Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
damage, which they should not, radionuclide release will be easily limited by the
containment. If, however, a core melt accident should occur, it has been found that the
debris can be retained within the reactor vessel under certain conditions. The TMI‐2
accident, which began as a small‐break LOCA and escalated into a large‐scale core melt,
showed that the debris could be cooled in‐vessel with water addition.
Primary system failures
Failure modes of the primary coolant pressure boundary include: piping leaks; piping
breaks; pump seal failures; steam generator tube failures; value failures or misalignment;
and pressure vessel failure.
Piping leaks ‐ Leaks in the primary system piping core occur from various causes,
including through‐wall cracks. Such leakage can be detected by abnormal radiation
readings, loss of coolant inventory, or direct observation of leakage. Detection and
investigation of any leakage is extremely important, because leaks can provide a warning
of an impending pipe break. The so‐called “leak‐before‐break” theory argues that leakage
will always precede a pipe break and that the leakage can be detected and action taken
before a break occurs, so that instantaneous double‐ended “guillotine” pipe breaks need
not be considered as design basis accidents. While this argument has technical merit in
many cases, it has not been generally accepted by regulatory bodies. Release of
radioactivity due to piping leaks in general is easily confined within the containment.
Piping breaks ‐ Breaks in the primary system piping give rise to one of the major
classes of design basis accidents (DBA). The LBLOCA is the classical DBA for the
design of the emergency core cooling systems and the containment. SBLOCA have also
been studied extensively, especially since the TMI‐2 accident, which was essentially a
SBLOCA, led to large‐scale core melting and radionuclide release to the containment.
The design requirement is to show that cladding temperature, cladding oxidation, and
containment pressure remain within acceptable bounds, so that fuel integrity and
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