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

Abstract

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.

Design of a 150-miniature detectors 3D core-mapping system for the CROCUS reactor

“…• 50 rectangular "rulers" of polyoxymethylene (POM-C) with a thickness of 4 mm guiding and positioning the neutron detectors at the intended position. Black POM-C was selected because of its good resistance in water, its good light opacity and its density comparable to that of water (1.41 g/cm 3 ). Depending on the lattice arrangement, rulers can be crossing the whole core or being split in two to accommodate the presence of the cruciform safety blades; • 52 horizontal supporting rods made of anodized aluminum for holding and positioning the rulers among fuel rods at 5 different axial layers.…”

“…In this framework, the zero-power research reactor CROCUS, operated at the École polytechnique fédérale de Lausanne (EPFL) was selected as a perfect candidate for the installation of a 3D full-core mapping system. A 3D measuring system in CROCUS will provide valuable insight on the propagation of neutron modulation in the core, thanks to the presence of several mechanical perturbation experiments, including the COLIBRI fuel rod oscillator [2], [3], the PISTIL rotating absorber [4], and a linear oscillating absorber currently under development. In addition, CROCUS is a readily available reactor with low technological uncertainties [5], where it would be possible to accurately position ~150 neutron detectors within the whole core volume upon the design of a proper mechanical supporting structure.…”

Power calibration methodology at the CROCUS reactor

“…The rods are maintained vertically by two octagonal aluminum grid plates spaced 1 m apart. In the current experimental configuration for the COLIBRI program [28]- [30], the grids have a 1-mm cadmium layer to limit axial neutron leakage to the environment, i.e. structures activation, with the active zone of the fuel starting in the middle of the lower cadmium layer.…”

Review of kinetic modulation experiments in low power nuclear reactors

“…Indeed, separating low/high frequency modulations or low/high power experiments relieves design and safety criteria over the drive system. Figure 8 shows a combination of two modulators that are functional at different frequency ranges, which were recently used in the AKR-2 reactor [11]. It should however be noted that a frequency range overlap was required to analyze experimental data recorded with different modulators (having different moderation strengths) as a whole for RTF determination.…”

“…Nonetheless, the method is being actively used and its new possible applications have been suggested [9]. To the authors' knowledge, multiple modulators have been built, tested [10,11] and operated [12][13][14][15][16] in the last 10 years.…”