The basic post-trip heat removal mechanical systems arrangements are shown in a simplified form in Fig 2.114.

The individual mechanical systems, which together with the associated essential electrical system and post — trip sequencing equipment comprise the complete post­trip heat removal system are:

* system

• Main gas circulators and inlet guide vanes.

• Circulator auxiliaries cooling systems.

• Circulator auxiliaries diverse cooling system.

• Decay heat boilers and decay heat boiler feed system.

• Decay heat boiler air cooling system.

• Control system for starting and standby feed pumps.

The X systems are seismically qualified and are de­signed to remain functional after all earthquakes up to the magnitude of the safe shutdown earthquake.

Y system

• Emergency boiler feed and main boilers, including LP vent system.

• Inlet guide vanes emergency drive.

• Reactor seawater system.

• Pressure vessel cooling system.

All the post-trip heat removal systems, except the diesel generators, are provided on a per-reactor basis, the follow’ing plant descriptions are for a single reactor unit.

g 2 1 Forced gas circulation systems (X)

Forced gas circulation is achieved by operating the main gas circulators at 450 r/min (15<?o of normal speed) with the inlet guide vanes (IGV) fully open. This speed reduction is achieved by supplying the motors at a reduced frequency by means of variable frequency converters, which are connected automati­cally by the post-trip sequencing equipment (PTSE); each eas circulator has its own independent frequency converter. For depressurisation faults, the speed of the gas circulators is signalled to increase as a func­tion of the reactor pressure until normal full speed is achieved.

An independent circulator auxiliaries cooling system (CACS) is provided for each quadrant pair of cir­culators to cool the main motors and the lubricating oil systems. For normal operation, heat is rejected from the CACS to the reactor seawater system via two ІОО^о-duty plate heat exchangers; during post­trip operation an additional heat sink to the atmos­phere is available via the circulator auxiliaries diverse cooling system (CADCS). The latter comprises a single loop per reactor with two forced draught air-cooled heat exchangers, cooled by four fans and four pumps circulating the cooling water to the four CACS sys­tems; the capacity of the system is only sufficient for the post-trip heat removal duty and it is shut­down during normal operation.