If we go back to the Risk Probability diagram,
like here, now we have an increase of the non acceptable area.
But it is not by systematic events.
Their normative requirements are not that strict than the DBA's.
There would be new elements for those severe accidents,
such as hydrogen recombiners, reinforced vents, and others.
But specially, new procedures are developed.
Remember that the Defense-in Depth can be described
as the philosophy that provides redundancy for installations
with both, active systems and multiple barriers.
Remember the Castle approach
against the release of fission products. As you can see here,
it is based on the following requirements.
The first one and the first barrier
is the fuel pellet itself. It has or it is a ceramic pellet
that functions as a retention of fission products
and some structural capacity of physical barrier.
Although it is usually combined with barrier 2.
That are the mechanical rods.
The third barrier is the primary system boundary,
the reactor cooling system up to the first valve.
These three barriers are protected by the criteria
of the system in normal operation
that combine the Swiss cheese model with Farmer’s diagram.
Which can be considered the first level trying to preserve
the release during normal operation.
Now, we reach...
Now, sorry, that we reached the first level,
we include also procedures for preventing deflection
during normal operation.
All the scenarios considered “abnormal operations”
have all those transients which are not that challenging
to be considered as accidents
in the Probabilistic Risk Assessment.
Here we have tube leak in the steam generators,
small breaks in the steam lines,
steam generator safety or relief valves opening, etc.
Third level are the control design basis accidents,
or control of design basis accidents.
Here we have all the emergency procedures
which have been designed to help the operators to solve
or to run through any design basis accident or DBA.
This means that there would not be any challenge to the containment barrier.
And most of the time, the facility would be driven, sorry,
to a safe shutdown state.
Here we can find steam line breaks, both inside and outside containment,
containment barrier bypass, Steam Generator tube ruptures
and others.
Thanks to the safety systems and the emergency procedures,
in the worst case scenario, the plant would be
in a long term cooling scenario
without challenging the containment barrier.
And all the doses and releases would be confined in containment.
The fourth physical barrier is the containment itself.
The containment are a set of barriers which avoids the release
of fission products to the outside world.
All those accident which challenge the fuel integrity,
or/and the containment barriers
are included in the forth level.
The fifth level implements the On-site emergency plan.
Or On-site or Off-site emergency plan.
So, we saw the requirements of the Defense-in-Depth.
First, Defense-in-Depth requires the application of conservative codes
and standards to establish substantial safety margins
in the design of the plant.
Also, they require high quality design, construction and operation
in order to reduce the probability of malfunctions
and to promote the use of automatic actuations.
Third, it recognizes that equipment can fail
and that the operator can make mistakes.
Hence, redundancy of safety components and systems is needed.
Also, it recognizes that, despite these precautions,
accidents with significant fuel damage may not be fully prevented.
Therefore, containment structures and other safety features
that prevent the release of fission products are required.
And finally, it also requires that proven emergency plans be in place
to ensure that the notification and protection actions
to the surrounding population can be effectively carried out.