The AP1000® (Advanced Plant) is a 1100-MWel-class pressurized water reactor with innovative passive-safety features and extensive plant simplifications that enhance construction, operation, maintenance, and safety. The passive-safety features of the plant use natural driving forces such as pressurized gas, gravity flow, natural circulation flow, and natural convection. These features do not rely on active components (such as pumps, fans, or diesel generators) or safety-grade support systems (such as alternative current (AC) power, component cooling water, service water, and heating, ventilating, and air conditioning heating, ventilating, and air conditioning (HVAC)).

This passive-safety approach allows the AP1000 plant to be uniquely equipped to withstand an extended station blackout scenario, such as the events following the earthquake and tsunami at the Fukushima Dai-ichi Nuclear Power Plant (NPP), and does not compromise core and containment integrity. Without AC power, the plant provides cooling for the core, containment, and spent-fuel pool for 72 h. During this time, there is no need for operator actions. Following this passive-coping period, minimal operator actions are needed to extend the operation of the passive features to seven days using installed equipment and can be extended to an indefinite coping period with additional actions following seven days. Major design features and parameters are shown in Table 1.

The AP1000 plant is equipped with two passive-safety systems (see Fig. 1) that focus on removing decay heat from the reactor and supplying the reactor with coolant during accident conditions. These two systems are the passive core cooling system (PXS) and the passive containment cooling system (PCS). The PXS is comprised of a reactor–coolant–injection subsystem and a core decay-heat removal subsystem; both are located within containment and integral with the plant **reactor coolant system (RCS).

The PXS uses three passive borated sources of cold-water during accident conditions to maintain core cooling through safety injection during a loss of coolant accident (LOCA). These injection sources include the core makeup tanks (CMTs), the accumulators, and the in-containment refueling water storage tank (IRWST). These injection sources are directly connected to two nozzles on the reactor vessel and are all injected through passive means including natural circulation, prepressurized gas, and gravity. The CMTs provide reactor-coolant makeup and boration and are located inside the containment at an elevation slightly above the reactor-coolant loops. The CMTs are connected to the RCS through a direct vessel-injection line and a cold-leg inlet-pressure balance line. Subsequently, an automatic depressurization system drops the pressure of the RCS, causing the accumulators to begin rapid injection of borated water into the reactor. As the automatic depressurization system continues to lower RCS pressure, the IRWST water is then injected into the reactor.

Residual heat removal is performed by natural circulation through the passive residual heat removal heat exchanger, which uses the IRWST as a heat sink. During passive residual heat removal heat exchanger operation, the IRWST will heat up and eventually boil. The steam released from the IRWST is then condensed on the containment shell by the PCS, which is passively cooling the containment vessel from the outside, and brought back to the IRWST through a gutter system. This recirculation of coolant from the containment shell back to the IRWST is also used during a loss of coolant accident for long-term cooling of the core.

The PCS cools the containment following any event, which results in energy release into the containment so that pressure inside containment is rapidly reduced without exceeding the containment-vessel design pressure (see Fig. 2). The steel containment vessel provides the heat-transfer surface for the heat from the PXS to be removed. Heat is removed from the containment vessel by the continuous, natural circulation of air up through the outside of the vessel. Initially, the normally operating air cooling is supplemented by evaporating water onto the outside of the containment steel shell. The water is provided from a large tank located on top of the containment shield building and drains by gravity.

Together, the PXS and PCS provide a passive, reliable, and simple means of mitigating accidents requiring core cooling and makeup with no AC power, active components, or operator actions required. Operators can take action for continuation of the passive systems after 72 h for long-term accident mitigation.

The AP1000 plant design has been deployed in China with two units at Sanmen in the Zhejiang province (see Fig. 3) and two units at Haiyang in the Shandong province (see Fig. 4). Sanmen 1 successfully completed all commissioning milestones and declared full commercial operation on Friday, Oct. 12, 2018. Haiyang Unit 1 and Sanmen 2 have reached 100% reactor thermal power, and Haiyang Unit 2 has achieved initial criticality.

The benefits of the AP1000 plant design objectives of standardization of systems, structures, and components, and simplicity of the passive-safety systems were realized in the short timeframe for commissioning. The standardization of components throughout the plant ensured consistent results from testing across multiple plants and the simplicity of the passive-safety systems accelerated commissioning by reducing the number of components requiring testing. As a result, Sanmen 1 reached commercial operation just 158 days after fuel load began on Apr. 25, 2018. This was a historic achievement for a first-of-a-kind reactor initial startup. The commissioning of Sanmen 1 also occurred without unplanned reactor trips. This highlights the unparalleled performance of the plant's advanced digital instrumentation and control systems. The lessons learned and experience gained on the commissioning of Sanmen 1 is being used to decrease the commissioning times for the follow-on units.

During the commissioning of Sanmen 1, special tests to further reinforce unique phenomenological performance parameters of the AP1000 plant design were performed. The first-plant-only-tests included an IRWST heatup test, comprehensive reactor-vessel-internals vibration testing and natural-recirculation decay-heat-removal tests. Additionally, due to the novel passive-safety features of the AP1000 plant design, CMT heated-recirculation and automatic-depressurization-system tests were performed at Sanmen 1 and 2, and Haiyang Unit 1. All of these first-of-a-kind tests were successfully completed and demonstrated the unparalleled passive-system performance of the AP1000 NPP. The performance of the passive-safety systems in these tests will support all future AP1000 units and is currently being credited for the AP1000 plant Vogtle 3 and 4 Project in the U. S. to gain overall efficiency in the commissioning of the two AP1000 plant units currently under construction.

The successful commissioning efforts at Sanmen 1 and the current progress on the remaining units in China are demonstrating the delivery certainty and proven innovative technology of the AP1000 plant design. All four units have completed or are progressing through commissioning without unplanned reactor trips or major issues with required testing of the novel passive-safety-design features. Thus, the passive-safety features of the design have been validated through performance testing, ensuring success for the next wave of AP1000 NPPs.