If we were going to build a nuclear power reactor
only with the criterion of energy production,
there would exist a set of probability-damage pairs
or probability severity pairs,
that could represent different outcomes in that power station.
It is important to note that the damage that can occur
has a physical limit. And that limit is huge,
but is not infinite.
So probably, Chernobyl was close to that limit.
If there were many events that have high probability
and a huge severity or damage,
then, that operation would be unacceptable.
So, we are going to study that using Farmer's Diagram
or Risk Assessment Matrix.
There, risk is defined as the product of the probability
of an event to happen and the severity or consequences.
So, the pairs probability-damage
to the right of the bisector,
if the damage and probability scales are equal,
are the most contributive to the risk of that installation.
As we see here, the ideal is to have more pairs to the left bottom.
So, less probability, less severity.
And less pairs to the right of that bisector.
So, we can draw a line.
As we see here, and that's a way to limit the risk
of any event
by eliminating all the points in the right of that line.
This is clearly an improvement.
But it is impossible to make by design or to eliminate by design,
all the points at the right of the line.
We are not controlling the total risk,
that is the accumulated risk of each event.
So, to make the process viable, the straight line can be replaced
by a stepped function.
This avoids having to calculate accurately the probability,
being enough to estimate
if the probability of the events is in a certain range.
We are going to have three different ranges.
The normal operation, the transients and the accidents.
In addition, as I have said, there are only three damage limits.
In each category, the Design Basis Events are chosen,
which is what defines the protections to be implemented
to limit the damage.
It's important to note that below a certain probability,
no damage limits are established. And therefore, in practice,
the limit is the physical limit.
We can see that below the accidents.
This is done because it is estimated
that such events are extremely unlikely.