Dispersion enhancement and damping by buoyancy driven flows in 2D networks of capillaries

The influence of a small relative density difference on the displacement of two miscible liquids is studied experimentally in transparent 2D networks of micro channels. Both stable displacements in which the denser fluid enters at the bottom of the cell and displaces the lighter one and unstable displacements in which the lighter fluid is injected at the bottom and displaces the denser one are realized. Except at the lowest mean flow velocity U, the average $C(x,t)$ of the relative concentration satisfies a convection-dispersion equation. The dispersion coefficient is studied as function of the relative magnitude of fluid velocity and of the velocity of buoyancy driven fluid motion. A model is suggested and its applicability to previous results obtained in 3D media is discussed

A reduced order model for investigating the dynamics of the Gen-IV LFR coolant pool

In the control field, the study of the system dynamics is usually carried out relying on lumped-parameter or one-dimensional modelling. Even if these approaches are well suited for control purposes since they provide fast-running simulations and are easy to linearize, they may not be sufficient to deeply assess the complexity of the systems, in particular where spatial phenomena have a significant impact on dynamics. Reduced Order Methods (ROM) can offer the proper trade-off between computational cost and solution accuracy. In this work, a reduced order model for the spatial description of the Gen-IV LFR coolant pool is developed for the purpose of being employed in a control-oriented plant simulator of the ALFRED reactor. The spatial modelling of the reactor pool is based on the POD-FV-ROM procedure, previously developed with the aim of extending the literature approach based on Finite Element to the Finite Volume approximation of the Naviera-Stokes equations, and building a reduced order model capable of handling turbulent flows modelled through the RANS equations. The mentioned approach is employed to build a ROM-based component of the ALFRED simulator for the coolant pool. The possibility of varying the input variables of the model has been also undertaken. In particular, the lead velocity at the Steam Generator outlet has been considered as a parametrized boundary condition since it can be a possible control variable. The results have turned out to be very satisfactory in terms of both accuracy and computational time. As a major outcome of the ROM model, it has been proved that its behaviour is more accurate than a 0D-based model without requiring an excessive computational cost

Effet getter de multicouches métalliques pour des applications MEMS. Etude de la relation Elaboration - Microstructure - Comportement

Whilst satisfying low-cost requirements, performances and lifetime of many MEMS can be enhanced by performing wafer-level packaging of devices under vacuum or controlled atmosphere conditions. However, this implies the use of non-evaporable getters (NEG) inside MEMS cavities for residual gases removal. Relationships between elaboration, microstructure and pumping behavior of NEG thin films are investigated in this thesis. After a literature review on MEMS hermetic sealing and getter effect, NEG thin films pumping behavior modification by metallic sub-layers addition is presented. Then, in order to explain this modification, elaboration parameters and thermal treatments influence on thin films microstructure is analyzed. Lastly, nitrogen gettering behavior of NEG is characterized, as well as activation and pumping mechanisms. From these results, some recommendations for NEG thin films integration in MEMS are finally proposed.L'objectif de cette thèse est d'établir les liens entre élaboration, microstructure et comportement des getters non-évaporables (NEG) en couches minces, en vue de leur utilisation dans le cadre du packaging collectif des MEMS sous vide ou sous atmosphère contrôlée. Après une étude bibliographique sur l'herméticité des MEMS et l'effet getter, la modification du comportement de piégeage de gaz par les NEG couches minces, engendré par l'ajout de sous-couches métalliques, est mise en évidence. Afin d'expliquer cette influence, la microstructure des couches minces est étudiée, notamment sa dépendance aux paramètres d'élaboration et aux traitements thermiques. Ensuite, le comportement macroscopique de piégeage de l'azote est caractérisé, de même que les mécanismes microscopiques d'activation et de pompage. Ces derniers permettent finalement d'élaborer quelques recommandations pour l'intégration des NEG couches minces dans les MEMS

Study of the getter effect for metallic materials thin films deposited by common processes of microelectronics

L'objectif de cette thèse est d'établir les liens entre élaboration, microstructure et comportement des getters non-évaporables (NEG) en couches minces, en vue de leur utilisation dans le cadre du packaging collectif des MEMS sous vide ou sous atmosphère contrôlée. Après une étude bibliographique sur l'herméticité des MEMS et l'effet getter, la modification du comportement de piégeage de gaz par les NEG couches minces, engendré par l'ajout de sous-couches métalliques, est mise en évidence. Afin d'expliquer cette influence, la microstructure des couches minces est étudiée, notamment sa dépendance aux paramètres d'élaboration et aux traitements thermiques. Ensuite, le comportement macroscopique de piégeage de l'azote est caractérisé, de même que les mécanismes microscopiques d'activation et de pompage. Ces derniers permettent finalement d'élaborer quelques recommandations pour l'intégration des NEG couches minces dans les MEMS.Whilst satisfying low-cost requirements, performances and lifetime of many MEMS can be enhanced by performing wafer-level packaging of devices under vacuum or controlled atmosphere conditions. However, this implies the use of non-evaporable getters (NEG) inside MEMS cavities for residual gases removal. Relationships between elaboration, microstructure and pumping behavior of NEG thin films are investigated in this thesis. After a literature review on MEMS hermetic sealing and getter effect, NEG thin films pumping behavior modification by metallic sub-layers addition is presented. Then, in order to explain this modification, elaboration parameters and thermal treatments influence on thin films microstructure is analyzed. Lastly, nitrogen gettering behavior of NEG is characterized, as well as activation and pumping mechanisms. From these results, some recommendations for NEG thin films integration in MEMS are finally proposed

Effet getter de multicouches métalliques pour des applications MEMS. Etude de la relation Elaboration - Microstructure - Comportement

L'objectif de cette thèse est d'établir les liens entre élaboration, microstructure et comportement des getters non-évaporables (NEG) en couches minces, en vue de leur utilisation dans le cadre du packaging collectif des MEMS sous vide ou sous atmosphère contrôlée. Après une étude bibliographique sur l'herméticité des MEMS et l'effet getter, la modification du comportement de piégeage de gaz par les NEG couches minces, engendré par l'ajout de sous-couches métalliques, est mise en évidence. Afin d'expliquer cette influence, la microstructure des couches minces est étudiée, notamment sa dépendance aux paramètres d'élaboration et aux traitements thermiques. Ensuite, le comportement macroscopique de piégeage de l'azote est caractérisé, de même que les mécanismes microscopiques d'activation et de pompage. Ces derniers permettent finalement d'élaborer quelques recommandations pour l'intégration des NEG couches minces dans les MEMS.Whilst satisfying low-cost requirements, performances and lifetime of many MEMS can be enhanced by performing wafer-level packaging of devices under vacuum or controlled atmosphere conditions. However, this implies the use of non-evaporable getters (NEG) inside MEMS cavities for residual gases removal. Relationships between elaboration, microstructure and pumping behavior of NEG thin films are investigated in this thesis. After a literature review on MEMS hermetic sealing and getter effect, NEG thin films pumping behavior modification by metallic sub-layers addition is presented. Then, in order to explain this modification, elaboration parameters and thermal treatments influence on thin films microstructure is analyzed. Lastly, nitrogen gettering behavior of NEG is characterized, as well as activation and pumping mechanisms. From these results, some recommendations for NEG thin films integration in MEMS are finally proposed.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Chapter 7 - Multi-scale simulations of liquid metal systems

The thermal-hydraulic behavior of complex, large-scale systems such as nuclear reactors is the product of a wide range of fluid-mechanics phenomena. In principle, these phenomena can be modeled directly (at least for single-phase flows) through the direct simulation of the Navier-Stokes equations. However, such a “reactor-scale” direct simulation approach remains infeasible today, given that such a model would need to span a range of scales. Where code developments up to today focused on resolving issues at a specific scale, challenges in which complex phenomena at various scales interact require a different approach. Some system thermal-hydraulic codes nowadays allow three-dimensional modeling. However, computational fluid dynamics codes are superior to that respect. On the other hand, such codes are computationally too expensive to allow modeling a complete system. To overcome this problem, different codes covering multiple scales can be coupled in a multiscale simulation approach. This chapter introduces this multiscale simulation concept, presents modeling choices to be made, explains the challenges, and provides guidelines. Finally, some examples of application of such approaches are shown

Probabilistic margin evaluation on accidental transients for the ASTRID reactor project

ASTRID is a technological demonstrator of Sodium cooled Fast Reactor (SFR) under development. The conceptual design studies are being conducted in accordance with the Generation IV reactor objectives, particularly in terms of improving safety. For the hypothetical events, belonging to the accidental category “severe accident prevention situations” having a very low frequency of occurrence, the safety demonstration is no more based on a deterministic demonstration with conservative assumptions on models and parameters but on a “Best-Estimate Plus Uncertainty” (BEPU) approach. This BEPU approach ispresented in this paper for an Unprotected Loss-of-Flow (ULOF) event. The Best-Estimate (BE) analysis of this ULOFt ransient is performed with the CATHARE2 code, which is the French reference system code for SFR applications. The objective of the BEPU analysis is twofold: first evaluate the safety margin to sodium boiling in taking into account the uncertainties on the input parameters of the CATHARE2 code (twenty-two uncertain input parameters have been identified, which can be classified into five groups: reactor power, accident management, pumps characteristics, reactivity coefficients, thermal parameters and head losses); secondly quantify the contribution of each input uncertainty to the overall uncertainty of the safety margins, in order to refocusing R&D efforts on the most influential factors. This paper focuses on the methodological aspects of the evaluation of the safety margin. At least for the preliminary phase of the project (conceptual design), a probabilistic criterion has been fixed in the context of this BEPU analysis; this criterion is the value of the margin to sodium boiling, which has a probability 95% to be exceeded, obtained with a confidence level of 95% (i.e. the M5,95percentile of the margin distribution). This paper presents two methods used to assess this percentile: the Wilks method and the Bootstrap method ; the effectiveness of the two methods is compared on the basis of 500 simulations performed with theCATHARE2 code. We conclude that, with only 100 simulations performed with the CATHARE2 code, which is a number of simulations workable in the conceptual design phase of the ASTRID project where the models and the hypothesis are often modified, it is best in order to evaluate the percentile M5,95 of the margin to sodium boiling to use the bootstrap method, which will provide a slightly conservative result. On the other hand, in order to obtain an accurate estimation of the percentileM5,95, for the safety report for example, it will be necessary to perform at least 300 simulations with the CATHARE2 code. In this case, both methods (Wilks and Bootstrap) would give equivalent results