Measurement of the Gas Velocity in a Water-Air Mixture in CROCUS Using Neutron Noise Techniques

VALIDATION OF AXIAL VOID PROFILE MEASURED BY NEUTRON NOISE TECHNIQUES IN CROCUS

Recently a joint project has been carried out between the Paul Scherrer Institut, the Ecole Polytechnique Federale de Lausanne and swissnuclear, an industrial partner, in order to determine the axial void distribution in a channel installed in the reflector of the zero power research reactor CROCUS, using neutron noise techniques. The main objective of the present paper is to report on the validation of the results against an alternative measurement technique using gamma-ray attenuation and simulations with the TRACE code. For the gamma-ray attenuation experiments, the channel used in CROCUS is installed out of the core in a Plexiglass water tank. The source and detector are fixed and the channel is moved axially to keep the geometry of the source/detector arrangement untouched. This is key to measure the void effect by gamma attenuation due to the low contrast of this technique. The paper compares the experimental results obtained with both techniques, with the outcomes of simulations carried out with the TRACE code. Even though the quantitative void fraction estimations are not consistent, the trends obtained with the simulation and experimental techniques are the same. The discrepancies between the various experimental techniques and the simulation outcomes are related to the heterogeneous distribution of the water-air mixture in the radial sections of the channel.

Local and high distance neutron and gamma measurements of fuel rods oscillation experiments

We report in the present article on the successful observation using noise analysis of the lateral oscillation of one fuel rod by ±2.5 mm around nominal at 0.1 Hz frequency, using an mm3 miniature neutron scintillator at the rod level, and a BGO gamma detector seven meters away from the reactor core center. The experiment was conducted as part of the COLIBRI program in the CROCUS reactor, which is dedicated to the investigation of reactor noise induced by fuel vibrations. It consists in experiments on rod lateral displacement (static) and oscillation (dynamic) with different rods’ numbers at various relevant amplitudes and frequencies. Its main motivation is the increased amplitudes in the neutron noise distributions recorded in ex- and in-core detectors that have been observed in recent years in Siemens pre-Konvoi type of PWR reactors. The obtained experimental data are used for the purpose of code validation, especially within the framework of the European project CORTEX on reactor noise applications. During the first phase of COLIBRI, the observation of a spatial dependence of the perturbation noise, also called neutron modulation, was demonstrated. In the second phase of COLIBRI starting 2021, it is planned to use a core mapping array of neutron detectors to record its propagation. It consists in about 150 miniature scintillators coupled to optical fibers and SiPM readouts, to be distributed in the reactor core. As a feasibility test, experiments were performed using a miniature scintillator prototype placed on a fuel rod, and oscillating the instrumented rod or the one directly adjacent to the detector. In addition, it is theoretically possible to measure branching or perturbation reactor noise using gamma radiation. Following recent developments on gamma measurements in CROCUS, the fuel oscillation was simultaneously recorded with a gamma detection array, LEAF. Its large BGO detectors were used by placing them at the maximum distance to the core, i.e. seven meters away with a clear line of sight using an experimental channel through the shielding of the reactor cavity.

ANALYSIS OF THE FIRST COLIBRI FUEL RODS OSCILLATION CAMPAIGN IN THE CROCUS REACTOR FOR THE EUROPEAN PROJECT CORTEX

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.

Neutron noise experiments in the AKR-2 and CROCUS reactors for the European project CORTEX

The present article gives an overview of the first experimental campaigns carried out in the AKR-2 and CROCUS reactors within the framework of the Horizon 2020 European project CORTEX. CORTEX aims at developing innovative core monitoring techniques that allow detecting anomalies in nuclear reactors, e.g. excessive vibrations of core internals. The technique will be mainly based on using the fluctuations in neutron flux, i.e. noise analysis. The project will result in a deepened understanding of the physical processes involved. This will allow utilities to detect operational problems at a very early stage, and to take proper actions before such problems have any adverse effect on plant safety and reliability. The purpose of the experimental campaigns in the AKR-2 and CROCUS reactors is to produce noise-specific experimental data for the validation of the neutron noise computational models developed within this framework. The first campaigns at both facilities consisted in measurements at reference static states, and with the addition of mechanical perturbations. In the AKR-2 reactor, perturbations were induced by two devices: a rotating absorber and a vibrating absorber, both sets in experimental channels close to the core. In CROCUS, the project benefited from the COLIBRI experimental program: 18 periphery fuel rods were oscillated at a maximum of ±2 mm around their central position in the Hz range. The present article documents the experimental setups and measurements for each facility and perturbation type.