Abstract :
Molten core material is considerably hotter than the melting point of the RPV steel; consequently, corium discharge could be accompanied by rapid ablation of the hole initially formed in the RPV by tube ejection or creep rupture. In rupture scenarios, the formation of a hole creates a localized stress concentration that might cause the initial failure to propagate to a much larger size. These processes not only accelerate corium discharge, but the resulting hole size has a direct influence on RCS blowdown, debris fragmentation, dispersal of debris from the cavity, and subsequent containment phenomena. The final hole size plays a central role in the quantification of the coherence ratio, which defines the coherency of melt dispersal and RCS blowdown in the two-cell equilibrium (TCE) model. This paper develops a model for the final hole size after ablation. The applicability of the model to both experiment and reactor analyses is addressed. The ablation model is validated against existing experiment data, but extrapolation outside the database is required for NPP applications. Sensitivity studies are performed over the extremes of the controlling parameters. Simple analysis suggests that RCS depressurization effectively eliminates any possibility of continued enlargement of a rupture once it is formed.
