High Resolution Modelling without Computation Slowdown for PETALE in CROCUS

International audienceThe PETALE experimental program was successfully carried out in the CROCUS reactor of EPFL in the fall of 2020, in a collaboration between EPFL and CEA. It consists of criticality and transmission experiments with four distinct metallic reflectors made of 304L stainless steel, iron, nickel, and chromium, to study the nuclear data of stainless steel. In addition to the experimental results, one of its outcomes is the production of C/E and their covariance matrices for the reaction rates of the dosimeters used in the transmission experiments, with the aim of allowing to constrain future nuclear data evaluations. For this purpose, the chosen methodology for uncertainty propagation is the computationally expensive Total Monte-Carlo method. A modified build of the Serpent2 Monte Carlo transport code is used to perform Variance Reduction and correlated sampling. In addition, high resolution modelling of the experiments is used to limit the presence of bias in the analysis, and to pave the way toward the submission of a high-quality benchmark to an NEA database. This paper presents advances in the modelling of the experiments, focusing on the metal reflectors, as installed at the periphery of CROCUS. It was possible to refine from a simple design model made of a few cuboids, to a fully detailed model, while keeping the loss in computational efficiency below 15%. Notably, all reflector sheets of PETALE were detailed into 121 voxels each, based on topological measurements, without impact on the calculation time, thanks to the use of Serpent2 3D lattices

Study of scandium targets for production of monoenergetic neutron fields with energies below 100 keV

In order to develop reference low-energy monoenergetic neutron fields, the 45Sc(p,n) reaction is being studied within the framework of a scientific cooperation between NPL, PTB, IRMM and IRSN. The first study is dedicated to the selection of the most suitable backing material for scandium targets. It must be able to sustain high proton beam currents to compensate for the low cross section of the 45Sc(p,n) reaction. Targets with backings made of Mo, Al, W, Ag, Pt and Ta were irradiated during several hours at a few tens of μA at the NPL neutron reference facility. Target thickness and composition were analysed with the RBS method at the AIFIRA facility before and after NPL irradiations leading to the selection of tantalum as the best choice for backing material

High Resolution Modelling without Computation Slowdown for PETALE in CROCUS

International audienceThe PETALE experimental program was successfully carried out in the CROCUS reactor of EPFL in the fall of 2020, in a collaboration between EPFL and CEA. It consists of criticality and transmission experiments with four distinct metallic reflectors made of 304L stainless steel, iron, nickel, and chromium, to study the nuclear data of stainless steel. In addition to the experimental results, one of its outcomes is the production of C/E and their covariance matrices for the reaction rates of the dosimeters used in the transmission experiments, with the aim of allowing to constrain future nuclear data evaluations. For this purpose, the chosen methodology for uncertainty propagation is the computationally expensive Total Monte-Carlo method. A modified build of the Serpent2 Monte Carlo transport code is used to perform Variance Reduction and correlated sampling. In addition, high resolution modelling of the experiments is used to limit the presence of bias in the analysis, and to pave the way toward the submission of a high-quality benchmark to an NEA database. This paper presents advances in the modelling of the experiments, focusing on the metal reflectors, as installed at the periphery of CROCUS. It was possible to refine from a simple design model made of a few cuboids, to a fully detailed model, while keeping the loss in computational efficiency below 15%. Notably, all reflector sheets of PETALE were detailed into 121 voxels each, based on topological measurements, without impact on the calculation time, thanks to the use of Serpent2 3D lattices

High Resolution Modelling without Computation Slowdown for PETALE in CROCUS

International audienceThe PETALE experimental program was successfully carried out in the CROCUS reactor of EPFL in the fall of 2020, in a collaboration between EPFL and CEA. It consists of criticality and transmission experiments with four distinct metallic reflectors made of 304L stainless steel, iron, nickel, and chromium, to study the nuclear data of stainless steel. In addition to the experimental results, one of its outcomes is the production of C/E and their covariance matrices for the reaction rates of the dosimeters used in the transmission experiments, with the aim of allowing to constrain future nuclear data evaluations. For this purpose, the chosen methodology for uncertainty propagation is the computationally expensive Total Monte-Carlo method. A modified build of the Serpent2 Monte Carlo transport code is used to perform Variance Reduction and correlated sampling. In addition, high resolution modelling of the experiments is used to limit the presence of bias in the analysis, and to pave the way toward the submission of a high-quality benchmark to an NEA database. This paper presents advances in the modelling of the experiments, focusing on the metal reflectors, as installed at the periphery of CROCUS. It was possible to refine from a simple design model made of a few cuboids, to a fully detailed model, while keeping the loss in computational efficiency below 15%. Notably, all reflector sheets of PETALE were detailed into 121 voxels each, based on topological measurements, without impact on the calculation time, thanks to the use of Serpent2 3D lattices

Experimental study of columnar recombination in fission chambers

International audienceIn this paper, we present experimental saturation curves of a small gap miniature fission chamber obtained in the MINERVE reactor. The chamber is filled with argon at various pressures, and the fissile material can be coated on the anode, cathode, or both. For analyzing the recombination regime, we consider a model of columnar recombination and discuss its applicability to our chamber. By applying this model to the data, it is possible to estimate the ratio between the recombination coefficient k and an effective column radius b, appearing in the model. From these results, a routine measurement of the recombination regime is proposed in order to detect gas leakage. This online diagnosis would be beneficial in terms of lifetime and reliability of the neutron instrumentation of nuclear reactors

Dosimetry modeling and experimental validation for the PETALE program in the CROCUS reactor

The PETALE experimental program in the CROCUS reactor intends to provide integral measurements on reactivity worth and dosimetry measurement to constrain nuclear data relative to stainless steel heavy reflectors. The experimental setup consists in eight successive plates of pure iron, pure nickel, pure chromium, or nuclear-grade stainless steel set at the close periphery of the core. The plates are interleaved with up to nine dosimeters that consist of thin activation foils with different possible materials to be sensitive to different ranges of the neutron spectrum. A precise measurement with a good estimation of the uncertainties and correlations is required, especially when comparing reaction rates, e.g. transmission measurement and/or spectral indices

In-core dosimetry for the validation of neutron spectra in the CROCUS reactor

The present article describes the preliminary validation study of simulated in-core and reflector n eutron spectra in preparation of oncoming experimental programs in the zeropower reactor CROCUS at EPFL. For this purpose, a set of activation foils were irradiated at three characteristic positions in the CROCUS reactor, and the subsequent activities were analyzed via γ spectrometry. The experimental setup was then modeled with the Monte Carlo neutron transport code Serpent2 and associated with an analysis tool to include the effect of the reactor power history during experiments. The comparison of calculated and measured reaction rates (C/E) indicates a general consistency (at 2σ) between calculated and measured spectra. However, offsets of C/E values were observed in (n, γ) reactions, up to 18% for 115In and 8% for 63Cu dosimeters. This could be caused by an unexpected isotopic composition, uncertainties in nuclear data, or the spectrometry analysis. In addition, a 100-groups spectrum unfolding was performed using the experimentally determined reaction rates and the Serpent2 spectra as the prior knowledge. The unfolded spectra were mainly adjusted in the thermal and fast ranges, while few modifications w ere m ade i n t he e pithermal r egion d ue t o the low contribution of epithermal neutrons in activation processes. Moreover, within energy groups where the capture reactions show resonant behavior, flux depletion (up to 38% as compared to the prior spectra) is observed due to the absence of self-shielding effect in the unfolding process. For this purpose, an unfolding method based on energy groups weighting is developed and tested

In-core dosimetry for the validation of neutron spectra in the CROCUS reactor

The present article describes the preliminary validation study of simulated in-core and reflector n eutron spectra in preparation of oncoming experimental programs in the zeropower reactor CROCUS at EPFL. For this purpose, a set of activation foils were irradiated at three characteristic positions in the CROCUS reactor, and the subsequent activities were analyzed via γ spectrometry. The experimental setup was then modeled with the Monte Carlo neutron transport code Serpent2 and associated with an analysis tool to include the effect of the reactor power history during experiments. The comparison of calculated and measured reaction rates (C/E) indicates a general consistency (at 2σ) between calculated and measured spectra. However, offsets of C/E values were observed in (n, γ) reactions, up to 18% for 115In and 8% for 63Cu dosimeters. This could be caused by an unexpected isotopic composition, uncertainties in nuclear data, or the spectrometry analysis. In addition, a 100-groups spectrum unfolding was performed using the experimentally determined reaction rates and the Serpent2 spectra as the prior knowledge. The unfolded spectra were mainly adjusted in the thermal and fast ranges, while few modifications w ere m ade i n t he e pithermal r egion d ue t o the low contribution of epithermal neutrons in activation processes. Moreover, within energy groups where the capture reactions show resonant behavior, flux depletion (up to 38% as compared to the prior spectra) is observed due to the absence of self-shielding effect in the unfolding process. For this purpose, an unfolding method based on energy groups weighting is developed and tested

Kinetic Parameter Measurements in the CROCUS Reactor Using Current Mode Instrumentation

This paper is an overview of developments and results regarding neutron noise measurements in current mode at the CROCUS zero power facility. Neutron noise measurements offer a noninvasive method to determine kinetic reactor parameters such as the prompt decay constant at criticality alpha = beta(eff)/Lambda, the effective delayed neutron fraction beta(eff), and the mean generation time Lambda for nuclear databases and code validation efforts. A newly developed current mode detection system using two fission chambers in the core reflector was used to accurately measure the kinetic parameters of CROCUS. Characteristics of the system and the performed noise measurements are presented. We implemented noise postprocessing of the current signals acquired at 900 mW using the autopower spectral density/cross-power spectral density (CPSD) method. The final CPSD estimated values were found to be alpha = (153.3 +/- 8.6) s(-1), beta(eff) = (778 +/- 14) pcm, and Lambda = (50.7 +/- 2.9) mu s. In order to comply with experimental uncertainty targets (<3%), the measurement time was 6 h. In comparison with code predictions using MCNP5-v1.6 employing the JEFF 3.1 and ENDF/B-7.1 cross-sectional libraries, we found differences up to 3%, confirming the need for accurate kinetic parameter measurements. In the future, the new measurement system will also be used for studies of spatial and external reactor noise