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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 />

8.5.6. Decrease <strong>in</strong> Reactor Coolant Inventory<br />

For a PWR, the <strong>in</strong>itiat<strong>in</strong>g events that can result <strong>in</strong> decrease <strong>in</strong> reactor coolant <strong>in</strong>ventory<br />

are as follows:<br />

• Inadvertent open<strong>in</strong>g of pressurizer relief valve or safety valve<br />

• Steam generator tube rupture (SGTR)<br />

• Postulated pipe breaks with<strong>in</strong> reactor coolant pressure boundary<br />

Among the events listed above, the first is an AOO and the rema<strong>in</strong><strong>in</strong>g two are accidents.<br />

As aforementioned, the last is def<strong>in</strong>ed as the LOCA and is discussed <strong>in</strong> another chapter.<br />

The major concern of SGTR is a radioactive material release bypass<strong>in</strong>g the conta<strong>in</strong>ment<br />

to the offsite, due to a direct release of RCS coolant to the secondary system. A typical<br />

sequence of the SGTR, assum<strong>in</strong>g the double ended rupture of a SG tube is presented here.<br />

A SGTR results <strong>in</strong> release of RCS coolant to the secondary system and potentially<br />

to the atmosphere if the secondary system pressure rises sufficiently to open the SG<br />

safety and/or relief valves. If normal operation of various plant control systems is<br />

assumed, the follow<strong>in</strong>g sequence of events for significant break flow exceed<strong>in</strong>g<br />

charg<strong>in</strong>g capability is expected:<br />

The pressurizer low pressure and low level alarm are actuated, and charg<strong>in</strong>g pump<br />

flow <strong>in</strong>creases <strong>in</strong> an attempt to ma<strong>in</strong>ta<strong>in</strong> the pressurizer level. On the secondary<br />

side, steam and feed flow mismatch occurs as feedwater flow to the affected SG is<br />

reduced as a result of break flow to the SG secondary side.<br />

Decrease <strong>in</strong> RCS pressure due to cont<strong>in</strong>ued loss of reactor coolant <strong>in</strong>ventory leads<br />

to a reactor trip signal.<br />

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