Scoping calculations of an in-vessel steam explosion. Castem-Plexus computations
Résumé
Steam explosion is considered as a potential risk for PWR nuclear power plants in case of a severe accident. The loss of coolant can cause the degradation of the core and its melting. By falling down in the water remaining in the lower plenum, the corium transfers fastly its energy to the water which vaporizes.
This paper presents parametrical computations of an in-vessel steam explosion, regarding the possible lower head vessel ruin resulting from a large central core degradation. The mechanical consequences of the steam explosion on the lower head are foreseen by the CASTEM-PLEXUS fast dynamic software, estimating approximately the thermodynamic data.
A 2D-axisymmetric representation is adopted because of the vessel symmetry. The vessel lower head is considered as an elastoplastic hemispheric shell without penetrations and coupled with coolant. Four parameters are examined: the corium location, the energy transferred from the corium to the water, the energy transfer kinetics and the water constitutive law.
During the explosion, we observe the propagation of a pressure wave from the corium zone towards the vessel and the core. The wave reflects against the vessel and the core support plate and then is sent again to the corium zone. After the wave passing, the water vaporizes massively and escapes in the free space in the core centre.
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