ASTEC model development for the severe accident progression in a generic AP1000-LIKE

Improvements in light water reactor (LWR) technologies in recent decades have led to the development of advanced LWRs, currently under construction, soon to join the international fleet of nuclear power plants (NPPs). Enhanced safety capabilities have been achieved in advanced LWRs through the utilization of passive safety systems and design improvements. The progression of severe accidents in advanced LWRs is not well described in the current open literature. To date severe accident analyses performed on the Westinghouse AP1000 have focused on evaluating the performance of the AP1000 safety systems in the prevention of severe accidents, with little focus on characterizing the progression of core degradation to identify the timing of key events such as core uncovery, relocation of corium to the lower plenum, and lower head failure.

An initial, generic model of an AP1000-LIKE reactor has been developed in the severe accident integral code ASTEC to characterize the progression of severe accidents in the AP1000-LIKE model and identify the representation of important physical phenomena and key events of core degradation progression. To evaluate the progression of severe accidents in the AP1000 an accident scenario without accident management measures (AMM), and a complete loss of primary flow is designed and simulated using the developed ASTEC model. The preliminary code results showed the applicability of the model to simulate severe accident scenarios. Without AMM, it is determined that reactor pressure vessel failure occurs nearly four hours after the start of the transient; after the loss of primary coolant flow, a large amount of hydrogen is produced (~800 kg), and a large mass of molten material forms (~150 tons) and relocates to the lower plenum. Future efforts will focus on developing a full plant model for a generic AP1000 and further characterizing the progression of severe accidents.