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Reactivity of Constitution vs. Crystallization Water Under Irradiation: Insights from Tobermorites
No full textInternational audienceUnderstanding radiolytic H2 production in irradiated cement is crucial for nuclear waste safety, yet the role of solid cement phases remains unclear. This study examines the behavior of model minerals -tobermorite 11 Å (Ca5Si6O17.5H2O) and tobermorite 9 Å (Ca5Si6O16(OH)2)-under electron irradiation. When fully dried, these minerals retain only crystallization water or structural hydroxyl groups, respectively. The results reveal that while crystallization water decomposes under irradiation, it does not lead to H2 formation, as hydrogen atoms react with radiation-induced defects to form SiO-H bonds. In contrast, tobermorite 9 Å produces H2 only when surface SiO-H bonds are present, indicating that radiolytic dihydrogen arises from surface bond breakage, while the cleavage of the bonds in the material does not ultimately lead to H2 production. These findings enhance our understanding of irradiation effects on cementitious materials, aiding in the assessment of their long-term stability in nuclear waste storage
Quantification de l'impact des risques climatiques sur le risque de crédit
No full textStress-tests are forward-looking risk assessment exercises aimed at evaluating the robustness of financial institutions under adverse but plausible macroeconomic scenarios. These tests, regularly performed voluntarily or required by the financial regulators, provide computations of financial risks' metrics along the provided scenarios. Among these, credit risk stress-testing focuses on estimating the default probabilities (PD) of counterparts in a bank's credit portfolio. However, integrating climate risks into stress-tests introduces unique challenges, such as the need for granular modeling, dynamic adaptation of portfolios, and long-term scenario horizons. The typically used credit risk stress-testing model, known as the Asymptotic Single Risk Factor (ASRF) model, fails to capture the specific dynamics of climate scenarios, namely the effects of transition risks driven by policy changes, technological shifts, and consumer sentiment.This thesis addresses these challenges by developing novel methods to quantify credit risk under Climate Stress-Tests. First, it proposes a probabilistic framework to model corporate business models and their adaptation under energy transition scenarios.Second, it extends this framework to compute scenario-conditional PDs by integrating stochastic processes thanks to a structural and path-dependent credit risk model where both sides of the balance sheet are modeled as stochastic processes and using Nested Monte Carlo simulations.Finally, it explores the impact of a single firm's misaligned anticipations of transition scenarios on credit risk, introducing a model that accounts for a potential re-evaluation of the anticipations at a later stage.The findings demonstrate that cost-based approaches reduce credit risk more effectively than static or reactive strategies, with up to 9 times lower default probabilities for high-emission firms. Forward-looking strategies perform better than others in delayed transition scenarios, leading to 6 times lower PDs compared to cost-agnostic methods. Notably, wrong anticipations do not always increase credit risk. In particular, they may result in improved PDs if they lead to greater relative carbon emissions than the perfect anticipations. In the opposite case, firms with initial misaligned and unfavorable anticipations consistently benefit from reassessing their strategies, reducing PDs by up to 20 times when corrective measures are applied early. These results provide actionable insights and robust methodologies to enhance the reliability and precision of credit risk climate stress-tests.Les tests de résistance sont des exercices prospectifs d'évaluation des risques visant à évaluer la robustesse des institutions financières face à des scénarios macroéconomiques adverses mais plausibles. Ces tests, réalisés régulièrement sur la base du volontariat ou exigés par les régulateurs financiers, permettent de calculer des métriques de risques financiers en fonction des scénarios fournis. Parmi eux, les tests de résistance au risque de crédit se concentrent sur l'estimation des probabilités de défaut (PD) des contreparties dans le portefeuille de credit d'une banque. Cependant, l'intégration des risques climatiques dans ces tests introduit des défis uniques, tels que la nécessité d'une modélisation granulaire, l'adaptation dynamique des portefeuilles et la prise en compte d'horizons de scénarios long-termistes. Le modèle couramment utilisé pour les tests de résistance au risque de crédit, connu sous le nom de Asymptotic Single Risk Factor model (ASRF, modèle asymptotique à un seul facteur de risque), ne parvient pas à capturer les dynamiques spécifiques des scénarios climatiques, notamment les effets des risques de transition liés aux changements de politiques, aux évolutions technologiques et au sentiment des consommateurs.Cette thèse relève ces défis en développant des méthodes novatrices pour quantifier le risque de crédit dans le cadre des tests de résistance climatique. Tout d'abord, elle propose un cadre probabiliste pour modéliser les modèles économiques des entreprises et leur adaptation aux scénarios de transition énergétique. Ensuite, ce cadre est étendu pour calculer les PD conditionnelles aux scénarios grâce à un modèle de risque de crédit structurel et path-dependent, où les deux côtés du bilan sont modélisés comme des processus stochastiques, en utilisant des simulations de Monte Carlo imbriquées. Enfin, elle explore l'impact des anticipations erronées d'une entreprise individuelle concernant les scénarios de transition sur le risque de crédit, en proposant un modèle qui prend en compte une éventuelle réévaluation des anticipations à une date ultérieure.Les résultats montrent que les approches basées sur les coûts réduisent plus efficacement le risque de crédit que les stratégies statiques ou réactives, avec des probabilités de défaut jusqu'à 9 fois plus petites pour les entreprises fortement émettrices. Les stratégies par anticipation parfaite du scénario surpassent les autres dans les scénarios de transition retardée, avec une réduction d'un facteur 6 des PD par rapport aux méthodes insensibles aux coûts. Notamment, les anticipations erronées n'augmentent pas toujours le risque de crédit. En particulier, elles peuvent améliorer les PD si elles conduisent à des émissions relatives de carbone plus faibles que celles prévues avec des anticipations parfaites. Dans le cas contraire, les entreprises ayant des anticipations initiales erronées et défavorables bénéficient systématiquement de la réévaluation de leurs stratégies, réduisant leurs PD jusqu'à 20 lorsque des mesures correctives sont appliquées rapidement. Ces résultats offrent des perspectives exploitables et des méthodologies robustes pour améliorer la fiabilité et la précision des tests de résistance climatique au risque de crédit
Evidence for the role of thermal and cloud merging in mesoscale convective organization
No full textInternational audienceObservations from airborne field campaigns are used to study the interplay between boundary-layer thermals and clouds in the trades. The size distributions of thermal and cloud-base chords inferred from turbulence and horizontal lidar-radar measurements are robustly described by the sum of two exponentials. Analytical calculations and statistical simulations demonstrate that the two exponentials result from objects merging, respectively representing the populations of merged- and unmerged-object chords. They also show how circulations induced by convective objects facilitate the merging process. The observed day-to-day variability of these populations at cloud base can thus be tied to the variability of thermal merging across the depth of the subcloud layer. Clouds rooted in unmerged thermals are small and shallow while those rooted in merged thermals are wider and deeper. An intricate interplay between thermal- and cloud-merging arises: when thermal merging is weak, thermal number density is high and cloud bases merge easily, leading to strong mesoscale mass fluxes and "Gravel" shallow mesoscale organizations. In contrast, when thermal merging is strong, clouds are fed by sparser but wider thermals, leading to longer cloud lifetimes but weaker cloud merging, weaker mesoscale mass fluxes, and "Flower" mesoscale organizations. This interplay between thermal- and cloud-merging imposes an upper bound on cloud coverage and suggests a negative feedback on the growth of mesoscale circulations. Thermal merging also controls observed size distributions of thermals in deep convective regimes. The merging process thus appears to be a fundamental player in the mesoscale organization of convection
Fermion parity switches imprinted in the photonic field of cavity embedded Kitaev chains
No full textInternational audienceThe entanglement of electronic states with quantum light in cavity embedded systems has opened new avenues to manipulate quantum materials. In this work we investigate the Kitaev chain coupled to a single mode photonic cavity. Using exact diagonalization we calculate the many-body energy spectrum of the electron-photon Hamiltonian in finite-length chains. We find two distinct types of ground states, one with a well defined parity and another with an alternating parity where a doubly degenerate ground state takes place at exceptional points, known as parity switching points. The double ground state hosts edge states weakly affected by the cavity coupling, even in the low frequency regime, in contrast with higher excited states showing strong dependence with the cavity coupling. Besides the electronic quantities, we also find that the photon number peaks at values of the chemical potential corresponding to parity switching points. Therefore, we suggest that quantum optics experiments could be employed to detect the double ground state hosting edge states weakly hybridized with light. Finally, calculations of photonic quadratures reveal squeezed states that are both captured by the exact diagonalization technique and mean field decoupling. However, within these two approaches differences in the photon probability in odd numbers of photons are reported
Total synthesis of photoactivatable latrunculin B for actin-targeting chemical biology
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Contact arithmetic
No full textThe irreducible factorization of polynomials over power series is central to several problems in computer algebra: integral bases, genus of a curve, Jacobian of a curve, Riemann-Roch spaces. Well-known applications include cryptography and algebraic geometry error-correcting codes. Towards solving these problems with quasi-optimal complexity, recent algorithms make use of the so-called "contact representation". When carrying out the Newton polygon method, this allows intermediate objects to be represented in a compact way with respect to the required relative precision. In this paper, we focus on the complexity of the corresponding "contact arithmetic" and present quasi-optimal algorithms for multiplication and division in the contact representation.</div
Baryon-antibaryon generalized distribution amplitudes and
No full text13 pages, 7 figuresInternational audienceBaryon-antibaryon generalized distribution amplitudes (GDAs) give an access to timelike gravitational form factors (GFFs) which are complementary to the spacelike ones which can be deduced from the hadronic generalized parton distributions (GPDs) measured in deep exclusive electroproduction processes. They allow to probe the GFFs of unstable baryons in the baryon octet, since the second moments of hadronic generalized distribution amplitudes (GDAs) lead to the timelike GFFs. These GDAs can be measured in the process , in the generalized Bjorken regime where the invariant mass of the pair is near threshold at high energy facilities, such as BESIII, Belle II, and the proposed Super Tau-Charm Facility. In this work, we investigate this process using the QCD collinear factorization framework, where the scattering amplitudes are expressed in terms of the baryon timelike electromagnetic (EM) FFs and Compton FFs. We also provide a numerical estimate of the cross sections with a model for baryon-antibaryon GDAs. Our work provides us a possibility to extract the timelike baryon GFFs from near future experimental measurements, and these GFFs may be further used to study longstanding questions in hadronic physics such as the baryon spin decomposition and D-term
Assurance indicielle sous contraintes de demande et de solvabilité
No full textIndex insurance is often proposed to reduce protection gaps, especially for emerging risks. Unlike traditional insurance, it bases compensation on a measurable index, enabling faster payouts and lower claim management costs. This approach benefits both policyholders, through quick payments, and insurers, through reduced costs and better risk control due to reliable data and robust statistical estimates. An important difference with the concept of Cat Bonds is that the feasibility of such coverage relies on the possibility of mutualization. Mutualization, in turn, is achieved only if a sufficiently high number of policyholders agree to subscribe. The purpose of this paper is to introduce a model for the demand for index insurance and to provide conditions under which the solvency of the portfolio is achieved. From these conditions, we deduce a product that combines index and traditional indemnity insurance in order to benefit from the best of both approaches. We illustrate our results with a practical example involving the design of an index insurance product in the field of cyber insurance.L’assurance indicielle est souvent proposée comme moyen de réduire les écarts de couverture, en particulier pour les risques émergents. Contrairement à l’assurance traditionnelle, elle base les indemnisations sur un indice mesurable, ce qui permet des paiements plus rapides et des coûts de gestion des sinistres réduits. Cette approche est avantageuse à la fois pour les assurés, grâce à la rapidité des versements, et pour les assureurs, en raison de la baisse des coûts et d’un meilleur contrôle des risques, rendu possible par des données fiables et des estimations statistiques robustes. Une différence importante avec le concept des obligations catastrophes (Cat Bonds) réside dans le fait que la faisabilité de ce type de couverture repose sur la possibilité de mutualisation. Or, cette mutualisation n’est réalisable que si un nombre suffisamment élevé de souscripteurs accepte d’y adhérer. L’objectif de cet article est d’introduire un modèle de la demande en assurance indicielle et de fournir les conditions dans lesquelles la solvabilité du portefeuille est assurée. À partir de ces conditions, nous déduisons un produit combinant assurance indicielle et assurance traditionnelle, afin de tirer parti des avantages des deux approches. Nous illustrons nos résultats à travers un exemple pratique portant sur la conception d’un produit d’assurance indicielle dans le domaine de l’assurance cyber
Modélisation de la formation de membranes induite par transport diffusif
No full textThis thesis focuses on the study of polymer membrane formation by phase separation, with the objective of better understanding the physical mechanisms driving the emergence of microstructures and identifying the key parameters that influence the final morphology. Chapter 1 presents the scientific context of this work.After highlighting the industrial importance of poly-mer membranes, the phase inversion process usedfor their fabrication is described through its three main methods (VIPS, TIPS, and NIPS), with a particular emphasis on non-solvent induced phase separation(NIPS), which constitutes the core of this thesis. The literature review underlines the strengths and limitations of existing models for process simulation, thereby justifying the approach adopted in this study.In the first part, a model based on the ternaryCahn–Hilliard equation is employed to analyze phaseseparation in the NIPS process. The study shows thatthe initial film composition plays a crucial role: some compositions lead to clear phase separation, othersdo not exhibit phase separation, while an intermediateregime displays a combination of both behaviors. Thecrucial role of species mobilities is highlighted: theirasymmetry significantly alters the dynamics and theresulting structures. The model is then extended to3D, and the corresponding simulations allow a betterrepresentation of the morphological complexity of real membranes.Second, coupling the Cahn–Hilliard and Navier–Stokes equations demonstrates that fluid flows accelerate microstructure growth, while a high visco-sity of the polymer-rich phase contributes to micro-structure stabilization.Finally, a study of the TIPS process is conducted by combining numerical simulations and experimental results in a binary system. The simulations allow estimation of the surface tension and exploration of a proposed hypothesis explaining the experimentally observed concentration gradients.Cette thèse a pour objectif de mieux comprendre les mécanismes physiques gouvernant l’émergence des microstructures lors de la formation de membranes polymères par séparation de phase et d’identifier les paramètres clés influençant la morphologie finale.Le chapitre 1 présente le cadre scientifique de l’étude.Après avoir souligné l’importance industrielle des membranes polymères, le procédé d’inversion de phase utilisé pour leur fabrication est décrit à travers ses trois méthodes principales (VIPS, TIPS et NIPS),en mettant l’accent sur la séparation de phase induite par non-solvant (NIPS), qui constitue le cœur de cette thèse. L’état de l’art met en évidence les atouts et li-mites des modèles existants pour la modélisation du processus et justifie l’approche adoptée dans ce travail.Dans un premier temps, un modèle basé sur l’équation de Cahn–Hilliard ternaire est employé pour analyser la séparation de phase dans le procédé NIPS.L’étude montre que la composition initiale du film joue un rôle prépondérant : certaines compositions conduisent à une séparation de phase nette, d’autres à l’absence de structuration, tandis qu’un troisième régime intermédiaire combine partiellement les deux comportements. Le rôle crucial des mobilités des espèces est mis en évidence : leur asymétrie modifie sensiblement la dynamique et les structures obtenues. Ensuite, une extension du modèle en 3D permet de mieux représenter la complexité morphologique des membranes réelles.Dans un second temps, le couplage Cahn–Hilliard/Navier–Stokes montre que les écoulements de fluide accélèrent la croissance de la microstructure, tandis qu’une viscosité élevée de la phase riche en polymère contribue à la stabilisation de la microstructure.Enfin, une étude est menée sur le procédé TIPS,combinant simulations numériques et résultats expérimentaux dans un système binaire. Les calculs permettent d’estimer la tension de surface et d’explorer une hypothèse proposée pour expliquer l’apparition de gradients de concentration observés expérimentalement
Validation of power output modelling of an agrivoltaic system with on-site measurements in Palaiseau, France
No full textInternational audienceThe need to address growing water, food, and renewable energies challenges has led to agrivoltaics, a photovoltaic (PV) application, to continue to gain importance since it increases food production while reducing water usage for farming and generates energy. Bifacial modules are uniquely positioned to contribute to the development of agrivoltaics, making it necessary to do extensive analysis of their on-site performance at different locations. Modelling makes it possible to analyze the impact of PV modules on the development of various crops and vice versa, bridging the gap between theory and practice. This work presents the results of modelling the operation of a bifacial module in an agrivoltaics installation located in Palaiseau, France. From the ideal tilt angle to reduce self-shading using backtracking, to its power output. Our findings show that while existing irradiance, temperature, and power models provide adequate estimations for a horizontal position, there is a variation in error when in backtracking mode. For the front irradiance there is an increase in relative mean bias error of 4.2%, from 3.64% to 7.84%. For the back irradiance the underestimation increases by 2.82%, from -5.28% to -8.10%. This change for the backface irradiance is impacted by an increase in albedo of 4% due to the presence of crops. The mean bias error for the module temperature was 0.94°C in a horizontal position and 0.64 °C in backtracking. For the power output, an effective irradiance calculated with modelled irradiances led to an overestimation of 11.82% when in a horizontal position compared to 16.08% in backtracking. For irradiance yield, the presence of crops during the months of July and August contributes to an increased yield from the backface of the module. In terms of shading, the lack of neighboring modules in the southern extremity of the string will result in a yield up to 4.6% higher than those located in the center. Due to shadows caused by the meteorological station located between PV rows, there is a loss of yield of up to 17.43% between central modules and those located on the northern extremity of the string. In terms of performance, there is an increase in performance ratio with respect to a monofacial module of 9% during summer and of 11% during winter