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Chapter 10. Specificities of PWR-SMRs
No full textInternational audienceSmall Modular Reactors that use pressurized water reactor technology have unique characteristics, particularly when considering integral reactors. Therefore, the thermal-hydraulic analysis of accidents, especially loss of coolant accidents, presents its own challenges. Additionally, reliance on passive safety systems is another aspect to consider in safety analyses. This chapter presents general considerations related to these new designs
A new multifluid method for dusty astrophysical flows: Application to turbulent protostellar collapses
No full textInternational audienceContext. Stars and planets form in collapsing clouds of gas and dust. The presence of dust grains and their local distribution play a significant role throughout the protostellar sequence, from the thermodynamics and the chemistry of molecular clouds to the opacity of collapsing protostellar cores and the coupling between the gas and the magnetic field and down to planet formation in young and evolved disks.Aims. We aim to simulate the dynamics of the dust, considering the whole range of grain sizes, from few nanometers to millimeters.Methods. We implemented a neutral pressureless multifluid that samples the dust size distribution in the RAMSES code. This multi-fluid is dynamically coupled to the gas via a drag source term and self-gravity, relying on the Eulerian approach.Results. We designed a Riemann solver for the gas and dust mixture that prevents unphysical dust-to-gas ratio variations for well coupled grains. We illustrated the capacities of the code by performing simulations of a protostellar collapse down to the formation of a first hydrostatic core, both for small and large dust grains. Grains over 100 microns significantly decouple from the gas. The spatial maps and the probability density functions indicate that dust enrichment within the first hydrostatic core and in some locations of the envelope increases as a function of the grain size and the level of initial turbulence.Conclusions. Thanks to the novel Riemann solver, we recovered the terminal velocity regime, even at low resolution. Moreover, we successfully extended it to regimes where the grain inertia matters. The multifluid module performs the coupling between the dust and the gas self-consistently all through the dynamical scales. The dust enrichment in the first hydrostatic core and the envelope have been revised here, assuming the initial turbulence and grain sizes. This enables us to probe new potential locations, epochs, and initial conditions for planet formation
Simple analytical modeling of residual flexural properties of hybrid PEEK thermoplastic composite laminate under kerosene flame exposure
No full textInternational audienc
Generative modelling of convergence maps based on predicted one-point statistics
No full textInternational audienceContext. Weak gravitational lensing is a key cosmological probe for current and future large-scale surveys. While power spectra are commonly used for analyses, they fail to capture non-Gaussian information from non-linear structure formation, which necessitates higher-order statistics and methods for an efficient map generation.Aims. We develop an emulator that generates accurate convergence (κ) maps directly from an input power spectrum and wavelet ℓ1-norm without relying on computationally intensive simulations.Methods. We used either numerical or theoretical predictions to construct κ maps by iteratively adjusting the wavelet coefficients to match the marginal distributions of the target and their inter-scale dependences by incorporating higher-order statistical information.Results. The resulting κ maps accurately reproduce the input power spectrum, and their higher-order statistical properties are consistent with the input predictions. They thus provide an efficient tool for weak-lensing analyses
Synthesis and characterization of stable non-fullerene acceptor polymers for All-Polymer Solar Cells
No full textInternational audienc
Magic-RR project overview: objectives, methodology and expected results
No full textInternational audienceMost research reactors (RRs) in Europe are over 60 years old, and there are only limited efforts underway (e.g., PALLAS and JHR projects) to partially replace this aging infrastructure. Continued safe operation (CSO) of these reactors is crucial to sustaining the EU’s leadership in nuclear materials development and qualification for advanced reactor designs and to ensuring a steady supply of medical isotopes. Extending the licenses for these reactors to ensure CSO requires comprehensive aging management reviews (AMRs) and time-limited aging analyses (TLAAs) of key structures and components. However, current challenges include a limited understanding of irradiation-induced degradation and corrosion mechanisms, a shortage of data on RR structural materials under high-fluence conditions necessary for CSO, the lack of predictive, physics-based models for irradiation damage in aluminum alloys, and insufficient surveillance specimens for some reactors. Additionally, there are no dedicated design codes for reactor vessels and core structures made of aluminum, and there is no standardized approach in Europe for aging management of operating RRs. To address these issues, a new project, Research on Ma terials A g e i ng and Stru c tural Integrity of R esearch R eactors (Magic-RR), was launched on 1 st of November 2024, funded by the EURATOM research and training program 2023 with contributions from several international partners including RR operators, new RR developers and technical universities. Magic-RR will leverage (1) available archive materials and data from the existing RRs, e.g. from surveillance programs and shut down reactors, (2) operational experience of RR operators and (3) advanced characterization and modelling techniques at universities and nuclear research centers, to achieve the following objectives to support the CSO of European RRs: – enhancing understanding of irradiation-induced damage in RR structural materials, particularly aluminum alloys, under high-fluence conditions. – Develop advanced multi-scale modeling techniques to predict irradiation effects on mechanical properties. – Investigating corrosion mechanisms and developing strategies for their prevention and mitigation. – Assessing and validating sub-size testing methods for surveillance programs. – Sharing operational knowledge on ageing management and structural integrity assessment of critical RR components and establishing guidelines for best practices. This paper provides comprehensive description of the objectives, methodology, expected results and impact of the Magic-RR project
When Pattern-by-Pattern Works: Theoretical and Empirical Insights for Logistic Models with Missing Values
No full textPredicting a response with partially missing inputs remains a challenging task even in parametric models, since parameter estimation in itself is not sufficient to predict on partially observed inputs. Several works study prediction in linear models. In this paper, we focus on logistic models, which present their own difficulties. From a theoretical perspective, we prove that a Pattern-by-Pattern strategy (PbP), which learns one logistic model per missingness pattern, accurately approximates Bayes probabilities in various missing data scenarios (MCAR, MAR and MNAR). Empirically, we thoroughly compare various methods (constant and iterative imputations, complete case analysis, PbP, and an EM algorithm) across classification, probability estimation, calibration, and parameter inference. Our analysis provides a comprehensive view on the logistic regression with missing values. It reveals that mean imputation can be used as baseline for low sample sizes, and improved performance is obtained via nonlinear multiple iterative imputation techniques with the labels (MICE.RF.Y). For large sample sizes, PbP is the best method for Gaussian mixtures, and we recommend MICE.RF.Y in presence of nonlinear features.</div
Observations of carbon radio recombination lines with the NenuFAR telescope: I. Cassiopeia A and Cygnus A
No full textInternational audienceContext. Carbon radio recombination lines (CRRLs) at decametre wavelengths trace the diffuse phase of the interstellar medium (ISM) of the Galaxy. Observations of these lines allow for physical parameters of this phase to be measured.Aims. We observed CRRLs with the recently commissioned New Extension in Nançay Upgrading LOFAR (NenuFAR) telescope towards two of the brightest sources at low-frequency (10–85 MHz): Cassiopeia A and Cygnus A (hereafter, Cas A and Cyg A, respectively). We then measured the density, ne, and temperature, Te, of the electrons in line-of-sight clouds.Methods. We used NenuFAR’s beam-forming mode and integrated several tens of hours on each source. The nominal spectral resolution was 95.4 Hz. We developed a reduction pipeline primarily aimed at removing the radio frequency interference (RFI) contamination and correcting the baselines. We then performed a first fitting of the spectral lines observed in absorption associated with the line-of-sight clouds.Results. Cas A is the brightest source in the sky at low frequencies and represents an appropriate test bench for this new telescope. On this source, we detected 398 Cα lines between the principal quantum numbers n = 426 and n = 826. Cyg A is also a bright source, however, the Cα lines were observed to be fainter. We stacked the signal by groups of a few tens of lines to improve the quality of our fitting process. For both sources, we reached a significantly higher signal-to-noise ratio (S/N) and spectral resolution than the most recent detections by the LOw Frequency ARray (LOFAR). The variation of the spectral line widths with the electronic quantum number provides constraints on the physical properties of the clouds: Te, ne, and the temperature, T0, of the radiation field, the mean turbulent velocity, νt, and the typical size of the cloud.Conclusions. Our final constraints differ from those inferred from LOFAR results, with ∼50% lower Te, ∼35% lower ne, and from 10 to 80% higher νt, on average. The NenuFAR observations sample a larger space volume than LOFAR’s towards the same sources due to the differences in instrumental beam sizes. These discrepancies highlight the sensitivity of low-frequency CRRLs as probes of the diffuse ISM, paving the way towards large area surveys of CRRLs in our Galaxy
Analysis of Cliff Effects and Thermal Hydraulic Instabilities in the PLANDTL-2 Sodium Experiment Transient Tests
No full textInternational audienceSeparate Effect Tests (SET) and Integral Effect Tests (IET) are commonly used in support of reactor designs, both to analyze physical phenomena and to validate simulation codes. In the framework of the Franco-Japanese collaboration on Research and Development for Sodium Fast Reactors (SFR) thermal hydraulics, transient tests were performed in the IET named PLANDTL-2 test facility in Japan. This IET's instrumented test section is composed of an electrically heated core and a hot pool with a Dipped Heat Exchanger (DHX). The Intermediate Heat Exchanger (IHX) and the Electro-Magnetic Pump (EMP) are located in a deported primary loop. Studied transients consist in transition from forced to natural convection, in the hot pool and in the primary circuit, under various decay heat removal operations using the DHX. It was observed that in the long term, a cliff effect occurs, meaning that the apparent steady natural convection is perturbed if a threshold is reached. Instabilities and flowrate oscillations from positive to negative values in the primary loop are observed after a period of smooth natural circulation. The unstable behavior results from the competition between IHX and DHX cooling, the latter leading to an increase in thermal stratification in the hot pool. This paper aims to analyze this phenomenon, bring a comprehensive criterion for the onset of unstable behaviors and give some general guidelines to avoid such effects for accidental transient management
Controlling the Polarization State of Plasma-Induced THz Waves
No full textInternational audienceTerahertz generation from two- and multi-color optical fields due to photoionization is a well-established way to produce intense, ultra-broadband THz and far-infrared radiation. THz radiation is the result of the transfer of electrons into the continuum and their field-induced acceleration. Recently, it was shown that this mechanism allows the production of different THz waveshapes, by controlling the polarization state of the pump pulses. This control was shown to be useful for imaging tunneling electronic wavepackets on the attosecond scales