1.81. The process of identification and classification of structures, systems and components that are items important to safety (Ref. [1], paras 5.1-5.3) directs the attention of designers, manufacturers and operators to all the features that are important for ensuring the safety of the plant and to the association of specific design requirements (e. g. the single failure criterion and appropriate codes and standards) with each structure, system and component.

1.82. Several safety classification systems for pressure retaining mechanical equipment use three nuclear safety classes and one non-nuclear safety class. The highest safety class is generally restricted to the components of the reactor coolant pressure boundary.

1.83. The containment pressure boundary, including penetrations and isolation valves, as well as pressure retaining parts of front line systems used for the management of energy and radionuclides in the primary containment during a design basis accident, are generally assigned to the second safety class.

1.84. The pressure retaining parts of systems for the management of energy and radionuclides in the secondary containment during a design basis accident, and of systems for the control of combustible gases during a design basis accident, are often assigned to the third safety class.

1.85. In so far as they are relied upon in design basis accidents, the containment systems are safety systems and should be classified as seismic class 1, the highest level of seismic classification. Electrical equipment of the containment systems, including equipment for emergency power supply, should be assigned to electrical class 1E, the highest level of safety classification for electrical instrumentation and control equipment.