원전 운전온도에서 감육결함부 국부손상기준의 신뢰성평가

Abstract

Local thickness reduce(wall thinning) phenomenon that happen in the piping of power plant or other chemistry plant, the oil and gas is becoming main cause that deteriorate integrity of piping. Therefore, the integrity evaluation about wall thinning defect is very important in terms of operating plants and safety security side of these piping. Failure criteria about wall thinning defect are divided into two types greatly. It is global failure criteria which consider the loss of the ability of supporting entire load by wall thinning defect in the pipe and local failure criteria which consider the increasing of the local stress or strain in the defect department. Local failure criteria are often applied local strain or local stress. Local strain criterion assumes that failure occurs when principal strain of defect department exceeds fracture strain of material, and local stress criterion assumes that failure occurs when stress of defect department exceeds critical stress of material. Many studies are achieving real piping test of nuclear power plant or the specimen test that imitate the state of the stress and are developing local failure criteria from result of finite element simulation about test. However, developed local failure criteria were presented at room temperature that does not consider nuclear plant operating temperature. These failure criteria are based on material property of nuclear plant's piping. These piping materials show other characteristics according to use environment. Specially, the change type of carbon steel properties by temperature is various. And the carbon steel piping in the secondary system of nuclear plant was placed in operating environment of around 290℃. Purpose of this study is to evaluate local failure criteria of wall thinning defect considering nuclear power plant operating temperature. This study performs tensile test of notched round bar in 289℃ where is a nuclear plant operating temperature, and then presents the local failure criteria from the finite element simulation. And the tensile test in the operating temperature is performed using grooved-flat plate specimen that imitate state of the stress of wall thinning defect. The finite element simulation for the test of grooved-flat plate specimen is achieved, and failure point can be predicted applying local failure criteria presented in notched round bar specimen test. In the comparison of notched round bar test and finite element simulation, load-displacement curve until the maximum load is almost identical, but the result of simulation is always higher than result of tensile test after the maximum load. The reason is that strain is different in the deformation region of standard tensile specimen and notched round bar specimen. Namely, material property changes due to that dynamic strain aging phenomenon occur partially in the operating temperature of nuclear plant. Specially, after the necking, the elongation decrease greatly in the notch region, because the result of simulation is always higher than result of tensile test in the load-displacement curve. Finite element analysis cannot simulate whole behavior of notched round specimen test, but it can simulate the behavior to maximum load of test. Therefore, local failure criterion for maximum load was deduced a conclusion from this study. From the result of simulation, it was considered that average stress triaxiality in the cross section of the center of the notch and to maximum load. Averaged stress triaxiality at maximum load decrease according to increase the notch radius, and averaged equivalent stress in the cross section indicate almost constant value(true-tensile strength) about the each notch radius. This result coincides with local failure criterion that is deduced with notched round bar specimen test result that is achieved under room temperature condition.