Natural circulation cooling during a small-break loss-of-coolant accident in a simulated pressurized water reactor

This paper presents experiment results from a simulated small-break loss-of-coolant accident (SBLOCA) involving natural circulation core heat rejection mechanisms. The experiment results are compared to pre-experiment RELAP5/MOD2 calculations, and finally are related to expected pressurized water reactor (PWR) behavior during SBLOCA conditions. The simulation conducted in the Semiscale MOD-2C scaled-experiment facility was a 0.5% cold leg centerline break performed at high temperature and pressure with initial conditions typical of a PWR (15.6-MPa pressure, 587-K hot leg fluid temperature). The Semiscale MOD-2C facility represents all major components of a large PWR; the facility includes active loops, steam generators, and a vessel with an electrically heated core. The transition from forced circulation to the various modes of natural circulation was characterized in the Semiscale simulation. The natural circulation cooling modes obseved include single-phase, two-phase, and reflux. Analysis included extensive decoupling of natural circulation heat rejection modes between the loops, and multi-dimensional effects within the steam genertor U-tubes. Comparison of RELAP5/MOD2 calculations with data shows correct qualitative calculation of the occurrence of single-phase, two-phae, and reflux cooling during the Semiscale transient. Analysis of similitude criteria for the Secmiscale facility suggests that the Semiscale results preserve the first order effects thought important for naturalmore » circulation phenomena expected in a PWR.« less