The boiling-water reactor, like the P^^, has multiple provisions for cooling the core in the event of an unplanned depressurization or loss of coolant within the reactor. A typical B^^ emergency core cooling system is illustrated in Figure 4.27 It is composed of four separate subsystems, namely the high-pressure corespray (HPCS) system, the automatic depressurization system (ADS), the low-pressure corespray (LPCS) system, and the low-pressure coolant injection (LPCI) system.

The HPCS pump takes water from the condensate storage tank and/or the pressure suppression pool as shown in Figure 4.27 The water in the system is piped into the vessel and feeds semicircular perforated rings that are designed to spray water regularly over the core and onto the fuel assemblies. This system operates over the full range of reactor pressure and is activated when the water level in the reactor drops below a preset level or the pressure in the contain­ment vessel reaches a high value.

If the HPCS cannot maintain the water level or if it fails to operate, the reac­tor pressure is reduced automatically by operation of the ADS, which dis­charges fluid from the vessel into the pressure suppression pool. The depressurization allows the LPCI and LPCS systems to come into operation, and these provide sufficient cooling. The LOCS pump takes its water from the sup­pression pool and discharges from a circular perforated pipe in the top of the reactor vessel above the core; it is actuated in much the same way as the HPCS. The LPCI system is used for residual heat removal on a long-term basis.