Core liquid level depression in 5% small break LOCAs: an investigation using subscale data

Core heatup has been shown to be possible, during the course of small break loss-of-coolant accidents (SBLOCAs), even when sufficient inventory is present to fully cover the rods with a two-phase mixture. Such behavior occurs when steam, trapped between the loop seal and the core inventory, moves coolant out of the core barrel region and uncovers the rod upper elevations. The net result is core liquid level depression. Core liquid level depression and subsequent core heatups are investigated using subscale data from the ROSA-IV Program's 1/48-scale Large Scale Test Facility (STF) and the 1/1705-scale Semiscale facility. The depression phenomena and factors which influence the minimum core level are described and illustrated using examples from the data. Analyses conducted using the TRAC-PF1/MOD1 version 12.7 thermal-hydraulic code are also described and summarized