Abrupt Ice Age Shifts in Southern Westerlies and Antarctic Climate Forced from the North

The Southern Hemisphere (SH) mid-latitude westerly winds play a central role in the global climate system via Southern Ocean upwelling, carbon exchange with the deep ocean, Agulhas Leakage, and Antarctic ice sheet stability. Meridional shifts in the SH westerlies have been hypothesized in response to abrupt North Atlantic Dansgaard-Oeschger (DO) climatic events of the last ice age, in parallel with the well-documented shifts of the intertropical convergence zone. Shifting moisture pathways to West Antarctica are consistent with this view, but may represent a Pacific teleconnection pattern. The full SH atmospheric-circulation response to the DO cycle, as well as its impact on Antarctic temperature, have so far remained unclear. Here we use five volcanically-synchronized ice cores to show that the Antarctic temperature response to the DO cycle can be understood as the superposition of two modes: a spatially homogeneous oceanic “bipolar seesaw” mode that lags Northern Hemisphere (NH) climate by about 200 years, and a spatially heterogeneous atmospheric mode that is synchronous with NH abrupt events. Temperature anomalies of the atmospheric mode are similar to those associated with present-day Southern Annular Mode (SAM) variability, rather than the Pacific South America (PSA) pattern. Moreover, deuterium excess records suggest a zonally coherent migration of the SH westerlies over all ocean basins in phase with NH climate. Our work provides a simple conceptual framework for understanding the circum-Antarctic temperature response to abrupt NH climate change. We provide observational evidence for abrupt shifts in the SH westerlies, with ramifications for global ocean circulation and atmospheric CO₂. These coupled changes highlight the necessity of a global, rather than a purely North Atlantic, perspective on the DO cycle

Low Void Effect (CFV) Core Concept Flexibility from Self-breeder to Burner Core

International audienceIn the frame of the French strategy on sustainable nuclear energy, several scenarios consider fuel cycle transition toward a plutonium multirecycling strategy in Sodium cooled Fast Reactor (SFR). Basically, most of these scenarios consider the deployment of a 60 GWe SFR fleet in two steps to renew the French PWR fleet. As scenarios do investigate long term deployment configurations, some of them require tools for nuclear phase-out studies. Instead of designing new reactors, the adopted strategy does focus on adaptation of existing ones into burner configurations. This is what was done in the frame of the EFR project at the end of the 90's using the CAPRA approach (French acronym for Enhance Plutonium Consumption in Fast Reactor). The EFR burner configuration was obtained by inserting neutronic penalties inside the core (absorber material and/or diluent subassembly). Starting from the preliminary industrial image of a SFR 3600MWth core based on Low Sodium Void concept (CFV in French), a CAPRA-like approach has been studied. As the CFV self-breeding is ensured by fertile blankets, a first modification consisted in the substitution of the corresponding depleted uranium by inert or absorber material leading to a natural burner core with only small impact on flux distribution. The next step toward CAPRA configuration was the substitution of 1/3 of the fuel pins by dummy pins (MgO pellets). The small spectrum shift due to MgO material insertion leads to an increase Doppler constant which exceeds the value of the reference case. As the core sodium void worth value is conserved, the CFV CAPRA core safety potential is quite similar to the one of the reference core.Fuel thermo-mechanical requirements are met by both nominal core power and fuel time residence reduction. However, these reduction factors are lower than those obtained for EFR core. The management of the enhanced reactivity swing is discussed

NEA SFR subassembly benchmark sensitivity/uncertainty propagation with depletion

International audienceFor the next generation of fast reactors, global objectives are required in terms of safety improvement, sustainability, waste minimization and non-proliferation. Concerning safety issues, particular efforts have been made in order to obtain core designs that can be resilient to accidental transients. Under the auspices of the Working Party on Scientific Issues of Reactor Systems (WPRS) an OECD/Nuclear Energy Agency (NEA) Expert Group Task Force on Uncertainty Analysis in Best-Estimate Modelling (UAM) for Design, Operation and Safety Analysis of Sodium-cooled Fast Reactors (SFR-UAM) has been initiated in 2015 with the objective to study the uncertainties in different stages of the next generation Sodium Fast Reactors. Among the identified topics of this expert group, a representative SFR fuel subassembly depletion benchmark has been set up in order to analyse the reactivity swing as well as the associated uncertainty level based on available nuclear data variance/covariance matrices coming from evaluations. The benchmark focuses on changes on number densities, on Doppler Effect and sodium void worth as well as kinetic parameters from the beginning of equilibrium cycle up to the end of cycle. For sensitivity/uncertainty estimation, two methods are tested a deterministic one based on sensitivities and a stochastic one using direct nuclear data sampling. In the deterministic approach, Boltzmann and Bateman equations are coupled at the sensitivity level with the help of the perturbation theory. This coupling is currently operational in ERANOS code system. The present implementation gives sensitivities for both reactivity coefficients and mass balance. For stochastic approach, relevant nuclear data set are directly sampled from variance/covariance data and used as input parameter for neutronic calculations. Both methods are found to be consistent

Neutronic and fuel cycle comparison of uranium and thorium as matrix for minor actinides bearing- blankets

International audienceMinor actinides transmutation is one of the three main axes defined by the 2006 French law for nuclear waste management, along with long-term storage and use of a deep geological repository. In the heterogeneous approach, minor actinides are loaded in specially designed targets assemblies which are located in the periphery of the core, in order to limit the impacts on core operations. In this paper, we compare the use of uranium and thorium dioxide as support matrix in which minor actinides are diluted in the target assemblies. Both UO$_2$ and ThO$_2$ exhibit sufficiently good irradiation behavior to withstand the long residence time associated with heterogeneous transmutation. Five different reprocessing strategies are compared in which some or all the elements in the blankets are reused after reprocessing. The impacts on core safety parameters and fuel cycle parameters are also evaluated for each case and it is found that using thorium as support matrix with reuse of uranium 233 leads to transmutation performances similar to the one obtained with the reuse of plutonium from uranium blankets with slightly lower global impacts on reactor operation and fuel cycle

Sensitivity analysis of minor actinides transmutation to physical and technological parameters

International audienceMinor actinides transmutation is one of the three main axis defined by the 2006 French law for management of nuclear waste, along with long-term storage and use of a deep geological repository. Transmutation options for critical systems can be divided in two different approaches- Homogeneous transmutation, in which minor actinides are mixed with the fuel. This exhibits the drawback of polluting the entire fuel cycle with minor actinides and also has an important impact on core reactivity coefficients such as Doppler Effect or sodium void worth for fast reactors.- Heterogeneous transmutation, in which minor actinides are inserted into transmutation targets which can be located in the center or in the periphery of the core. This presents the advantage of decoupling the management of the minor actinides from the conventional fuel and not impacting the core reactivity coefficients.In both cases, the design and analyses of potential transmutation systems has been carried out in the frame of generation 4 fast reactor using a perturbation approach in which nominal power reactor parameters are modified to accommodate the loading of minor actinides. However, when designing such a transmutation strategy, parameters from all steps of the fuel cycle must be taken into account, such as spent fuel heat load, gamma or neutron sources or fabrication feasibility. Considering a multi-recycling strategy of minor actinides, an analysis of relevant estimators necessary to fully analyze a transmutation strategy has been performed in this work and a sensitivity analysis of these estimators to a broad choice of reactors and fuel cycle parameters has been carried out.No threshold or percolation effects were observed. Saturation of transmutation rate with regards to several parameters has been observed, namely the minor actinides volume fraction and the irradiation time. Estimators of interest that have been derived from this approach include the maximum neutron source and decay heat load acceptable at reprocessing and fabrication steps, which influence among other things the total minor actinides inventory , the overall complexity of the cycle and the size of the geological repository. Based on this analysis, a new methodology to assess transmutation strategies is proposed

A 2D/1D Method for Consistent Burnup Parametrization of Cross Sections in SFR Fuel Depletion Calculations

International audienceThe accuracy of neutronic calculations in reactor physics is determined by the quality of the averaged cross sections used to solve the Boltzmann transport equation. As the reactor burns its fuel, a change occurs in the neutronic properties of the media so the averaged cross sections become time-dependent. Several transport calculations are therefore required at the cross section generation stage to perform a burnup (or fluence) parametrization of the data. This paper proposes to use the now well known 2D/1D method to perform this parametrization. The idea of such a strategy is to avoid computationally expensive 3D simulations while overcoming the drawbacks of standard 2D models. The algorithm is applied to produce time-dependent effective cross sections for a SFR fuel assembly with CFV design. The fuel depletion analysis is then conducted in a ``core environment' and results are compared to independent Monte Carlo simulations. Good performances are found both for the evolution of the spatial distribution of isotopic concentrations and the assembly reactivity loss

Analysis of the impacts of homogeneous minor actinides loading in low void effect sodium fast reactor cores

International audienceMinor actinides transmutation is a solution to decrease the long-term radiotoxicity of nuclear waste and limit their short-term decay heat. In the homogenous approach, minor actinides are mixed in the fuel and loaded in the core in order to turn them into fission products. This leads to a neutron spectrum hardening in the core, which has a negative impact on the core integral feedback coefficients such as the Doppler Effect or the sodium void worth. Analysis of these impacts was used in the past to establish limit on maximal minor actinides loading in a reactor core. Low-void cores (CFV in French) have been recently developed by CEA to achieve negative sodium void worth by adding axial heterogeneities in the core layout in the form of an upper sodium plenum and an inner fertile blanket. In this paper, the impacts of minor acti-nides loading in such a core are analyzed and a comparison is carried out with earlier homogeneous core designs. In a first time, the impacts on integral feedbacks coefficients are evaluated, and in a second time, the perturbations of the core behavior during selected representative transients are analyzed. It is shown that even if the impacts on integral coefficients are similar between the two core designs, the low-void core behavior during loss-of-flow transient is not negatively impacted by minor actinides loading. For reactivity insertion transient, the two core designs behave similarly. It can be concluded that considering heterogeneous cores, the use of integral coefficients is insufficient to characterize the impact of minor actinides loading on the core for loss-of-flow transient. However, the impacts of minor actinides loading on the Doppler integral feedback coefficient can be used as reliable estimator for the modification of the core behavior during a reactivity insertion transient

A multiscale model for magneto-elastic couplings

At the macroscopic scale two different phenomena illustrate the couplings between the elastic and magnetic behaviours of ferromagnetic materials : first, magnetisation induces a deformation mechanism called magnetostriction, and, second, stresses have an effect on the magnetic behaviour. The complexity of the non-linear relations between these phenomena is such that few realistic macroscopic constitutive equations have been proposed to model the coupled magneto-elastic behaviour of magnetic materials. Magnetisation and magnetostriction are macroscopic manifestations of the complex magnetic domain structure that is modified by applied mechanic and magnetic loads. Herein, it is proposed to use homogenisation methods to deduce the macroscopic behaviour of single crystals and polycrystals from a statistical description of the magnetic domain structure. Therefore, the macroscopic couplings naturally arise from the expression of the free energy written at the level of the magnetic domains