The Defense-in-Depth concept has limitations.
And due to that limitations an evolution happened.
For example, the Design Basis Accidents do not always provide proper protection
for minor events of the same class.
We can say that there is an issue, for example, regarding the small LOCAs.
This issue is that the evolution is very different from a big LOCA
where there is a huge depressurization:
And therefore, the response of the safety systems
is going to be very different for a big LOCA or for a small LOCA.
This was stated by the WASH-1400,
which was the first rigorous study of the Probabilistic Safety Assessment.
But until Three Mile Island happened, it was not taken seriously.
Also, there are events out of the Risk-Probability diagram
with high risk contribution.
Let's say two of them mainly.
The ATWSs, that is an Automatic Trip Without Scram,
that will happen
during Transients with reactor trip failures.
And also the Station Black Out,
that means a loss of Off-site and On-site power.
All of those are incorporated, but not with the requirements
of the other events within the base design barrier.
Some elements are incorporated for severe accidents.
Like additional equipment, let's say hydrogen recombiners,
reinforced vents, etc.
But above all, we have procedures and also we have rules.
All of them are associated level of requirement
lower than the events within the envelope.
A curious fact about all this is that Fukushima,
according to the Japanese regulator,
did not consider the Station Black Out
because it was very unlikely.