Structural Characterization of Charcoal Exposed to High and Low pH: Implications for 14C Sample Preparation and Charcoal Preservation

Chemical and structural similarities between poorly preserved charcoal and its contaminants, as well as low radiocarbon concentrations in old samples, complicate 14C age determinations. Here, we characterize 4 fossil charcoal samples from the late Middle Paleolithic and early Upper Paleolithic strata of Kebara Cave, Israel, with respect to the structural and chemical changes that occur when they are subjected to the acid-base-acid (ABA) treatment. Differential thermal analysis and TEM show that acid treatment disrupts the structure, whereas alkali treatment results in the reformation of molecular aggregates. The major changes are ascribed to the formation of salt bridges at high pH and the disruption of the graphite-like crystallites at low pH. Weight losses during the treatments are consistently greater for older samples, implying that they are less well preserved. Based on the changes observed in vitro due to pH fluctuations, various methods for removing contamination, as well as a mechanism for preferential preservation of charcoal in nature, are proposed.Anthropolog

On the explosivity of a molten drop submitted to a small pressure perturbation

Under some circumstances that we aim to determine, a hot molten fuel drop flowing into a volatile liquid coolant and submitted to a small pressure wave, can be destabilized and explode in a few milliseconds. We propose a new approach to address this problem: in contrast with previous studies, we do not try to model the complete phenomenon but concentrate on its initiation. This way, we can differentiate favourable and non-favourable conditions with applications to PWR/BWR safety. We do the hypothesis that the occurrence of contacts between the two fluids is the criterion of explosion and phenomena occurring up to the contacting event are modelled, including the vapour film oscillations and the amplification of Rayleigh-Taylor instabilities at its interface. The latter feature receives a particular attention with a transient modelling adapted for variable acceleration cases. The fragmentation process itself is not studied in details but we give arguments supporting the fact that contacts between both liquids should induce a strong destabilization of the drops and initiate fragmentation. In this way, we can characterize the explosivity, i.e. the ability for the drop to explode, as a function of the various physical properties (e.g. pressure, temperatures). The model so deduced is qualified by comparison with the explosivity maps provided by Nelson and Duda [Nelson, L.S., Duda, P.M., 1985. Steam explosion experiments with single drops of iron oxide: Part II: parametric studies, NUREG CR-2718, April 1985]. Results obtained with this model confirm the experimental trends regarding the role of ambient pressure and liquid temperature. The influence of other parameters as the drop and the trigger characteristics are also investigated. We conclude this paper with some consideration on the implications for nuclear safety. © 2008 Elsevier B.V. All rights reserved

Eulerian modelling of melt solidification impact during fuel-coolant interaction

International audienceMelt droplet solidification is one major phenomenon acting on the mitigation of the strength of vapour explosions in situations related to nuclear safety. The modelling of solidification effects is then an important challenge for the evaluation of fuel-coolant interaction. We present an attempt for modelling the crust that might develop around melt droplets and its effect on the fine fragmentation within an Eulerian formulation of the flow equations, with application to the MC3D code. A physically-based modelling of the melt droplets temperature profile and a fragmentation criterion for partly solidified droplets are presented. The implementation of these models in the specific case of Eulerian modelling of the melt droplet phase is described. Two additional transported model parameters based on the most important droplets features regarding the fuel-coolant interaction behaviour are derived. The first property to transport is the crust stiffness because it enables the correct prediction of the amount of droplets participating in the droplet fragmentation process. The second is related to the energy transfer from the droplet interior to the surface. This enables to improve the surface temperature determination and reflects the history of the droplet cooling. Also the potential effect of solidification on the vapour explosion strength is shown. © 2015 Elsevier Ltd All rights reserved

Réacteurs de recherche. Accidents de type Borax

La présentation aborde un transitoire accidentel, de type accident grave de réactivité, concernant les réacteurs expérimentaux de type piscine à cœur fortement enrichi et plaques à base d’aluminium. A partir d’éléments historiques (origine : Etats-Unis, décennie 1950 et 1960), on présente dans un premier temps les caractéristiques générales des transitoires de type Borax. On présente ensuite les principaux enseignements des essais américains (campagnes d’essais Borax / Spert). On entre ensuite dans la physique du transitoire en se focalisant tout particulièrement sur les effets de couplage thermohydraulique / neutronique. On présente enfin les approches de modélisation du transitoire avec illustration à un benchmark AIEA en cours (CRP 1496)

Analysis of film boiling heat transfer during fuel-coolant interaction

International audienceIn the paper the applicability of the heat transfer models for the typical fuel-water and fuel-sodium interaction conditions are analysed and discussed. The important differences of sodium properties compared with water require not just adequate modelling of the film boiling regime but also adequate modelling of the transition boiling regime. The aim of the paper is to discuss the heat transfer modelling approaches applicable for the fuel-coolant interaction codes. First, the applicability of the adjusted Epstein-Hauser correlation for the film boiling modelling is analysed either comparing the correlation to the experiments with water or analysing the sodium properties. In the paper two modes of the film boiling regime are discussed. Indeed, experiments performed with the sub-cooled water have indicated an increase in the heat transfer between the stable and unstable mode. The numerical correction factors of the adjusted Epstein-Hauser correlation are proposed for the stable and unstable modes of the film boiling regime. Second, for the stable mode of the film boiling regime a modified Epstein-Hauser correlation is proposed to enable better general matching with the experimental data at significant sub-cooling. Next, for the unstable mode an alternative correlation is proposed and assessed. Finally, the empirical correlation for defining the transition from the film boiling regime towards the transition boiling regime is used to discuss the relevance of the unstable mode or transition boiling regime modelling in sodium. For sodium, additional experiments should be performed. © 2016 Elsevier Lt

The challenge of modeling fuel-coolant interaction Part II - Steam explosion

International audienceIn the course of a severe accident in a nuclear power plant cooled or moderated by water, the core might melt and flow down into the water. Under certain circumstances, a steam explosion might develop during the mixing of the melt and the water. Such an explosion, if occurring in the reactor pit of a PWR or BWR, might challenge the containment integrity and is thus an important issue for nuclear safety. This paper aims at presenting both a status of research and understanding of the phenomenon and the main characteristics of the models developed in the 3-dimensional computer code MC3D. We make a particular emphasis on the underlying difficulties, uncertainties and needs for further improvements. We discuss more particularly the two major phenomena that are the fine fragmentation and the pressurization process. We also give insights on the impact of melt solidification on the fragmentation and on the issue of oxidation. The verification basis of the models is discussed and finally, an example of 3D calculation is presented to highlight the current code capabilities. © 2014 Elsevier B.V. All rights reserved

A novel approach for modeling mixed convection film boiling for a vertical flat plate

International audienceA numerical model is developed to study mixed convection film boiling over a vertical flat plate. The integral form of conservation equations for each phase along with the appropriate interface conditions due to phase change is transformed into ordinary differential equation (ODE)-form. The length scale used in the model is based on Rayleigh-Taylor instability wave at the liquid-vapor interface. The heat transfer associated in the process is assessed and results are validated successfully for different available experimental results for natural convection and mixed convection film boiling. The mixed convection film boiling is characterized in terms of relevant nondimensional parameters for each phase. © 2014 Taylor and Francis Group, LLC

Fragmentation of a liquid metal droplet falling in a water pool

International audienceThis paper focuses on the experimental investigation of breakup regimes of a molten fusible metallic droplet in water, at intermediate Weber numbers with emphasis on mass and energy balance. The experiment consists in impacting perpendicularly a molten drop onto the interface of a deep water pool, at a controlled temperature. Using a drop-ondemand device and high speed shadowgraph, a single drop can be visualized during its evolution. There is a noticeable velocity jump when the droplet crosses the interface that can be modeled using unsteady Bernoulli equation. As observed for liquid-gas systems, the drop experiences different regimes of fragmentation, depending on its Weber number: oscillations, bag oscillations, prolate drop stretching breakup and then bowl-shaped bag breakup. However opposite to the gas-liquid case, Rayleigh-Taylor instability mechanism seems to be absent and this seems related to the bowl-shaped bag breakup mechanism when compared to the dome-shaped gas-liquid case. Statistics of the daughter droplets are then given, using either image analysis for large droplets size distribution or sieving and weighting of the solidified fragments for measurement of Sauter Mean Diameter (SMD) and surface energy creation. Lastly, a simple relation between the SMD and the Weber number is presented based on energy and mass balances. When comparing with previous higher Weber number results, a viscous transition corresponding to a strong increase in the energy loss is also shown to occur for the higher Weber number