Abstract
It has been thought that wall thinning on the secondary side piping in nu-clear power plants is mostly caused by Flow-Accelerated Corrosion (FAC). Recently, it has been seen that wall thinning on the secondary side piping carrying two-phase flow is caused by not only FAC but also Liquid Droplet Impingement Erosion (LDIE). Moreover, it turns out that LDIE in nuclear power plants does not result from a single degradation mechanism but also from the simultaneous happenings of LDIE and FAC. This paper presents a comparison of the mass loss rate of the tested materials between carbon steel (A106 B) and low alloy steel (A335 P22) resulting from degradation effect. An experimental facility was set up to develop a prediction model for clarifying multiple degradation mechanisms that occur together. The experimental facility allows examining liquid droplet impingement erosion in the same conditions as the secondary side piping in nuclear power plants by generating the magnetite on the surface of the test materials. The magnetite is formed by controlling the water chemistry and the temperature of fluid inside the facility. In the initial stage of the experiments, the mass loss rate of A106 B was greater than that of A335 P22. However, after a certain period of time, the mass loss rate of A335 P22 became greater than that of A106 B. It is presumed that the results are caused by the different yield strengths of the test materials and the different degrees of buffer action of the magnetite deposited on their surfaces. The layer of magnetite on the surface of A106 is thicker than that of A335 P22, due to the different amount of chrome content. In nuclear power plants, carbon steel piping having experienced wall thinning degradation is generally replaced with low-alloy steel piping. However, the materials of pipes carrying two-phase flow should be selected considering their susceptibility to LDIE.
Highlights
It has been thought that the wall thinning on the secondary side piping in nuclear power plants is mostly caused by Flow-Accelerated Corrosion (FAC)
Characterization of the Droplets In order to determine the optimal experimental conditions for the droplet size and the Droplet Number Density (DND), which are the key factors to Liquid Droplet Impingement Erosion (LDIE), the droplet sizes were analyzed using a strobe light source and an ultrahigh-speed camera, and the Stand-Off Distance (SOD), the nozzle size, and the nozzle pressure differences were tested in advance of the tests for the multiple degradations
The surface of the specimen A106 B is much darker than the surface of the specimen A335 P22, which means that the formed magnetite on the surface of the A106 B was more than that of the surface of the A335 P22 in the same time
Summary
Damage caused by LDIE in South Korea consists of leakage from the vent header connected to the high pressure feed water heater in Wolsong Unit 1 (2008) [1], leakage from the branch line connected to the high pressure turbine in Hanul Unit 2 (2010), and leakage from the end cap connected to the reheater vent line in Hanbit unit 2 (2011) [2]. In all of these cases, magnetite was formed on the surface of the damaged piping, and the surface was pitted with pockmarks. The test materials were carbon steel (A106 B) and low-alloy steel (A335 P22), which are widely used in the secondary system of nuclear power plants
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