Containment design and load carrying capacity

The new containment has to withstand not only the classical design basis accidents but also the much more severe loads due to postulated core-melt accidents (static pressure of 20 bar at 200deg C and dynamic pressure in the order of 100 bar and 5 ms duration). The inner steel containment shell alone cannot withstand these loadings. Rather it is necessary that it comes into contact with surrounding supporting structures, which then help to carry the loading. Additionally, the environmental air flows in natural circulation through the annular gap between the inner steel shell and the outer concrete shell and thus removes the decay heat. Leakages from the containment must not pass without filtering into this air flow. Taking into account only mechanical aspects, a spherical containment shell would be most suitable. However, a cylindrical containment with a doubled wall thickness in the cylindrical domain would allow for more flexibility with respect to internal space requirements or to external cooling requirement, respectively. Both types of containment can bear the classical design loads as free standing structures. This feature requires a relatively large gap between inner steel shell and the external supporting structures only after moderate, but acceptably small plastic strains. In case of high dynamic pressure loadings the containment shell crashes against ribs designed to yield plastically. Thus, the consequences of a hydrogen detonation can be coped with at moderate plastic strains in the containment shell, without questioning its tightness. (orig./DG)