Abstract
The thermal-hydraulic phenomena of Direct Vessel Injection (DVI) line Small-Break Loss-of-Coolant Accident (SBLOCA) in pressurized water reactor, APR1400, were investigated. To understand the thermal-hydraulic phenomena during the SBLOCA transient, the reduced-height and reduced-pressure integral test loop, SNUF (Seoul National University Facility), was constructed with scaling down the prototype. For the appropriate test conditions in the experiment of SNUF, the energy scaling methodology was suggested as scaling the coolant mass inventory and thermal power for the reduced-pressure condition. From the RELAP5 analysis, the energy scaling methodology was confirmed to show the reasonable transient when ideally scaled-down SNUF model was compared to the prototype model. In order to overcome the limitation of power in actual SNUF, the modified-power curve was utilized without simulating the forced flow by pump, so that those corrections did not affect the major phenomena during transient. Geometric distortion of actual SNUF also did not strongly disturb the thermal-hydraulic behaviors, especially occurrence of the downcomer seal clearing. In the experiments according to the conditions determined by energy scaling methodology, the phenomenon of downcomer seal clearing had a dominant role in decrease of the system pressure and increase of the coolant level of core. It occurred when the steam injected from cold legs penetrated the coolant in upper downcomer toward the broken DVI line. The experimental results was utilized to validate the calculation capability of RELAP5, especially for the downcomer seal clearing phenomenon, and to estimate the scale-up capability of RELAP5 code according to the scaling methodology.
