4.211. Materials for coverings and coatings (such as paint, sealant and epoxy resin) should be selected to ensure that they do not interfere with any normal operations or safety functions, for example by deteriorating and causing clogging of the filters of sumps, or as a result of the formation of organic iodine. Appropriate paints and coatings should be used to facilitate decontamination of the walls.

4.212. If organic liners are applied to increase the leaktightness of the containment structure, they should be selected to withstand the thermal loads and pressure loads, as well as the environmental conditions in the containment, without losing their safety function. Provision for managing the ageing of these organic liners should be made, including provision for maintenance and surveillance.

4.213. Painting and coating materials should be selected so as not to pose a fire hazard.

4.214. In the selection of painting and coating materials, consideration should be given to the effect of the dissolution of their solvents in the sump on the volatility of iodine.

I—19. The weir wall pressure suppression containment system (Fig. I-6) in boiling water reactors consists of three different structures: the dry well, the containment envelope and the reactor building.

FIG. I-6. Schematic diagram of a weir wall pressure suppression containment system (the reactor building with its confinement function is not shown) for a boiling water reactor: 1, containment; 2, dry well; 3, suppression pool (weir well type); 4, containment spray system; 5, suppression pool cooling system; 6, hydrogen control system; 7, filtered air discharge system; 8, liner.

I-20. The function of the dry well structure is to enclose the reactor pressure vessel completely, and to create a pressure boundary to separate the reactor pressure vessel and its recirculation system from the containment vessel and the main body of the suppression pool. The dry well structure vents the steam — air mixture to the suppression pool. It also provides radiation shielding from the reactor and the piping of the nuclear steam supply system. The weir wall portion of the dry well structure functions as the inner wall of the suppression pool and serves to channel the steam released by a postulated LOCA through horizontal submerged vents into the suppression pool for condensation.

I—21. One of the functions of the reactor building is to provide protection against external missiles for the containment envelope, personnel and equipment. It also provides shielding from the fission products in the secondary confinement envelope, functions as a secondary containment barrier and provides a means for the collection and filtration of leaks of fission products from the steel containment vessel following a LOCA.

I—22. In postulated LOCA conditions, the pressure rise in the dry well reduces the water level between the weir wall and the wall of the dry well structure, uncovering the vents in the wall of the dry well structure, and forces the steam— air mixture in the dry well structure through the vents and into the suppression pool. The steam is condensed in the suppression pool water. Fission product noble gases and other non-condensables from the dry well structure escape from the surface of the pressure suppression pool into the containment envelope.

I—23. In the long term, an active spray system is used to reduce pressure and to reduce the concentration of airborne radionuclides within the containment envelope. This system takes water from the suppression pool by suction through a heat exchanger, following which the water is pumped to spray headers located in the dome of the containment envelope.

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).

4.112. Some energy management systems use the external recirculation of sump water or wet well water through heat exchangers to remove the residual heat from the containment over the medium term (after about one hour). These external recirculation loops are part of the containment envelope. They should be subject to specifications for structural integrity and leaktightness comparable with those of the containment structure itself.

4.113. The specification of the volume of water to be stored in the sump and the design of the suction points should be such that an adequate net pump suction head will be available to the recirculation pumps at any time. The possibility of water boiling in the sump should be considered in the design of the recirculation system if relevant.

4.114. The recirculation loops and their support systems should be redundant so as to satisfy the single failure criterion, and they should be spatially separated so as to reduce the potential for common cause failure. The devices at which suction takes place should be designed to minimize cavitation and to prevent the ingress of foreign material (such as thermal insulation), which could block or damage the recirculation system.

4.115. To avoid the clogging of screens and filters, special care should be taken in the design of piping, component insulation and the intake screens and filters themselves, and consideration should be given to the behaviour under accident conditions of organic paints and coating materials.

4.116. The recirculation loops should be equipped with leakage detection and isolation devices outside the containment and close to the containment penetrations so as to be able to isolate any leaks in the external recirculation loops and therefore to maintain a sufficient water inventory for cooling. Any leakage between the containment penetration and the isolation valve should be prevented by design, for example (a) by means of the provision of a guard pipe or by locating the isolation valve close to the penetrations; (b) by means of quality control in the production of devices to prevent leaks. Strict inspections, maintenance and test controls should be instituted.

4.117. An intermediate cooling system should be provided for heat transport to the ultimate heat sink. This cooling system should be equipped with features to detect and isolate leaks within the recirculation loop heat exchangers. This system should be classified as a safety system.

4.118. Some containment designs do not make use of containment atmosphere cooling systems such as spray or air cooler systems. In the event of a LOCA, they rely on passive heat dissipation and on the release of steam from the reactor coolant systems to the containment atmosphere, limited in time by means of safety injection systems (especially hot leg injection) of appropriate designs. In such cases, it should be demonstrated that energy management for the containment in the medium term and long term can be provided by means of sump recirculation cooling performed by the safety injection system.

4.119. The design of the safety injection system should be such that the release of steam from a broken pipe is sufficiently limited in time, with account taken of the available passive heat sinks provided by the containment and its internal structures.

5.19. Periodic structural tests should be conducted to demonstrate that the containment structure continues to perform as intended in the design. The test pressure should be the same as in the pre-operational test and as required by the applicable design codes. In the design, attention should be paid to the additional stresses imposed by the tests, and margins should be included to prevent the tests from causing any degradation of the containment structure. A leak test should be performed during any structural integrity test. Additional guidance is provided in Ref. [5].[12]

5.20. The design should provide the capability for periodic in-service testing of the leak rate to prove that the leak rate assumed in the safety analysis is maintained throughout the operating lifetime of the plant. The in-service leak rate tests may be made at either:

(a) A pressure that permits a sufficiently accurate extrapolation of the measured leak rate to the leak rates at the accident pressures considered in the safety analysis; or

(b) The containment design pressure.

5.21. There are also methods available to provide a continuous estimate of the overall containment leak rate during plant operation and to derive rough indications of containment leak rates in accident conditions. Such approaches are generally based on variations in the containment pressure or the mass balance during normal operation of the plant. In some cases, the use of these methods together with extensive local leak rate tests during shutdown for refuelling may justify a reduction in the frequency of the global tests.

5.22. The design should permit leak tests of isolation devices, air locks, penetra­tions and containment extensions (para. 5.12).

5.23. The design should facilitate local testing by providing access to penetra­tions and incorporating necessary connections and isolation valves.

5.24. To permit greater precision in measuring the leak rate and to improve the detection of leaking valves, a capability for testing individual valves should be provided. This may require the provision of additional isolation valves.

5.25. Design provisions should be made to permit testing of the secondary confinement envelope (the secondary containment and the surrounding building). Local leak tests of isolation devices, air locks and penetrations should also be considered.

5.26. In containments with a pressure suppression pool, features should be provided for periodically assessing any leakage that might lead to bypassing of the pool, so as to ensure that the bypass rate of the pool is consistent with the value considered in the safety analysis.

5.27. The design should permit the functional testing of the equipment in containment systems during normal plant operation.

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.

4.158. A hydrogen monitoring or sampling system should be provided within the containment for determining the hydrogen concentrations at represent­ative points over time in accident conditions, especially those caused by a LOCA. If mixing of the containment atmosphere cannot be guaranteed, proper location is essential for the monitoring or sampling devices to be representative of the areas and locations where hydrogen might accumulate.

4.159. If the systems for hydrogen monitoring or sampling could transport radionuclides outside the containment, they should be considered extensions of the containment and should be designed to meet the same criteria as the containment itself.

4.160. Systems for hydrogen removal, deliberate ignition, homogenization or inerting should be provided so as to avoid reaching the hydrogen ignition limit globally or locally inside the containment at any time during or after a postulated LOCA.

4.161. If hydrogen control systems could transport radionuclides outside the containment, they should be considered extensions of the containment and should be designed to meet the same criteria as the containment itself.

A.9. Storage tanks and the drain sump of each safety system as well as the condensate collector of each air cooler should be provided with a water level indicator.

Balance of fluid flow

A.10. The periodic calculation of a mass balance can show quantitatively the amount of identified and/or unidentified small leaks in a given volume. For the calculation of a mass balance, fluid flows should be measured to establish the mass balances in the different systems. Measurements of temperature, pressure and humidity are combined to monitor for leaks from the containment in most operational states by enabling the periodic calculation of the mass of the containment atmosphere.

4.80. Reinforced concrete structures are generally used for the outer wall of double wall containments. The leak rate for the outer wall should be low enough to ensure that there is an underpressure in the annulus for the purposes of leak collection. The maximum leak rate should be defined with account taken of the most severe loads in the annulus associated with design basis accidents and of external environmental parameters (especially extreme wind speeds). The secondary containment structure should be designed to prevent the direct impact of external missiles onto the primary containment, or at least to limit the associated loads.

4.215. To provide defence in depth and to enhance the general reliability of the containment systems, instrumentation should be provided for the purposes of:

(a) Detecting deviations from normal operation,

(b) Monitoring the stability of the containment structure,

(c) Leakage testing and integrity testing,

(d) Monitoring the availability of the containment systems,

(e) Providing actuation signals for containment systems,

(f) Post-accident monitoring.

Detection of deviations from normal operation

4.216. Specific design recommendations regarding instrumentation for monitoring the containment for the early detection of deviations from normal operation are provided in Appendix I. See also Section 6 on instrumentation for the detection and monitoring of severe accident conditions.

Control of the containment structure

4.217. Appropriate instrumentation should be incorporated inside the containment in order to monitor closely any deformation (radial, vertical or circumferential) or movement of the containment structures or the containment walls.

4.218. For prestressed concrete walls, means to detect loss of the prestressing should be provided. The concrete compression and rigidity parameters (such as Young’s modulus) should be defined, and they should be verified by such means as acoustic measurements. The temperature in concrete singularities should also be measured to aid the interpretation of the results of pressure proofing tests.

4.219. Appropriate instrumentation for measurements relating to earthquakes should be provided on the basemat of the containment or on a suitable floor.