Energy measurement of fast neutron fields with a Recoil Proton Telescope using active pixel sensors

The spectrometer ATHENA (Accurate Telescope for High Energy Neutron metrology Applications), in development at the LNE-IRSN, characterizes energy and fluence of fast neutron fields. The detector is a Recoil Proton Telescope and measures neutron energies in the range of 5 to 20 MeV. The system is intended to become a primary standard for both energy and fluence measurements. The most innovative part of ATHENA is made of three CMOS pixel sensors, thinned down to 50 microns thickness, allowing an accurate tracking of the recoil. The use of CMOS sensors and a thick silicon diode increase the intrinsic efficiency of the detector by a factor of ten compared with conventional designs. In this paper, we demonstrate the ability of the detector to measure neutron energies in the range of 5 to 20 MeV. Experimental investigations, using mono-energetic neutron fields produced by the AMANDE facility, indicate a good reconstruction of neutron energies. The present design is still under development. The accuracy of measuring 5 MeV and 14 MeV neutron energies was 15% and 5%, respectively

A spectrometer using active pixels sensors for the metrology of neutron fields

La métrologie fondamentale est la garante de la pérennité des systèmes de mesure et est en charge de fournir les étalons de références. En ce qui concerne la métrologie des rayonnements ionisants et, en particulier la métrologie des neutrons, des détecteurs étalons sont utilisés pour caractériser les champs de références, en énergie et en fluence. Les dosimètres ou détecteurs de particules sont étalonnés. Cette thèse présente le développement d’un spectromètre neutron candidat au statut d’étalon primaire pour la caractérisation de champs neutroniques dans la gamme 5-20 MeV. Le spectromètre utilise le principe du télescope à protons de recul comme moyen de détection ; la technologie CMOS, au travers de trois capteurs de positions, est mise à profit pour réaliser la trajectographie du proton de recul. Un détecteur Si(Li) est en charge de la mesure de l’énergie résiduelle du proton. Les simulations des dispositifs, réalisées sous MCNPX, ont permis d’estimer les performances du dispositif et de valider la procédure de reconstruction de l’énergie des champs neutroniques. Une étape essentielle de caractérisation des éléments du télescope et en particulier des capteurs CMOS est également proposée afin de garantir la validité de mesures expérimentales postérieures. Les tests réalisés aussi bien en champs mono-énergétiques qu’en champs étendus témoignent des très bonnes performances du système. La quantification des incertitudes indiquent une mesure de l’énergie avec une précision de plus de 1.5 % pour une résolution de moins de 6 %. La mesure de la fluence neutronique est quand a elle réalisée avec une incertitude de 4 à 6 %.The fundamental metrology is responsible for the sustainability of the measurement systems and handles to supply the reference standards. Concerning the metrology of ionizing radiations and, in particular the neutron metrology, detectors standards are used to characterize reference fields, in terms of energy and fluence. The dosimeters or particle detectors are calibrated on these reference fields. This thesis presents the development of a neutron spectrometer neutron candidate to the status of primary standard for the characterization of neutron fields in the range from 5 to 20 MeV. The spectrometer uses the recoil proton telescope as detection principle; the CMOS technology, through three sensor positions, is taking advantage to realize the tracking of protons. A Si(Li) detector handles the measure of the residual proton energy. The device simulations, realized under MCNPX, allow to estimate its performances and to validate the neutron energy reconstruction. An essential step of characterization of the telescope elements and in particular of CMOS sensors is also proposed to guarantee the validity of posterior experimental measurements. The tests realized as well in mono-energy fields as in radionuclide source show the very good performances of the system. The quantification of uncertainties indicates an energy estimation with 1.5 % accuracy and a resolution of less than 6 %. The fluence measurement is performed with an uncertainty about 4 to 6%

A spectrometer using active pixels sensors for the metrology of neutron fields

La métrologie fondamentale est la garante de la pérennité des systèmes de mesure et est en charge de fournir les étalons de références. En ce qui concerne la métrologie des rayonnements ionisants et, en particulier la métrologie des neutrons, des détecteurs étalons sont utilisés pour caractériser les champs de références, en énergie et en fluence. Les dosimètres ou détecteurs de particules sont étalonnés. Cette thèse présente le développement d’un spectromètre neutron candidat au statut d’étalon primaire pour la caractérisation de champs neutroniques dans la gamme 5-20 MeV. Le spectromètre utilise le principe du télescope à protons de recul comme moyen de détection ; la technologie CMOS, au travers de trois capteurs de positions, est mise à profit pour réaliser la trajectographie du proton de recul. Un détecteur Si(Li) est en charge de la mesure de l’énergie résiduelle du proton. Les simulations des dispositifs, réalisées sous MCNPX, ont permis d’estimer les performances du dispositif et de valider la procédure de reconstruction de l’énergie des champs neutroniques. Une étape essentielle de caractérisation des éléments du télescope et en particulier des capteurs CMOS est également proposée afin de garantir la validité de mesures expérimentales postérieures. Les tests réalisés aussi bien en champs mono-énergétiques qu’en champs étendus témoignent des très bonnes performances du système. La quantification des incertitudes indiquent une mesure de l’énergie avec une précision de plus de 1.5 % pour une résolution de moins de 6 %. La mesure de la fluence neutronique est quand a elle réalisée avec une incertitude de 4 à 6 %.The fundamental metrology is responsible for the sustainability of the measurement systems and handles to supply the reference standards. Concerning the metrology of ionizing radiations and, in particular the neutron metrology, detectors standards are used to characterize reference fields, in terms of energy and fluence. The dosimeters or particle detectors are calibrated on these reference fields. This thesis presents the development of a neutron spectrometer neutron candidate to the status of primary standard for the characterization of neutron fields in the range from 5 to 20 MeV. The spectrometer uses the recoil proton telescope as detection principle; the CMOS technology, through three sensor positions, is taking advantage to realize the tracking of protons. A Si(Li) detector handles the measure of the residual proton energy. The device simulations, realized under MCNPX, allow to estimate its performances and to validate the neutron energy reconstruction. An essential step of characterization of the telescope elements and in particular of CMOS sensors is also proposed to guarantee the validity of posterior experimental measurements. The tests realized as well in mono-energy fields as in radionuclide source show the very good performances of the system. The quantification of uncertainties indicates an energy estimation with 1.5 % accuracy and a resolution of less than 6 %. The fluence measurement is performed with an uncertainty about 4 to 6%

Un spectromètre à pixels actifs pour la métrologie des champs neutroniques

The fundamental metrology is responsible for the sustainability of the measurement systems and handles to supply the reference standards. Concerning the metrology of ionizing radiations and, in particular the neutron metrology, detectors standards are used to characterize reference fields, in terms of energy and fluence. The dosimeters or particle detectors are calibrated on these reference fields. This thesis presents the development of a neutron spectrometer neutron candidate to the status of primary standard for the characterization of neutron fields in the range from 5 to 20 MeV. The spectrometer uses the recoil proton telescope as detection principle; the CMOS technology, through three sensor positions, is taking advantage to realize the tracking of protons. A Si(Li) detector handles the measure of the residual proton energy. The device simulations, realized under MCNPX, allow to estimate its performances and to validate the neutron energy reconstruction. An essential step of characterization of the telescope elements and in particular of CMOS sensors is also proposed to guarantee the validity of posterior experimental measurements. The tests realized as well in mono-energy fields as in radionuclide source show the very good performances of the system. The quantification of uncertainties indicates an energy estimation with 1.5 % accuracy and a resolution of less than 6 %. The fluence measurement is performed with an uncertainty about 4 to 6%.La métrologie fondamentale est la garante de la pérennité des systèmes de mesure et est en charge de fournir les étalons de références. En ce qui concerne la métrologie des rayonnements ionisants et, en particulier la métrologie des neutrons, des détecteurs étalons sont utilisés pour caractériser les champs de références, en énergie et en fluence. Les dosimètres ou détecteurs de particules sont étalonnés. Cette thèse présente le développement d’un spectromètre neutron candidat au statut d’étalon primaire pour la caractérisation de champs neutroniques dans la gamme 5-20 MeV. Le spectromètre utilise le principe du télescope à protons de recul comme moyen de détection ; la technologie CMOS, au travers de trois capteurs de positions, est mise à profit pour réaliser la trajectographie du proton de recul. Un détecteur Si(Li) est en charge de la mesure de l’énergie résiduelle du proton. Les simulations des dispositifs, réalisées sous MCNPX, ont permis d’estimer les performances du dispositif et de valider la procédure de reconstruction de l’énergie des champs neutroniques. Une étape essentielle de caractérisation des éléments du télescope et en particulier des capteurs CMOS est également proposée afin de garantir la validité de mesures expérimentales postérieures. Les tests réalisés aussi bien en champs mono-énergétiques qu’en champs étendus témoignent des très bonnes performances du système. La quantification des incertitudes indiquent une mesure de l’énergie avec une précision de plus de 1.5 % pour une résolution de moins de 6 %. La mesure de la fluence neutronique est quand a elle réalisée avec une incertitude de 4 à 6 %

PIN POWER CALCULATION SCHEME FOR REACTOR PRESSURE VESSEL FAST NEUTRON FLUENCE ESTIMATION

The estimation of the neutron fluence at the Reactor Pressure Vessel (RPV) is classically carried out by a two-step approach. The first step is to estimate the full core neutron source term whether the second step of the calculation consists in the transport of neutrons from the core (source term) to the RPV using the neutron fission distribution determined in the previous step. For this purpose, the neutron fission distribution is to be accurately determined at the fuel pin level for the assemblies on the border of the core. To achieve this goal, two methods are evaluated in this study. The first method considered is a full core 2D Monte Carlo calculation using the MNCP6 code. The second method is based on a deterministic approach using the CASMO5 multi-segment option, allowing a full 2D transport calculation at the pin level with an expected accuracy similar to a stochastic method. The comparison of the two methods shows an overall good agreement with differences within the statistical uncertainty for different cores: homogeneous UOX core, mixed UOX-MOX loading and the effect of the hafnium rods used in the assemblies in the periphery of the core. The modelling limitation and the associated calculational time are discussed for the comparison of the two approaches

PIN POWER CALCULATION SCHEME FOR REACTOR PRESSURE VESSEL FAST NEUTRON FLUENCE ESTIMATION

The estimation of the neutron fluence at the Reactor Pressure Vessel (RPV) is classically carried out by a two-step approach. The first step is to estimate the full core neutron source term whether the second step of the calculation consists in the transport of neutrons from the core (source term) to the RPV using the neutron fission distribution determined in the previous step. For this purpose, the neutron fission distribution is to be accurately determined at the fuel pin level for the assemblies on the border of the core. To achieve this goal, two methods are evaluated in this study. The first method considered is a full core 2D Monte Carlo calculation using the MNCP6 code. The second method is based on a deterministic approach using the CASMO5 multi-segment option, allowing a full 2D transport calculation at the pin level with an expected accuracy similar to a stochastic method. The comparison of the two methods shows an overall good agreement with differences within the statistical uncertainty for different cores: homogeneous UOX core, mixed UOX-MOX loading and the effect of the hafnium rods used in the assemblies in the periphery of the core. The modelling limitation and the associated calculational time are discussed for the comparison of the two approaches

Impact of core power variations on the fast neutron flux incident on pressurized water reactor vessels

International audienceAs many French nuclear 900 MWe Pressurized Water Reactors (PWRs) approach the end of their design lifetime (40 years), it is of great importance, in the perspective of a possible extension of operation, to be able to accurately characterize the structural integrity of the reactor pressure vessel for safety reasons. Therefore, the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) consider the possibility to develop a best-estimate calculation scheme for the fast neutron fluence (neutrons with energies greater than 1 MeV) at the vessel of PWRs, which is used to predict the material embrittlement. However, before building this type of model, one prior key point is to determine which parameters need to be considered. The aim of this paper is to assess the need to take into account the power history of the reactor core and the induced variation of its operational parameters (core power, boron concentration and control rods insertion) in order to perform fast neutron fluence calculations. The analysis has been performed using a calculation scheme coupling a deterministic approach (CASMO5 and PARCS codes) to evaluate the core fission distribution and a Monte-Carlo modeling (MCNP6 code) to estimate the attenuation of the neutrons source from the core to the vessel

APPLICATION OF THE SPH EQUIVALENCE TECHNIQUE TO THE CABRI FULL CORE IN NON-FUNDAMENTAL MODE

We developed a SPH equivalence technique in non-fundamental mode condition between a CABRI full-core model solved with the method of characteristics (MOC) in 2D and a simplified full-core model solved with the simplified P3 (SP3) method, linear anisotropic sources and discretized with Raviart-Thomas finite elements over a pure Cartesian mesh. The MOC and SP3 calculations are performed with DRAGON5 and DONJON5 codes, respectively. A three-parameter database is generated by DRAGON5 and is interpolated in DONJON5 as a function of the core condition. An objective function is set as the root mean square (RMS) error (MOC-SP3 discrepancy) on absorption distribution and leakage rates defined over the macro-geometry in DONJON5. Our algorithm is a quasi-Newtonian gradient search based on the Limited memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) method. Numerical results are presented with Hafnium bars withdrawn or inserted

Sensitivity of Pressurized Water Reactors Vessels Aging to Fuel Composition

International audienceThe operation of many French nuclear 900 MWe Pressurized Water Reactors (PWR) is being extended beyond their design lifetime. In the perspective of further extension, the evaluation of reactor pressure vessels (RPV) aging is of great importance. The embrittlement of the vessel is mainly induced by fast neutron irradiation (neutrons with energies above 1 MeV). Consequently, the fast neutron fluence is one of the quantities used by safety authorities to characterize the structural integrity of the RPV. Estimates of this quantity are based on evaluations of the fast neutron flux that can be performed by a two-step approach. The first step estimates the full core fission neutron source term and the second step models the transport of neutrons from the core to the RPV using the fission neutron distribution determined in the previous step. Such fast neutron flux evaluations are subject to bias and uncertainties, which must be precisely analyzed in order to enhance nuclear safety. Especially, a lack of knowledge in fuel composition can have a significant impact on the quantification of the fast neutron flux, since it may impact both steps of the calculation. Therefore, this study proposes to discuss the sensitivity of fast neutron flux assessments to perturbations of nuclear fuel composition. The analysis has been performed using a deterministic code (CASMO5) to compute the fission neutron source term and a Monte-Carlo model (MCNP6) to compute the neutron attenuation