Reliability Analysis For Corroded Nuclear Pipe In Second Looping System

The system of nuclear pipes is the important part of nuclear power plant. The corrosion of nuclear pipes made of low carbon steel is inevitable. The pipeline serve in complicated environment of high-temperature, high-pressure and high-radiation. Nuclear power has developed in our country for decades. Many nuclear pipes have entered the later period of service. The damage of nuclear pipes could cause not only economical loss but also severely environmental pollution, especially in the harsh radiation environment. The earthquake has great randomness and uncertainty. In recent years, the earthquake happens in our country frequently. It is vital to calculate the reliability of corroded nuclear pipes in earthquake and predict the remaining life of pipeline. In order to maintain the safety of the corroded pipe, this paper has done the following research in the view of reliability analysis.(1) The nuclear pope is inevitable to be corroded in service. It is made of low carbon steel. At the same time, the liquid inside will make the situation worse. This situation is called the flow accelerated corrosion (FAC). In order to know the cause of corrosion fundamentally, this paper discuss and researches the characteristic of FAC, the contact of FAC and metals corrosion, the difference between FAC and metals corrosion and the formation mechanism of FAC.(2) In order to study the influence of corrosion model on the reliability of the second looping system in nuclear power plant, the limit state equation is established for nuclear pipe under the malfunctioned condition based on the maximum internal pressure. The power function corrosion model and the exponential function corrosion model are introduced in the calculation of the reliability of corroded pipes, compared with the commonly used linear model. The influence of the corrosion models on the corroded nuclear pipe is acquired. According to the advantages and the reliability calculation, the power function model is recommended for reliability calculation of the corroded nuclear pipe.(3) In order to study the seismic response of corroded nuclear pipe in second looping system, finite element model is established with the FE software ABAQUS. The influence of corrosion geometry is researched on the pressure capability for nuclear straight pipes and elbows. The site of nuclear power plant is assumed to be class Ⅰ. Based on the model of nuclear power plant provided by Doctor Zhao Chunfeng, the acceleration response of the power plant is acquired under the action of earthquake wave RG1.60. The acceleration response is used to conduct the time history analysis for nuclear straight pipe and elbow. The earthquake response of nuclear straight pipe and elbow is calculated with different corrosion depth, length and width. At last, the influence of input angle of the acceleration response on the response of nuclear elbow is studied.(4) In order to research the reliability of pipes under the earthquake in secondary loop piping system in nuclear power plant. Based on the failure pressure models for corroded pipes and the RCC-M code for the PWR nuclear islands, the ultimate stress failure equation is derived to calculate the seismic time-dependent reliability for corroded pipes in nuclear secondary loop piping system. The result of FE analysis together with linear corrosion model and power function corrosion model is used to calculate the seismic reliability of corroded nuclear pipe and elbow in second looping system. Changes of reliability index of corroded nuclear pipe and elbow are acquired based on both corrosion models.(5) The random corrosion model is constructed with the Markov chain theory to describe the random process of FAC corrosion for nuclear pipe. The probability distribution of effective remaining area of pipe section after corrosion is calculated using the random corrosion model. The simplified calculation model of suspended nuclear pipe is provided to derived the axial displacement and stress under earthquake. The seismic reliability index is calculated for corroded nuclear pipe to undertake the action of earthquake in the calculating period. This can offer some benefit help to engineers in the design process of nuclear pipe.