This type of passive safety system uses an elevated pool as a heat sink. Steam vented in the containment will condense on the containment condenser tube surfaces to provide pressure suppression and containment cooling. Three variations of the concept are presented in Figures 10 to12. In the first variation of the concept, Figure 10, an air heat exchanger (HEX) is connected with a pool located on the top of the containment. Single phase liquid is expected to flow inside the HEX driven by gravity gradient caused by the inclination of the same HEX. Experiments have been performed to prove the validity of the solution. In the second variation of the concept, Figure 11, a closed loop filled with single phase liquid connects an air HEX and a pool-type HEX. Natural circulation and heat removal capability are generated when the air HEX receives heat from the containment: this occurs

through liquid heating and stratification that produces a difference between densities in the rising and descending leg of the pool-type HEX. In the third variation of the concept, Figure 12, two different zones of the containment, typically characterized by different pressures in case of accident (pressure is the same during normal operation), are connected with the rising and the descending side of a pool — type HEX. In this case, the steam-air mixture is the working fluid with condensate in the descending leg. Driving forces may be lower than in the previous cases and working condition may not be stable over a reasonably wide range of conditions. Positive driving forces may be low in all three cases and careful system engineering is needed. These passive safety systems are of Categories B and D.

FIG. 10. Containment pressure reduction and heat removal following a LOCA using steam condensation on condenser tubes.

3.2.
Passive containment spray systems

Figure 13 shows a design that implements a natural draft air cooled containment. Subsequent to a LOCA, steam in contact with the inside surface of the steel containment is condensed. Heat is transferred through the containment wall to the external air. An elevated pool situated on top of the containment provides a gravity driven spray of cold water to provide cooling in a LOCA scenario. The air flow for the cooling annulus, that is generated by a chimney-like type effect, is a Category B passive safety system. The containment vessel sprays are a Category D passive safety system.

FIG. 13. Containment pressure reduction and heat removal following a LOCA using a passive containment spray and natural draft air.

The geometrical and layout characterization of passive safety systems constitutes the key subject for Sections 2 and 3. The thermal-hydraulic characterization for the same systems requires the consideration of phenomena, i. e. passive safety system phenomena.

Therefore the aim of Section 4 is twofold:

• To classify thermal-hydraulic phenomena for passive systems;

• To establish a correlation between systems (as described in Sections 2 and 3)

(bullet above).