4.48. The design pressure and the design temperature are the two fundamental parameters used for determining the size of the containment structure (Tables 2 and 3).

4.49. The design pressure should be determined by increasing by at least 10% the peak pressure that would be generated by the design basis accident with the most severe release of mass of material and energy. The calculated peak pressure should be determined on the basis of conservative assumptions in relation to the thermohydraulic characteristics.

4.50. The strength of the containment structure, as tentatively determined on the basis of the design pressure and the design temperature, should be verified for all load combinations and should comply with the corresponding acceptance criteria for the integrity and leaktightness of the containment.

4.51. The design temperature should be specified as the maximum temperature to be anticipated in the structure of the containment, and should be determined by analysing all design basis accidents. The containment structure and systems should be able to maintain their functionality and specified performance when operating below the design temperature.

4.52. All values of pressure and temperature used in the load combinations should be determined with sufficient margins, which should take into account:

— Uncertainties in the amounts of fluids released and in the release rates in terms of both mass and energy, including chemical energy from metal- water reactions;

— Structural tolerances;

— Uncertainties in relation to the residual heat;

— The heat stored in components;

— The heat transferred in heat exchangers;

— Uncertainties in the correlations of heat transfer rates;

— Conservative initial conditions.