[en] The aim of the study presented in this paper was to give some advice to the designers in order to improve the post-buckling stiffness of the roof slab's bottom plate during the HCDA. An experimental approach has been performed on representative test plates and the following parameters have been tested: (1) Number of pads between two stiffeners and then their density. (2) Distance between pads and bottom plate (d). (3) Level of residual pressure applied on the plate. The experimental results have been confirmed with 2D and 3D numerical approaches. It appears that for an important number of equidistant pads and a distance d = 5 mm, the post-buckling behaviour of the plate is significantly improved. On the other hand, sensitivity studies of the behaviour of the structures under dynamic loads (as compared to the static experiments and calculations) have recommended pads fixed with a 330 mm step. This arrangement is the minimum arrangement, it is possible to manufacture. As the buckling and post-buckling behaviour of the bottom plate of the roof slab is considered satisfactory for such an arrangement, it represents a solution for the conceptual design of a LMFBR roof slab. (author)