4.88. The free volume of the space within the containment envelope is the primary physical parameter determining peak pressures after postulated pipe rupture events. It can thus be used as an inherently safe and reliable design feature. If the volume of the containment is subdivided into compartments that are provided with collapsing panels or louvres that open in the event of LOCAs, these collapsing panels or louvres should be designed to open quickly at the predetermined pressure so as to achieve fast equalization of the pressures in the various compartments and to utilize the full free volume of the containment.

4.89. The containment structure and its internal structures, as well as the water stored within the containment, act as a passive heat sink. In the postulated conditions of a pipe rupture accident, the rate of transfer of heat to structures is an important parameter in determining pressures and temperatures. The primary mechanism for heat transfer is the condensation of steam on exposed surfaces, and the thermal conductivity of the structure plays an important part in determining the rate of heat transfer. All conditions that could affect the transfer of heat to the structures, such as the effects of coatings or gaps, should be considered in a conservative manner in the design, and adequate margins should be applied.