Design Change Effect of Automatic PHT Pump Trip on Small Break LOCA for Wolsong Unit 1

In Wolsong Unit 2,3,4 design, the primary heat transport (PHT) pumps are automatically tripped on low reactor outlet header pressure (less than 2.5 MPa(a)) to prevent the pumps from cavitation. Low pressure in the PHT pump casing would allow cavitation and cause vibration. Then, the excessive vibration could damage the PHT pump or piping. However, there is no automatic PHT pump trip logic in Wolsong Unit 1 protection system and these pumps can be tripped only by the operators. If these PHT pumps are not tripped properly, they could be damaged by the excessive vibration. Thus, the automatic pump trip logic is adopted as one of design change items for Wolsong Unit 1 refurbishment program. This design change is the addition of a PHT pump trip parameter. The new parameter trips all four PHT pumps on sustained low reactor outlet header pressure. In this paper, the design change effect of the automatic PHT pump trip for Wolsong Unit 1 small break LOCA is presented