Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
• Decrease in feedwater temperature;
• Increase in feedwater flow;
• Increased main steam flow;
• Inadvertent opening of a steam generator relief or safety valve
• Steam system piping failures inside and outside containment.
Among the events listed above, first four events are AOOs and the last, assuming the
double ended break of the steam line, is an accident. A typical event sequence of steam
line breaks is as follows:
A steam line break causes excess steam release from the secondary system, and
results in overcooling of the RCS. In the presence of strong negative MTC, typical
of EOC condition, core power increases to equalize the thermal load caused by the
steam blowdown. To compensate for degraded margin‐to‐DNB, the Reactor
Protection System (RPS) generates a reactor trip, causing insertion of the control
rods to make core sub‐critical.
Due to the continued steam blow‐down through the break from one affected and
other unaffected SGs, early in this phase the SG pressure falls below steam line
isolation signal actuation pressure, and then the path for steam from unaffected SG
to reach the break location is blocked.
When RCS pressure decreases continuously and reach the setpoint of the Safety
Injection (SI) actuation signal, the boric acid is entered into the RCS, decreasing
the core reactivity. The cool‐down and associated shrinkage of RCS fluid is
interrupted after the affected SG blowdown into the containment has been
completed.
The RCS energy balance is established where the heat sources (core decay heat,
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