Now, let's see three concepts
on which the Defense-in-Depth philosophy is based.
The first one is what we call the first level of safety.
And it states that the design should be inherent safe.
So, it should have great tolerance to malfunctions,
focusing on quality, but specially on redundancy.
When we speak of redundancy, we are saying
that there may be multiple systems to perform the same safety function.
Then, we can go to the second level of safety.
There we state that incidents may occur, there is no perfect system.
And as we have mentioned before, systems are more and more complex.
So, we have to be ready for failure.
The Defense-in-Depth systems would be provided to avoid or to limit
damage during incidents or accidents.
And finally, we have the third level of safety.
And we can say that there will be additional safety systems
for highly unlikely accidents,
assuming concurrent failure of a safety system whose mission is
to avoid or mitigate the consequences of such an accident.
Those systems should be diverse, that means, diverse in their actuations.
Let's say diverse in signals, diverse in actuation logic,
wiring, power supply. But also in software and hardware designs.
The Defense-in-Depth systems or components philosophy
rests on the following bases:
The first one is the Standards that we have to comply with.
The second one is the Regulatory Guides.
We have to take into account that most of the countries
in the Western world have their regulatory guides
based or related to the USA regulatory guides.
But each country will have its own characteristics.
And finally, on the top, we will find the 10CFR50.
So, that's the pyramid of the Defense-in-Depth philosophy.
If we go to the 10CRF50 Attachment A, we will see
that there are different cases
that will be grouped in different annexes.
We will have general requirements.
We will have multiple barriers protections.
Or what we call "The Kassel approach".
We'll have reactivity protection and control systems.
We will have fluid systems, containment systems,
fuel and radioactivity control.
And they will be with the following criteria.
And this can be said to be the basis for the Defense-in-Depth philosophy.