BWR Natural Circulation Experiments at ROSA-III under Low-Power and Reduced Inventory Conditions

Abstract

The natural circulation core cooling is important during small break loss-of-coolant accidents and transients involving loss of forced-circulation core cooling. This paper presents results of natural circulation experiments conducted in the ROSA-III (Rig of Safety Assessment No. 3) facility under reduced mass inventory conditions. Three distinct modes of BWR natural circulation were identified in the experiments : the primary circulation mode, the internal circulation mode and the open-loop (core uncovery) mode. One-dimensional quasi-steady analytical model was developed for the internal natural circulation mode to determine the core uncovery criterion. The model predicted well the two-phase mixture level inside the core shroud for quasi-steady state natural circulation at pressures of 7.35 and 2.06 MPa, and scaled core powers below 20%, and was used to predict BWR natural circulation behaviors.