Improvement and evaluation of ATWS Protective Signal and mitigation system for ACPR1000 nuclear power plant

At the event of Loss of Normal Feed Water-An Anticipated Transient without Scram (LOFW-ATWS) in ACPR1000 unit, there exists the primary overpressure risk probability when the regulation function of temperature regulation (R) rods and power regulation (G) rods cannot work timely or lost. In order to reduce primary pressure and avoid the overpressure risk in this transient process, this paper proposes the improvement scheme of ATWS protective signal and mitigation measure which adds shutting-down action of coolant pumps, and then adopts THEMIS program to analyze ATWS transient to verify this scheme. The verification results indicate that the improvement scheme can effectively reduce the peak value of primary pressure and eliminate the overpressure risk in LOFW-ATWS event. This improvement scheme has been adopted in the design of Yang Jiang Nuclear Power Plant (NPP) 5&6 units in China.