Miniature and Minimalistic Neutron Detectors for Online High-Resolution Experiments in the Zero-Power Reactor CROCUS

The continuous strive for more efficient and reliable nuclear power plants is leading to increasing complexity in the design and operations of reactors. As a consequence, comprehensive analyses are required to assess the reactor's safety during normal and accidental conditions. Validated simulation tools with high-fidelity capabilities, i.e., at the pin or sub-pin scale, are necessary to predict local neutronics effects in the core. However, the scarcity of high-resolution in-core experimental data imposes limitations to the validation of such high-fidelity pin-resolved neutronics codes. The challenges in performing online high-resolution in-core experiments are numerous: from the accessibility of in-core locations to the availability of neutron detection technologies with adequate dimensions and online capabilities. In the present thesis, the development of a novel miniature and minimalistic (MiMi) neutron detection technology is presented, allowing unprecedented spatial resolution for the online study of the neutron flux in zero-power research reactors. The MiMi neutron detectors are tested in the EPFL zero-power reactor CROCUS and used to build a data set of high-resolution neutronics experiments in CROCUS, including measurements of local gradients and directionality of neutron flux, and the local impact of a fuel rod displacement. As the next level of development, a three-dimensional (3D) full-core mapping system named SAFFRON, consisting of 149 MiMi neutron detectors distributed in-core, is designed and installed in CROCUS. Static thermal neutron flux maps are measured in absolute terms and between different core configurations, e.g., water level vs. control rod operation at criticality. The obtained results open up the investigation of a variety of space-dependent neutronics phenomena in CROCUS.

A mm(3) Fiber-Coupled Scintillator for In-Core Thermal Neutron Detection in CROCUS

Pavel Frajtag, Vincent Pierre Lamirand, Jean-Baptiste Mosset, Oskari Ville Pakari, Andreas Pautz, Fanny Vitullo

This article presents the development, full-characterization, and in-core testing of a miniature neutron detector for the investigation of highly localized in-core neutronics effects in the zero-power reactor CROCUS, operated at the Ecole Polytechnique federale de Lausanne (EPFL), Switzerland. A ZnS:(LiF)-Li-6 screen, mixing inorganic scintillator and neutron converter, with a surface of 1 mm(2) and a thickness of 0.2 mm, was coupled with a silicon photomultiplier (SiPM) via a 10-m optical fiber. Analog readout electronics were adopted in this prototype version to process the SiPM output signal. A first campaign was carried out to assess the detection system capabilities in terms of neutron detection and counting in mixed neutron-gamma radiation fields. The miniature detector was thoroughly characterized with a Pu-Be source installed in the CARROUSEL facility and was subsequently tested inside a control rod guide tube of the CROCUS reactor in different reactor conditions, i.e., at shutdown, startup, and critical. The detector shows a linear response with a reactor power increase up to 6.5 W (i.e., around 108 cm(-2) . s(-1) total neutron flux) and excellent neutron counting capabilities, when compared to other localized detection systems available in CROCUS, such as the miniature fission chambers and single-crystal vapor deposited (sCVD) diamond detectors. In addition, the exposure of the miniature detector to a Co-60 source with an activity of similar to 250 GBq, combined with the computation of the gamma flux in CROCUS with the Serpent 2 Monte Carlo code, confirmed the gamma insensitivity of the system in a mixed neutron-gamma field.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2020

Analysis of the first COLIBRI fuel rods oscillation campaign in the CROCUS reactor for the European project CORTEX

Carlo Fiorina, Pavel Frajtag, Daniel Godat, Mathieu Hursin, Vincent Pierre Lamirand, Axel Guy Marie Laureau, Oskari Ville Pakari, Andreas Pautz, Adolfo Rais

The Horizon2020 European project CORTEX aims at developing an innovative core monitoring technique that allows detecting anomalies in nuclear reactors, such as excessive vibrations of core internals, flow blockage, or coolant inlet perturbations. The technique will be mainly based on using the fluctuations in neutron flux recorded by in-core and ex-core instrumentation, from which the anomalies will be differentiated depending on their type, location and characteristics. The project will result in a deepened understanding of the physical processes involved, allowing utilities to detect operational problems at a very early stage. In this framework, neutron noise computational methods and models are developed. In parallel, mechanical noise experimental campaigns are carried out in two zero-power reactors: AKR-2 and CROCUS. The aim is to produce high quality neutron noise-specific experimental data for the validation of the models. In CROCUS, the COLIBRI experimental program was developed to investigate experimentally the radiation noise induced by fuel rods vibrations. In this way, the 2018 first CORTEX campaign in CROCUS consisted in experiments with a perturbation induced by a fuel rods oscillator. Eighteen fuel rods located at the periphery of the core fuel lattice were oscillated between ±0.5 mm and ±2.0 mm around their central position at a frequency ranging from 0.1 Hz to 2 Hz. Signals from 11 neutron detectors which were set at positions in-core and ex-core in the water reflector, were recorded. The present article documents the results in noise level of the experimental campaign. Neutron noise levels are compared for several oscillation frequencies and amplitudes, and at the various detector locations concluding to the observation of a spatial dependency of the noise in amplitude.

2021

Analysis of the first COLIBRI fuel rods oscillation campaign in the CROCUS reactor for the European project CORTEX

Carlo Fiorina, Pavel Frajtag, Daniel Godat, Mathieu Hursin, Vincent Pierre Lamirand, Axel Guy Marie Laureau, Oskari Ville Pakari, Andreas Pautz, Adolfo Rais

2021