Until now, U.S. safety regulations have been based on ensuring plants are designed to withstand certain specified failures or abnormal events, or 'design-basis-events'-- such as equipment failures, loss of power, and inability to cool the reactor core -- that could impair critical safety functions. However, four decades of analysis and experience have demonstrated that reactor core-damage risks are dominated by 'beyond-design-basis events,' the report says. The Fukushima Daiichi, Three Mile Island, and Chernobyl accidents were all initiated by beyond-design-basis events. The committee found that current approaches for regulating nuclear plant safety, which have been based traditionally on deterministic concepts such as the design-basis accident, are clearly inadequate for preventing core-melt accidents and mitigating their consequences. A more complete application of modern risk-assessment principles in licensing and regulation could help address this inadequacy and enhance the overall safety of all nuclear plants, present and future.