Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
Normal operation.
Normal operating transients.
Abnormal operating transients.
Safety analysis therefore consists of ensuring the validity of each of these barriers and
their correct operation under normal and accident reactor operating conditions. This kind
of analysis emphasizes the progressive nature of safety by distinguishing three successive
but interrelated stages:
Prevention.
Monitoring.
Mitigating action.
This method is deterministic, since it attests to the possibility of a certain number of
accident situations. If the definition of the first barrier is simple despite its extent, this is
not true for the other two barriers. The reactor coolant pressure boundary is clearly
defined within the reactor building. It branches out, however, in a fairly complex manner
in the auxiliary buildings. The spent fuel pit has the same function, despite its free
surface. The reactor building containment is not the only place containing spent fuel or
primary coolant. Delimitation of the third barrier is thus also fairly complex. Finally and
most importantly, this succession of three barriers implies one markedly important fact:
the steam generator tubes with a considerable total surface area and a very thin wall
simultaneously fulfill the function of primary coolant enclosure and containment (second
and third barriers).
These reflections have contributed to the evolution of safety thinking from the barrier
method to the defense in depth concept. This concept in fact includes the barrier method,
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