This paper proposes a method for determining a set of safety factors taking account of multiple failure modes and their interactions. The purpose of the work is to materialize the System Based Code concept for the applications to fast breeder reactors. Current structural codes prevent failure by limiting primary stress, excessive strain, buckling and fatigue damage. However, the relationships between safety factors in these criterion and failure modes are not necessarily clear. For example, safety factors in the limitation of primary stress are considered to cover not only ductile fracture but also fracture due to crack like defects. When the System Based Code concepts, one of the most important of which is designing to target reliability, it is essential to determine safety factors so that they explicitly correspond to particular failure modes. This paper deals with the most important two failure modes to be prevented in fast breeder reactors (FBRs), that is, primary stress due to seismic load and secondary stress due to creep-fatigue. Safety factors that are consistent for design code and fitness-for-service code are derived by the following steps: 1) Formulations of continuous evaluation of reliability are derived for both fracture by primary stress and creep-fatigue crack initiation and propagation due to secondary stress, with their interaction taken into account, 2) Reliability is calculated for various combinations of loading conditions, 3) Safety factors corresponding to various levels of target reliabilities are investigated and compared with currently used ones. The safety factors thus determined not only have firm physical basis but also contribute to enlarge design windows for fast breeder reactor components. Items to be further investigated for the methodology to be implemented in current code are also discussed.

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