1/29/2009 Colloquium - Roy Freud
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Roy FreudNuclear Research Center – Negev, Beer-Sheva, Israel |
Event Info
Title: Collapsing Criteria for Vapor film around solid Spheres as a Fundamental Stage leading to Vapor Explosion
Date: Jan 29, 2009
Location: 3105 Etcheverry Hall
Time: 4-5pm
Abstract
Following a partial fuel-melting accident, a Fuel-Coolant Interaction (FCI) can result with the fragmentation of the melt into tiny droplets. A vapor film is then formed between the melt fragments and the coolant, while preventing a contact between them. Triggering, propagation and expansion typically follow the premixing stage.
When the vapor film collapses and the coolant re-establishes contact with the dry surface of the hot melt, it may lead to a very rapid and rather violent boiling.
The film boiling and its possible collapse is a fundamental stage leading to vapor explosion. If the interaction of the melt and the coolant doesn’t result in a film boiling, no explosion occurs.
The present experimental study examines the minimum temperature that is required to maintain a film boiling around metal spheres immersed into a liquid (subcooled distilled water) reservoir. In order to simulate fuel fragments that are small in dimension and has mirror-like surface, small spheres coated with anti-oxidation layer were used.
Correlations for the minimum temperature and the minimum heat flux necessary to maintain film boiling were established in terms of the subcooling level, the size of the spheres and their material.
The minimum temperature to maintain film boiling was used as the principle criteria for the occurrence of vapor explosion. Other criteria, for the intensity of the vapor film collapse was derived from the maximum heat flux following the vapor film collapse, and the audible sound (which is generated by the shock wave). It is assumed that a high intensity of the vapor film collapse will result in a more efficient propagation stage and enhancement of the vapor explosion.
