1.59. The layout of the containment should be defined with account taken of several factors that are dealt with in this Safety Guide and that are summarized below:

— Optimization of the location of the entire primary system, with particular attention paid to the enhancement of cooling of the core by natural circulation;

— Provision of separation between divisions of safety systems;

— Provision of the necessary space for personnel access and the monitoring, testing, control, maintenance and movement of equipment;

— Placement of the equipment and structures so as to optimize biological shielding;

— Location of penetrations in areas of the containment wall so as to ensure accessibility for inspection and testing;

— Ensuring an adequate single free volume in the upper part of the containment to improve the efficiency of the containment spray (if any);

— Ensuring an adequate free volume and adequate cooling flow paths for passively cooled containments;

— Limitation of the compartmentalization of the containment volume so as to minimize differential pressures in the event of a LOCA and to promote hydrogen mixing, thus preventing the local accumulation of hydrogen.

1.60. The lower part of the containment should be designed to facilitate the collection and identification of liquids leaked, and also the channelling of water to the sump in the event of an accident. The annulus between the primary and secondary containments should form a single volume to the extent possible, in order to maximize the mixing and dilution of any radioactive material released from the primary containment in the event of an accident.

1.61. Containment systems should be designed to have high functional reliability commensurate with the importance of the safety functions to be performed.

1.62. The functions of containment systems should be available on demand and should remain available in the long term following a postulated initiating event until the specific safety function is no longer needed. Periodic testing of the systems should be performed in order to verify that the assumptions made in the design, including the probabilistic safety assessment if applicable, about the levels of reliability and performance are justified throughout the operating lifetime of the plant.

1.63. The single failure criterion[4] is required to be applied to each safety group incorporated in the design (Ref. [1], para. 5.34). Containment systems that, in and after design basis accidents, perform safety functions for energy management, radionuclide management, containment isolation and hydrogen control should be designed according to the single failure criterion.

1.64. The containment structure and the passive fluid retaining boundaries of its appurtenances should be of sufficiently high quality (ensured, for example, by means of rigorous design requirements, proper selection of bounding postulated initiating events, conservative design margins, construction to high standards of quality, and comprehensive analysis and testing of performance) that the failure of the containment structure itself and the failure of the passive fluid retaining boundaries of its appurtenances need not be postulated.

1.65. The containment systems should, to the extent possible, be independent of process systems or other safety systems. In particular, the failures of other systems that have caused an accident should not prevent the containment from fulfilling its required safety functions during the accident.

1.66. Consideration should be given to the use of passive systems and intrinsic safety features, which may, in some cases, be more suitable than active systems and components.

1.67. The structures, systems and components of the containment systems should be qualified to perform their safety functions in the entire range of environmental conditions that might prevail during and following a design basis accident, or should otherwise be adequately protected from those environmental conditions.

1.68. Components of the containment systems that can be shown to be unaffected by the design basis accident conditions need no environmental qualification.

1.69. The environmental and seismic conditions that may prevail during and following a design basis accident, the ageing of structures, systems and components throughout the lifetime of the plant, synergistic effects, and safety margins should all be taken into consideration in the environmental qualifi­cation of the containment systems.

1.70. Environmental qualification should be carried out by means of testing, analysis and the use of expertise, or by a combination of these.

1.71. Environmental qualification should include the consideration of such factors as temperature, pressure, humidity, radiation levels, the local accumu­lation of radioactive aerosols, vibration, water spray, steam impingement, flooding and contact with chemicals. Margins and synergistic effects (in which the damage due to the superposition or combination of effects may exceed the total damage due to the effects separately) should also be considered. In cases where synergistic effects are possible, materials should be qualified for the most severe effect, or the most severe combination or sequence of effects.

1.72. Non-metallic materials, such as elastomeric seals and concrete, should be qualified for ageing on the basis of sample ageing tests, operating experience in the nuclear or non-nuclear industry, or published test data for the same or similar materials under the same qualification conditions. All ageing mechanisms that are significant and relevant in the expected conditions should be considered in the qualification. Techniques to accelerate the testing for ageing and qualification may be used, provided that there is proper justifi­cation. The same applies to the possibility of testing for separate effects rather than the superposition of effects.

1.73. For components subject to the effects of ageing by various mechanisms, a design life and, if necessary, the replacement frequency should be established. In the qualification process for such components, samples should be aged to simulate the end of their design lives before being tested under design basis accident conditions.

1.74. Components that have been used for qualification testing should generally not be used for construction purposes unless it can be shown that the conditions and methods of testing do not themselves lead to an unacceptable degradation of safety performance.

1.75. Qualification data and results should be documented as part of the design documentation.