Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
quantity of fission products towards the containment. The phase of progressive in‐vessel
heating‐up and melting establishes the initial conditions for the assessment of the thermal
and mechanical loads that may ultimately threaten the integrity of the containment.
The ex‐vessel progression of severe accidents is affected by the mode and timing of the
failure of the reactor pressure vessel, the pressure in the reactor coolant system at vessel
failure, the composition, amount and nature of the molten core debris expelled, the type
of concrete used in the construction of the containment, and the availability of water in
the reactor cavity. Some highly energetic phenomena may be caused by severe accidents.
Such phenomena could cause the ultimate load bearing capacity of containments
constructed by means of existing technologies to be exceeded, and consequently lead to a
large early release of radionuclides to the environment.[3]
Therefore, when the core is degraded, it is essential that containment leaktightness is
maintained for as long as possible. This is the purpose of the Severe Accident
Management Guideline (SAMG) in US or some of the U procedures in France. If the
accident cannot be controlled, failure of containment is unavoidable in the long term.
The different sequence stages for a severe accident are summarized in Fig. 16.4.3.
Following initiators (breaks, equipment failure, etc) accumulated as a result of safety
system failures, four main stages can be distinguished in the development of the accident,
if it is not controlled [6]:
1 st stage: reactor coolant system draining;
2 nd stage: core water depletion and degradation;
3 rd stage: vessel melt through;
4 th stage: basemat erosion by the corium.
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