Stress Index Development for Special Elbows

The reactor coolant piping system is designed to be in compliance with the requirements of Class 1 piping of Section III of the ASME Boiler and Pressure Vessel Code. Stress indices are required to evaluate the nuclear Class 1 piping. The reactor coolant system of Kori 1 nuclear power plant consists of a 2-loop system, each having two special elbows: one is the reducing elbow with a non-uniform thickness, and the other is the elbow with a splitter installed inside the elbow. However, stress indices for special elbows are not specified in NB-3683.7 of the ASME Code. In this paper, we computed the stress indices of special elbows for pressure and moment loads by the finite element analysis. The linear elastic method was used for the analysis with the finite element program, ANSYS, and the solid element (SOLID 45) was selected to model the geometry. The finite element model of the special elbows included a straight segment of pipe on each end of the elbows. The uniformly distributed internal pressure was applied to the pipe and elbow. The equivalent axial blow-off load was applied to one end, and the translational degree of freedom was fixed at the other end of the model. The moment was applied to one end, and the boundary condition was fixed at the other end of the model. Based on the analysis results, it is concluded that stress indices, except B₁, in NB-3683.7 of the ASME Code can be used for special elbows conservatively. More analyses are required to apply B₁ index to the special elbows.