Improved solidification influence modelling for Eulerian fuel–coolant interaction codes

Abstract

Steam explosion experiments revealed important differences in the efficiency between simulant alumina and oxidic corium melts. The experimentally observed differences are importantly attributed to the differences in the melt droplets solidification and void production, which are limiting phenomena in the steam explosion process and have to be adequately modelled in fuel–coolant interaction codes. This article focuses on the modelling of the solidification effect. An improved solidification influence modelling approach for Eulerian fuel–coolant interaction codes was developed and is presented herein. The solidification influence modelling in fuel–coolant interaction codes is strongly related to the modelling of the temperature profile and the mechanical effect of the crust on the fragmentation process. Therefore the first objective was to introduce an improved temperature profile modelling and a fragmentation criterion for partly solidified droplets. The fragmentation criterion was based on the established modified Weber number, which considers the crust stiffness as a stabilizing force acting to retain the crust under presence of the hydrodynamic forces. The modified Weber number was validated on experimental data. The application of the developed improved solidification influence modelling enables an improved determination of the melt droplet mass, which can be efficiently involved in the fine fragmentation during the steam explosion process. Additionally, also the void production modelling is improved, because it is strongly related to the temperature profile modelling in the frame of the solidification influence modelling. Therefore the second objective was to enable an improved solidification influence modelling in codes with an Eulerian formulation of the droplet field. Two additional transported model parameters based on the most important droplets features regarding the fuel–coolant interaction behaviour, were derived. First, the crust stiffness was considered as an important property, because it enables the correct prediction of the amount of droplets participating in the fine fragmentation process during the explosion phase. Second, the heat flux from the droplet interior to the surface was considered as an important feature, because it enables to improve the surface temperature determination and reflects the history of the droplet's cooling. The last objective was to implement the improved solidification influence modelling into the Eulerian code MC3D. The first demonstrative simulations with the implemented modelling are promising and are showing improvements in the simulation capability.