Probabilistic characterization of primary stress index B2 for nuclear pipe bends subjected to in-plane opening moment

This paper provides a probabilistic characterization of the stress index B₂ which can be used for developing reliability-based design guidelines for nuclear bend pipes. The design equations for pipe bends involve stress indices (B₁ and B₂) that are used to modify the stress parameters used for straight pipes. These indices allow a designer to use the same basic stress limits as for a straight pipe in the design of these bends. The most commonly used code for the design of nuclear piping systems (including pipe bends) is the ASME Boiler and Pressure Vessels Code Section III, which is based on deterministic design guidelines. A need for modifying the design guideline to a reliability-based one is acknowledged by the research community worldwide. Although recent research works have looked into the probabilistic design aspects for the straight pipes, no such recommendation is available for pipe bends. The primary objective of this paper is to quantify the uncertainties involved in the primary stress index B₂ for a 90° bend pipe in a nuclear piping system. This is intended as a preliminary step towards developing detailed reliability-based design specifications for pipe bends as well. Uncertainties in geometric properties of the piping system are considered in this work. The uncertainty characteristics for B₂ are obtained, using Monte Carlo simulations, subjected to in-plane opening bending moments. The limit state/performance function is considered under the service level B (upset condition). Finally a mathematical expression is provided for obtaining the probabilistic characterization of B₂ by considering uncertainties in geometric variables.