In many new concepts, the containment consists of an inner steel shell for providing leak tightness and an outer reinforced concrete shell for protection against external hazards. It should be noted that compared to the concrete wall the steel shell is more sensitive to the fast or local thermal loads which can be produced by direct containment heating or fission product deposition phenomena. For this containment configuration, the passive containment cooling system (PCCS) utilises the steel shell as a heat transfer surface. In case of an accident the inner containment shell will heat-up and drive an air flow, that enters the gap between inner and outer containment shell through windows at a lower position and leaves for the environment through filtered air outlet at the top. The air-flow will cool the steel shell by natural convection. Steam produced inside the containment due to accident conditions will be condensed on the inner surface of the steel shell and the condensate will collect in the containment sump.

In some PWRs (e. g. AP600, EP1000, AC600/1000), between the inner steel shell of the containment and the outer reinforced concrete shell an air baffle forms an annular wind duct. The PCCS utilises the steel shell as a heat transfer surface. In case of an accident the containment heats up and drives an upward air-flow in the gap between the steel shell and the air baffle [5]. Water from a water storage tank is sprayed onto the top of the steel shell. It flows down the outer surface as a water film counter-current to the air, intensifying heat removal by evaporation. Steam will be condensed on the inner surface of the steel shell with the condensate being collected in the containment sump. The capacity of the water storage tank is sufficient for 72 hours [6]. In the very unlikely case that the operator would not be able to replenish the water in the water tank when it empties after 72 h, the natural convection of air will be sufficient to prevent containment failure although the design pressure will be slightly exceeded [4]. A similar PCCS concept without the seismic issue of an elevated tank utilises an external ground level water storage tank with an insulated floating lid on the water level. The top of this tank is connected to an internal vessel with a low boiling point fluid. If the containment heats up this fluid starts to boil and pressurises the external tank by pushing the floating lid. Therefore the coolant is sprayed onto the top of the containment [7]. Another concept uses the lower part of the annulus between the steel and concrete shell as a water pool. Air enters the gap trough windows above the water level and leaves it at the top. When the steel shell heats up, the temperature of water will increase up to boiling. The heat will be removed by single-phase convection of water and air or by evaporation, depending on the containment conditions [7].

In Russian WWER-640/V-407 concept [8], at the outer surface of the containment steel shell, rectangular pockets are arranged in rows and columns. The pockets of each column are inter­connected by vertical lines. A NC flow of cooling water from an external pool near the roof will be established. Steam is condensed at the cooled parts of the inner surface of the steel shell. The condensate is collected in the sump to allow for post accident recirculation.

A passive containment cooler (PCC) which is used in some designs primarily for severe accident prevention purposes will also mitigate severe accident consequences as it is foreseen in the design of the ESBWR, and some advanced PWR and CANDU designs. The PCC is a condenser immersed in a special PCC pool above the containment, using the design principles of an isolation condenser. It consists of a tube bundle connecting inlet (top) and outlet (bottom) collectors. The steam-gas mixture from the containment enters the inlet collector of the PCC and will be condensed inside the tubes. The condensate drains towards the RPV and the wet well [9].

In a PCC system for an advanced heavy water reactor design the condensate is collected in a water storage tank for gravity driven injection, and non-condensables are vented to a suppression pool [10]. A PCC proposed for application in a PWR (e. g. the CP-1300) utilises an external pool in a high elevation as a heat sink. The steam-H2 mixture passes H2-ignitors before entering the heat exchanger. The condensate is collected in the IRWST and/or used for passive containment spray [7].

In some designs (e. g. some BWRs and PWRs with double concrete containment) a building condenser (BC) is used for passive containment cooling. A BC consists of an internal (inside containment) heat exchanger (HX) in combination with an external (outside containment) pool containing the cooling water. The HX is composed of a number of finned condenser tubes between an inlet — and an outlet collector. It is placed at an upper position inside the containment. The HX tubes are slightly inclined. Thus the outlet is somewhat higher than the inlet. The collectors are connected with the external pool (penetrations). The outlet line ends at a higher elevation in the pool than the inlet line. Cooling water will flow by natural circulation from the pool through the BC back to the pool. The condensate drops down from the outer surface of the HX to a core flooding pool (BWR) or to the containment sump (PWR). A skirt is used in PWR design to separate the condensate from non-condensable gases. As a kind of BC, the modular PCCS [11] can be considered for PWRs with double concrete containment. Modular PCCS uses an internal HX with a skirt (similar to BC) connected with a pool in the gap between the containment shells. Before the cooling water enters the pool it passes through an external HX where heat is transferred to an air-flow. The air-flow will enter the gap through outer concrete shell windows, pass the HX, move over the water surface in the pool and leave the building through a chimney. Condensation of steam and separation of non-condensables functions as in a BC system.

Passive containment cooling can also be ensured by internal plate condensers (PCs) that are fitted to the containment wall and connected with an external heat sink. One example for a PC utilises cooling elements made from ductile cast iron, containing fins at the side facing the containment atmosphere. These fins can absorb loads from fragments impact without any effects on the cooling system. The cooling water flows through steel pipes in bores along the fins. Natural circulation is established by an external draught cooling tower. PCs were found to be a valuable passive energy sink particularly under convective heat transfer conditions, i. e. beyond its design conditions [12].

Containment spray systems are widely used in current reactor designs as well as in future concepts to reduce the concentration of fission products (e. g. for iodine precipitation and aerosol wash out). Containment spray systems also contribute to containment pressure and temperature reduction. Although containment spray systems in most cases utilise pumps (active), at the secondary side of the cooler natural circulation driven systems can be used. Nevertheless, there are completely passive concepts with a spray water supply from elevated [7] or pressurised [4] tanks, that can contain special chemicals e. g. for iodine precipitation. Another passive concept uses the condensate of a PCC system to feed the spray [7]. An alternative passive system to drive a spray mass flow uses an ejector-condenser (E-C) system [13]. Its principle is based on the dynamic form of natural convection utilising inertia forces instead of gravity for fluid circulation. Steam speeds up to a high velocity in a Laval nozzle. Then it mixes in a mixing chamber with cold water that takes off its mass, heat and kinetic energy. The resulting two-phase mixture gets a supersonic flow, the kinetic energy of which turns into potential form in a diffuser. Thus the outlet pressure can exceed essentially the inlet steam and water pressure. In containment spray application water is taken from the sump, cooled by an external cooler (e. g. draught cooling tower) and fed by the E-C to the spray nozzles. The driving steam for the E-C is taken from a heat exchanger that evaporates part of the supplied water mass flow (bypass) by the heat of the containment atmosphere [14]. For start of the system an external start-up tank is necessary.