Session: MF-12-01 Leak Before Break

Paper Number: 127787

127787 - Effect of End-Restraint Condition on Leak Before Break Evaluation 

In the current Leak-Before-Break (LBB) procedure (i.e., SRP 3.6.3), the pipe ends are considered to be “free” whereas in reality the pipe ends are restrained due to the presence of large components such as RPV and SG.  The restraint of the pipe can limit the behavior of a crack and consequently result in a decrease in the applied load.  This reduction in applied load may vary depending on the loading condition such as the ratio of primary over secondary stresses.  The current LBB procedure with “free ” pipe ends overestimates the COD value (leading to smaller leakage crack size) and underestimates the allowable load (leading to smaller critical crack size) which in turn affect the LBB margin. Moreover, the degree of end-restraint effect depends on the piping configuration, the pipe size and location of the crack.

In this paper, an existing analytical solution to incorporate the effect of end-restraint conditions on LBB evaluation has been updated to extend the applicability of larger circumferential through wall crack size. The end-restraint conditional was applied on a straight pipe segment with more realistic boundary condition at both ends (in lieu of free boundary condition). Three pipe sizes were used in the current analysis namely large, medium and small diameter pipes with a crack located in the middle (axial direction) of the pipe with two boundary conditions – free ends and restrained ends. For smaller pipe size, another variation was investigated by inserting a crack towards the end of the pipe (instead of middle of the pipe) to vary the degree of end-restraint conditions. A 3D finite element model with a circumferential crack was also created to validate the analytical model. A moment reduction factor due to the effect of end-restraint condition was determined from both analytical and finite element model as a function various circumferential crack sizes. The as-determined moment reduction factor was also applied in calculating crack opening displacement (COD). Eventually, critical and leakage crack size were calculated for all three pipe sizes for both free and end-restraint conditions. Finally, the effect of end-restraint condition was evaluated by comparing LBB margins for free end and end-restraint conditions.

Presenting Author: Mo Uddin Structural Integrity Associates, Inc.

Presenting Author Biography: Dr. Mo Uddin has been working as a Manager of Nuclear Analytical Services at Structural Integrity Associates for over 3 years. Prior to that Dr. Uddin worked at Engineering Mechanics Corporation of Columbus (Emc2) for over 11 years. Dr. Uddin received his PhD in Aeronautics and Astronautics at Purdue University, West Lafayette, IN. His research mainly focuses on Structural integrity assessment, Computational and experimental fracture mechanics, Probabilistic fracture mechanics, Leak-Before-Break (LBB) analysis of nuclear piping systems.

Authors:

Mo Uddin Structural Integrity Associates, Inc.
Garivalde Dominguez Structural Integrity Associates, Inc.
Dilip Dedhia Structural Integrity Associates, Inc.