I-35. In the passive simplified pressurized water reactor concept (Fig. I-11), the containment vessel consists of a metallic shell surrounding the nuclear steam supply system. While the operational systems rely on proven pressurized water reactor technology, the safety systems for such reactors work passively, and do not depend on active components and safety grade support systems.

I-36. In accidents, the residual heat is transferred via steam to the containment atmosphere, either through the leak or through the passive core cooling system, which uses the in-containment refuelling water storage tank as a heat sink. The in-containment refuelling water storage tank is also used as a water source to provide the safety injection in the event of a LOCA, and flooding of the reactor cavity for external cooling of the reactor pressure vessel in the event of a severe accident.

I-37. Containment energy management is provided by passive external containment cooling, either by means of passive air circulation in the annulus or supported by external gravity spraying of the containment vessel. The design features of the containment promote flooding of the containment cavity region in accidents and submersion of the lower head of the reactor pressure vessel in water. The liquid effluents released during a LOCA through the break are also directed to the reactor cavity. After collection of the water in the lower part of the containment during an accident, a water level is reached that ensures that the water is drained back via sump screens into the reactor coolant system.

FIG. 1-11. Schematic diagram of a passive simplified pressurized water reactor: 1, in­containment refuelling water storage tank; 2, primary circuit depressurization system; 3, air baffle; 4, passive containment cooling system: gravity drain water tank; 5, containment vessel gravity spray; 6, natural convection air discharge; 7, natural convection air intake.