Analysis of the First Colibri Fuel Rods Oscillation Campaign in the Crocus Reactor for the European Project Cortex

Abstract

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.

Ten springs of experiments in CROCUS

“…The experiments consists in the lateral oscillation of one to 18 fuel rods in the outer fuel zone of CROCUS, to measure the induced noise and its eventually spatial dependence over the core. It required the development, qualification and licensing of a unique in-core device for fuel displacement [18]- [20]. It aims at the validation of the noise simulation codes, and was used in particular in three experimental campaigns [21]- [26] within the framework of the Horizon2020 project CORTEX [27]- [29].…”

“…It required the development, qualification and licensing of a unique in-core device for fuel displacement [18]- [20]. It aims at the validation of the noise simulation codes, and was used in particular in three experimental campaigns [21]- [26] within the framework of the Horizon2020 project CORTEX [27]- [29].…”

First in-core gamma spectroscopy experiments in a zero power reactor

“…All fuel rods are cladded with aluminum and are maintained in a vertical position by the upper grid and lower grid plates, spaced 100 cm apart. In the current COLIBRI configuration [22]- [25], both grid plates incorporate a cadmium layer with a thickness of 1 mm to limit axial thermal leakage to surrounding structures. This is also the configuration for the herein presented results.…”

“…1) Cerium Bromide (CeBr 3 ) detectors When the two control rods are removed, their guide tubes are a prominent location for in-core measurements, e.g. used for delayed gamma fraction estimation [28], intrinsic gamma noise [29], and induced neutron/gamma noise measurements [22], [23]. Cerium(III) Bromide (CeBr 3 ), as a comparatively new scintillation material [30], [31], was chosen as the most suitable material for a high photon flux zone with a comparatively fast decay time of 20 ns at a density of 5.2 g/cm 3 [32].…”

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

“…During the first phase of COLIBRI, the observation of a spatial dependence of the perturbation noise, also called neutron modulation, was demonstrated [13]. In the second phase of COLIBRI starting 2021, it is planned to use a core mapping array of neutron detectors to record its propagation [14].…”