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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 />

8.8.1.1. Before the reactor trip<br />

The flow rate of the coolant cont<strong>in</strong>ues to change as time progressed. The break flow<br />

<strong>in</strong>creased to a peak <strong>in</strong> a short time and then decreased as RCS pressure decreased. The<br />

flow regime at break changed from the <strong>in</strong>itially subcooled state to the saturated two‐phase<br />

mixture and the fraction of steam expands gradually. At most, the critical break flow<br />

exists throughout the transient, i.e. flow can be determ<strong>in</strong>ed by the pressure and fluid<br />

enthalpy upstream and not dependent on the pressure downstream. Therefore, the break<br />

flow break flow rate can be predicted with a accurate calculation of critical flow.<br />

8.8.1.2. Reactor trip<br />

Figure 8.8‐2 Pressure behavior dur<strong>in</strong>g SBLOCA<br />

As RCS pressure decreased, the high temperature coolant beg<strong>in</strong>s to flash to steam. Dur<strong>in</strong>g<br />

this process, the reactor is shutdown automatically. Boil<strong>in</strong>g of the coolant <strong>in</strong> the reactor<br />

core changes the depressurization rate of RCS and the mass flux per unit area decreases<br />

as the break flow changed to two‐phase mixture. The pattern of depressurization is<br />

dependent on the size and location of the break. Similar to LBLOCA case, the behaviors<br />

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