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

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

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.

E D P SCIENCES2020

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

Development of a Novel Finite Volume Methodology for Multi-Dimensional Fuel Performance Applications

Alessandro Scolaro

Fuel performance codes are an essential tool for ensuring the safe and economic opera-tion of nuclear reactors. Traditionally, these codes have been developed following a simple 1.5-D modelling approach, where the fuel behavior is simplified by assuming axisymmetric and plain strain conditions and where phenomenological correlations are largely privileged over the use of accurate physical and mathematical models. However, in recent decades the advent of modern high-performance computing has awakened a growing interest towards higher-fidelity tools with multi-dimensional and multi-physics capabilities. The main development efforts have been carried out employing the popular finite element method, while the alternative approach offered by the finite volume method has not been explored by the fuel performance community. Interested in the potential of this numerical scheme for fuel analysis and multi-physics applica-tions, this PhD thesis aims to develop a finite volume methodology and associated software for the high-fidelity analysis of fuel behavior. As a result, a novel finite volume fuel performance code named OFFBEAT is developed. The code is based on the open-source C++ library OpenFOAM and it is envisioned as a multi-dimensional, readily-available and flexible tool, which is straightforward to extend and modify, and open to multi-physics simulations. The thesis presents the development strategy of OFFBEAT, discussing the numerical framework, the treatment of the gap as well as the structure of the code with its main models and overall so-lution scheme. In this context, a novel contact methodology is developed that introduces a semi-implicit discretization of the contact stresses, improving the convergence properties of many con-tact scenarios. To test the robustness and accuracy of the code, extensive efforts are carried out to verify and validate OFFBEAT against numerical benchmark and experimental data. The novel methodology is applied to the 3-D analysis of the effect of eccentricity on fuel disc irra-diation test campaigns performed in the past in the Halden Boiling Water Reactor and character-ized by rods with large gap and high conductive fuel. This represents an interesting example of how multi-dimensional capabilities can be used to study separate-effect tests, which are becoming increasingly more relevant in nuclear fuel research and are often characterized by unconventional features. Additionally, the thesis describes a set of methodologies and tools that complement OFFBEAT ena-bling its interaction with other codes relevant for the fuel performance community. First, a cou-pling between OFFBEAT and the Monte Carlo neutron transport code Serpent2 is developed to allow one to obtain a higher-fidelity solution for the neutron flux and for the fuel isotopic composi-tion. This is particularly relevant for fuel rod configurations outside of the range of application of traditional simplified neutronics models such as for new fuel types, experimental reactors, or fuel rods with strong absorbers. Second, a coupling strategy is developed and implemented between the legacy code TRANSURANUS and OFFBEAT. Taking advantage of the strength of both codes, the main aim is to use a validated and fast 1.5-D code to simulate the base irradiation and set accu-rate initial conditions for a multi-dimensional transient performed with OFFBEAT.

EPFL2021

Development of a Novel Finite Volume Methodology for Multi-Dimensional Fuel Performance Applications

Alessandro Scolaro

EPFL2021