"BWR Systems 
 Says ""During the evolution of the boiling water reactors, three major types of containments were built. The major containment designs are the Mark I (page 3-16), Mark II (page 3-17), and the Mark III (page 3-18). Unlike the Mark III, that consists of a primary containment and a drywell, the Mark I and Mark II designs consist of a drywell and a wetwell (suppression pool).""  
 and has diagrams of each.    

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