The development of the AP-1000 design and of its precursor the AP-600 started in the aftermath of the Chernobyl event (1986) when the need came from the scientific and technological community for a resilient system against deliberate threats by humans. The “passive system” design concept became relevant. The first AP-1000 entered into operation around 3 decades after that event. This paper discusses the issue of how much the progress in nuclear science and technology since the end of the 1980s has affected the AP-1000 design. Five interconnected areas are identified: (1) reliability of passive systems, (2) scaling and uncertainty, (3) coupling between three-dimensional neutron physics and thermal hydraulics, (4) consideration of large-break loss-of-coolant accidents, and (5) simulation of instrumentation and control systems. All these areas are relevant for the AP-1000 and standard pressurized water reactors; however, the areas (1) and (2) have specific applicability for the AP-1000 and constitute the main concerns of this paper. The conclusion from qualitative investigation is that the safety demonstration of the AP-1000 did not take full benefit from progress in these areas, namely, inadequacies characterize the scaling database and the processes for determining the reliability of thermal-hydraulic passive systems did not receive proper attention.
