Investigation of the Downcomer Boiling Phenomena During the Reflood Phase of a Postulated Large-Break LOCA in the APR1400

Hydraulic phenomena in the downcomer of a conventional pressurized water reactor have an important effect on the transient evaluations of a postulated large-break loss-of-coolant accident (LBLOCA). In particular, safety analyses using best-estimate codes show that downcomer boiling is one of the important phenomena in the postulated LBLOCA because it can degrade the hydraulic head in the downcomer and consequently affect the reflood flow rate for core cooling. To experimentally identify the thermal-hydraulic behavior in the downcomer, a downcomer-boiling test facility was constructed for simulating downcomer boiling in the reflood phase of a postulated LBLOCA.

The test facility was designed by adopting a full-pressure, full-height, and full-size downcomer-gap approach but with the circumferential length reduced 47.08-fold. The test was divided into two phases: (a) visual observation and acquisition of the global two-phase flow parameters and (b) measurement of the local two-phase flow parameters.

This paper presents the test results from Phase I. The major measured parameters were the axial void fraction and the fluid temperatures and pressures in the test section. The measured data were used to evaluate a safety analysis code, MARS 2.1b, to investigate its modeling accuracy and identify weaknesses of the thermal-hydraulic models therein.