Nuclear Energy for New Europe 2009

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No: 411

Conference: Nuclear Energy for New Europe 2009

Title: Influence of Melt Superheat on Droplets Crust Growth during Steam Explosion Premixing Phase

Theme: Severe Accidents

Author(s): Matjaž Leskovar, Mitja Uršič

Contact : Matjaž Leskovar

E-mail: matjaz.leskovar@ijs.si

Address: Institut "Jožef Stefan" R4
1001 Ljubljana

Country: Slovenia

A steam explosion is a physical phenomenon in which the internal energy of a hot liquid is rapidly transferred to a colder and more volatile liquid, which as a result vaporizes at high pressure and expands against the inertial constraint of the surrounding structure as well as the mixture. It may occur during a severe nuclear reactor accident if the molten core comes into contact with the coolant water. A strong enough steam explosion in a nuclear power plant could jeopardize the containment integrity and so lead to a direct release of radioactive material to the environment.
Steam explosion experiments have indicated important differences in behaviour between simulant and prototypic melts, i.e. the energy efficiency for prototypic corium melts is about one order of magnitude lower than for simulant alumina melts. These differences are mainly attributed to the differences in the crust growth on the melt droplets during the premixing phase. The formed solid crust inhibits the fine fragmentation process, which drives the steam explosion, and if the crust is thick enough it completely prevents it. The crust growth depends on a number of factors, e.g. droplets size, material properties, heat transfer conditions, melt temperature, etc. In this paper we will focus on the influence of the melt superheat on the crust growth. Superheat denotes how much the melt temperature is above its liquidus temperature.
A melt droplet crust growth model, which treats the melt droplet as an opaque symmetrical sphere, was developed. The model assumes that the heat inside the droplet is transferred only by conduction and that the heat losses from the droplets surface are uniform. The heat flux from the droplets surface was determined as the sum of the film boiling heat flux and the radiation heat flux. The temperature profile inside the melt droplet was calculated numerically with the finite differences method. For a number of conditions, considering also different corium compositions, the lifetime of active melt droplets (i.e. droplets which are liquid enough that they can fine fragment during the explosion phase and so participate in the team explosion) was calculated as a function of melt superheat. Based on the results of the performed analysis and the review of past performed experiments, a high melt superheat steam explosion experiment was proposed, which would provide important missing experimental data for computer code validation, necessary for reliable extrapolation of experimental findings to reactor conditions.

No:	411
Conference:	Nuclear Energy for New Europe 2009
Title:	Influence of Melt Superheat on Droplets Crust Growth during Steam Explosion Premixing Phase
Theme:	Severe Accidents
Author(s):	Matjaž Leskovar, Mitja Uršič
Contact :	Matjaž Leskovar
E-mail:	matjaz.leskovar@ijs.si
Address:	Institut "Jožef Stefan" R4 1001 Ljubljana
Country:	Slovenia

A steam explosion is a physical phenomenon in which the internal energy of a hot liquid is rapidly transferred to a colder and more volatile liquid, which as a result vaporizes at high pressure and expands against the inertial constraint of the surrounding structure as well as the mixture. It may occur during a severe nuclear reactor accident if the molten core comes into contact with the coolant water. A strong enough steam explosion in a nuclear power plant could jeopardize the containment integrity and so lead to a direct release of radioactive material to the environment. Steam explosion experiments have indicated important differences in behaviour between simulant and prototypic melts, i.e. the energy efficiency for prototypic corium melts is about one order of magnitude lower than for simulant alumina melts. These differences are mainly attributed to the differences in the crust growth on the melt droplets during the premixing phase. The formed solid crust inhibits the fine fragmentation process, which drives the steam explosion, and if the crust is thick enough it completely prevents it. The crust growth depends on a number of factors, e.g. droplets size, material properties, heat transfer conditions, melt temperature, etc. In this paper we will focus on the influence of the melt superheat on the crust growth. Superheat denotes how much the melt temperature is above its liquidus temperature. A melt droplet crust growth model, which treats the melt droplet as an opaque symmetrical sphere, was developed. The model assumes that the heat inside the droplet is transferred only by conduction and that the heat losses from the droplets surface are uniform. The heat flux from the droplets surface was determined as the sum of the film boiling heat flux and the radiation heat flux. The temperature profile inside the melt droplet was calculated numerically with the finite differences method. For a number of conditions, considering also different corium compositions, the lifetime of active melt droplets (i.e. droplets which are liquid enough that they can fine fragment during the explosion phase and so participate in the team explosion) was calculated as a function of melt superheat. Based on the results of the performed analysis and the review of past performed experiments, a high melt superheat steam explosion experiment was proposed, which would provide important missing experimental data for computer code validation, necessary for reliable extrapolation of experimental findings to reactor conditions.