The NSSS for BWRs are exclusively provided by General Electric. It is most convenient to differentiae the BWR containments by the way they were developed to reflect improvements and efficiency in design, construction, and operation.
Early BWR Containments.
As shown in Table 1.1, Dresden 1 was part of the Round 1 of the AEC’s demonstration program. Big Rock Point was constructed in Round 3 of the AEC’s demonstration program. The containments of both these plants can be considered as large dry containments. They did not have the pressure suppression feature, which is the part of the later BWRs. The containment for the two plants consisted of large dry spherical steel containments, comparable to that of the earlier PWR steel containments.
Humboldt Bay was the first U.S. BWR plant with a pressure suppression feature and has been termed as Pre-Mk I containment. It consisted of two chambers; the cylindrical, reinforced concrete drywell enclosing
FIG. 1.6 PWR ICE CONDENSER CONTAINMENT 
the reactor pressure vessel and primary system components, and the exterior annular cylindrical suppression chamber that contained a large quantity of water used for pressure suppression, and as a heat sink. The drywell vented into the suppression chamber by means of large diameter horizontal vent headers connected to vertical standpipes that were provided to discharge steam below the surface of the suppression pool. NUREG/CR-5640 (1990)  provides a detailed discussion of these features.
The vital auxiliary systems of all operating BWR containments are (1) the pressure suppression system, (2) the containment ventilation system, and (3) the standby gas treatment system. The pressure suppression system will be discussed in respective containment category (e.g., BWR Mk I). The other systems are briefly discussed in the following paragraphs.
During operation, the BWR primary containment is closed and isolated with the containment cooling provided by a recirculating ventilation system. The areas enclosed by the reactor building (sometimes termed as secondary containment) are continuously ventilated and exhausted through an elevated stack. Following an accident, the containment isolation system causes isolation valves to close in certain lines penetrating the containment boundary. The secondary containment exhaust is directed to the Standby Gas Treatment System for processing prior to release through the elevated stack.