STUDY ON IMPACTS OF ADS ON AP1000 DURING SB-LOCA

AP1000 model, including Reactor Coolant System (RCS), Engineering Safety Features (ESFs) and containment, is built by using the modular accident analysis program. A severe accident scenario of small-break loss-of-coolant accident (SB-LOCA) at cold leg concurrent with different conditions of the automatic depressurization systems (ADS) was simulated and the calculation was mainly during the in-vessel phase, the effect of the ADS1-4 on the AP1000 was analyzed and evaluated. The analysis results showed that the execution of ADS 1-3 was effective to decrease the pressure of the primary system at the early stage and lead to quick and effective injection of cold water from core make-up tank (CMT) and accumulators into the core, but it was impossible to decrease the pressure of primary system effectively to allow the injection from the internal refueling water storage tank (IRWST) to the core and the core was uncovered. The zirconium-water reaction will produce the hydrogen. The execution of ADS4 accelerated the injection from the accumulators to the core and decreased the pressure of the primary system quickly so that the IRWST injection was available and the core was still covered with water. It was of great importance to the safety of AP1000. The passive containment cooling system (PCS) was an effective measure to control the pressurization of the containment.