Comparison of the allowable primary membrane stress (Sm) for unirradiated 316L(N) stainless steel (based on RCC-MR code), ferritic/martensitic Fe-9Cr-1MoVNb (based on RCC-MR code) and V-4Cr-4Ti (based on 1/3 of the minimum ultimate tensile strength, which is taken as the average/1.15).