Our focus, can be explained in three different points.
First, we can say that people tend to,
perform the duties successfully, when the reasons to do the tasks,
are clear, and shared by everybody involved in the job.
To do this, safety has developed what we call safety analysis.
In the nuclear sector it is essential,
to meet each and every one of the safety requirements.
The safety analysis is performed,
in a set of simplifications and limitations that everybody must know.
Those requirements, are based on those simplifications and limitations.
And they are quite rigorous and clear.
And finally, we must be reminded,
that there are a redundancy and variety,
of barriers, and levels of safety.
So, a breach of a specific requirement,
will not generate a problem inmediatly.
This can generate the feeling that,
those barriers are really "unnecessary".
When this is not true.
The multiplicity of safety features like barriers, procedures,
behaviors, and so on, it's essential part of the safety culture.
So, now we are going to expand our knowledge,
in the defense-in-depth concept.
That we have already mentioned during the introduction of this NOOC.
So, a question arises, that is What are the defense-in-depth?.
In general, defense-in-depth is a concept use in many fields.
From computing, to nuclear, even oil ,and gas industry use it.
It is also called, the castle approach.
Because, it is based on multiple layers of safety and security controls.
Also known as barriers or defenses.
That are placed all around the system, we want to protect.
So, we can have from moats, to towers, to walls and of course the workers.
That are the bearers of the safety culture.
And, we have a complete castle.
But, based on these ideas,
there have been multiple industries,
that have adapted it to their needs.
At the early 60s, of the last century,
it was a started to be used also, in the nuclear in industry.
So it was used in the concept of ensuring the nuclear reactor safety.
and it was based on the concept that,
there was a lack of knowledge of the nuclear reactor operations,
and the consequences of potential accidents.
So, basically,
While both, experience with the operation of nuclear reactor,
and the knowledge base of potential,
and the actual, consequences have increased since then,
the concept continues to be highly relevant today.
As we mention before, the idea is to create multiple barriers.
Independent and redundant.
Creating different layers of protection and response,
to challenges from minor to major.
An example of this multiplicity and redundancy,
are the protection systems.
the new PWR designs come with many protection systems,
and backup systems.
They acted with different set-points, variables,
and even their hardware is diverse to prevent failure,
of the safety actuations.
Now, we are going to see again the Swiss cheese model,
and expand our knowledge.