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Regional Basic Professional Training Course in Korea

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<strong>Regional</strong> <strong>Basic</strong> <strong>Professional</strong> <strong>Tra<strong>in</strong><strong>in</strong>g</strong> <strong>Course</strong> (BPTC) on Nuclear Safety<br />

(2) Refill phase<br />

The refill period beg<strong>in</strong>s at the time accumulator <strong>in</strong>jection flow is <strong>in</strong>itiated and ends when<br />

the mixture level <strong>in</strong> the lower plenum reaches the core <strong>in</strong>let.<br />

Cold ECC water mixes with steam <strong>in</strong> cold leg generat<strong>in</strong>g oscillations due to direct contact<br />

condensation phenomena. Once <strong>in</strong> the downcomer ECC water can flow by gravity down<br />

the annulus or be swept out through the break by the escap<strong>in</strong>g upward steam flow, that is,<br />

due to the counter‐current flow limitation (CCFL) phenomenon as shown <strong>in</strong> Figure 8.7‐3.<br />

When subcooled ECC fluid gets <strong>in</strong> contact with the superheated solid structures <strong>in</strong> the<br />

downcomer, steam is generated by nucleate boil<strong>in</strong>g. This reduces the gravitational head<br />

of the fluid <strong>in</strong> the downcomer and penetration <strong>in</strong> lower plenum. ECC liquid that has<br />

penetrated <strong>in</strong>to the lower plenum can be swept out aga<strong>in</strong>. Direct contact condensation of<br />

steam on the subcooled ECC fluid cont<strong>in</strong>ues.<br />

Figure 8.7‐3 Sketch of ECC bypass <strong>in</strong> a two‐loop PWR<br />

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