Modeling of in-vessel gap cooling and its validation against LAVA, ALPHA, and LMP200 experiments

Abstract

The gap cooling phenomenon is a key issue to explain why the 19 tons of melt retained in the RPV lower head in the TMI accident did not significantly damage the vessel. This study developed the model for gap cooling phenomenon and was validated against the several gap cooling experiments with 30 kg, 50 kg, 70 kg of Al2O3 melt (LAVA and ALPHA experiments), and 220 kg and 360 kg of Al2O3 + Fe melt (LMP200 experiments). To estimate the thermal behavior of the vessel during the gap cooling, we modeled heat transfer from the melt and crust to the vessel and heat removal by water penetrating the gap. The gap size which determines the flow rate of water penetrating the gap was evaluated considering thermal interaction between the melt and water; Inverse-Leidenfrost effect, thermal fracture of the crust, thermal deformations of crust and vessel. In addition, a three-regime model widely used to analyze quenching heat transfer was applied to simplify the calculation compared to the calculation with the boiling curve of previous studies. A sensitivity study for the discretization dimension of the vessel showed that the 1D calculation can substitute the 2D calculation for analyzing the gap cooling experiments. From the sensitivity study, the node size and the time step were proposed as 3 mm and 0.1 s to obtain converged results. Through the extensive validation against the gap cooling experiments with the large-scaled melt mass to 360 kg (LMP200 experiments) as well as the small-scaled melt mass of 30, 50 kg of melt (LAVA and ALPHA experiments), we introduced the correction factors which account for the uncertainties of the degree of local contacts between the melt and reactor vessel and the effect of the solidified debris penetration in the gap on CCFL. We found out that the current model predicted the peak temperatures and the peak times with the error ranges of -15 similar to 15% and -50 similar to 70%, respectively. Finally, the current model was compared with other gap cooling calculation codes.