One of the most disputed issues raised by molten corium concrete interaction (MCCI) is how the 2D cavity ablation in an oxidic pool evolves: why is the ablation anisotropic with siliceous concretes and isotropic with carbonaceous concretes. The work performed in the frame of the SARNET2 WP6 group during the last 4 years has enabled significant progress on this topic. This paper summarizes this progress using the analysis of recent 2D real material experiments in an oxidic pool and from analytical simulant experiments on 2D heat convection in a bubbling pool, including calculations and recalculations with MCCI codes available in Europe.Firstly, the effective heat transfer coefficients from the bulk pool to the bottom and lateral pool interfaces deduced from MCCI experiments lead to a range of a few 100 W/m(2)/K. By contrast, a detailed review of possible 2D convection mechanisms shows that the individual heat convection mechanisms (without taking a crust into account) such as gas bubbling convection and solutal convection overestimates the overall heat transfer coefficient, and does not account for the main trends of 2D ablation deduced from MCCI tests, which are very dependent on the composition of concrete components and aggregates. This fact, in turn, points to the effect of more complex pool/concrete interface structures. On the basis of a thorough interpretation of the experimental database and of a detailed comparison of MCCI code predictions, a set of the most realistic and consistent assumptions are identified and major remaining uncertainties are listed. (C) 2014 Elsevier Ltd. All rights reserved.

Within SARNET, the corium topic covers all the behaviors of corium from early phase of core degradation to in or ex-vessel corium recovery with the exception of corium interaction with water, direct containment heating and fission product release. The corium topic regroups in three work packages the critical mass of competence required to improve significantly the corium behavior knowledge. The spirit of the SARNET networking is to share the knowledge, the facilities and the simulation tools for severe accidents, so to reach a better efficiency and to rationalize the R&D effort at European level. Extensive benchmarking has been launched in most of the areas of research. These benchmarks were mainly dedicated to the recalculation of experiments, while, in the next periods, a larger focus will be given to integral experiments or reactor applications. Eventually, all the knowledge will be accumulated in the ASTEC severe accident simulation code through physical model improvements and extension of validation database. This paper summarizes the progress that has been achieved in the frame of the networking activities. A special focus is placed on the melt pool and debris coolability and corium-concrete interaction, in which, the effects due to multidimensional geometries and heterogeneities has been shown, during SARNET, to play a crucial role and for which further research is still needed.

In a hypothetical nuclear reactor severe accident, corium spreading is one possible mitigation measure that has been selected for the EPR design. A post-test benchmark exercise has been organized on the VULCANO VE-U7 corium spreading experiment. In this test, a prototypic corium mixture representative of what could be expected at the opening of EPR reactor-pit gate has been spread on siliceous concrete and on a reference channel in inert refractory ceramic. The spreading progression was not much affected by the presence of concrete and sparging gases. The procedure used to estimate the corium physical properties from its composition and temperature provided a satisfactory data set. The CORFLOW, LAVA and THEMA codes provide satisfactory calculations of the spreading front evolution and of its final length. LAVA and THEMA estimations of the substrate temperatures, which are the initial conditions for longer term Molten Core Concrete Interaction or Corium Ceramic Interaction computations, are also close to the measured data, within the experimental uncertainties.

The Project ECOSTAR (5. EC Framework Programme) on Ex-Vessel Core Melt Stabilisation Research is oriented towards the analysis and mitigation of severe accident sequences that could occur in the ex-vessel phase of a postulated core melt accident. Spreading of the corium melt on the available basement surface is an important process, which defines the initial conditions for concrete attack and for the efficiency of cooling in case of water contact, respectively. The transfer and spreading of the melt on the basement is one of the major issues in ECOSTAR. This is addressed here by a spreading code benchmark involving a large-scale spreading experiment that is used for the validation of the existing spreading codes. The corium melt is simulated by a mixture of Al{sub 2}O{sub 3}, SiO{sub 2}, CaO and FeO with a sufficiently wide freezing interval. In the 3-dim benchmark test ECOKATS-1 170 litres of oxide melt are poured onto a 3 m by 4 m concrete surface with a low flow rate of about 2 l/s. From the results of an additional 2-dim channel experiment some basic rheological data (e.g. initial viscosity) are obtained in order to minimise the uncertainty in material properties of the melt. The participatingmore » spreading codes CORFLOW (Framatome ANP/FZK), LAVA (GRS), and THEMA (CEA) differ from each other by their focus of modelling and the assumptions made to simplify the relevant transport equations. In a first step both experiments (3-dim/2-dim) are calculated blindly by the participating codes. This serves for an overall assessment of the codes capabilities to predict the spreading of a melt with rather unknown material properties. In a second step the 3-dim experiment ECOKATS-1 is recalculated by the codes with the more precise knowledge of the rheological behaviour of the oxide melt in the 2-dim experiment. This, in addition, serves for the validation of the codes' capabilities to predict the spreading of a melt with well-known material properties. Based on the benchmark results and taking the specific validation process for each of the three codes applied into account, it is recommended that the spreading issue for reactor safety research be considered closed. (authors)« less

In the context of severe accidents, large R&D efforts throughout the world are currently directed towards ex-vessel cerium behaviour. Among the mitigation means which can be envisaged, the European industries and utilities are considering the implementation of a core-catcher outside the reactor pressure vessel in order to prevent basemat erosion and to stabilize and control the cerium within the containment.The CSC project focused on two key phenomena for external core-catcher efficiency, reliability and safety : spreading and coolability. An experimental programme, covering different scenarios and including both simulant and real materials provided a lot of results which constitute now a large database and which enabled the qualification of 2 computer codes. After a brief presentation of the project, this paper presents the results obtained.

Summary In the late phase of Molten Core Concrete Interaction (MCCI) the ablation rate becomes low and the gas flow rate issued from the concrete decomposition is reduced. Moreover the oxide phase becomes lighter than the metal phase due to the addition of the light oxide from the concrete. A large density difference and a low gas flow rate ensure that a stratified configuration will occur. The stratified configuration has then to be taken into account in the MCCI simulation tools. Only few experimental programs were conducted with stratified pools; among them the BETA tests were performed at FZK with a large test matrix. The COMET-L2 and L3 experiments were recently conducted at FZK in a stratified configuration. In the frame of SARNET WP11.2 a benchmark was organised in order to compare the MCCI code results regarding these experiments. COMET-L2 was used for a post-test simulation and after that COMET-L3 was used as a blind simulation. The participants and codes were AREVA with COSACO, CEA with TOLBIAC-ICB, EDF with TOLBIAC-ICB, FZK with WECHSL, GRS with MEDICIS and WEX, IRSN with MEDICIS, UPM with MELCOR and VTT with MELCOR. The results show a large discrepancy between the code results, due to the different models used in the stratified regime.

Summary Within SARNET, the CORIUM topic covers all the behaviour of corium from earl y phase of core degradation to in or ex-vessel corium recovery with the exception of corium interaction with water, direct containment heating and fission produ ct release. The CORIUM topic regroups in three work packages the critical mass of competence to improve significantly the corium behaviour knowledge. The spirit of the SARNET networking is to share the knowledge, the facilities and the simulation tools for severe accidents, so to reach a better ef ficiency and to rationalise the R&D effort at European level. Extensive benchmarking has been launched in most of the areas of research. These benchmarks were mainly dedicated to the recalculation of experiments, while, in the next periods, a larger focus will be given to integral e xperiments or reactor applications. Eventually, all the knowledge will be accumulated in the ASTEC severe accident simulation code through physical model improvements and extension of validation database. This paper summarizes the progress that has been ac hieved in the frame of the networking activities and underlines also the orientation of t he CORIUM work packages for the next period. A special focus is placed on the melt pool and debris coolability and corium-concrete interaction, in which, the effects due to multidimensional geometri es and heterogeneities has been shown, during SARNET, to play a crucial role and further r esearch is still needed.