On March 11, 2011, a magnitude 9.0 earthquake struck Japan. It was one of the most powerful earthquakes on record, unleashing a tsunami that ravaged 430 miles of coastline, destroying communities, and killing nearly 16,000 people. The combined effects of the earthquake and tsunami overwhelmed on and offsite power systems at the Fukushima Daiichi nuclear power plant, leading to the meltdown of three reactors and the release of radioactive contaminants into the surrounding environment. Five years on, the decommissioning and clean-up at Fukushima remains in the early stages and will likely take decades to be completed. In the aftermath of this multi-unit accident, the global nuclear community has been reassessing certain safety assumptions about nuclear reactor plant design, operations and emergency actions, particularly with respect to extreme events that might occur.