Cause Analysis for Wall Thinning of Small-Bore Piping in Nuclear Power Plant by ToSPACE, FLUENT and Theoretical Evaluation

It has been known that FAC, LDIE, cavitation and flashing are the damage mechanisms that can cause the pipe thickness of the secondary system of nuclear power plants thinner. Severe wall thinning was found in the MSR drain pipes at a Korean nuclear power plant a decade ago, and all the affected pipes were replaced with low alloy steel with higher chromium contents. Therefore, this study was conducted to reduce the possibility of similar thinning cases that may occur in the future by identifying the exact cause of thinning. ToSPACE and FLUENT codes and theoretical evaluation method were applied to analyze the causes of thinning. ToSPACE and FLUENT analyses and theoretical evaluation including all the operating conditions show a relatively large pressure drop and a pressure lower than the saturated vapor pressure in common at the end of the pipe entering the condenser. This means that flashing occurs at the end of the pipe under all operating conditions, and the effect can be greater than that of other parts. As a result, since severe wall thinning occurred at the end of the pipeline entering the condenser, it was evaluated that flashing by the high-velocity two-phase fluid was the direct cause of the wall thinning in the MSR drain pipes. The results of this study will contribute to establishing appropriate countermeasures in the event of pipe wall thinning in the future.

Flow-Accelerated Corrosion in Pipe Wall Downstream of Orifice for Water and Air-Water Bubble Flows

An orifice is used widely as a flow meter or a contraction device in pipeline systems in hydro-power plants, thermal power plants, and chemical plants because of its simple construction, high reliability, and low cost. However, it is well known that flow-accelerated corrosion (FAC) occurs on the pipe wall downstream of the orifice. Some of the authors have examined FAC through experimental and numerical analyses and have reported that one of the major governing parameters of FAC for single-phase water flow is the pressure fluctuation p’ on the pipe wall, and also that pipe wall thinning rate TR can be estimated by p’. In addition, they have presented the effects of the ori-fice geometry on p’ or TR, and have described a method for suppressing p’ or TR. In the present study, FAC for a two-phase air-water bubble flow is examined and compared with the single-phase water flow experimentally. Further, it is shown that because p’ is also considered a governing parameter of FAC for a two-phase air-water bubble flow, TR can be estimated using p’. It is also indicated that, by using a downstream pipe with a smaller diameter than that of the upstream pipe, p’ or TR can be suppressed.

Analysis of Material Loss Behavior According to Long-Term Experiments on LDIE-FAC Multiple Degradation of Carbon Steel Materials

Recently, damage caused by liquid droplet impingement erosion (LDIE) in addition to flow-accelerated corrosion (FAC) has frequently occurred in the secondary side steam piping of nuclear power plants, and the damage-occurring frequency is expected to increase as their operating years’ increase. In order to scrutinize its causes, therefore, an experimental study was conducted to understand how the behavior of LDIE-FAC multiple degradation changes when the piping of nuclear power plants is operated for a long time. Experimental results show that more magnetite was formed on the surface of the carbon steel specimen than on the low-alloy steel specimen, and that the rate of magnetite formation and extinction reached equilibrium due to the complex action of liquid droplet impingement erosion and flow-accelerated corrosion after a certain period of time. Furthermore, it was confirmed at the beginning of the experiment that A106 Gr.B specimen has more mass loss than A335 P22 specimen. After a certain period of time, however, the mass loss tends to be the opposite. This is presumed to have resulted from the magnetite formed on the surface playing a role in suppressing liquid droplet impingement erosion. In addition, it was confirmed that the amount of erosion linearly increases under the conditions in which the formation and extinction of magnetite reach equilibrium.

Comparison between FAC Analysis Result Using ToSPACE &CHECWORKS Programs and Experimental Result

A number of piping components in the secondary system of nuclear power plants (NPPs) have been exposed to aging mechanisms such as FAC (Flow-Accelerated Corrosion), cavitation, flashing, LDIE (Liquid Droplet Impingement Erosion), and SPE (Solid Particle Erosion). Those mechanisms may lead to thinning, leaking, or the rupture of components. Due to the pipe ruptures caused by wall thinning of Surry Unit 2 in 1986 and Mihama Unit 3 in 2004, pipe wall thinning management has emerged as one of the most important issues in the nuclear industry. To manage the wall thinning of pipes caused by FAC and erosion, KEPCO-E & C has developed ToSPACE program. It can predict both FAC & erosion phenomena, and also be utilized in the pipe wall thinning management works such as susceptibility analysis, UT (Ultrasonic Test) data evaluation as well as establishment of long-term inspection plan. Even though the ToSPACE can predict the five aging mechanisms mentioned above, only the FAC prediction result using ToSPACE was compared herein with the experimental result using FACTS (Flow Accelerated Corrosion Test System) to verify the ToSPACE’s capability. In addition, the FAC prediction result using ToSPACE was also compared with that of CHECWORKS that is widely used all over the world.

‘Size effect’ related bending formability of thin-walled aluminum alloy tube

Aluminum alloy （Al-alloy） thin-walled （D/t ＆gt; 20, diameter D, wall thickness t） bent tubes have attracted increasing applications in many industries with mass quantities and diverse specifications due to satisfying high strength to weigh ratio requirements of product manufacturing. However, due to nonlinear nature of bending with coupling effects of multiple factors, the similarity theory seems not applicable and there occurs a challenge for efficient and reliable evaluation of the bending formability of thin-walled tube with various bending specifications. Considering the unequal deformation and three major instabilities, the bending formability of thin-walled Al-alloy tube in changing tube sizes such as D and t are clarified via both the analytical and FE modeling/ simulations. The experiments of rotary draw bending are conducted to validate the theoretical models and further confirm ＇size effect＇ related bending formability. The major results show that （1） The anti-wrinkling capability of tube decreases with the larger D and smaller t, and the effect significance of t is larger than that of D even under rigid supports; （2） The wall thinning increases with the larger D and smaller t, and this tendency becomes much more obvious under rigid supports; （3） The cross-section deformation increases with the larger D and smaller t according to the analytical model obtained intrinsic relationship, while this tendency becomes opposite due to the nonlinear role of mandrel die; （4） The size factor D/t can be used as a nondimensional index to evaluate both the bending formability regarding the wall thinning and cross-section deformation.