Regional Basic Professional Training Course in Korea

8.8.1.6. Loop Seal Formation and Clearing
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8-B. Deterministic Accident Analysis (LOCA)
Core level is affected by the difference between the break flow and the Safety Injection
flow. However, the more important factor is the difference in hydrostatic pressure of the
SG U‐tubes. This is due to the difference of the void fraction and mixture density between
the countercurrent flow at the upward junction and the cocurrent flow at the downward
junction of SG U‐tube. Due to high density of steam at the hot side region of the U‐tube, a
significant hydrostatic pressure is acting. This differencial pressure is propagated to the
two‐phase level of the core through hot leg. It may suppress water surface of the core and
uncover the core. The barrier playing a role to maintain the pressure difference between
two regions is called as “loop seal.” The loop seal in this case is a water captured in the
region between the SG and the suction part of reactor coolant pump. Normally, loop seal
at the broken loop is cleared first. After loop seal clearing at the broken loop, water at the
upper downcomer of reactor vessel flows into the core and core water level increases
rapidly.
Figure 8.8‐5 Loop seal clearing following SBLOCA of 2‐loop pant