Civil Engineering Reference
In-Depth Information
5.3.2.4 Fourth Safety Level: Measures Taken to Reduce Damage
if Design Basis Is Exceeded—Accident Management Measures
On the fourth safety level, accident management measures take credit of existing
design margins up to failure of in-plant systems and of additionally installed
components which will be used in case of failure of the plant protection system.
Accident management measures also take credit of existing in-plant systems
which are not classified as safety systems.
Measures are taken to minimize the potential damage caused by plant-internal
accidents beyond the design basis and external impacts, e.g. airplane crash, chem-
ical explosion or beyond design earthquakes [
4
].
5.4 Design Basis Accidents
The reactor plant and the protection systems must be designed and built on the basis
of the design basis accident concept. This includes a number of design basis
accidents which must be accommodated safely, even if a fault independent of the
original cause of accident initiation occurs. The safety must be demonstrated by
advance calculation (design basis accident analysis). This analysis must be based on
conservative assumptions wherever uncertainties exist.
Selected design basis accidents require proof to be supplied, that certain limits
(temperatures of the fuel elements, pressures, stresses and strains in components of
the primary cooling systems) are not reached, with the provision that this requires
no manual measures to be taken in the first 30 min. The dose levels listed in the
Radiation Protection Ordinance for accidental radioactivity releases (Sect.
5.6.6
)
must not be exceeded.
5.4.1 Events Exceeding the Design Basis
Sequences of events exceeding the design basis and leading to severe accidents—
despite measures taken by severe accident management measures—must have a
probability of occurrence of less than 10
5
to 10
6
per reactor year [
4
-
6
]. This must
be shown by probabilistic safety analysis (Sect.
5.4.2
).
5.4.2 Probabilistic Safety Analyses (PSA)
Probabilistic safety analyses are not part of the valid licensing procedures, which
use only deterministic criteria. However,
they have proved their value as
