DBA (Design Basis Accident) is an accident that a nuclear facility is designed to withstand.

BDBA (Beyond Design Basis Accident) is an accident that was not fully considered in the design process because it was judged to be too unlikely.

Let’s try to quantify how unlikely.

There are complex methods to do that based on fault tree analysis, but we’ll do rough approximation instead. There were 3 serious accidents (Three Mile Island, Chernobyl, and Fukushima) in approximately 15,000 reactor-years, corresponding to return period of 5,000 years and probability $2 \cdot 10^{-4}$ per year.

So a nuclear power plant like Indian Point with two reactors has BDBA probability of about 1% in 25 years. This is actually a pretty high number. For comparison, $10^{-4}$ is probability of fatal injury in one year among construction workers.

Apparently this is not the reason Indian Point will be closed by April 2021, but it should be.