In accordance with the international seismic regulations
and their applicational practices, the earthquake should be contemplated
in the Defense-in-Depth
with different levels of severity.
A maximum level, which is really the base design earthquake,
and generally called SSE in its English terminology,
it's the Safe Shutdown Earthquake.
And it's the maximum earthquake that is considered that it could happen
in a certain location.
At this given site, the SSE could cause
the maximum movement of soil,
and was adopted for the design of the installation.
It has a very low probability of being exceeded
during the entire life of the nuclear power facility.
And thus, it's associated with the highest safety requirements.
In the event of this earthquake, the structures, systems,
and the equipment and the components of the plant
that are necessary to guarantee nuclear safety
would remain in operation.
Also, we have the OBE, or Operating Basis Earthquake,
that is the largest earthquake that is reasonably expected to happen
at the site throughout the operational life of the plant.
And usually it has half the intensity of the SSE.
To achieve the objectives of the seismic categories,
it is necessary to carry out specific evaluation studies
of the geographical seismotechtonic and seismic hazard characteristics
both, of the geographical area, in which the nuclear installation is placed
and of the local area in which the facility is located.
And here we can see the difference of intensity in accelerations
and frequency of the OBE and SSE
for horizontal and vertical accelerations.
Now we are going to see the three different seismic categories.
First, we have Seismic Category I
that is the highest category and are all those items
systems, extractors, components, etc.,
that are required to maintain safe shutdown conditions
and to prevent or mitigate accidents that could provoke radioactive releases.
Also, we have Category II that are all those items
not included in Category I,
that are needed to prevent or mitigate accidents
in long term conditions.
And finally, our Seismic Category III, that are all those items
buildings, components, etc., that have no seismic category.