DISCONTINUOUS GALERKIN DISCRETIZATION AND HP-REFINEMENT FOR THE RESOLUTION OF THE NEUTRON TRANSPORT EQUATION

International audienceThis paper presents a hp−refinement method for a first order scalar transport- reaction equation discretized by a discontinuous Galerkin method. First, the theoretical rates of convergence of h− and p−refinement are recalled and numerically tested. Then, in order to design some meshes, we propose two different estimators of the local error on the spatial domain. These quantities are analysed and compared depending on the regularity of the solution so as to find the best way to lead the refinement process and the best strategy to choose between h− and p−refinement. Finally, the different possible refinement strategies are compared first on analytical examples and then on realistic applications for neutron transport in a nuclear reactor core

Vaccine breakthrough hypoxemic COVID-19 pneumonia in patients with auto-Abs neutralizing type I IFNs

Life-threatening `breakthrough' cases of critical COVID-19 are attributed to poor or waning antibody response to the SARS- CoV-2 vaccine in individuals already at risk. Pre-existing autoantibodies (auto-Abs) neutralizing type I IFNs underlie at least 15% of critical COVID-19 pneumonia cases in unvaccinated individuals; however, their contribution to hypoxemic breakthrough cases in vaccinated people remains unknown. Here, we studied a cohort of 48 individuals ( age 20-86 years) who received 2 doses of an mRNA vaccine and developed a breakthrough infection with hypoxemic COVID-19 pneumonia 2 weeks to 4 months later. Antibody levels to the vaccine, neutralization of the virus, and auto- Abs to type I IFNs were measured in the plasma. Forty-two individuals had no known deficiency of B cell immunity and a normal antibody response to the vaccine. Among them, ten (24%) had auto-Abs neutralizing type I IFNs (aged 43-86 years). Eight of these ten patients had auto-Abs neutralizing both IFN-a2 and IFN-., while two neutralized IFN-omega only. No patient neutralized IFN-ss. Seven neutralized 10 ng/mL of type I IFNs, and three 100 pg/mL only. Seven patients neutralized SARS-CoV-2 D614G and the Delta variant (B.1.617.2) efficiently, while one patient neutralized Delta slightly less efficiently. Two of the three patients neutralizing only 100 pg/mL of type I IFNs neutralized both D61G and Delta less efficiently. Despite two mRNA vaccine inoculations and the presence of circulating antibodies capable of neutralizing SARS-CoV-2, auto-Abs neutralizing type I IFNs may underlie a significant proportion of hypoxemic COVID-19 pneumonia cases, highlighting the importance of this particularly vulnerable population

COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study

Background: The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms. Methods: International, prospective observational study of 60 109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms. Results: ‘Typical’ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (≤ 18 years: 69, 48, 23; 85%), older adults (≥ 70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each P < 0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country. Interpretation: This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men

Mass transfer between a continuous oxide phase (U-O-Zr) and a steel droplet at liquid state: potential impact on corium pool behaviour during in-vessel melt retention

International audienceIn the frame of light water reactors' severe accidents (SA), the success of a mitigation based on the core melt retention in the vessel lower head largely depends on the corium pool phases stratification. Such a melt is composed of molten oxide materials relocated from the core and molten steel progressively "incorporated" into the pool during the transient. Because of the miscibility gap in the U-O-Zr-steel thermodynamic system at liquid state, such a pool can go through various stratified states during a transient with an heavy metal phase at the bottom or a light metal one at the top.Actually, there is a significant knowledge gap related to steel migration to the pool that influences the stratification transient and can lead in some cases to a large uncertainty on the heat flux that the pool imposed on the vessel lower head. In particular, when the ablated vessel wall is considered, the molten steel relocation, either as a continuous lighter phase above the pool or as dispersed phase into the oxide pool, depends on the behaviour of the refractory crust at the pool/vessel interface (that can undergo dissolution or mechanical failure). In the case of a dispersed metal phase into the oxide pool, determining the subsequent state of the lower head pool requires the evaluation of the thermochemical interaction between both materials and their possible hydrodynamic separation. To the best of our knowledge, no model described these coupled phenomena in integral SA codes. In this paper, a model is proposed to better assess the phenomena at stake in such a case and clarify the need (or not) to enhance stratified pool models in SA codes. To do so, this integral model, constructed on "first-order" hypotheses, has been implemented in the PROCOR software platform and used for a parametric analysis that provide insightful order of magnitudes and trends. The main parameters are the droplet size, the corium chemical composition and the model closures associated with interfacial mass transfer.The results reported here show that mass transfer, "competing" with droplet hydrodynamics, probably plays a role only for droplets of millimetric size that can, depending on the pool composition, relocate under the oxide under the form of a continous heavy metal layer. For larger droplets, it is a light metal layer above the pool that can be expected. The prolonged existence of a dispersed metal phase that would remain mixed into the oxide phase seems to be only possible for a very narrow range of pool composition near the stratification inversion threshold

Mass transfer induced Rayleigh-Taylor instabilities between two immiscible liquids: the unique case of corium

International audienceLiquid pools composed of molten multicomponent materials are often multiphase systems and their behavior is complex because the associated non-miscible phases can be chemically reactive. At the interfaces between phases, mass transfer occurs and can have a significant impact on the overall pool thermalhydraulics. The case of interest for this paper pertains to the behavior of molten core materials "so-called corium" in a nuclear reactor during a severe accident like at the Fukushima Daiichi plant. These materials form a liquid pool in which inter-phase mass transfer occurs between an oxygen-rich phase (mainly composed suboxidized U and Zr elements form the fuel and cladding materials of the core) and an oxygen-poor phase (initially made of molten steel elements) that tend to segregate under the force of gravity. The specificity of the underlying (O, U, Zr, steel) thermodynamic system is that the density difference between both phases under equilibrium can change of sign depending on the tie line under consideration. Accordingly, interface mass transfer can induce local density changes in such a way that, under certain conditions, Rayleigh-Taylor instabilities (RT) can occur, grow and lead to modification of the phase topology that are referred to as "stratification transients". To the author's best knowledge, such a coupling between mass transfer and hydrodynamics in a system composed of two immiscible liquid phases is peculiar.Because of the materials at play (corrosive liquids at very high temperature), so-called prototypical experiments (where depleted uranium is used instead of enriched uranium) to characterize the corium pool behavior are scarce. In any case, they are carried out at a spatial scale that is at least one order of magnitude (in length) smaller than the reactor one. Accordingly, the experimental data about coupling between mass transfer and hydrodynamics are very limited and the analysis of the corium pool behavior highly relies on modelling and numerical simulations. Two different types of models can be distinguished. Integral models are used in lumped parameter codes that simulate the overall pool behavior during a phase of the severe accident while distributed parameter models are to be coupled with Computational Fluid Dynamics for finer analysis of stratification transients in order to calibrate or validate integral models.Such modelling is an active research avenue in the severe accident community, in particular at CEA. On the one hand, recent works in collaboration with Ecole Polytechnique have been focused on the development and the analysis of a diffuse interface model of Cahn-Hilliard type and the "proof of principle" has been made that it can treat the coupling between interfacial mass transfer and RT instabilities growth. On the other hand, an integral model that explicitly (albeit under simplifying hypotheses) takes into account the dispersed phase dynamics in addition to inter-phase mass transfer was found useful to qualitatively analyze different possible stratification transients for which experiments does not exist.In this work, a critical review of these coupled phenomena is proposed based on both state-of-the-art experimental data and very recent simulation results

Interfaces en physique des réacteurs nucléaires: Contribution à la modélisation et au développement de méthodes numériques associées en neutronique et physique du corium

Dans le cadre industriel des réacteurs nucléaires de fission électrogènes, cette synthèse d’une grandepartie de mes activités de recherche est faite sous le thème de la modélisation et du développement deméthodes pour la simulation numérique de phénomènes associés aux interfaces entre différents matériaux.Ces travaux s’inscrivent dans deux domaines distincts de la physique des réacteurs nucléaires : d’une part,la neutronique qui s’intéresse à la caractérisation de la population des neutrons au sein d’un réacteurde fission, d’autre part, la physique du corium qui porte sur le comportement des matériaux fondus(combustible, structure, ...) du réacteur au cours d’un accident grave de fusion du cœur.En neutronique, les besoins en simulation relatifs aux réacteurs à neutrons rapides refroidis au sodiumm’ont amené à participer au développement d’un solveur 3D pour l’équation de transport des neutronsbasé sur la méthode Galerkin discontinue. Dans ce cadre, j’ai contribué :• à l’analyse et au développement de méthodes de raffinement adaptatif hp (en ordre et en espace)associées à ce schéma de discrétisation spatiale ;• à l’extension de la théorie standard des perturbations (qui permet de relier les variations de réactivitéaux variations locales des paramètres régissant l’état du système) à cette discrétisation adaptative ;• à l’amélioration du traitement de l’interface entre le cœur et le réflecteur d’un réacteur à neutronsrapides vis-à-vis de l’homogénéisation énergétique.Pour ce qui est de la physique du corium, en support à l’étude de la stratégie de rétention du corium encuve pour les réacteurs à eau légère, mes travaux ont porté essentiellement sur la modélisation tant à uneéchelle macroscopique que mésoscopique de certains phénomènes associés au comportement du corium.En particulier, j’ai contribué :• à l’amélioration de la modélisation thermohydraulique de la couche métallique supérieure du bainde corium ;• au développement d’une modélisation mésoscopique de la diffusion multicomposant multiphasiqueassociée à la cinétique de stratification des phases liquides du bain ;• à l’analyse du couplage des modèles de thermohydraulique et thermochimie pour le corium vis-à-visde la cohérence des données thermodynamiques mises en jeu.Par-delà la diversité des “physiques”, modèles et méthodes numériques mis en jeu dans les travauxrapportés ici, y est illustrée la position que j’ai essayé d’occuper dans ces processus de recherche à l’interfacepluridisciplinaire entre application industrielle et recherche académique

Développement de la méthode des caractéristiques pour le calcul de réseau

Les méthode des caractéristiques -- Les méthodes d'accélération -- Préconditionnement par une méthode de algebraic collapsing acceleration -- Gmres dans contexte de la méthode des caractéristiques -- Cadre de développement et méthodologie de test -- Benchmarks BWR -- Benchmark Candu NG -- Benchmark PWR

Butler-based thermodynamic modelling of interfacial energies for in-vessel corium systems

International audienceThe modelling of transient phase segregation phenomena in corium multicomponent thermodynamic systems is limited especially in the miscibility gap at liquid state that leads to a phase separation (stratification of the liquid phases for corium) which is of tremendous importance. The modelling of the associated phase separation transients in corium stratification is limited by deficiency in the interfacial energy data which are difficult to measure experimentally for these high-temperature systems. The limitations of summarizing the interfacial energies as constant coefficients were observed in a recent work on simulation of the stratification kinetics for a corium pool and the necessity to improve the estimation of interfacial energies. This work tests the efficacy of a thermodynamic approach to evaluate the interfacial tensions, initially proposed for metallic systems. This method is based on the Butler equation and makes use of existing thermodynamic databases for multicomponent systems constructed by the CALPHAD method. In practice, the application of this method to corium systems was performed using the open-source code openIEC. To do so, this code was modified to suit our requirements both functionally and methodically. In particular, the OpenCalphad Gibbs energy minimizer has been interfaced in this framework and code verification against previous results was carried out. Then, interfacial energies were calculated for two relevant corium systems (U-O and U-O-Zr-Fe) using two different existing databases (TAF-ID, NUCLEA). The first results thus obtained were analyzed in terms of general trends and compared with existing theoretical aspects. Large differences observed depending on the thermodynamic database were also investigated

Lettre de Michel Le Tellier (chancelier de France) à Romain Dalon (avocat général au parlement de Guyenne) datée du 14 avril 1680, à Saint-Germain-en-Laye

Lettre de Michel Le Tellier (chancelier de France) à Romain Dalon (avocat général au parlement de Guyenne) datée du 14 avril 1680, à Saint-Germain-en-Laye. In: Correspondance administrative sous le règne de Louis XIV, recueillie et mise en ordre par G. B. Depping. Tome IV et dernier. Travaux publics – Affaires religieuses – Protestants – Sciences, lettres et arts – Pièces diverses. Paris : Imprimerie nationale, 1855. p. 326