Développement et optimisation de méthodes de mesures neutroniques par chambre à fission auprès de réacteurs expérimentaux. Maîtrise , traitement et réduction des incertitudes

Nuclear measurement, and in particular neutronic measurement, plays a key role in nuclearresearch and industry. Neutrons, when detected, are able to provide capital pieces ofinformation on the behavior of, for example, a nuclear reactor core. This allows, amongothers things, a safe operating of the reactor, qualifying calculation tools used for theconception of future reactors (such as the JHR or the 4th generation reactors) and makingprogress in fundamental research by improving nuclear data libraries. The result of eachmeasurement is affected by an uncertainty which depends on many factors. Its estimation isa necessity and its reduction is one of the major challenges taken up to by the CEA.Neutrons are not charged particles and are therefore unable to directly ionize the gas of agas filled detector; therefore their detection using this kind of measurement tool requires aconversion reaction, which is, in the case of the fission chamber detector, the induced fissionreaction. The reduction and mastery of the uncertainties affecting the fission chambersmeasurements are the core of the thesis subject. This work was achieved within theExperimental Program Laboratory (LPE) of the Experimental Physics Section (SPEx) at CEACadarache. It is divided into four parts:· the first one consists in a state of the art of fission chambers measurements within theframework of the zero-power experimental reactors. It compiles knowledge aboutmeasurement techniques, technologies and physics used for neutron detection.· the second part study the optimization of two of the key parameters defining thedesign of a fission chamber:o the fissile deposit thickness. The results, obtained thanks to simulation,allowed a better understanding of this parameter’s impact on measurementswhich lead to an improvement of the future detectors design.o the filling gas type and pressure. A deep experimental parametric study wascarried out in the MINERVE facility which enables understanding the impact ofboth filling gas characteristics on results. New filling standards have beendiscovered and are now taken into account when designing new detectors.Those standards allow dividing by two the measurement uncertainties due topressure variations and enable using fission chambers in more variousexperimental setups.· the third part of this works is focused on the improvement of the electronic equipmentand post-treatments used for fission chambers measurements. Three innovativeacquisition devices were chosen for testing in MINERVE. The results obtained enablegiving a set of short term and long term recommendations considering the update ofthe instrumentation used in the SPEx zero power reactors. In addition, a new deadtime correction method was developed during the thesis and is presented in this part.Its positive impact on rod-drop measurement is given for illustration as the gapbetween experimental results and expected values is divided by four thanks to thisinnovative correction method.· the last part is about the optimization of spectral indices measurement. The mostimportant parameters regarding spectral indices assessment are studied, their impacton spectral index is quantified and their respective acquisition methods are optimized.The study was mainly concentrated on the calibration data acquisition. This work ledto significant improvement, most notably concerning the « 238U fission / 235U fission»spectral index measured in the MINERVE core. The gap between calculation andexperimental results has been greatly reduced (from 35.70% to 0.17%) and theassociated uncertainty has also been diminished (from 15.7% to 5.6%). Those resultsalso allowed explaining abnormal gaps between calculation and experimentationobserved in measurement performed in the MINERVE facility in 2004. (...)Le rôle de la mesure nucléaire, et en particulier celui de la détection neutronique est fondamental dans la recherche et l’industrie nucléaire. Les neutrons fournissent des informations capitales sur le comportement d’un milieu expérimental de type réacteur nucléaire. Leur mesure permet, entre autres, d’assurer l’exploitation « au quotidien » des installations nucléaires de base par le respect des critères de sûreté, de qualifier les codes de calcul utilisés notamment pour la conception des coeurs du futur (comme les réacteurs de génération 4 ou le Réacteur Jules Horowitz) et de faire des progrès en recherche fondamentale, notamment en améliorant les bibliothèque de données nucléaires. Le résultat de chaque mesure est affecté d’une incertitude dont les contributions sont parfois complexes mais dont la prise en compte est systématiquement nécessaire pour leur interprétation. Leur réduction est un des challenges majeurs du CEA.Les neutrons sont des particules non chargées dont la détection par chambre d’ionisation passe nécessairement par une réaction de conversion. Le capteur de type « chambre à fission », utilise la réaction de fission induite. La maîtrise et la réduction des incertitudes affectées aux mesures utilisant ce détecteur constituent la thématique de cette thèse qui s’est déroulée au sein du Laboratoire des Programmes Expérimentaux (LPE) du Service de Physique Expérimentale (SPEx) du CEA Cadarache. Ils se divisent en quatre thématiques :• la première consiste en un état de l’art de la mesure neutronique par chambre à fission en réacteur maquette. Elle fait le point sur les techniques de mesure, la technologie et les processus physiques mis à profit lors de la détection des neutrons,• la deuxième thématique porte sur l’optimisation de deux paramètres intrinsèques du détecteur :o l’épaisseur du dépôt de matière fissile. Les résultats obtenus par simulation ont permis de mieux appréhender l’impact de ce paramètre sur les mesures et d’optimiser la conception des détecteurs futurs,o la pression et la nature du gaz de remplissage. Ces travaux ont permis d’étudier expérimentalement l’impact de cette grandeur sur le comportement de la chambre à fission et d’en optimiser le remplissage. De nouveaux standards ont été découverts et mis en place, adaptés au matériel utilisé aujourd’hui, qui permettent d’une part une division par deux des incertitudes liés aux variations de la pression de remplissage et une utilisation plus souple des détecteurs d’autre part,• la troisième thématique s’intéresse à l’amélioration de l’électronique de mesure et des post-traitements utilisés. Trois chaînes d’acquisition innovantes ont été testées de manière à couvrir les grandes lignes du panorama actuel de l’électronique de mesure. Il en résulte des recommandations dorénavant prises en compte pour l’équipement des maquettes critiques du SPEx. Ces travaux de thèses ont également été l’occasion d’introduire une méthodologie de correction de temps mort innovante et d’en illustrer l’impact positif (division par quatre des écarts entre la mesure et le résultat attendu) sur des mesures réalisées sur la maquette MASURCA,• la quatrième et dernière thématique s’intéresse à l’optimisation des mesures d’indice de spectre par chambre à fission. Chaque paramètre influençant la mesure voit son impact quantifié et sa détermination optimisée, un soin tout particulier étant apporté aux mesures d’étalonnage des capteurs. Il en résulte des améliorations majeures,notamment sur l’indice de spectre « fission 238U / fission 235U » mesuré au centre du coeur de MINERVE, caractérisé par une réduction des écarts « calcul / expérience »(passant de 35.70% à 0.17% dans le meilleur des cas) et une diminution des incertitudes de mesure (passant de 15.7% à 5.6%). Ces résultats ont également permis d’expliquer et de réduire drastiquement les écarts anormaux entre le calcul et l’expérience constatés lors de mesures réalisées en 2004 sur le réacteur MINERVE. (...

Development and optimisation of fission chambers neutron measurements applied on experimental nuclear reactor. Uncertainties thorough understanding, processing and reduction

Le rôle de la mesure nucléaire, et en particulier celui de la détection neutronique est fondamental dans la recherche et l’industrie nucléaire. Les neutrons fournissent des informations capitales sur le comportement d’un milieu expérimental de type réacteur nucléaire. Leur mesure permet, entre autres, d’assurer l’exploitation « au quotidien » des installations nucléaires de base par le respect des critères de sûreté, de qualifier les codes de calcul utilisés notamment pour la conception des coeurs du futur (comme les réacteurs de génération 4 ou le Réacteur Jules Horowitz) et de faire des progrès en recherche fondamentale, notamment en améliorant les bibliothèque de données nucléaires. Le résultat de chaque mesure est affecté d’une incertitude dont les contributions sont parfois complexes mais dont la prise en compte est systématiquement nécessaire pour leur interprétation. Leur réduction est un des challenges majeurs du CEA.Les neutrons sont des particules non chargées dont la détection par chambre d’ionisation passe nécessairement par une réaction de conversion. Le capteur de type « chambre à fission », utilise la réaction de fission induite. La maîtrise et la réduction des incertitudes affectées aux mesures utilisant ce détecteur constituent la thématique de cette thèse qui s’est déroulée au sein du Laboratoire des Programmes Expérimentaux (LPE) du Service de Physique Expérimentale (SPEx) du CEA Cadarache. Ils se divisent en quatre thématiques :• la première consiste en un état de l’art de la mesure neutronique par chambre à fission en réacteur maquette. Elle fait le point sur les techniques de mesure, la technologie et les processus physiques mis à profit lors de la détection des neutrons,• la deuxième thématique porte sur l’optimisation de deux paramètres intrinsèques du détecteur :o l’épaisseur du dépôt de matière fissile. Les résultats obtenus par simulation ont permis de mieux appréhender l’impact de ce paramètre sur les mesures et d’optimiser la conception des détecteurs futurs,o la pression et la nature du gaz de remplissage. Ces travaux ont permis d’étudier expérimentalement l’impact de cette grandeur sur le comportement de la chambre à fission et d’en optimiser le remplissage. De nouveaux standards ont été découverts et mis en place, adaptés au matériel utilisé aujourd’hui, qui permettent d’une part une division par deux des incertitudes liés aux variations de la pression de remplissage et une utilisation plus souple des détecteurs d’autre part,• la troisième thématique s’intéresse à l’amélioration de l’électronique de mesure et des post-traitements utilisés. Trois chaînes d’acquisition innovantes ont été testées de manière à couvrir les grandes lignes du panorama actuel de l’électronique de mesure. Il en résulte des recommandations dorénavant prises en compte pour l’équipement des maquettes critiques du SPEx. Ces travaux de thèses ont également été l’occasion d’introduire une méthodologie de correction de temps mort innovante et d’en illustrer l’impact positif (division par quatre des écarts entre la mesure et le résultat attendu) sur des mesures réalisées sur la maquette MASURCA,• la quatrième et dernière thématique s’intéresse à l’optimisation des mesures d’indice de spectre par chambre à fission. Chaque paramètre influençant la mesure voit son impact quantifié et sa détermination optimisée, un soin tout particulier étant apporté aux mesures d’étalonnage des capteurs. Il en résulte des améliorations majeures,notamment sur l’indice de spectre « fission 238U / fission 235U » mesuré au centre du coeur de MINERVE, caractérisé par une réduction des écarts « calcul / expérience »(passant de 35.70% à 0.17% dans le meilleur des cas) et une diminution des incertitudes de mesure (passant de 15.7% à 5.6%). Ces résultats ont également permis d’expliquer et de réduire drastiquement les écarts anormaux entre le calcul et l’expérience constatés lors de mesures réalisées en 2004 sur le réacteur MINERVE. (...)Nuclear measurement, and in particular neutronic measurement, plays a key role in nuclearresearch and industry. Neutrons, when detected, are able to provide capital pieces ofinformation on the behavior of, for example, a nuclear reactor core. This allows, amongothers things, a safe operating of the reactor, qualifying calculation tools used for theconception of future reactors (such as the JHR or the 4th generation reactors) and makingprogress in fundamental research by improving nuclear data libraries. The result of eachmeasurement is affected by an uncertainty which depends on many factors. Its estimation isa necessity and its reduction is one of the major challenges taken up to by the CEA.Neutrons are not charged particles and are therefore unable to directly ionize the gas of agas filled detector; therefore their detection using this kind of measurement tool requires aconversion reaction, which is, in the case of the fission chamber detector, the induced fissionreaction. The reduction and mastery of the uncertainties affecting the fission chambersmeasurements are the core of the thesis subject. This work was achieved within theExperimental Program Laboratory (LPE) of the Experimental Physics Section (SPEx) at CEACadarache. It is divided into four parts:· the first one consists in a state of the art of fission chambers measurements within theframework of the zero-power experimental reactors. It compiles knowledge aboutmeasurement techniques, technologies and physics used for neutron detection.· the second part study the optimization of two of the key parameters defining thedesign of a fission chamber:o the fissile deposit thickness. The results, obtained thanks to simulation,allowed a better understanding of this parameter’s impact on measurementswhich lead to an improvement of the future detectors design.o the filling gas type and pressure. A deep experimental parametric study wascarried out in the MINERVE facility which enables understanding the impact ofboth filling gas characteristics on results. New filling standards have beendiscovered and are now taken into account when designing new detectors.Those standards allow dividing by two the measurement uncertainties due topressure variations and enable using fission chambers in more variousexperimental setups.· the third part of this works is focused on the improvement of the electronic equipmentand post-treatments used for fission chambers measurements. Three innovativeacquisition devices were chosen for testing in MINERVE. The results obtained enablegiving a set of short term and long term recommendations considering the update ofthe instrumentation used in the SPEx zero power reactors. In addition, a new deadtime correction method was developed during the thesis and is presented in this part.Its positive impact on rod-drop measurement is given for illustration as the gapbetween experimental results and expected values is divided by four thanks to thisinnovative correction method.· the last part is about the optimization of spectral indices measurement. The mostimportant parameters regarding spectral indices assessment are studied, their impacton spectral index is quantified and their respective acquisition methods are optimized.The study was mainly concentrated on the calibration data acquisition. This work ledto significant improvement, most notably concerning the « 238U fission / 235U fission»spectral index measured in the MINERVE core. The gap between calculation andexperimental results has been greatly reduced (from 35.70% to 0.17%) and theassociated uncertainty has also been diminished (from 15.7% to 5.6%). Those resultsalso allowed explaining abnormal gaps between calculation and experimentationobserved in measurement performed in the MINERVE facility in 2004. (...

3-D localization of radioactive hotspots via portable gamma cameras

International audienceA portable stereo gamma camera has been built, based on two hybrid pixelated detectors, each of them comprising a 1 mm thick CdTe substrate bump-bonded to the Timepix readout chip, a pixelated CMOS ASIC consisting of 256 × 256 pixels with 55 µm pitch.The technique described in this paper allows the automatic estimation of the distances between the prototype and the located radioactive sources by using triangulation based on epipolar geometry. The 3-D Cartesian coordinates of the sources are provided even when they are hidden behind an occluding material, such as a wall, or inside a barrel. Furthermore, the combination of the prototype with a depth sensor enables a wide range of applications, including the 3-D volumetric reconstruction of the scene under study as well as the capability of autonomously determining if an object is occluding the located radioactive sources.We discuss here the main features of the whole methodology developed and we also present the experimental results obtained at laboratory scale

Development of a hybrid gamma camera based on Timepix3 for nuclear industry applications

International audienceA prototype of hybrid gamma imager has been developed, based on a single Timpex3 readout chip hybridized with a 1 mm thick cadmium telluride semiconductor. This prototype combines coded-aperture imaging spectrometry and Compton imaging techniques to perform hybrid gamma imaging. It associates and takes advantage of both techniques to locate gamma emitters. The prototype is designed to be portable and compact in order to ease its field use. In this work, we present experimental results obtained with this prototype in coded-aperture imaging spectrometry mode, Compton imaging mode, and hybrid gamma imaging

Evaluation of Timepix3 Si and CdTe hybrid-pixel detectors spectral performances on X and gamma-rays

International audienceThe Timepix3 hybrid-pixel readout chip consists in a matrix composed of 256 × 256 square-shaped pixels with 55 µm pitch, which can be hybridized to several semi-conductors, such as Si or CdTe of thicknesses up to 5 mm. Working as an event based readout chip, it simultaneously records in each pixel the Time-over-Threshold (ToT) that gives access to deposited energy, and the time-of-arrival (ToA) with a time resolution of 1.5 ns. In this paper, we present the energy calibration of Timepix3 bump bonded with a 300 µm thick silicon sensor, and of Timepix3 bump bonded with a 1 mm thick cadmium telluride sensor. Both detectors are calibrated with a so-called per-pixel calibration using the monochromatic SOurce of Low-Energy X-rays (SOLEX from LNHB at CEA Saclay) and a set of laboratory sealed radioactive sources. Evaluation were carried out over an energy range from 6 keV to 122 keV for Timepix3 Si and from 20 keV to 1.3 MeV for Timepix3 CdTe. Results of this calibration allowed determining an energy resolution of 3.97 keV at 59.5 keV for Timepix3 Si, and 5.6 keV, 27.24 keV, and 47.4 keV at respectively 59.5 keV, 661.7 keV and 1.332 MeV for Timepix3 CdTe. It is the first time that CdTe bump-bonded Timepix3 spectral performances are evaluated for gamma-rays above MeV. The reconstruction of the total absorption peak for such high energies is possible by means of the ToA measurement which allows the identification of each interaction (scattering and absorption) of an incoming gamma-ray in the semi-conductor

On-line Fission Products measurements during a PWR severe accident the French DECA-PF project

International audienceFollowing the Fukushima accident, a lot of recommendations was drawn by international organizations (IAEA, OECD, NUGENIA network ) in order to improve the safety in such accidental conditions and mitigate their consequences. One of these recommendations was to improve the robustness of the instrumentation, which was dramatically lacking at Fukushima, as well as to better determine the Source Term involved in nuclear accident.The DECA-PF project (Diagnosis of a degraded reactor core through Fission Product measurements) was elaborated in this context and selected as one of 21 collaborative RandD projects in the field of nuclear safety and radioprotection, funded in May 2013 by the French National Research Agency.Over the months following the Fukushima accident, a CEA crisis team was held in order to analyze on-line the situation taking into account the data delivered by TEPCO and other organizations. Despite the difficulties encountered concerning the reliability of these data, the work performed showed the high capacity of Fission Products (FP) measurements to get a diagnosis relative to the status of the reactors and the spent fuel pools (SFP). Based on these FP measurements, it was possible to conclude that the main origin of the releases was coming from the cores and not from the SFP, in particular for SFP-4 which was of high concern, and that the degradation level of the reactors was very large, including probably an extensive core melting. To improve the reliability of this kind of diagnosis, the necessity to get such measurements as soon as possible after the accident and as near as possible from the reactor was stressed.In this way the present DECA-PF project intends to develop a new and innovative instrumentation taking into account the design of the French nuclear power plants on which sand bed filters have been implemented for severe accident management. Three complementary techniques, devoted to measure the FP release on-line, are being studied Gamma spectrometry, with an industrial objective to build a prototype aimed at improving the capacity of the present radiation monitoring system,Gas chromatography, for the quantification of the fission gases (Xe, Kr) as well as potential carbon oxides produced in case of Molten Corium Concrete Interaction,Optical absorption spectroscopy, the objective of this most innovative technique being to quantify the tetra-oxide of ruthenium, which could be produced in case of lower head failure, and the gaseous forms of iodine (molecular and organic) released in the environment.A global description and the present status of this project is presented, focusing on the Source Term establishment at the outlet stack of the sand bed filters and on the perspectives of implementation of the on-line gamma spectrometry equipment

A versatile calibration procedure for portable coded aperture gamma cameras and RGB-D sensors

International audienceThe present paper proposes a versatile procedure for the geometrical calibration of coded aperture gamma cameras and RGB-D depth sensors, using only one radioactive point source and a simple experimental set-up. Calibration data is then used for accurately aligning radiation images retrieved by means of the -camera with the respective depth images computed with the RGB-D sensor. The system resulting from such a combination is thus able to retrieve, automatically, the distance of radioactive hotspots by means of pixel-wise mapping between gamma and depth images. This procedure is of great interest for a wide number of applications, ranging from precise automatic estimation of the shape and distance of radioactive objects to Augmented Reality systems. Incidentally, the corresponding results validated the choice of a perspective design model for a coded aperture -camera

Reliability improvement for anisotropic biased compensated α/β contamination meter

International audienceNuclear instruments such as alpha/beta contamination meter are frequently used in a compensated mode where the contribution of gamma radiation background is compensated by a guard detector. The signal of interest is then the subtraction of counting from both channels. In practice, the noise signal measured by the guard detector is not strictly equal to the noise contribution into the first detector due to anisotropic biases.The random error (under Poisson assumption) is taken into account to build a hypothesis test. The system is also designed to minimize the systematic error but in some cases, this bias could not be completely removed. The measurement system then shows different behavior when the surrounding environment changes exhibiting inopportune false alarms.A method allowing the false alarms to be suppressed is addressed in this study for compensated measurement. An improvement in terms of reliability has been proven

Evaluation of Timepix3 Si and CdTe Hybrid-Pixel Detectors’ Spectrometric Performances on X- and Gamma-Rays

International audienceThe Timepix3 hybrid-pixel readout chip consists of a matrix made of 256 × 256 square-shaped pixels with a 55-μm pitch, which can be hybridized to several semiconductors, such as silicon (Si) or cadmium telluride (CdTe) with thicknesses up to 5 mm. Working as an event-based readout chip, it simultaneously records the time-over-threshold (ToT) in each pixel, measuring the deposited energy, as well as the time-of-arrival (ToA), with a time resolution of 1.5 ns. In this article, we present the energy calibration of two Timepix3 chips: the first one was bump-bonded with a 300-μm-thick silicon sensor and the second one with a 1-mm-thick CdTe sensor. Both detectors were calibrated with per pixel calibration, using the monochromatic SOurce of Low-Energy X-rays [SOLEX from Henri Becquerel National Laboratory (LNHB) at CEA Saclay] and a set of calibration-grade sealed radioactive sources. Evaluations were carried out over the energy range 6-122 keV for Timepix3 Si and 20 keV- 1.332 MeV for Timepix3 CdTe. Based on this experimental procedure, an energy resolution of 2.6 keV (3.4%) at 59.5 keV was observed for Timepix3 Si. For Timepix3 CdTe, this parameter was 5.6 keV (9.4%) at 59.5 keV, 27.24 keV (4.1%) at 661.7 keV, and 47.4 keV (3.5%) at 1.332 MeV. It is the first time that CdTe bump-bonded Timepix3 spectral performances were evaluated for gamma rays above 1 MeV. The reconstruction of the full-energy peak for such high energies is possible due to the ToA information, which allows the identification of all interactions caused by a given gamma ray within the detector

A panoramic coded aperture gamma camera for radioactive hotspots localization

International audienceA known disadvantage of the coded aperture imaging approach is its limited field-ofview (FOV), which often results insufficient when analysing complex dismantling scenes such as post-accidental scenarios, where multiple measurements are needed to fully characterize the scene. In order to overcome this limitation, a panoramic coded aperture γ-camera prototype hasbeen developed. The system is based on a 1 mm thick CdTe detector directly bump-bonded to a Timepix readout chip, developed by the Medipix2 collaboration (256 × 256 pixels, 55 µm pitch, 14.08 × 14.08 mm2 sensitive area). A MURA pattern coded aperture is used, allowing for background subtraction without the use of heavy shielding. Such system is then combined with a USB color camera. The output of each measurement is a semi-spherical image covering a FOV of 360 degrees horizontally and 80 degrees vertically, rendered in spherical coordinates (θ,φ). The geometrical shapes of the radiation-emitting objects are preserved by first registering and stitching the optical images captured by the prototype, and applying, subsequently, the same transformations to their corresponding radiation images. Panoramic gamma images generated by using the technique proposed in this paper are described and discussed, along with the main experimental results obtained in laboratories campaigns