[en] The Department of Thermal and Mechanical Studies (DEMT/SMTS/RDMS) and the RAMSES Committee of CEA have developed a simplified method to determine the critical instantaneous buckling load under primary loading. This method was introduced as a simplified analytical rule in the RCC-MR (Design and Building Rules for Mechanical Equipment of LMFBR). The RCC-MR imposes buckling design rules for Class I components in paragraph RB 3270, and for class II components in paragraph RC 3270. The practical analysis methods are presented in volume Z, appendix A7 of the same code. It takes account of the lowering of the critical buckling load due to plasticity, to shape imperfections and to the stable of unstable character of the buckling given by charts. These charts can be used for all types of structure, independent of the loading mode or of the temperature (for type 316 stainless steel) and cover imperfections ranging from 0 to 5 times the wall thickness. The RCC-MR method, based on the analysis of the critical load of a beam under axial compression, was validated by more than 60 experimental results obtained on representative mock-ups of thin shells subjected to various loadings, such as internal and external pressure, axial compression and shear, and whose imperfections vary from 0 to 4 times the shell thickness. The RCC-MR rule remains available for structures with defects size from 0. to several thickness like the LMFBR large and thin structures. The comparisons made with the ASME rules, section III, for the structures that these rules consider, are carried ou for cylinders under axial compresison, cylinders without external pressure, and spheres under external pressure. 16 refs.