Cause Analysis of Flow Accelerated Corrosion and Erosion-Corrosion Cases in Korea Nuclear Power Plants

Abstract

Significant piping wall thinning caused by Flow-Accelerated Corrosion (FAC) and Erosion-Corrosion (EC)continues to occur, even after the Mihama Power Station unit 3 secondary pipe rupture in 2004, in whichworkers were seriously injured or died. Nuclear power plants in many countries have experienced FACand EC-related cases in steam cycle piping systems. Korea has also experienced piping wall thinning casesincluding thinning in the downstream straight pipe of a check valve in a feedwater pump line, the downstreamelbow of a control valve in a feedwater flow control line, and failure of the straight pipe downstreamof an orifice in an auxiliary steam return line. Cause analyses were performed by reviewing thickness datausing Ultrasonic Techniques (UT) and, Scanning Electron Microscope (SEM) images for the failed pipe,and numerical simulation results for FAC and EC cases in Korea Nuclear Power Plants. It was concludedthat the main cause of wall thinning for the downstream pipe of a check valve is FAC caused by watervortex flow due to the internal flow shape of a check valve, the main cause of wall thinning for the downstreamelbow of a control valve is FAC caused by a thickness difference with the upstream pipe, and the maincause of wall thinning for the downstream pipe of an orifice is FAC and EC caused by liquid dropletsand vortex flow. In order to investigate more cases, additional analyses were performed with the reviewof a lot of thickness data for inspected pipes. The results showed that pipe wall thinning was also affectedby the operating condition of upstream equipment. Management of FAC and EC based on these cases willfocus on the downstream piping of abnormal or unusual operated equipment.