Load combination

4.59. Identified loads should be combined with account taken of:

— Load type (i. e. static or dynamic, global or local);

— Whether loads are consequential or simultaneous (e. g. LOCA pressure and temperature loads);

— Time history of each load (to avoid the unrealistic superposition of load peaks if they cannot occur coincidently);

— Probability of occurrence of each load combination.

4.60. In general, load combinations for normal operations and for design basis accidents are taken into account in the relevant design codes. The inclusion of selected severe accidents in the load combination should be considered (para. 6.8).

4.61. At the end of the analysis the number of load combinations may be reduced by grouping them appropriately. The analysis will be performed only for the most demanding cases.

Acceptance criteria

4.62. For each load combination, appropriate acceptance criteria should be determined in terms of allowable stresses, deformations and leaktightness, where applicable. Definitions of allowable stresses and deformations are specific to each design standard and to each type of containment material.

4.63. Codes for the structural design of containment systems provide allowable stress limits for the ‘design’ load combination and test stress limits for the ‘test’ load combination (Table 3). Acceptance criteria for these load combinations should be derived from the structural design code applied.

4.64. For all other load combinations, acceptance limits should be defined according to the expected performance. Design margins should be provided by either:

— Limiting stresses to some fraction of the ultimate limit for that material; or

— Use of the load factor approach (i. e. increasing the applied loads by a certain factor).

4.65. A limited number of acceptance criteria (levels) should be defined for structural integrity and leaktightness as proposed below. This approach is general and applicable to containments of all types.

4.66. For the structural integrity of the containment, the following levels should be considered:

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— Level I: elastic range. No permanent deformation of, or damage to, the containment structure occurs. Structural integrity is ensured with large margins.

— Level II: small permanent deformations. Local permanent deformations are possible. Structural integrity is ensured, although with margins smaller than those for Level I.

— Level III: large permanent deformations. Significant permanent deforma­tions are possible, and some local damage is also expected. Normally this level is not considered in analysing design basis accidents (see paras 6.8­6.11 for consideration of severe accidents).

4.67. For leaktightness, the following levels should be considered:

— Level I: leaktight structure. Leakages from the containment are below the design value and can be correlated with the internal pressure.

— Level II: possible limited increase of leak rate. The leak rate may exceed the design value, but the leaktightness can be adequately estimated and considered in the design.

— Level III: large or very large increase of leak rate. Leaktightness cannot be ensured owing to large deformations of the containment structure. Structural integrity may still be ensured.

4.68. Acceptance levels for structural integrity and leaktightness should be indicated for each load combination included in the design basis. The acceptance levels should be selected according to the expected performance determined by safety considerations.