Flux neutronique | EPFL Graph Search

Un flux neutronique désigne une densité volumique de neutrons ayant la même vitesse, multipliée par cette vitesse : Φ = n • v . Il caractérise l'interactivité de la population des neutrons en déplacement avec les atomes du milieu. Une population de densité n / 2 se déplaçant à la vitesse v aura la même interactivité avec les atomes du milieu qu'une population de densité n allant à la vitesse v / 2. Il se mesure en . L'unité pratique est le neutron par centimètre carré et par seconde, . La probabilité d'interaction d'un neutron varie en fonction de sa vitesse, c'est-à-dire de son énergie. Par exemple, un neutron lent a beaucoup plus de chance de provoquer une réaction de fission nucléaire qu'un neutron rapide. C'est la raison pour laquelle, en neutronique, on s'intéresse à des populations de neutrons ayant la même vitesse c'est-à-dire la même énergie. La probabilité d'interaction des neutrons de vitesse donnée avec les noyaux du milieu est caractérisée par la section efficace. Or

CROCUS Concrete Activation Study by γ-ray Spectrometry and TLD Neutron Flux Measurements

Isabelle Tanseri

The semester project was under the course Physics lab IVb at EPFL. At EPFL site, the experimental fission reactor CROCUS is shielded by concrete which becomes radioactive as a result of neutron interaction. The activation is of interest to study for validation of models and cal- culations, and to quantify the amount of radioactive waste when the reactor will be recycled. In this project, measurements on concrete ac- tivation by NaI- and HPGe-detectors were performed on an in-cavity concrete block, together with TLD measurements on neutron flux in- side the core of CROCUS and in the cavity. The concrete activation and neutron flux allows for comparison and validation of models. The in-cavity concrete block had a low activation, trace elements of 60Co and 65Zn seem to be existing. The relative flux was the highest for structures close to the core and at the ceiling. In future projects, more experimental data on both concrete activation and neutron flux would be performed to acquire uncertainty estimations and correction factors.

2017

Extension of miniature neutron scintillators features in CROCUS: novel acquisition scheme and extended flux range

Colin Rochat

The goal of this work was the improvement of the capabilities of a miniature neutron detector, for easing its use, and extending its range of applicability. Firstly, the Red Pitaya STEMlab 125-14 was the object of an exploratory study in order to propose an alternative method to the two current acquisition electronics based on photon counting, using analog and digital means. The study allowed to establish a new methodology based on the direct use of the SiPM pre-amplifier output, requiring the hypothesis that neutron events can be discriminated directly by pulse amplitude. It was first tested in the low neutron flux of the CARROUSEL facility, and the results were compared to the analog electronics. The agreement in count rate, spectrum, and linearity demonstrates the validity of this new methodology. Moreover, its application in CROCUS allowed demonstrating that it is working in reactor condition up to a flux of 1.13∗108 s−1cm−2. Secondly, an array of six miniature neutron detectors with different sensitivities was designed and tested. The qualification study in CARROUSEL allowed to show that all detectors behave normally, and that the sensitivity of the detectors increases as a function of the scintillator’s volume at low neutron flux, as expected. The conclusion is limited by the knowledge in effective dimensions of each scintillator. In a first experiment in CROCUS using the same setup, including digital threshold (50), a loss of linearity was observed simultaneously for all detectors at a power ofabout 4W(i.e. 7∗107s−1cm−2). As the cause was expected to be a higher gamma background, the experiment was repeated with a higher threshold (120): consistently with the CARROUSEL test, count rate and loss of linearity depends on the scintillation’s volume. Further studies are needed to conclude on the phenomenon at hand, but since the main parameter for gamma discrimination proved efficient, the gamma contribution increased at higher powers seems a good candidate. In conclusion, the results are globally satisfying for the new methodology with Red Pitaya STEMlab 125-14. However, the experimental campaign of this work was only a feasibility study. The results have to be confirmed with further studies. Moreover, this system could be improved. In particular, it can be adapted firstly for both RF inputs of Red Pitaya and secondly for dynamical measurements. The results of the multi-sized array of miniature neutron detector were also encouraging. However, the behaviour of certain detectors prevent to deduce with certainty some expected trends. Further studies have to be carried out by multiplying the number of detectors of each dimension.

2021

Characterisation of the activation in Crocus

The aim of this study is to characterize the activation of the crocus cavity. For this purpose, the concrete plug of the irradiation channel crossing the cavity shielding was used to measure the activation by gamma spectrometry. In a first time measurements were made after a long cooling time to determine the residual activation. Then, the plug was irradiated at 40Watts for 2 hours to provide a sufficient irradiation to the concrete. At the same time, measurements were led thanks to the dosimetry by thermoluminescence in the cavity. TLDs measurements gave reaction rates in Li6 at strategical positions detailed on this report. TLDs positions were defined in a Cartesian system centred on the centre core. 2D plans based on previous plan were designed to improve the calculation accuracy of the neutrons flux simulations. The analysis shown a very low activity of the concrete plug, below the low level waste thresholds. However results provided by TLDs and the Nagra’s calculations allowed to highlight a greater neutrons fluence in the upper part of the cavity. So, it might be relevant to pursue the gamma ray spectrometry measurements with concrete plugs from other cavity areas where the neutron flux should be higher.

2018

Extension of miniature neutron scintillators features in CROCUS: novel acquisition scheme and extended flux range

Colin Rochat

2021