma contribution. In order to reduce the gamma contribution to the signal, it was proposed to use the fission chambers in Campbelling mode. An irradiation experiment in the BR2 reactor with a <sup>242</sup>Pu and a <sup>235</sup>U fission chamber, both equipped with a suitable cable for measurements in Campbelling mode, proved the effectiveness of the suppression of the gamma-induced signal component by the Campbelling mode: gamma contribution reduction factors of 26 for the <sup>235</sup>U fission chamber and more than 80 for the <sup>242</sup>Pu fission chamber were obtained. The experimental data also prove that photofission contributions are negligibly small. Consequently, in typical MTR conditions the gamma contribution to the fission chamber Campbelling signal can be neglected.
    IEEE Transactions on Nuclear Science 05/2011; · 1.22 Impact Factor
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    ABSTRACT: Flux monitoring is of great interest for experimental studies in material testing reactors. Nowadays, only the thermal neutron flux can be monitored on line, e.g., using fission chambers or self-powered neutron detectors. In the framework of the Joint Instrumentation Laboratory between SCK-CEN and CEA, we have developed a fast neutron detector system (FNDS) capable of measuring on line the local high-energy neutron flux in fission reactor core and reflector locations. FNDS is based on fission chambers measurements in Campbelling mode. The system consists of two detectors, one detector being mainly sensitive to fast neutrons and the other one to thermal neutrons. On line data processing uses the CEA depletion code DARWIN in order to disentangle fast and thermal neutrons components, taking into account the isotopic evolution of the fissile deposit. The first results of FNDS experimental test in the BR2 reactor are presented in this paper. Several fission chambers have been irradiated up to a fluence of about 7 × 10(20) n∕cm(2). A good agreement (less than 10% discrepancy) was observed between FNDS fast flux estimation and reference flux measurement.
    The Review of scientific instruments 03/2011; 82(3):033504. · 1.52 Impact Factor
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    ABSTRACT: Fission chambers are widely used in the nuclear industry. As an example, they play a major role in the control of any fission reactor and are thus regarded as a key component for ensuring their safety. They are also employed in the material testing reactors for monitoring irradiations. We have recently started a research program, the objective of which is to improve the performance of those neutron detectors in terms of lifetime, calibration, and online diagnosis. In this paper, we present several studies carried out in order to model the signal delivered by a fission chamber. First, the simulation of the deposit evolution allowed us to select the most appropriate fissile material for a given spectrum and fluence. Second, we studied the impact of the bias voltage and filling gas characteristics on the charge collection time. Finally, the simulation of a pulse signal prior to amplification showed how it is important to have a satisfactory knowledge of the energy for creating ion pairs to accurately assess the signal in current or Campbelling mode.
    IEEE Transactions on Nuclear Science 01/2011; · 1.22 Impact Factor
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    ABSTRACT: With the development of innovative nuclear systems and new generation neutron sources, the nuclear instrumentation should be adapted. Since several years, we developed microscopic fission chambers to study the transmutation of minor actinides in high thermal-neutron fluxes. The recent developments done to fulfill the drastic conditions of irradiations are described in this paper together with the feedback from the measurements. Two installations were used: the HFR of the ILL for its highest thermal neutron flux of the world and the MEGAPIE target which was the first 1 MW liquid Pb-Bi spallation target in the world.
    Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2009 First International Conference on; 07/2009
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    ABSTRACT: Fission chambers are widely used for on-line monitoring of neutron fluxes in irradiation reactors. A selective measurement of a component of interest of the neutron flux is possible in principle thanks to a careful choice of the deposit material. However, measuring the fast component is challenging when the flux is high (up to 10<sup>15</sup> n/cm<sup>2</sup>/s) with a significant thermal component. The main problem is that the isotopic content of a material selected for its good response to fast neutrons evolves with irradiation, so that the material is more and more sensitive to thermal neutrons. Within the framework of the FNDS (Fast Neutron Detector System) project, we design tools that simulate the evolution of the isotopic composition and fission rate for several deposits under any given flux. In the case of a high flux with a significant thermal component, <sup>242</sup>Pu is shown after a comprehensive study of all possibilities to be the best choice for measuring the fast component, as long as its purity is sufficient. If an estimate of the thermal flux is independently available, one can correct the signal for that component. This suggests a system of two detectors, one of which being used for such a correction. It is of very high interest when the detectors must be operated up to a high neutron fluence.
    Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2009 First International Conference on; 07/2009
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    ABSTRACT: The Dosimetry Command control and Instrumentation Laboratory (LDCI) at CEA/Cadarache is specialized in the development, design and manufacturing of miniature fission chambers (from 8 mm down to 1.5 mm in diameter). LDCI fission chambers workshop specificity is its capacity to manufacture and distribute special fission chambers with fissile deposits other than U235 (typically Pu242, Np237, U238, Th232). We are also able to define the characteristics of the detector for any in-core measurement requirements: sensor geometry, fissile deposit material and mass, filling gas composition and pressure, operating mode (pulse, current or Campbelling) with associated cable and electronics. The fission chamber design relies on numerical simulation and modeling tools developed by the LDCI. One of our present activities in fission chamber applications is to develop a fast neutron flux instrumentation using Campbelling mode dedicated to measurements in material testing reactors.
    Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2009 First International Conference on; 07/2009
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    ABSTRACT: For the on-line monitoring of high fast neutron fluxes in the presence of a strong thermal neutron component, SCK·CEN and CEA are jointly developing a Fast Neutron Detector System, based on <sup>242</sup>Pu fission chambers as sensors and including dedicated electronics and data processing systems. Irradiation tests in the BR2 reactor of <sup>242</sup>Pu fission chambers operating in current mode showed that in typical MTR conditions the fission chamber currents are dominated by the gamma contribution. In order to reduce the gamma contribution to the signal, it was proposed to use the fission chambers in Campbelling mode. An irradiation experiment in the BR2 reactor with a <sup>242</sup>Pu and a <sup>235</sup>U fission chamber, both equipped with a suitable cable for measurements in Campbelling mode, proved the effectiveness of the suppression of the gamma-induced signal component by the Campbelling mode: gamma contribution reduction factors of 26 for the <sup>235</sup>U fission chamber and more than 80 for the <sup>242</sup>Pu fission chamber were obtained. The experimental data also prove that photofission contributions are negligibly small. Consequently, in typical MTR conditions the gamma contribution to the fission chamber Campbelling signal can be neglected.
    Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2009 First International Conference on; 07/2009
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    ABSTRACT: A fission chamber with a Pu242 deposit is the best suited detector for on-line measurements of the fast component of a high neutron flux (∼1014ncm-2s-1 or more) with a significant thermal component. To get the fast flux, it is, however, necessary to subtract the contribution of the thermal neutrons, which increases with fluence because of the evolution of the isotopic content of the deposit.This paper presents an algorithm that permits, thanks to measurements provided by a Pu242 fission chamber and a detector for thermal neutrons, to estimate the thermal and the fast flux at any time. An implementation allows to test it with simulated data.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2009; 603(3):415–420. · 1.14 Impact Factor
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    ABSTRACT: Fission cross sections of Minor Actinides are of great importance for the reduction of long-term nuclear waste radiotoxicity by transmutation. In this paper we present the results of measurements done on the fission cross sections of three minor actinides: 238Np, 242gs-mAm and 245Cm, in the thermal energy range. These cross sections participate significantly to the incineration of 237Np, 241Am and 244Cm isotopes and show some discrepancies with nuclear data libraries or previous experiments.
    04/2008;
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    ABSTRACT: The FNDS project aims at developing fission chambers to measure on-line the fast component of a high neutron flux ( or more) with a significant thermal component. We identify with simulations the deposits of fission chambers that are best suited to this goal. We address the question of the evolution of the deposit by radiative capture and decay. A deposit of 242Pu appears as the best choice, with a high initial sensitivity to fast neutrons only slowly degrading under irradiation. The effect of unavoidable impurities was assessed: small concentrations of 241Pu and 239Pu can be tolerated.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2008; · 1.14 Impact Factor
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    ABSTRACT: In the framework of nuclear waste transmutation studies, the Mini-INCA project has been initiated at CEA/DSM to determine optimal conditions for transmutation and incineration of Minor Actinides (MA) in high intensity neutron fluxes in the thermal region. Our experimental tool is based on alpha- and gamma-spectroscopy of irradiated samples and microscopic fission-chambers. It can provide both microscopic information on nuclear reactions (total and partial cross sections for neutron capture and/or fission reactions) and macroscopic information on transmutation and incineration potentials. The 232Th, 237Np, 241Am, and 244Cm transmutation chains have been explored in details, showing some discrepancies in comparison with evaluated data libraries but in overall good agreement with recent experimental data.
    http://dx.doi.org/10.1051/ndata:07612. 01/2008;
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    ABSTRACT: The MEGAPIE project is one of the key experiments towards the feasibility of Accelerator Driven Systems. On-line operation and post-irradiation analysis will provide the scientific community with unique data on the behavior of a liquid spallation target under realistic irradiation conditions. A good neutronics performance of such a target is of primary importance towards an intense neutron source, where an extended liquid metal loop requires some dedicated verifications related to the delayed neutron activity of the irradiated PbBi. In this paper we report on the experimental characterization of the MEGAPIE neutronics in terms of the prompt neutron (PN) flux inside the target and the delayed neutron (DN) flux on the top of it. For the PN measurements, a complex detector, made of 8 microscopic fission chambers, has been built and installed in the central part of the target to measure the absolute neutron flux and its spatial distribution. Moreover, integral information on the neutron energy distribution as a function of the position along the beam axis could be extracted, providing integral constraints on the neutron production models implemented in transport codes such as MCNPX. For the DN measurement, we used a standard 3He counter and we acquired data during the start-up phase of the target irradiation in order to take sufficient statistics at variable beam power. Experimental results obtained on the PN flux characteristics and their comparison with MCNPX simulations are presented, together with a preliminary analysis of the DN decay time spectrum.
    12/2007;
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    ABSTRACT: The MEGAPIE project is a key experiment on the road to Accelerator Driven Systems and it provides the scientific community with unique data on the behavior of a liquid lead-bismuth spallation target under realistic and long term irradiation conditions. The neutronic of such target is of course of prime importance when considering its final destination as an intense neutron source. This is the motivation to characterize the inside neutron flux of the target in operation. A complex detector, made of 8 micro fission-chambers, has been built and installed in the core of the target, few tens of centimeters from the proton/Pb-Bi interaction zone. This detector is designed to measure the absolute neutron flux inside the target, to give its spatial distribution and to correlate its temporal variations with the beam intensity. Moreover, integral information on the neutron energy distribution as a function of the position along the beam axis could be extracted, giving integral constraints on the neutron production models implemented in transport codes such as MCNPX.
    05/2007;
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    ABSTRACT: First prototypes of the industrial version of the CEA sub-miniature fission chambers (1.5 mm outer diameter) for in-core detection of high thermal neutron fluxes (up to 4 × 10 14 n/(cm².s)), manufactured by the PHOTONIS company and called CFUZ53, were tested in the CALLISTO loop of the BR2 reactor in PWR-like conditions. In this paper we present a first analysis of the recently obtained experimental results: neutron sensitivity, linearity to thermal neutron flux, current/voltage characteristics, gamma contribution, temperature effects and long term behaviour (mechanical integrity, burn-up of uranium, …). We also compare the experimental data with calculation results from a fission chamber theoretical model. The preliminary analysis indicates that the CFUZ53 signals show consistent signals in PWR conditions up to thermal neutron fluences beyond 2 × 10 20 n/(cm².s).
    01/2005;
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    ABSTRACT: Fission chambers are widely used for on-line measurements of neutron fluxes in irradiation reactors. When the flux is high with a significant thermal component, the measurement of the fast component is a concern and implies a careful choice of the deposit material. Indeed, most fissile materials have a cross section much higher with thermal neutrons than with fast ones. Moreover, a deposit chosen to favour fissions by fast neutrons will make radiative captures in the thermal domain, leading to a gradual modification of the isotopic compositions of the deposit that may dramatically increase the fissions with thermal neutrons. Within the framework ot the FNDS (Fast neutron Detector System) project, we performed simulations of the evolution of the isotopic composition and fission rate for several deposits under high fluxes. We show that Pu242 is the best choice to extract the fast component from a flux with a significant thermal component, provided that the impurities in Pu241 and Pu239 are small enough. However, an independent measurement of the thermal component appears to be necessary to compensate for the evolution of the Pu242 deposit.
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    ABSTRACT: The Mini-INCA project is dedicated to the measurement of incineration-transmutation chains and potentials of minor actinides in high-intensity thermal neutron fluxes. In this context, new types of detectors and methods of analysis have been developed. The 241 Am and 232 Th transmutation-incineration chains have been studied and several capture and fission cross sections measured very precisely, showing some discrepancies with existing data or evaluated data. An impact study was made on different based-like GEN-IV reactors. It underlines the necessity to proceed to precise measurements for a large number of minor-actinides that contribute to these future incineration scenarios.