The steel containment vessel is an integral part of the containment system, which serves to limit releases in the event of an accident and also to provide the ultimate heat sink. Current steel containment vessels in United States generally fall into three categories : 1) cylindrical steel containments, 2) spherical steel containments, and 3) BWR Mark I/II containments which typically have a drywell with the shape of an inverted light bulb or a cone, connected to a wetwell with the shape of a torus or a cylinder. Vertical or circumferential stiffeners may be attached to the exterior of the vessel. The steel containment is generally protected by a 24 in to 36 in reinforced concrete cylindrical shield building in order to provide shielding against release of radioactive material and for external missile protection. Chapter 1 provides some additional details on steel containments.
Cylindrical steel containment vessels are common due to relative ease of fabrication and erection. Most cylindrical steel containments have a hemispherical, torispherical, or ellipsoidal dome made of steel. The lower section of such containment could either be a hemispherical, torispherical, or ellipsoidal steel head similar to its dome, or a hybrid steel cylinder with concrete basemat with a leak tight steel liner.. The main advantage of a spherical containment is that it requires only about one-half the wall thickness of a cylindrical vessel for the same internal pressure loading. However, fabrication and erection of a spherical containment vessel is relatively costly and time consuming, compared to a single-curved cylindrical containment. The thickness of cylindrical steel containment generally varies between 2.5 and 3.7 cm (1 to 1.5 in.) which may be thickened around the transition regions at the top and bottom head and around openings and penetrations to account for stress concentrations.
Penetrations through the steel containment shell are provided for piping, instrumentation, control lines and electrical power. Large diameter openings such as equipment hatches and personnel access should be evaluated for buckling, which will be further discussed in Section 3.2.4. The reactor building polar crane can either be supported by the locally thickened containment vessel, or by an interior structure.
Controlling loads for the design of a steel containment vary depending upon its reactor type (such as BWR vs. PWR) and the seismic region where the nuclear plant is located. But because of their relatively low mass, steel containments induce less seismic loads as compared to concrete containments. However, localized pressure transient loads and buckling failure modes are much more critical in the design of a steel containment than in the case of a concrete containment, which has significantly higher built in structural stiffness.
Design of steel containments in the United States is carried out under ASME BPV Code Section III, Div 1 Subsection NE — Class MC Components  as supplemented by RG 1.57 . For the aforementioned hybrid steel cylinder and concrete basemat, the design provisions for the lower (concrete) portion of the containment is governed by ASME BPV Code Section III, Div 2  for concrete containments.