4.80. Reinforced concrete structures are generally used for the outer wall of double wall containments. The leak rate for the outer wall should be low enough to ensure that there is an underpressure in the annulus for the purposes of leak collection. The maximum leak rate should be defined with account taken of the most severe loads in the annulus associated with design basis accidents and of external environmental parameters (especially extreme wind speeds). The secondary containment structure should be designed to prevent the direct impact of external missiles onto the primary containment, or at least to limit the associated loads.