There is more variability in the design of PWR containments because it was the responsibility of the architect engineer (AE) rather than the NSSS vendor and in the US there were a number of different AEs. However, the general characteristics are similar in that the containments consist of either a strong steel-lined reinforced or pre-stressed concrete shell, or a strong steel shell (with a reinforced concrete building surrounding this). These provide both strength and leak tightness. A typical PWR large dry containment is shown in Fig. 10.17.

The pressure within the containment is generally reduced below atmospheric to inhibit leakage but one subset of dry containments uses this as a design feature. The sub-atmospheric containment is maintained at a negative pressure (~ 5 psi or 35 kPa) with respect to the outside atmosphere. This negative pressure provides some additional margin for response to design basis accidents, and therefore the design pressure and/or volume can be reduced accordingly.

A variation adopted by Framatome for the French 1300-MW plants was the use of two unlined concrete structures. The inner one is pre-stressed and provides the pressure-retaining capability while the outer reinforced concrete structure provides protection from external hazards. Leakage from the unlined inner structure is reduced by painting the walls with an epoxy paint and the interspace between the two containments is maintained at a reduced pressure and is filtered.

Although the containments are essentially passive in the short term they are provided with systems to remove heat and fission products from their atmospheres. This is normally done using spray systems (see for example Fig. 10.14). The spray water will both remove heat and help to remove fission products. To maintain fission product iodine in a non-volatile form the spray water is dosed to make it alkaline by adding chemicals either directly to the spray water or to the recirculation sumps. Some plants use fan coolers to remove heat from the containment, either as an alternative to the use of containment sprays or as a diverse heat removal route. The heat is rejected by passing air through a cooling matrix, cooled by the service water system.