I—31. The containment of passive simplified boiling water reactors is constructed of reinforced concrete with an internal steel liner (Fig. I-10). The containment is usually subdivided into a dry well and a pressure suppression

FIG. I-9. Schematic diagram of a full pressure double wall containment system for a pressurized water reactor with provision for mitigation of the consequences of a severe accident: 1, in-containment emergency core cooling system (ECCS) water storage; 2, ECCS; 3, primary depressurization device; 4, core catcher; 5, containment heat removal system; 6, annulus filtered air extraction system.

pool, which acts as a heat sink in accident conditions and provides water for active make-up for the reactor pressure vessel.

I-32. Passive cooling and core flooding features are commonly provided by core flooding pools, which act as heat sinks for the passive emergency condensers and also for the safety relief valve system. In addition, the flooding pool water is used for passive flooding of the reactor core following depressuri­zation of the reactor pressure vessel in the event of a LOCA. Energy management in the containment is provided by passive containment cooling

FIG. I-10. Schematic diagram of a passive simplified boiling water reactor: 1, pressure suppression pool; 2, core flooding pool; 3, dryer-separator storage pool; 4, emergency condensers; 5, core flooding lines; 6, containment cooling condenser; 7, vent pipes; 8, overflow pipes; 9, H2 vent pipes; 10, safety relief valves; 11, dry well flooding line; 12, active residual heat removal system.

condensers that transfer the heat to the dryer-separator storage pool on top of the containment and transfer the condensate back into the core flooding pools.

I-33. For severe accident control, passive simplified boiling water reactors rely on external cooling of the reactor pressure vessel. The lower part of the dry well is flooded from the core flooding pools, and natural circulation inside the insulation of the reactor pressure vessel ensures the transfer of steam to the containment cooling condensers.

I-34. The containment is inerted during power operation to prevent the risk of hydrogen combustion. Hydrogen collected in the upper part of the containment is flushed through dedicated vent pipes into the wet well to avoid impairment of the function of the containment cooling condensers.