It's more or less a tautology, because the most severe accident the reactor is thought to withstand is the design-basis event. AFAIK in the instances of Fukushi

It's more or less a tautology, because the most severe accident the reactor is thought to withstand is the design-basis event. AFAIK in the instances of Fukushima and Chernobyl/RBMK it was later thought that the design basis was simply incorrect.