1.13. The main functional requirement for the overall containment system derives from its major safety function: to envelop, and thus to isolate from the environment, those structures, systems and components whose failure could lead to an unacceptable release of radionuclides. For this reason, the envelope should include all those components of the reactor coolant pressure boundary, or those connected to the reactor coolant pressure boundary, that cannot be isolated from the reactor core in the event of an accident.

1.14. The structural integrity of the containment envelope is required to be maintained and the specified maximum leak rate is required not to be exceeded in any condition pertaining to design basis accidents and it should not be exceeded in any condition pertaining to severe accidents considered in the design. This is required to be achieved by means of containment isolation, energy management and structural design (Ref. [1], paras 6.43-6.67). Features for the management of radionuclides should be such as to ensure that the release of radionuclides from the containment envelope is kept below authorized limits.

1.15. In operational states, the containment systems should prevent or limit the release of radioactive substances that are produced in the core, that are produced by neutron or gamma radiation outside the reactor core or that may leak from the systems housed within the containment envelope. Specific systems may be necessary for this purpose, such as the ventilation system, for which requirements are outlined in Ref. [1] (paras 6.93-6.95). Furthermore, the containment systems should enable the reduction of temperature and pressure within the containment when necessary.

1.16. In operational states, most containment systems are in standby mode. During plant shutdown the containment may be intentionally opened (such as via air locks, equipment hatches or spare penetrations) to provide access for maintenance work on systems and components or to provide the necessary servicing space.

1.17. The structural part of the containment envelope is usually a steel or concrete building. The containment is required to be designed to withstand the pressures, thermal and mechanically induced loads, and environmental conditions that result from the events included in the design basis (Ref. [1], para. 6.45).

1.18. Containment isolation features include the valves and other devices that are necessary to seal or isolate the penetrations through the containment envelope, as well as the associated electrical, mechanical and instrumentation and control systems. The design should be such as to ensure that these valves and other devices can be reliably and independently closed when this is necessary to isolate the containment.

1.19. The energy management features[1] should be designed to limit the internal pressures, temperatures and mechanical loading on the containment as well as those within the containment envelope to levels below the design values for the containment systems and for the equipment within the containment envelope. Examples of energy management features are: pressure suppression pools, ice condensers, vacuum chamber systems for pressure relief, structural heat sinks, the free volume of the containment envelope, the capability for the removal of heat through the containment wall, spray systems, air coolers, recir­culation water in the sump, and the suppression pool and cooling systems.

1.20. The features for radionuclide management should operate together with the features for the management of energy and combustible gases and the containment isolation system to limit the radiological consequences of postulated accident conditions. Typical features for the management of radio­nuclides are double containment systems, suppression pools, spray systems and charcoal filters, and high efficiency particulate air (HEPA) filters.

1.21. The features for the control of combustible gases should be designed to eliminate or reduce the concentration of hydrogen, which can be generated by water radiolysis, by metal-water reactions in the reactor core or, in severe accident conditions, by interactions of molten core debris with concrete. Features used in various designs include hydrogen recombiners (i. e. passive recombiners or active igniters), large containment volumes for diluting hydrogen and limiting the hydrogen concentration, features for mixing the containment atmosphere, features for inerting and devices for ensuring that any burning of hydrogen is controlled.

1.22. Energy, combustible gases and features for radionuclide management should be evaluated on the basis of conservative estimates according to their relevance to safety functions.

1.23. Several different designs are used for containment systems. Annex I provides general guidance about the most commonly used containment designs.

1.24. In severe accident conditions, high energetic loading could jeopardize the structural integrity of the containment. Either high energetic loading should be dealt with adequately in the containment design (Ref. [1], Section 6) or features should be incorporated for preventing or limiting such loading (see Section 6 of this Safety Guide for detailed design considerations for severe accidents).