Coolability of a corium pool in a debris bed – Critical heat flux (CHF) dependency with tilting angle, debris size and steam flowrate
Résumé
In case of severe accident in a light water reactor, large debris beds may form inside the core. If, in some part of the debris bed, the temperature increases up to the
fuel melting point, it will lead to the formation and expansion of a molten pool which becomes more and more difficult to cool down as it grows. Therefore, it is of
primary interest to determine the maximum size of a molten pool surrounded by debris which may be stabilized under water either after reflooding the vessel (as in
TMI-2) or in case of a debris bed formed in a flooded reactor pit. One of the key parameters for that issue is the maximum heat flux (CHF) that may be extracted from
the pool by water flowing within the debris bed. To determine that CHF, a series of tests have been conducted at IRSN. A heated copper cylinder generates a heat flux
at one of its ends which simulates the boundary of the pool and is placed in contact with a debris bed made of monodisperse steel balls. Temperature measurements
along the cylinder allow to calculate the heat flux transmitted to the debris bed through the cylinder tip.
Several experimental parameters have been investigated:
- The tilting angle of the heated surface, to take into account the shape of the molten pool.
- The diameter of the balls.
- The upwards steam flowrate simulating the steam coming from the part of the core located below the molten pool.
- The upwards liquid flowrate simulating the natural circulation of water along the debris bed.
As it is usual for CHF experiments, a significant dispersion of the measurements was observed, due to unavoidable evolutions of the contact zone between the
copper surface and the balls. However, the impact of that random dispersion could be reduced by performing a large number of tests. This allows to identify clear
tendencies on the CHF for the 3 studied parameters, with a rather good degree of confidence.
From those tendencies, it was possible to derive a general CHF correlation with the tilting angle and the steam flowrate. When the liquid velocity increases, the
CHF increases in most cases, except for some cases with low velocity. However it is more difficult to find a clear correlation with the liquid velocity or flow rate.
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Licence : CC BY NC ND - Paternité - Pas d'utilisation commerciale - Pas de modification
Licence : CC BY NC ND - Paternité - Pas d'utilisation commerciale - Pas de modification
