Seismic Integrity Characteristics of Uncracked and Through Wall Circumferential Cracked Pipes under Beyond Design Basis Earthquake

Abstract

With technological improvements in design and construction, the magnitude of safety shutdown earthquake (SSE) is increasing for several new nuclear power plants (NPPs). However, in recent years we have witnessed a few prominent examples of beyond design basis events near nuclear power plants (NPPs), including the Fukushima in Japan and the North Anna in U.S.A.. As a follow-up to the Fukushima accident, old NPPs in EU and Korea are being subjected to stress tests to evaluate the integrity of their safety shutdown function under extreme conditions. In this procedure, the potential magnitude of an earthquake is reevaluated through a probabilistic approach. In light of these, it is becoming difficult to ignore the probability of earthquake exceeding the design basis. Furthermore with the expected evolution of cracks in pipes of NPPs, it has become very important to monitor the integrity of the plant structure. In particular, in weldments of piping in the pressurized water reactor (PWR) primary system, such as pressurizer surge line nozzles, cracks are likely to initiate and grow with time owing to various environmental effects. Presently, certain non-destructive examination methods are applied for detecting such cracks in pipes

however, these can only be applied at 10 years of the inspection period. In addition, to avoid the unnecessary replacement of components, the American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME B&PV) Code Sec. XI specifies an allowable flaw size. Although we noted that some unexpected cracks were detected. Therefore, from a long-term viewpoint, it is essential to consider cracks in pipe analysis. Toward this end, the present thesis focuses on seismic analysis for uncracked and cracked pipes to understand the dynamic behavior of the structure under a beyond design basis earthquake.

According to the ASME B&PV Code Sec. III, a pipe under seismic loading is subjected to two types of loads: the seismic inertial moment (MSI) due to vibration and the seismic anchor motion moment (MSAM) due to relative displacement between multiple anchors. The response spectrum analysis can be used to calculate MSI, and seismic anchor motion analysis can be used to calculate MSAM. These analyses are general procedures, but they can be used to provide the values of these two loads separately. In contrast, a time history analysis can be used to consider two loads simultaneously and provide a more realistic solution. This study aimed to (i) understand the characteristics of each method and then (ii) compare the dynamic behavior of uncracked and cracked pipes using time history analysis using ABAQUS that is a commercial finite element analysis tool.

First, an uncracked pipe was analyzed using general seismic analysis and time history analysis to understand the characteristics of each method. It was found that time history analysis generally produced a less conservative solution. Then, various conditions were considered in cracked pipe analysis—pipe length, crack position, and excitation mode. The crack was simulated using hinge element which is one of connector element in ABAQUS. It was confirmed that the applied load on the pipe can decrease by 4-70% owing to cracks

however, it is difficult to find a clear trend that can explain all cases. Additional computations were performed using a simplified model and conditions, and a qualitative interpretation of the complicated cracked pipe analysis result was performed. The main factors that can affect the change in the safety margin under seismic load are (i) the magnitude of the effect of a crack evolution on the change in stiffness and (ii) the relation between the natural frequency of the structure and the applied vibration.

Since the Fukushima accidents the evaluation of the structural integrity of NPP pipings is moving from a deterministic approach to a probabilistic analysis. To calculate the probability of pipe rupture, the exact prediction of the dynamic behavior of a pipe under particular conditions may be a key point. Therefore, the ultimate application of this thesis results is defined to provide complete measures for probabilistic fracture mechanics.