Méthodes d'intégration et algorithmes d'optimisation accélérés

We show that accelerated optimization methods can be seen as particular instances of multi-step integration schemes from numerical analysis, applied to the gradient flow equation. In comparison with recent advances in this vein, the differential equation considered here is the basic gradient flow and we show that multi-step schemes allow integration of this differential equation using larger step sizes, thus intuitively explaining acceleration results

European collaborative efforts to achieve effective, safe, and cost-controlled dismantling of nuclear facilities

This paper aims to give an overview of very recent European coordinate efforts to implement technologies of the “4.0 Industry” in the nuclear deconstruction sector. This objective aims to benefit from the lever of efficiency and reliability represented by innovative technologies on all the value chain of the dismantling, from early characterization to the dismantling operations themselves through engineering studies, waste management, project management and coordination of multiple stakeholders of each project. The outcomes of five projects (INNO4GRAPH, LD-SAFE, PLEIADES, CLEANDEM and INSIDER) are summarized here. They result in a unique data and knowledge common base, as well as in a significant sharing of experience based on dismantling projects already carried out or to come. They also result in designing new tools or methods natively taking into account the needs of a maximum of dismantling operators, as well as new test facilities. This will allow the undertaken joint work and collaboration to be continued. All of this paves the way to further collaborative projects and developments, in order to continue to implement reliable new technologies and processes in European dismantling projects to make future dismantling operations more efficient, safer and more cost-effective

Can lepton flavor violating interactions explain the atmospheric neutrino problem?

We investigate whether flavor changing neutrino interactions (FCNIs) can be sufficiently large to provide a viable solution to the atmospheric neutrino problem. Effective operators induced by heavy boson exchange that allow for flavor changing neutrino scattering off quarks or electrons are related by an $SU(2)_L$ rotation to operators that induce anomalous tau decays. Since $SU(2)_L$ violation is small for New Physics at or above the weak scale, one can use the upper bounds on lepton flavor violating tau decays or on lepton universality violation to put severe, model-independent bounds on the relevant non-standard neutrino interactions. Also $Z$-induced flavor changing neutral currents, due to heavy singlet neutrinos, are too small to be relevant for the atmospheric neutrino anomaly. We conclude that the FCNI solution to the atmospheric neutrino problem is ruled out.Comment: 16 pages, no figures, Late

Le corium de Fukushima Daiichi : formation, état actuel et R&D en vue de sa récupération

International audienceLe 11 mars 2011, un tsunami de grande amplitude a atteint les cotes japonaises moins d’une heure après le séisme de magnitude 9 de Tohoku et a entraîné un accident grave sur la centrale de Fukushima Daiichi. Les cœurs des tranches 1, 2 et 3 ont fondu et formé un matériau dénommé corium. On pense que ce corium se répartit actuellement entre la cuve du réacteur et le puits de cuve (piédestal). Le gouvernement japonais a décidé le démantèlement de ces centrales accidentées, ce qui implique en particulier la découpe et la récupération du corium.Une équipe française a été chargé de la R&D sur la technologie de découpe laser du corium et ses principaux résultats sont présentés

Méthodes d'intégration et algorithmes d'optimisation accélérés

We show that accelerated optimization methods can be seen as particular instances of multi-step integration schemes from numerical analysis, applied to the gradient flow equation. In comparison with recent advances in this vein, the differential equation considered here is the basic gradient flow and we show that multi-step schemes allow integration of this differential equation using larger step sizes, thus intuitively explaining acceleration results

Development and validation of agglomeration model for CFD simulations of aerosol dispersion during Fukushima fuel debris retrieval

International audienceThe general context of this article is related to demonstrate the feasibility of the use of the laser cutting technique for the fuel debris retrieval on the damaged reactors of Fukushima Dai-ichi. IRSN is involved in a project led by ONET, in collaboration with CEA, to bring relevant elements to analyze the risk occurred by the dispersion of aerosols emitted by the dismantling operations. During the laser cutting operations for retrieving the fuel debris in air condition, particles will be produced, inducing a potential risk of dispersion into the environment. Hence, evaluating the amount of aerosols able to deposit on the walls and those able to be released into the environment is one of the safety key issues in the dismantling actions of reactors of Fukushima Dai-ichi. For that, IRSN performed computational fluid dynamics (CFD) simulations of dispersion, agglomeration and deposition of particles whose size distribution was measured during laser cutting operations of inactive fuel simulants (Chagnot et al., 2018, Porcheron et al., 2018).These simulations were conducted with the ANSYS CFX CFD code into which a moment method called DQMOM and a deposition model previously developed by IRSN (Nerisson et al., 2011) were implemented. The first numerical results (Gelain et al., 2018) showed a quite good agreement with the experimental ones for a standard particle size distribution (lognormal with one mode), but some improvements are needed for less classical distributions such bimodal ones.The work presented here proposes an alternative method to the previous one allowing taking into account different kinds of particle size distribution in the CFD simulations of particle agglomeration. This method uses the real experimental distribution to calculate the distribution moments which are then directly implemented in the simulation instead of using a lognormal fit. The previous CFD simulations were calculated again by using this method and showed a better agreement with the experimental results issued from aerosol source term characterization for both absolute values and time evolutions

Fukushima Daiichi fuel debris simulant materials for the development of cutting and collection technologies

International audienceCutting fuel debris (solidified corium) is an important issue for the decommissioning of Fukushima Daiichi Nuclear Power Station. The main reasons for developing and using fuel debris simulants are presented. The relative merits of the various types of materials (stainless steel, zirconium, sintered alumina-zirconia, cast fused zirconia, metal+zirconia, melted inactive simulants, prototypic fuel debris simulant, irradiated fuel debris simulant) that can be used to simulate fuel debris cutting have been assessed against criteria relevant for the cutting technique itself (hardness, melting temperature, elastic modulus, toughness, heterogeneity) as well as relevant to (radioactive) aerosol and combustible gas generation. It appears that simplified simulants can be used for the development of fuel debris cutting techniques but have some limitations in terms of representativity so that melted inactive fuel debris simulant must be used to assess the cutting performance. Concerning combustible gas generation, zirconium plates will provide an upper bound in term of underwater generation of hydrogen. Finally, for aerosol and dust generation, it appears that non-radioactive simulant cannot correctly represent the aerosol formation during cutting. Prototypic fuel debris simulant, using depleted uranium and natural isotopic composition for the fission product elements are the best available option for determination of cutting secondary outlet

Development and validation of agglomeration model for CFD simulations of aerosol dispersion during Fukushima fuel debris retrieval

International audienceThe general context of this article is related to demonstrate the feasibility of the use of the laser cutting technique for the fuel debris retrieval on the damaged reactors of Fukushima Dai-ichi. IRSN is involved in a project led by ONET, in collaboration with CEA, to bring relevant elements to analyze the risk occurred by the dispersion of aerosols emitted by the dismantling operations. During the laser cutting operations for retrieving the fuel debris in air condition, particles will be produced, inducing a potential risk of dispersion into the environment. Hence, evaluating the amount of aerosols able to deposit on the walls and those able to be released into the environment is one of the safety key issues in the dismantling actions of reactors of Fukushima Dai-ichi. For that, IRSN performed computational fluid dynamics (CFD) simulations of dispersion, agglomeration and deposition of particles whose size distribution was measured during laser cutting operations of inactive fuel simulants (Chagnot et al., 2018, Porcheron et al., 2018).These simulations were conducted with the ANSYS CFX CFD code into which a moment method called DQMOM and a deposition model previously developed by IRSN (Nerisson et al., 2011) were implemented. The first numerical results (Gelain et al., 2018) showed a quite good agreement with the experimental ones for a standard particle size distribution (lognormal with one mode), but some improvements are needed for less classical distributions such bimodal ones.The work presented here proposes an alternative method to the previous one allowing taking into account different kinds of particle size distribution in the CFD simulations of particle agglomeration. This method uses the real experimental distribution to calculate the distribution moments which are then directly implemented in the simulation instead of using a lognormal fit. The previous CFD simulations were calculated again by using this method and showed a better agreement with the experimental results issued from aerosol source term characterization for both absolute values and time evolutions

Récupération du corium des réacteurs de Fukushima Dai-ichi : efficacité d'épuration des aérosols produits lors de la découpe au laser sous eau de simulants de corium

International audienc