When the suppression chamber pressure increases by the FSC operation and the drywell gas transfer into the suppression chamber, the vacuum breaker valves automatically open and the gas in the suppression chamber returns to the drywell.

There is a primary containment atmosphere control system, a non-ESF system, which maintains a low oxygen concentration in the PCV during normal operations, by replacing the PCV atmosphere with nitrogen gas.

By doing so (replacing the containment atmosphere by nitrogen gas during normal operations), detonation can be prevented in case hydrogen is generated in a LOCA This contributes greatly to the PCV integrity.

5. Standby gas treatment system (sgts)

Figure 2.7.13 shows the configuration of the SGTS. The SGTS consists of two independent lines and each has a moisture separator, HEPA filters, charcoal filters for iodine removal, and exhaust fans. Two redundant active components are used. This system is automatically actuated by a signal implying an accident like a LOCA It can maintain the negative reactor building pressure and has a capacity to treat daily the 50% or 100% atomsphere of the reactor building air volume.

The SGTS of a BWR is installed in the PCV and the reactor building and this arrangement works as

a double containment

The fission products leaked from the PCV to the reactor building are treated by the SGTS and the treated atmosphere is discharged from the stack to the environment; this discharge is monitored.

The SGTS is connected to the emergency power supply and it can be operated and controlled under the loss-of-offsite-power conditions.

6. Reactor containment facility of an abwr

The reactor containment facility of an ABWR adopts a reinforced concrete structure with steel liners. Figure 2.7.14 illustrates a reinforced concrete containment vessel (RCCV). Its design peak pressure is 0.314 Mpa [gauge].

The PCV consists of a cylindrical drywell surrounding the RPV, cylindrical suppression chamber, base mat and other structures. Inside the RCCV, there are a reinforced concrete diaphragm floor partitioning the drywell and the suppression chamber, and the RPV support, in which horizontal vent steel piping is embedded, connecting the drywell and the suppression chamber. The RCCV is equipped with vacuum breakers, RVCV penetrations and isolation valves, too.

In a LOCA, steam-water mixtures released to the drywell are transferred to the suppression pool water through the vent piping for cooling and condensing. The drywell pressure can be controlled and the released radioactive materials

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pressure suppression chamber

reactor building

from reactor building annex

to stack

to reactor building HVAC

from reactor building annex

Figure 2.7.13 SGTS system configuration

Figure 2.7.14 RCCV cross section (elevation)

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