The emergency core cooling systems (ECCS) provide core cooling under loss of coolant accident conditions to limit fuel cladding damage. The emergency core cooling systems consist of two high pressure and two low pressure systems. The high pressure systems are the high pressure coolant injection (HPCI) system and the automatic depressurization system (ADS). The low pressure systems are the low pressure coolant injection (LPCI) mode of the residual heat removal system and the core spray (CS) system.
The manner in which the emergency core cooling systems operate to protect the core is a function of the rate at which reactor coolant inventory is lost from the break in the nuclear system process barrier. The high pressure coolant injection system is designed to operate while the nuclear system is at high pressure. The core spray system and low pressure coolant injection mode of the residual heat removal system are designed for operation at low pressures. If the break in the nuclear system process barrier is of such a size that the loss of coolant exceeds the capability of the high pressure coolant injection system, reactor pressure decreases at a rate fast enough for the low pressure emergency core cooling systems to commence coolant injection into the reactor vessel in time to cool the core.
Automatic depressurization is provided to automatically reduce reactor pressure if a break has occurred and the high pressure coolant injection system is inoperable. Rapid depressurization of the reactor is desirable to permit flow from the low pressure emergency core cooling systems so that the temperature rise in the core is limited to less than regulatory requirements.
If, for a given break size, the high pressure coolant injection system has the capacity to make up for all of the coolant loss, flow from the low pressure emergency core cooling systems is not required for core cooling protection until reactor pressure has decreased below approximately 100 psig.
The performance of the emergency core cooling systems as an integrated package can be evaluated by determining what is left after the postulated break and a single failure of one of the emergency core cooling systems. The remaining emergency core cooling systems and components must meet the 10 CFR requirements over the entire spectrum of break locations and sizes. The integrated performance for small, intermediate, and large sized breaks is shown on below.