Nuclear Energy for New Europe 2009

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No: 406

Conference: Nuclear Energy for New Europe 2009

Title: Modelling of Solidification Influence on Steam Explosion Efficiency in MC3D

Theme: Severe Accidents

Author(s): Mitja Uršič, Matjaž Leskovar

Contact : Mitja Uršič

E-mail: mitja.ursic@ijs.si

Address: Institut "Jožef Stefan" R4
1001 Ljubljana

Country: Slovenia

Steam explosion experiments have indicated important differences in behaviour between simulant and prototypic melts. The steam explosion energy efficiency for oxidic corium melts is significantly lower than for simulant alumina melts. Differences in the material properties are probably the reason for the observed differences. The experimentally observed differences are importantly attributed to differences in the melt solidification, i.e. differences in crust formation during the premixing phase. The crust formation is believed to be one of the most decisive consequences of the material properties regarding the limitation of the strength of the steam explosion. Namely, the crust inhibits the fine fragmentation process and if the crust is thick enough it completely prevents it. Therefore, the crust formation during the premixing phase could explain the observed differences in the steam explosion efficiency between simulant alumina and corium melts.
The melt droplet solidification modelling approaches in current FCI (fuel – coolant interaction) codes are based on rather basic models. Current FCI codes do not take crust formation into account in enough detail to be able to predict solidification effects on FCI and consequently to predict the observed differences in the steam explosion efficiency between different melt materials. In the MC3D code, which is a reference FCI computer code in the SERENA and SARNET programmes, the melt droplets solidification is modelled in the simplest possible way, i.e. by the comparison of the droplet’s bulk temperature with the melt solidification temperature. In reality the temperature profile inside a melt droplet is not flat and consequently a solid crust can form on the droplet’s surface much earlier than the droplet’s bulk temperature decreases below the melt solidification temperature.
It is believed that an improvement in the accuracy of the melt droplet solidification modelling would enable to reduce the significant uncertainties in the prediction of pressure loads during a steam explosion in reactor conditions. Therefore, a reasonably complex melt droplets solidification model, considering the melt material properties, was developed. Appropriate fragmentation criteria were proposed to take into account the influence of the formed solid crust on the fragmentation during the premixing phase and on the fine fragmentation during the explosion phase. Adequate transport equations were derived for the most important model parameters in conservative form to enable the use of the developed model and criteria in FCI computer codes with an Eulerian description.
The proposed solidification model and criteria were implemented into the comprehensive FCI Eulerian computer code MC3D, which is being developed by IRSN, France, and is used for nuclear reactor safety analyses. In the paper the first results of the simulations performed with the upgraded code MC3D with the implemented model and criteria will be presented and discussed.

No:	406
Conference:	Nuclear Energy for New Europe 2009
Title:	Modelling of Solidification Influence on Steam Explosion Efficiency in MC3D
Theme:	Severe Accidents
Author(s):	Mitja Uršič, Matjaž Leskovar
Contact :	Mitja Uršič
E-mail:	mitja.ursic@ijs.si
Address:	Institut "Jožef Stefan" R4 1001 Ljubljana
Country:	Slovenia

Steam explosion experiments have indicated important differences in behaviour between simulant and prototypic melts. The steam explosion energy efficiency for oxidic corium melts is significantly lower than for simulant alumina melts. Differences in the material properties are probably the reason for the observed differences. The experimentally observed differences are importantly attributed to differences in the melt solidification, i.e. differences in crust formation during the premixing phase. The crust formation is believed to be one of the most decisive consequences of the material properties regarding the limitation of the strength of the steam explosion. Namely, the crust inhibits the fine fragmentation process and if the crust is thick enough it completely prevents it. Therefore, the crust formation during the premixing phase could explain the observed differences in the steam explosion efficiency between simulant alumina and corium melts. The melt droplet solidification modelling approaches in current FCI (fuel – coolant interaction) codes are based on rather basic models. Current FCI codes do not take crust formation into account in enough detail to be able to predict solidification effects on FCI and consequently to predict the observed differences in the steam explosion efficiency between different melt materials. In the MC3D code, which is a reference FCI computer code in the SERENA and SARNET programmes, the melt droplets solidification is modelled in the simplest possible way, i.e. by the comparison of the droplet’s bulk temperature with the melt solidification temperature. In reality the temperature profile inside a melt droplet is not flat and consequently a solid crust can form on the droplet’s surface much earlier than the droplet’s bulk temperature decreases below the melt solidification temperature. It is believed that an improvement in the accuracy of the melt droplet solidification modelling would enable to reduce the significant uncertainties in the prediction of pressure loads during a steam explosion in reactor conditions. Therefore, a reasonably complex melt droplets solidification model, considering the melt material properties, was developed. Appropriate fragmentation criteria were proposed to take into account the influence of the formed solid crust on the fragmentation during the premixing phase and on the fine fragmentation during the explosion phase. Adequate transport equations were derived for the most important model parameters in conservative form to enable the use of the developed model and criteria in FCI computer codes with an Eulerian description. The proposed solidification model and criteria were implemented into the comprehensive FCI Eulerian computer code MC3D, which is being developed by IRSN, France, and is used for nuclear reactor safety analyses. In the paper the first results of the simulations performed with the upgraded code MC3D with the implemented model and criteria will be presented and discussed.