Interprétation des essais ICB 2D en matériaux réels avec le code ASTEC/MEDICIS

Titre du congrès :MCCI Seminar 2007 Ville du congrès :Cadarache Date du congrès :10/10/2007

Most available 2D MCCI experiments with real material have been analysed and recalculated using the ASTEC/MEDICIS code. The ASTEC/MEDICIS code is shown here to be a flexible tool permitting to check the relevance of various physical assumptions and models. Main findings obtained from these experiments concern the thermal resistance of the pool/concrete interfaces, the 2D ablation behaviour versus the concrete type and indirectly the structure of the pool/concrete interface. From a general point of view, the thermodynamic equilibrium at the pool/crust interface is only partially reached in real material experiments because of transient conditions, the high gas bubbling and the high liquid corium viscosity due to the presence of ablated silica ; no direct information on the crust existence and stability can be derived from these experiments and no available mechanistic model is able to reproduce the observed pool/concrete interface behaviour. Therefore it is attempted here to explain trends observed in 2D MCCI tests using phenomenological models for describing the pool/concrete interface structure. A basic assumption used in the preliminary approach presented here is the following: no stable crust appears in most cases but rather a mushy zone exists at the pool/concrete interface and the heat is convected by gas bubbling across this pool mushy zone up to a lateral boundary, beyond which the solid fraction is high enough to suppress the heat convection and heat is transported only by conduction up to the concrete ablation interface. The temperature of the boundary between pool conductive and convective zones, called the solidification temperature, determines in this thermal resistance type model the local convective heat flux. In the CCI2 experiment, reasons of the almost uniform ablation are related to the limestone-sand concrete high gas content, which causes on one side a fine break-up of aggregates and a fast mixing with the pool melt leading to the build-up of refractory phases all along the pool/concrete interface and on the other side hinders a solid chunk accumulation at the bottom interface. In order to model this situation with ASTEC/MEDICIS, the same assumptions on the solidification temperature value and the heat transfer coefficient in the slag layer are applied on all pool/concrete interfaces. The solidification temperature is chosen near but somewhat below the melt liquidus temperature, simulating qualitatively the build-up of the above-mentioned refractory phases at the pool/concrete interface. These assumptions permit to reproduce satisfactorily the temperature evolution, the axial and lateral ablation kinetics and the final eroded cavity shape in the CCI2 test. In the CCI3 experiment, the prevailing lateral ablation can be explained only by introducing a dissymmetry respectively at the bottom and lateral interfaces explained as follows: due to the low gas content the concrete aggregates might remain non fragmented during ablation and the lateral wall ablation will release big solid aggregates falling down and feeding a solid chunk accumulation at the bottom interface promoted by the low gas bubbling. At the lateral interface, the pool melt solidification might be hindered by the low melting point of concrete cement / corium melt mixture ; in order to simulate this situation approximately with ASTEC/MEDICIS code, a solid crust is assumed to be present at the bottom interface increasing the thermal resistance of this interface while at the lateral interface the crust built-up is suppressed increasing the heat flux to this latter interface. These assumptions permit to reproduce rather well the pool temperature evolution, which show a fast decrease below the melt liquidus temperature, the axial and lateral ablation kinetics and the final eroded cavity shape. Assumptions and model choices consistent with those used for MCCI-OECD CCI2 and CCI3 experiments are kept in the MEDICIS applications to VULCANO VB-U5 and VB-U6 tests in order to reproduce main ablation results obtained from these tests. This interpretation work of existing MCCI-OECD and VULCANO experiments confirmed and completed by the analysis of future tests should permit to build improved and more reliable models for predicting the 2D MCCI behaviour of an homogeneous melt pool.