BUSCADOR RECOLECTA

[EN] CORTEX is a EU H2020 project (2017-2021) devoted to the analysis of ¿reactor neutron noise¿ in nuclear reactors, i.e. the small fluctuations occurring around the stationary state due to external or internal disturbances in the core. One important aspect of CORTEX is the development of neutron noise simulation codes capable of modeling the spatial variations of the noise distribution in a reactor. In this paper we illustrate the validation activities concerning the comparison of the simulation results obtained by several noise simulation codes with respect to experimental data produced at the zero-power reactors AKR-2 (operated at TUD, Germany) and CROCUS (operated at EPFL, Switzerland). Both research reactors are modeled in the time and frequency domains, using transport or diffusion theory. Overall, the noise simulators managed to capture the main features of the neutron noise behavior observed in the experimental campaigns carried out in CROCUS and AKR-2, even though computational biases exist close to the region where the noise-inducing mechanical vibration was located (the so-called "noise source"). In some of the experiments, it was possible to observe
the spatial variation of the relative neutron noise, even relatively far from the noise source. This was achieved through reduced uncertainties using long measurements, the installation of numerous, robust and efficient detectors at a variety of positions in the near vicinity or inside the core, as well as new post-processing methods.

For the numerical simulation tools, modeling the spatial variations of the neutron noise behavior in zero power research reactors is an extremely challenging problem, because of the small magnitude of the noise field; and because deviations from a point-kinetics behavior are most visible in portions of the core that are especially difficult to be precisely represented by simulation codes, such as experimental channels. Nonetheless the limitations of the simulation tools reported in the paper were not an issue for the CORTEX project, as most of the computational biases are found close to the noise source., The CORTEX research project has received funding from the Euratom research and training program 2014 2018 under grant agreement No 754316.

[EN] Vibrations of fuel assemblies are an important issue in the safe operation of nuclear reactors, because they can challenge the integrity of the fuel with potential for radioactive releases. Reactor neutron noise-based techniques for monitoring vibrations are valuable for core diagnostic since they are not intrusive and make use of ordinary neutron flux measurements from ex-core and in-core detectors. The application of these techniques involves the solution of inverse problems that require numerical simulations capable of estimating the reactor neutron noise, given a model of the vibrations. For this purpose, several novel reactor neutron noise solvers have been developed in the CORTEX project using either Monte Carlo or deterministic methods, such as the discrete or-dinates method, the method of characteristics, and the diffusion approximation. In the current work, these solvers have been scrutinized by computing the neutron noise induced by vibrations of one or multiple fuel pins in a simplified UOX fuel assembly benchmark, via proper variations of macroscopic neutron cross sections. The comparison of these neutron noise solutions obtained from the different methods shows novel insights into the simulation of neutron noise induced by mechanical vibrations, such as the challenges posed by the Monte Carlo method, the impact of the angular discretization on the application of the discrete ordinates method, and the accuracy of the diffusion approximation assessed via the higher-order neutron transport methods., The research leading to these results received funding from the Euratom Research and Training Program 2014-2018 under grant agreement no. 754316. Part of the computations and data handling was enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at C3SE and at UPPMAX, partially funded by the Swedish Research Council through grant agreement no. 2018-05973. TRIPOLI-4 (R) and APOLLO3 (R) are registered trademarks of CEA. A. Rouchon and A. Zoia thank EDF and Framatome for partial financial support.

[ES] En el conjunto de estudios del estado del núcleo del reactor que deben realizarse
para asegurar la seguridad e integridad del mismo a lo largo de un ciclo de operación, debe prestarse
especial atención a los fenómenos producidos por la inserción de reactividades como, por ejemplo,
los debidos a los movimientos de los grupos de barras de control, los cuales suelen estar predefinidos
por el proveedor de la tecnología en las Especificaciones Técnicas de Funcionamiento.
El presente trabajo realiza un estudio acerca de en qué medida la elección de un determinado criterio
para el movimiento y selección de los grupos de barras de control puede provocar una determinada
inserción de reactividad. De esta forma, se ha realizado un análisis sensibilidad de la reactividad
provocada por la extracción o inserción de un determinado grupo de barras de control y su influencia
en el estado del núcleo del reactor e incluso en su posible evolución en un transitorio.
El estudio de los diferentes casos contemplados se ha realizado mediante el uso del código acoplado
termohidráulico-neutrónico TRACE v5.0 patch 3/PARCS v3.0, siendo el modelo escogido
representativo de los datos de planta que podríamos encontrar en un reactor comercial PWR.

[EN] In a nuclear reactor, even operating at full power and steady-state conditions, fluctuations are detected in the recording of any process parameter. These fluctuations (also called noise) could be of various origins, such as, turbulence, mechanical vibrations, coolant boiling, etc. The monitoring and complete comprehension of those parameters should thus allow detecting, using existing instrumentation and without introducing any external perturbation to the system, possible anomalies before they have any inadvertent effect on plant safety and availability.
In order to reproduce and study the induced neutron noise in a nuclear reactor core, it is compulsory to develop suitable tools. Existing time-domain codes were originally not developed for this type of calculations. Modifications of those codes and the development of an associated intricate methodology are necessary for enabling noise calculations. This involves, in some cases, changes in the source code and the development of new auxiliary tools to ensure accurate reproductions of the core behavior under the existence of a neutron noise source.
In the proposed work, the time-domain neutron diffusion code PARCS is used to model the effect of stationary perturbations representative of given neutron noise sources. In order to validate the feasibility of the time-dependent methodology thus developed, comparisons with the results of simulations performed in the frequency domain, using the CORE SIM tool, developed at Chalmers University of Technology, are performed.
The development of a few test cases based on a real reactor model are undertaken as the basis for such comparisons and a methodology aimed at assessing the time-domain simulations versus the frequency-domain simulations is established. It is demonstrated that PARCS, although not primarily developed for neutron noise calculations, can reproduce neutron noise patterns for reasonable frequencies. However, it is also observed that unphysical results are occasionally obtained., This work was carried out under the pre-doctoral contract FPI Subprogram 1 and mobility aids of the Universitat Politècnica de València and the support of the Spanish Ministerio de Ciencia e Innovación under the project ENE2017-89029-P. The research leading to these results was also partially funded from the Euratom research and training programme 2014 2018 under grant agreement No 754316 (CORTEX project).

[ES] En un reactor nuclear, se observan fluctuaciones en el registro de cualquier parámetro
del proceso. La monitorización y la comprensión completa de esos parámetros permitirían detectar,
utilizando la instrumentación existente y sin introducir perturbaciones externas al sistema, posibles
anomalías antes de que tengan algún efecto indeseado en la seguridad y disponibilidad de la planta.
Para el estudio y la simulación del ruido, es obligatorio desarrollar herramientas adecuadas. Sin
embargo, los códigos basados en el dominio del tiempo existentes no se desarrollaron originalmente
para este tipo de cálculos, por lo que resultará necesario realizar modificaciones en dichos códigos,
así como el desarrollo de una intrincada metodología asociada para permitir dicho cálculo.
En el trabajo propuesto, el código de difusión neutrónica basado en el dominio del tiempo PARCS se
utiliza para simular el efecto de las perturbaciones estacionarias representativas de las fuentes de
ruido neutrónico consideradas. Para la validación de la metodología desarrollada, se realizan
comparaciones con los resultados obtenidos en el dominio de la frecuencia, mediante el uso de la
herramienta CORE SIM, desarrollada en la Universidad de Tecnología de Chalmers.
Se desarrollarán diversos casos de prueba basados en un modelo de reactor comercial para tales
comparaciones y se establecerá una metodología que permita comparar las simulaciones en el
dominio del tiempo frente a las simulaciones en el dominio de la frecuencia., Este trabajo se llevó a cabo en virtud del contrato predoctoral FPI Subprograma 1 y ayudas de
movilidad de la Universitat Politècnica de Valencia y el apoyo del Ministerio de Ciencia e Innovación
español en el marco del proyecto ENE2017-89029-P. La investigación que condujo a estos resultados
también fue parcialmente financiada por el programa de investigación y capacitación Euratom 2014-
2018 bajo el acuerdo de subvención no 754316 (proyecto CORTEX).

[ES] El objetivo del presente trabajo ha sido la mejora en la comprensión sobre la
fenomenología del ruido neutrónico presente en reactores de agua ligera. Que en el caso de los
diseños KWU, además de desconocerse su origen, puede provocar oscilaciones en el flujo neutrónico
capaces de alterar la operación normal de la planta.
Con este fin, se han separado señales reales de potencia de un reactor nuclear en los distintos
armónicos que las componen. Posteriormente, se han realizado transitorios debidos a la introducción
de fluctuaciones en la densidad del moderador y la temperatura del combustible, con la forma de los
armónicos obtenidos anteriormente, para un amplio rango de amplitudes. De forma que, se ha podido
observar cual es la contribución de cada uno de los modos analizados al comportamiento de la
potencia total.
Se pretende analizar la influencia que tiene la excitación de diversos modos en el interior del reactor
debido a fluctuaciones generadas en el mismo, así como su influencia en el ruido registrado. También
se desea estudiar la aptitud del código acoplado TRACE y PARCS para la modelización de la
transmisión del ruido neutrónico en el interior de la vasija, mediante la simulación de perturbaciones
confinadas a un único elemento combustible.

[EN] Mechanical vibrations of core internals are among the main perturbations that induce oscillations in the neutron flux field, also known as neutron noise. In this work, different simulation models for the study of the influence of the mechanical vibrations of fuel assemblies on the neutron flux in the reactor core have been discussed. These methodologies employ the diffusion approximation, with or without a previous homogenization model, to simulate the neutron noise in the time or the frequency domain. The diffusion-based approach is expected to be less accurate in the vicinity of the vibrating fuel assemblies, but correct when considering distances larger than a few diffusion lengths away from the perturbation. All methodologies provide consistent results and can reproduce typical features of the neutron noise induced by mechanical vibrations of core components. First, FEMFFUSION can perform simulations in both the time and frequency domains. Second, CORE SIM + can be used to study various neutron noise scenarios in realistic three-dimensional reactor configurations. The third methodology is centred on using commercial codes as CASMO-5, SIMULATE-3 and SIMULATE-3K. This methodology allows time domain simulations of the neutron noise induced by different neutron noise sources in a nuclear reactor. Finally, a model for time-dependent geometry is implemented for the code system ATHLET/QUABOX-CUBBOX employing a cross-section-based approach for encoding water gap width variations at the reflector., The research conducted was made possible through funding from the Euratom research and training programme 2014-2018 under grant agreement No. 754316 for the CORe Monitoring Techniques And EXperimental Validation And Demonstration (CORTEX) Horizon 2020 project, 2017-2021.

[EN] The mechanical vibrations of fuel assemblies have shown to give high levels of neutron noise, triggering in some circumstances the necessity to operate nuclear reactors at a reduced power level. This behaviour can be modelled using the neutron noise diffusion approximation in the frequency-domain. This work presents an extension of the finite element method code FEMFFUSION, to simulate mechanical vibrations in hexagonal reactors in the frequency domain. This novel strategy in neutron noise simulation is based on introducing perturbations on the edges of the cells associated with the vibrating fuel assemblies, allowing to model the movement of these fuel assemblies accurately and efficiently, without the necessity of using locally refined meshes. Numerical results verify the edge-wise methodology in the frequency-domain against the usual cell-wise frequency-domain model and the time-domain model. The edge-wise frequency-domain methodology has also been compared to other neutronic codes, as CORESIM and PARCS., This project has received funding from the Euratom research
and training program 2014-2018 under grant agreement No
754316.

[EN] This work outlines an approach for localizing anomalies in nuclear reactor cores during their steady state operation, employing deep, one-dimensional, convolutional neural networks. Anomalies are characterized by the application of perturbation diagnostic techniques, based on the analysis of the so-called ¿neutron-noise¿ signals: that is, fluctuations of the neutron flux around the mean value observed in a steady-state power level. The proposed methodology is comprised of three
steps: initially, certain reactor core perturbations scenarios are simulated in software, creating the respective perturbation datasets, which are specific to a given reactor geometry; then, the said datasets are used to train deep learning models that learn to identify and locate the given perturbations within the nuclear reactor core; lastly, the models are tested on actual plant measurements. The overall methodology is validated on hexagonal, pre-Konvoi, pressurized water, and VVER-1000 type nuclear reactors. The simulated data are generated by the FEMFFUSION code, which is extended in order to
deal with the hexagonal geometry in the time and frequency domains. The examined perturbations are absorbers of variable strength, and the trained models are tested on actual plant data acquired by the in-core detectors of the Temelín VVER-1000 Power Plant in the Czech Republic. The whole approach is realized in the framework of Euratom¿s CORTEX project., The research conducted was made possible through funding from the Euratom research and training programme 2014-2018 under grant agreement No. 754316 for the "CORe Monitoring Techniques And EXperimental Validation And Demonstration (CORTEX)" Horizon 2020 project, 2017-2021.

[EN] The mechanical vibrations of fuel assemblies have been shown to give rise to high levels of neutron noise, triggering in some circumstances the necessity to operate nuclear reactors at a reduced power level. This work analyses the effect in the neutron field of the oscillation of one single fuel assembly. Results show two different effects in the neutron field caused by the fuel assembly vibration. First, a global slow variation of the total reactor power due to a change in the criticality of the system. Second, an oscillation in the neutron flux in-phase with the assembly vibration. This second effect has a strong spatial dependence that can be used to localize the oscillating assembly. This paper shows a comparison between a time-domain and a frequency-domain analysis of the phenomena to calculate the spatial response of the neutron noise. Numerical results show a really close agreement between these two approaches., This project has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 754316. Also, this work has been partially supported by Spanish Ministerio de Economia y Competitividad under project BES-2015-072901 and financed with the help of a Primeros Proyectos de Investigation (PAID-06-18), Vicerrectorado de Investigacitin, Innovation y Transferencia of the Universitat Politecnica de Valencia (UPV).

[EN] The mechanical vibrations of core internals such as fuel assemblies (FAs) cause oscillations in the neutron flux that require in some circumstances nuclear power plants to operate at a reduced power level. This work simulates and analyzes the changes of the neutron flux throughout a nuclear core due to the oscillation of a single FA without considering thermal-hydraulic feedback. The amplitude of the FA vibration is bounded to a few millimeters, and this implies the use of fine meshes and accurate numerical solvers due to the different scales of the problem. The results of the simulations show a main oscillation of the neutron flux with the same frequency as the FA vibration along with other harmonics at multiples of the vibration frequency much smaller in amplitude. Also, this work compares time domain analysis and frequency domain analysis of the mechanical vibrations. Numerical results show a close match between these two approaches for the fundamental frequency., This project has received funding from the Euratom research and training programme 2014-2018 under grant agreement number 754316. Also, this work has been partially supported by Spanish Ministerio de Economia y Competitividad under project BES-2015-072901 and financed with the help of Primeros Proyectos de Investigacion (PAID-06-18), Vicerrectorado de Investigacion, Innovacion y Transferencia of the Universitat Politecnica de Valencia (UPV).

[EN] The early detection of anomalies through the analysis of the neutron noise recorded by in-core and ex-core instrumentation gives the possibility to take proper actions before such problems lead to safety concerns or impact plant availability. The study of the neutron fluctuations permits detecting and differentiate anomalies depending on their type and possibly to characterize and localize such anomalies. This method is non-intrusive and does not require any external perturbation of the system. To effectively use the neutron noise for reactor diagnostics it is essential to accurately model the effects of the anomalies on the neutron field. This paper deals with the development and validation of a neutron noise simulator for reactors with different geometries. The neutron noise is obtained by solving the frequency-domain two-group neutron diffusion equation in the first order approximation. In order to solve this partial differential equation a code based on a high order finite element method is developed. The novelty of this simulator resides on the possibility of dealing with rectangular meshes in any kind of geometry, thus allowing for complex domains and any location of the perturbation. The finite element method also permits automatic refinements in the cell size (h-adaptability) and in its polynomial degree (p-adaptability) that lead to a fast convergence. In order to show the possibilities of the neutron noise simulator developed a perturbation in a hexagonal two-dimensional reactor is investigated in this paper., This project has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 754316. Also, this work has been partially supported by Spanish Ministerio de Economía y Competitividad under project BES-2015-072901 and financed with the help
of a Primeros Proyectos de Investigacin (PAID-06-18), Vicerrectorado de Investigación, Innovación
y Transferencia of the Universitat Politecnica de València (UPV).

KWU pre-Konvoi PWRs (SIEMENS design) are commonly exhibiting high neutron noise levels which canlead to costly operational issues (i.e. activation of SCRAM system, operation of unrated core power, etc.).The frequency region of interest of neutron noise is below 1 Hz, which is the typical frequency range ofthermal-hydraulic phenomena. This feature seems to indicate that, coolant flow and temperature oscil-lations can have a key impact on neutron noise phenomena. Moreover, an increasing neutron noise trend(in term of normalized root mean square) was recently observed in many KWU-PWRs. This increasingtrend has been speculated to be correlated with the introduction of a new fuel design type in theKWU-PWRs. This fact indicates that there should be some correlation between neutron noise spectralcharacteristics and fuel assemblies’ performance. In order to advance in understanding this phenomenon,the transient nodal code SIMULATE-3K (S3K) has been used to simulate mechanical vibrations of fuelassemblies and thermal-hydraulic fluctuations of the core inlet flow and temperature. The simulatedneutron detectors responses are analysed with noise analysis techniques and compared to real plant data.This analysis indicates that the cross-feedback between the mechanical and thermal-hydraulic distur-bances complicate the identification of the origin of the perturbation source. The simulated results indi-cate that the neutron noise spectral characteristics can be associated separately to different causes. In thissense, the results of this work seem to indicate that the spectral features of the neutron noise are a con-sequence of both mechanical perturbations and thermal-hydraulic fluctuations.