LOCA Stages

Length of Blow-Down Phase

0-20 s

Length of Refill Phase

20-40 s

Length of Reflood Phase

40-250 s

Long term cooling phase

250+ s

Blow-Down Phase of LOCA

Rupture of the primary coolant pressure boundary, which leads to the trigger of a rapid depressurization. System signals and flow stagnation in the vessel are also triggered by this pressure boundary rupture.

Refill Phase of LOCA

The Emergency Core Cooling System activates to inject water to the lower plenum. Countercurrent steam flow escapes from the core and causes bypass to sweep injected water directly through the break.

Reflood Phase of LOCA

ECCS liquid reaches the bottom of the active fuel rod and starts to submerge and rewet the core. The cladding temperatures hit a higher peak which is driven by core uncovery before turning out.

Long-Term Cooling Phase

Once the core is fully submerged and temperatures are brought under control, the system transitions to decay heat removal. ECCS pump suction is realigned from external storage to containment.

LOCA?

Loss of Coolant Accident is a LOCA, and occurs when there is a breach in the primary coolant pressure boundary.

LOFA?

Loss of Flow Accident is a LOFA. This occurs when there is a loss of pumping power, but the Pressure Boundary remains intact.

LOCA vs LOFA

The difference between the two is that LOCA and LOFA have different inventory mass in the system. For a LOCA, the inventory changes, but for a LOFA, it stays the same. In a LOCA, the flow is still occurring, but in a LOFA, there is a change to the way the coolant is moving through the system.