I-4. A typical full pressure double wall containment (Fig. I-2) consists of:

— A steel or concrete shell, basically cylindrical or spherical in shape (the containment);

— A concrete shell surrounding this containment (the secondary confinement);

FIG. I-2. Schematic diagram of a full pressure double wall containment system for a pressurized water reactor: 1, full pressure containment; 2, secondary confinement; 3, annulus; 4, annulus evacuation system; 5, filtered air discharged system.

— An air extraction system for the annulus (the space between the containment and the secondary confinement).

I-5. The principle of the primary containment is similar to that of the full pressure dry containment in pressurized water reactors (paras I-2 and I-3). The secondary confinement fulfils the following three functions:

— In combination with the containment, it provides radiation shielding for plant personnel and the environment in normal operation and under accident conditions.

— It protects the systems and components that it contains against external postulated initiating events.

— It captures leakage from the containment in the annulus between the two shells.

I-6. Safety systems such as the emergency core cooling system and the high pressure boron injection system may be located in the annulus between the two shells if they can withstand the thermal loads and pressure loads calculated for design basis accidents. Leaks from the containment into the annulus are extracted and filtered under accident conditions by an air removal system, and their emission through the plant stack is controlled.