4.75. An analysis should be performed to identify the ultimate capability of the containment. The bulk behaviour of the containment structure under static (pressure, temperature and actions of pipes) and dynamic (seismic) loads should be considered, and proper attention should be paid to local effects such as penetrations and structural singularities.

4.76. Failure modes such as liner tearing, penetration failures and tendon failures should be analysed. To the extent possible, a failure should not be catastrophic and should not cause additional damage to systems and components for retaining radioactive material.

4.77. It should be demonstrated in the analysis that the acceptance criteria for structural integrity and leaktightness of the containment are met with an adequate margin so as to avoid ‘cliff edge’ effects[8].

4.208. Thermal insulation materials should not compromise any safety functions in the event of their deterioration. They should be installed and affixed to prevent loosening and the possible clogging of sieves and valves as a consequence.

4.209. In particular, materials used to insulate pipes and tanks inside the containment should be selected and designed to achieve the following:

(a) To minimize the production of debris that can accumulate on containment floors and clog sumps or damage recirculation pumps,

(b) To ensure easy decontamination if the need arises,

(c) To avoid giving rise to fire hazards,

(d) To minimize the release of toxic gases during their heating at startup.

4.210. Ageing mechanisms that affect thermal insulation materials should be assessed and appropriate replacement intervals should be established (para. 4.39).

I-14. The pressure suppression containment system (Fig. I-5) in boiling water reactors is divided into two main compartments: a dry well housing the reactor coolant system and a wet well partly filled with water, whose function is to condense steam in the event of a LOCA. The two compartments are connected by pipes that are submerged in the water of the wet well. Spray systems are usually installed in both the dry well and the wet well. The reactor building surrounding the containment forms a secondary confinement which captures leaks from the containment. The containment envelope usually consists of either a concrete structure with a steel liner for leaktightness or a steel shell.

I-15. The purpose of the pressure suppression system is to reduce the pressure if a pipe in the reactor coolant system ruptures. The steam from a leak in these pipes enters the dry well and is passed through pipes into the water of the suppression pool (wet well), where it condenses, and the pressure in the dry well is reduced. The pressure suppression system helps in reducing the concen­trations of airborne radioiodines by scrubbing radionuclides from the steam.

I-16. The wet well is also used as a heat sink for the automatic pressure relief system. This serves to limit the pressure rise in the reactor coolant system when the reactor cannot discharge steam to the turbine condenser system. The steam still produced by residual heat after shutdown of the reactor is passed into the water in the wet well via safety relief valves connected to the steam pipes within the dry well.

I-17. The concrete or steel structure of the reactor building surrounding the containment serves as protection against external events.

I-18. The reactor building is held at a slightly negative gauge pressure in both operational states and accident conditions. In the event of an accident, leaks from the dry well into the reactor building are extracted and filtered by an air removal system to permit the use of controlled emission from the plant stack.

GENERAL

1.11. The containment systems should be designed to ensure or contribute to the achievement of the following safety functions:

(a) Confinement of radioactive substances in operational states and in accident conditions,

(b) Protection of the plant against external natural and human induced

events,

(c) Radiation shielding in operational states and in accident conditions.

1.12. The safety functions of the containment systems should be clearly identified for operational states and accident conditions, and should be used as a basis for the design of the systems and the verification of their performance.

4.111. For designs in which the pressure of the containment is lower than atmospheric pressure, a pressure relief system, a vacuum building and associated vacuum equipment provide the front end energy management, to relieve the pressure generated in the reactor building by a LOCA. The pressure relief valves isolating the vacuum building respond to an increase of pressure by opening to connect the reactor building to the vacuum building via ducts; the steam-air mixture resulting from a LOCA is thus drawn into the vacuum building. In some designs, panels are provided to isolate each reactor building from the common duct in normal operation and to open if the pressure in the reactor building rises. The panels should open reliably and should have an adequate flow area. The vacuum system should be capable of maintaining the vacuum at the design value. The design of the pressure relief valves should be such as to ensure that:

— The vacuum building is isolated from the pressure relief duct in normal operation;

— In the event of any pipe break in the reactor coolant system, a sufficient flow area is opened to prevent pressurization of the reactor buildings and the relief duct beyond their design pressure;

— The overpressure can be relieved fast enough to keep radioactive releases from the containment below specified limits;

— A sufficient filtered vent flow of a controllable nature is provided to return the containment promptly to operation at subatmospheric pressure.

5.15. Periodic in-service tests and inspections should be performed to demonstrate that the containment systems continue to meet the requirements for design and safety throughout the operating lifetime of the plant.

5.16. The test methods and intervals for in-service tests should be specified so as to reflect the importance to safety of the items concerned. In devising test methods and determining the frequency of testing, consideration should be given to the necessary levels of performance and reliability of the containment systems individually and as a whole.

5.17. Appropriate features should be provided for performing commissioning and in-service testing for containment pressure and leaktightness, and the correlated loads should be considered for the purposes of structural design.

5.18. General guidance on in-service inspection is provided in Ref. [16]. The remainder of Section 5 provides additional guidance specific to containment systems.

1.76. In the design and layout of containment systems, sufficient space and shielding should be provided to ensure that maintenance and operations can be carried out without causing undue radiation exposure of personnel. The point of access to the containment should be inside the controlled area and access should be subject to the approval of the radiation protection officer.

1.77. Consideration should be given to the potential exposure to radiation associated with operations that are planned to be conducted after an accident, or with operations that it may be necessary to conduct following the emergency procedures as well as with the recovery actions following an accident. Evaluations should include the consideration of access paths, such as possible open doors and hatches. If the doses due to such exposures exceed the applicable dose limits and dose constraints, additional shielding or even the repositioning of components should be considered.

1.78. Maintenance related factors considered in the containment design should include the provision of adequate working space, shielding, lighting, air for breathing, and working and access platforms; the provision and control of proper environmental conditions; the identification of equipment; the provision of hazard signs; the provision of visual and acoustic alarms; and the provision of communication systems.

1.79. The accessibility of both the containment and the systems contained within it should be considered for all operational states. The ability to ensure that radiation doses to operators remain within the acceptable dose limits will determine whether access can be allowed to the primary and/or the secondary containment (if applicable) during power operation, or whether plant shutdown is required for permitting such access.

1.80. If entry into the primary or secondary containment during power operation for the purposes of unplanned maintenance or even for routine (planned) maintenance is envisaged, proper provision should be made to ensure the necessary radiological protection and industrial safety of plant staff. This provision should include the application of the principle of keeping exposure as low as reasonably achievable, the provision of the necessary communication systems and alarms, and proper monitoring of the containment atmosphere, especially in the case of inerted containments or containments at subatmospheric pressure. At least two emergency escape routes from the containment should be provided. In addition, security provisions for controlling access to the containment should be considered.

Generation of hydrogen

4.156. Hydrogen and oxygen are generated during normal operation of a plant as a result of the radiolysis of water in the core. After a LOCA, a mixture of hydrogen and air might be formed in the containment atmosphere as a consequence of:

— Radiolysis of the water in the core,

— Radiolysis of the water in the sump or the suppression pool,

— Metal-water reactions in the core,

— Chemical reactions with materials in the containment,

— Degassing of hydrogen dissolved in the primary coolant,

— Releases from the hydrogen tanks used for control of the primary coolant chemistry.

All these contributions to the generation of hydrogen should be evaluated.

4.157. The amount of hydrogen generated should be calculated for normal operation and for LOCA conditions. The uncertainties in the various possible mechanisms for hydrogen generation should be taken into account by the use of adequate margins.

A.6. Leakage of fluids such as compressed air, nitrogen or water may be the cause of pressure increases. To detect leaks, measurements of the ambient pressure in the appropriate compartments in the containment building should be obtained. These measurements should account for variations in other parameters such as temperature, humidity, or levels of ionizing or electromag­netic radiation. The pressure measurements should be recorded to show trends.

Humidity in the containment building

A.7. Humidity is a highly significant factor for the detection of leaks from the primary circuit. Parameters that indicate changes in humidity include:

— The dew point temperature of the containment atmosphere,

— Electrical parameters (such as impedance or resistance) of sensors,

— The amount of condensate in the air coolers of the containment building.

A.8. Humidity levels should be monitored in appropriate compartments in the containment buildings (in the primary containment, and in the secondary

containment if applicable), and the measured values should be recorded to show trends.

4.78. Consideration should be given to the possibility of large releases of mass and energy, and the need for the internal structures to withstand the pressure differentials that could arise between different compartments so as to prevent any collapse. For each compartment, the most unfavourable location for a break should be considered. Openings between compartments should be considered by means of a conservative approach at the design stage and should be verified to be free of unintended obstructions after construction has been completed.

4.79. Consideration should be given to the need for the internal structures to withstand the loadings associated with design basis accidents, and so to withstand the hydrodynamic loads that are caused by water flowing from the discharge line of the safety valves and the relief valves into the suppression pool, the swelling of the pool water, the oscillation of condensate water, chugging and any other relevant hydraulic phenomena.