Effect of interfacial transfer and wall heat transfer constitutive correlations in a model of PWR ECC bypass

The Emergency Core Cooling (ECC) bypass/refill process in a PWR downcomer, following a postulated large rupture of a cold leg pipe (LOCA), is of particular importance for the PWR thermal-hydraulic safety. In the unlikely event of such accident, due to reactor vessel rapid depressurization and blowdown at the break, coolant flashing and voiding of the reactor core occurs. To prevent fuel assembly overheating, the ECC subcooled water is injected into the reactor vessel. However, instead of penetrating the lower plenum, the ECC water, driven by the steam, flows azimuthally around the core barrel, bypasses through the downcomer and gets expelled out at the break. Mathematical modeling of such complex thermal-hydraulic phenomenon is accompanied with a difficult task of selecting an appropriate set of constitutive correlations. In this paper, using two-dimensional transient diabatic two-phase model of lower plenum ECC refilling and downcomer bypass flow, numerical calculations are performed to study the effect of interphase mass and momentum transfer, and wall heat transfer on lower plenum refilling initiation and rate. The results confirm that the interfacial momentum transfer by interfacial friction has dominant influence on the transient, and that the model is specially sensitive to annular interfacial friction correlation. Considerable difference in refilling predictions was obtained when various annular interfacial friction correlations were assessed in the model. It has been confirmed that with the Popov-Rohatgi correlation, the model refilling predictions are in very good agreement with the experimental data