5.39. For existing plants, the ultimate load bearing capacity (structural integrity Level III) and retention capacity (leaktightness Level II) of the containment structure should not be exceeded in severe accidents, to the extent that this can be achieved by practicable means. Furthermore, the molten core material and core debris should be stabilized within the containment.

5.40. To determine the ultimate load bearing capacity and retention capacity beyond the design pressure, it should be considered whether to make a global evaluation of the structural behaviour of the containment in order to identify the most limiting components so as to evaluate margins, and to study the failure mode of the structure. Local effects, thermal gradients and details of component design should also be considered so as to identify possible mechanisms for large leaks. In this regard, special attention should be paid to the behaviour of piping penetrations, soft sealing materials and electrical penetrations.

5.41. For new plants, the integrity and leaktightness of the containment structure should be ensured for those severe accidents that cannot be practically eliminated (para. 6.5). The long term pressurization of the containment should be limited to a pressure below the value corresponding to Level II for structural integrity.

5.42. Load combinations for severe accidents are design specific and should be considered in addition to the load combinations for design basis accidents. Appropriate combinations, including loads such as those due to the pressures, temperatures and pipe reactions resulting from the severe accidents that are considered in the design basis, should be taken into account. For these combi­nations the structural integrity criteria for Level II should be met (see para. 4.66 for the definitions of acceptance criteria). For combinations that also include local effects derived from severe accidents, the structural integrity criteria of Level III should be met. Level II criteria for leaktightness should be met for load combinations including dead loads, live loads, prestressing (if applicable), test temperatures and accident pressures.

5.43. Consideration should be given to incorporating into the plant design the

following provisions to enhance the capability to cool molten core material and

core debris, and to mitigate the effects of its interaction with concrete:

(a) A means of flooding the reactor cavity with water to assist in the cooling process or of providing enough water early in an accident to immerse the lower head of the reactor vessel and to prevent breach of the vessel;

(b) Protection for the containment liner and other structural members with concrete, if necessary;

(c) Sufficient floor space on the basemat to spread core debris and to increase the capability of cooling the debris by means of flooding with water;

(d) Design features of the containment and the reactor cavity to reduce the amount of core debris that reaches the upper containment (i. e. ledges, baffles and subcompartments);

(e) A reinforced sump or cavity to catch and retain molten core material and core debris (a core catcher);

(f) Use of a type of concrete for the containment floor that minimizes adverse effects due to interactions between molten core material and core debris and concrete.