Session: MF-12-01 Leak Before Break

Paper Number: 107685

107685 - Evaluation of the Inherent LBB Behavior of Small Diameter Class 1 and 2 Nuclear Piping Systems 

Traditional Leak-Before-Break (LBB) evaluations, such as those following the procedure in NRC’s Standard Review Plan (SRP) 3.6.3, are generally easy to meet the acceptance criteria for large diameter primary pipe loops but become more difficult to satisfy as the pipe diameter decreases.  The SRP 3.6.3 methodology includes several conservative assumptions in the approach.  An advanced LBB method, the Engineering Mechanics Corporation of Columbus (Emc²) Robust LBB procedure, was implemented for a representative 3-inch and 4-inch nominal pipe size pipe systems that are employed at a U.S. pressurized water reactor (PWR) and boiling water reactor (BWR), respectively.  The Robust LBB procedure uses a finite element model of a piping system which accounts for plasticity from the applied forces and moments, rather than the elastic design stresses used in the traditional SRP 3.6.3 approach.  

In the Robust LBB procedure, it is necessary to input the seismic loading time-history inputs due to the plasticity making the traditional design elastic response-spectrum analyses unusable.  The inertial and SAM contribution can be based on the maximum allowable elastic stress limits for reaching Service Level D loading (or higher loading if desired).  The SAM stresses in a dynamic displacement-time stress analysis are dependent on the difference between the applied and natural frequencies of the pipe system, i.e., if there is an exciting frequency right at the natural frequency to reach the SL-D inertial stress limit, the displacement amplitudes are small and consequently the SAM stresses are small.  Several different frequencies were employed to have differences in the SAM loading.

The analysis procedures were:  first, the displacement-time input at the anchor points with different frequencies around the first natural frequency of the pipe system was done with elastic uncracked analyses to reach the moment corresponding to 3S_m inertial stresses.  Secondly, the uncracked pipe analysis is performed with the nonlinear stress-strain curve to calculate the reduction of the applied moments compared to the design elastic limits with the same forcing function.  A circumferential through-wall crack (TWC) of a small size was then inserted at the high stress location using a cracked-pipe-element (CPE) methodology.  The CPE was initially developed and validated in the 1990’s and provides computational efficiency to the solution, and can readily be put in the ABAQUS FE model to represent the moment-rotational capacity of the cracked pipe cross-section as calculated by external elastic-plastic fracture mechanics analysis.  The crack size was increased until it was at least 75% around the circumference or pipe severance/rupture was reached.

The findings to date for the two pipe-system geometries involving 3-inch and 4-inch diameter A106B and TP304 stainless steel pipes, showed that when doing the FE time-dependent analyses at the maximum SL-D inertial stress loading, the circumferential cracks were stable for TWC lengths greater than 75-percent of the circumference.  Simply introducing plasticity into the piping system has a significant impact on the peak applied moment for the same displacement-time history.  As the crack size is increased in the peak applied moment continues to decline due to flexibility changes in the pipe system.  The decrease in moment as a function of crack size shows that the piping system under seismic inertial loading is acting more like it is under displacement-controlled loading rather than load-controlled loading.  With these same loading conditions, the traditional SRP 3.6.3 LBB analysis shows that no crack could be tolerated.  

Presenting Author: Elizabeth Twombly Engineering Mechanics Corporation of Columbus

Presenting Author Biography: Elizabeth Kurth has worked with Emc2 for 15 years and has performed many analyses in support of leak-before-break including confirmatory analyses, critical crack size determinations, leakage rate calculations, and transition break size calculations. Ms. Kurth has extensive finite element experience including model development, and analyses including heat transfer, stress, frequency, weld analysis, and specialized tools within the finite element environment.

Authors:

Elizabeth Twombly Engineering Mechanics Corporation of Columbus
Lance Hill Engineering Mechanics Corporation of Columbus
Gery Wilkowski Engineering Mechanics Corporation of Columbus
Frederick (Bud) Brust Engineering Mechanics Corporation of Columbus
Bruce Lin U.S. Nuclear Regulatory Commission
Robert Tregoning U.S. Nuclear Regulatory Commission