Different approaches for stress index calculation in elbows of a fusion plant piping system

In the framework of the final verification for the mechanical assessment of a nuclear piping system, critical cross sections, which turned out to belong to an elbow, have been first selected: a study has been subsequently developed, focused on the evaluation of stress index and safety margins for primary stress, starting from what is set out by ASME B31.3 for Process Piping and by RCC-MRx design Code for mechanical components of nuclear installations. The aim was to move to the critical evaluation of the application of the approach “by rules” when finite elements analyses provide both results in terms of internal forces to obtain the nominal stress, and the detailed numerical evaluation of the local stress state and stress concentration. The results show the differences in stress computation that can be obtained by adopting the finite element modeling or by straight application of codes rules and allows to identify the different levels of conservatism and to choose the suitable criteria to be used in an elbow stress assessment.