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

Abstract

Abstract Traditional Leak-Before-Break (LBB) evaluations 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. An advanced LBB method, the Engineering Mechanics Corporation of Columbus (Emc2) Robust LBB procedure, was implemented for 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. 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 seismic anchor motion (SAM) contribution can be based on the maximum allowable elastic stress limits for reaching Service Level D loading. 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. 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 3Sm inertial stresses. Second, 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 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, show 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.