Power Plant Piping Research Articles

Development of a LDIE Prediction Theory in the Condition of Magnetite Formation on Secondary Side Piping in Nuclear Power Plants

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.

発電プラントの健全性確保に求められる欠陥検出確率の検討（損傷確率に及ぼす欠陥検出確率の影響評価）

This paper discuss about suitable damage size that should be detected by ultrasonic testing (UT) for nuclear power plant piping when consider about propagation of IGSCC. The power plant piping loop is inspected using UT as an in-service inspection (ISI). The reliability of UT result is affected by examination personnel. In this paper, the reliability of UT is defined simplified POD (Probability of detection) curve. The curve is defined by only three parameters, minimum detectable crack depth (a0), most detectable crack depth (a1) and maximum POD(m). The reliability of plant piping with different diameters are estimated by using this POD curve. The Failure probabilities were calculated with Monte Carlo approach. In case of low carbon stainless steel with IGSCC, a1 and m are less than 6 mm and 0.96 respectively, the piping loop failure probability is under 1x10-7. In another case of sensitized stainless steel with IGSCC, a1 and m are less than 6 mm and 0.96, the piping loop failure probability is under 1x10-3. It was found that the parameters of a1 and m do not affect plant reliability when a1 is between 3 to 6mm, and m is between 1.00 to 0.96. This result can be used to decide the requirement for examination personnel reliability.

Effect of Grain Size on Corrosion Fatigue Behaviors of Primary Coolant Pipes of Nuclear Power Plants

Corrosion fatigue is one of the most important failure modes of primary coolant pipe used in nuclear power plant, due to the thermal cyclic stress caused by a variety of start-up and shutdown as well as transients when severed in the high temperature and pressure water environment. And 316LN stainless steel is one of the main materials used in pressurized water reactor in nuclear power plant. The mechanisms of fatigue crack initiation and propagation were elucidated by investigating the composition properties of oxide films formed in simulated service environment. The effects of grain size on the fatigue life and crack initiation mechanism of 316LN stainless steel in 320°C water environment were also investigated. The results indicated that the specimens with fine grains (30μm) own the longest fatigue lives than those with intermediate (80μm) and coarse grains (210μm). The fatigue stress amplitudes of the specimens increased with the grain refinement, although the fatigue lives of the specimens with intermediate and coarse grains were close. Fatigue cracks were often initiated at the persistent slip bands on the surfaces of the specimens with intermediate and coarse grains, while it was not easy to be initiated on the fine grain specimens. Finally, combined with the experimental results and the actual production of industry, a suggestion for the production of the primary coolant pipe is put forward.

Effect of tempering time on the mechanical properties of P91 flux cored wire weld metal

ASME Grade 91 is one of the main materials for heavy-wall pipes in thermal power plants. Matching flux cored wires for welding P91 have already been available for several years and become more and more popular as flux cored arc welding (FCAW) offers several technical and economic advantages. To achieve high toughness of the weld metal at ambient temperature, temperature and/or time of post-weld heat treatment (PWHT) can be increased. As the temperature has to be kept below the transformation temperature Ac1, the range of suitable temperature is very small. In this contribution, the effect of increased tempering time on the mechanical properties at ambient temperature and elevated temperatures is investigated and the influence on long-term properties is discussed. Longer PWHT increases toughness of all-weld metal. MatCalc simulations and scanning electron microscopy (SEM) investigations indicate a coarsening effect of precipitates without detrimental effect on long-term creep properties.

Ultrasonic Inspection of Electrofusion Joints of Large Polyethylene Pipes in Nuclear Power Plants

High-density polyethylene (HDPE) pipe has many advantages such as good flexibility, corrosion resistance, and long service life. It has been introduced into nuclear power plants for transportation of cooling water in U.S. and Europe. Recently, four HDPE pipelines (PE4710) were used in essential cooling water system with operating pressure of 0.6 MPa and operating temperature of no more than 60 °C in a newly established AP1000 nuclear power plant in Zhejiang, China. The outside diameter and thickness are 30 in. and 3.3 in., respectively, which are much larger and thicker than traditional HDPE pipe for natural gas. This brought forward a challenge for nondestructive testing (NDT) and safety assessment of such pipes. In this paper, a solution for inspecting electrofusion (EF) joints of thick-walled HDPE pipes is presented, and the results of an on-site inspection of the nuclear power plant are revealed. To expand the thickness up-limit of previously developed ultrasonic-phased array instrument, an optimization method was proposed by calculating weighing effects of different testing parameters and introducing the concept of overall performance according to practical requirement, by comprehensively considering sensitivity, penetration, signal-to-noise ratio (SNR), resolution, and accuracy. Typical defects were found in field inspection. The result shows that the presented technique is capable of inspecting EF joints for connecting large-size HDPE pipes used in nuclear power plants.

Analysis of internal crack in a six-ton P91 ingot

P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack of feeding in the ingot center often generates many defects, such as porosity and crack. A six-ton P91 ingot was cast and sliced, and a representative part of the longitudinal section was inspected in more detail. The morphology of crack-like defects was examined by X-ray high energy industrial CT and reconstructed by 3D software. There are five main portions of defects larger than 200 mm3, four of which are interconnected. These initiated from continuous liquid film, and then were torn apart by excessive tensile stress within the brittle temperature range (BTR). The 3D FEM analysis of thermo-mechanical simulation was carried out to analyze the formation of porosity and internal crack defects. The results of shrinkage porosity and Niyama values revealed that the center of the ingot suffers from inadequate feeding. Several criteria based on thermal and mechanical models were used to evaluate the susceptibility of hot crack formation. The Clyne and Davies’ criterion and Katgerman’s criterion successfully predicted the high hot crack susceptibility in the ingot center. Six typical locations in the longitudinal section had been chosen for analysis of the stresses and strains evolution during the BTR. Locations in the defects region showed the highest tensile stresses and relative high strain values, while other locations showed either low tensile stresses or low strain values. In conclusion, hot crack develops only when stress and strain exceed a threshold value at the same time during the BTR.

Characterization of solid particles sampled from condensates in boiling water reactor

The growth, activation and deposition of corrosion products are the primary sources of radiation buildup on the surface of out-of-core piping in nuclear power plants. The buildup of radiation can have negative effects on the performance of the facility and cause harm to staff during maintenance outages for refueling. This paper reports on the crystalline and amorphous structures of corrosion products sampled in the boiling water reactors in nuclear power plants of Kuo-Sheng and identified using an acid dissolving technique. X-ray diffraction, scanning electron microprobe and inductively coupled plasma-atomic emission spectroscopy were used to analyze the samples. The results indicate that the quantity of amorphous iron oxide at inlet of the condensate demineralizer in Unit 2 is higher than that in Unit 1. The proportion of crystalline to amorphous corrosion products can affect the efficiency of removal. Thus, these results can be used to explain the difference in removal efficiency of condensate demineralizers in different units. Moreover, the iron oxide structures with various properties were observed in different operational periods. It is probable that the higher proportion of amorphous structures with a smaller particle size would reduce efficiency in the removal of condensate demineralization in Unit 2.

Development of Wall-Thinning Evaluation Procedure for Nuclear Power Plant Piping—Part 1: Quantification of Thickness Measurement Deviation

Pipe wall thinning by flow-accelerated corrosion and various types of erosion is a significant and costly damage phenomenon in secondary piping systems of nuclear power plants (NPPs). Most NPPs have management programs to ensure pipe integrity due to wall thinning that includes periodic measurements for pipe wall thicknesses using nondestructive evaluation techniques. Numerous measurements using ultrasonic tests (UTs; one of the nondestructive evaluation technologies) have been performed during scheduled outages in NPPs. Using the thickness measurement data, wall thinning rates of each component are determined conservatively according to several evaluation methods developed by the United States Electric Power Research Institute. However, little is known about the conservativeness or reliability of the evaluation methods because of a lack of understanding of the measurement error. In this study, quantitative models for UT thickness measurement deviations of nuclear pipes and fittings were developed as the first step for establishing an optimized thinning evaluation procedure considering measurement error. In order to understand the characteristics of UT thickness measurement errors of nuclear pipes and fittings, round robin test results, which were obtained by previous researchers under laboratory conditions, were analyzed. Then, based on a large dataset of actual plant data from four NPPs, a quantitative model for UT thickness measurement deviation is proposed for plant conditions.

Loading history effect on creep-fatigue crack growth in pipe bend

This study is concerned with assessing the structural integrity of power plant pipes that operate under creep and fatigue conditions. In the process of manufacturing pipe bends, it is difficult to avoid thickening on the inner radius of the pipe bend and thinning on the outer radius of the pipe bend. The cross-section of the bend also becomes non-circular due to the bending process, which tends to cause ovality in pipe bends. The acceptability of pipe bends is based on the induced level of shape imperfections considered. The influence of internal pressure, operation time and shape imperfections of a pipe bend on stress-strain redistributions and creep-fatigue crack growth rates was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bends was performed for different stages of lifetime while considering various loading conditions. As a result, it was determined that the combined effect of ovality and thinning cross-sections is significant and is a function of creep time. The tensile and creep tests were performed for determination of the main mechanical properties of the material of the pipe bends after loading history. The smooth and compact specimens were cut out from four tension and compression critical zones of the same pipe bend with given ovality and thinning cross-sections, as well as operating time. The creep-fatigue crack growth rate tests were performed on a specially designed program test-cycle. A comparison demonstrates that the tensile properties of 12Cr1MoV steel were degraded by long-term high-temperature service, whereas a comparison of the creep-fatigue crack growth rate characteristics indicates that they are a function of the critical zone positions in the considered pipe bend. A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature was used in the form of a creep stress intensity factor.

A Numerical Study on Flow-Accelerated Corrosion in Two Adjacent Elbows

Flow-Accelerated Corrosion (FAC) is a well-known degradation mechanism that attacks the secondary piping in nuclear power plants. Since the Surry Unit 2 event in 1986, most nuclear power plants have implemented management programs to deal with damages in carbon and low-alloy steel piping. Despite the utmost efforts, damage induced by FAC still occurs in power plants around the world. In order to predict FAC wear, some computer programs were developed such as CHECWORKS, CICERO, and COMSY. Various data need to be input to these programs; the chemical composition of secondary piping, flow operating conditions and piping geometries. CHECWORKS, developed by the Electric Power Research Institute (EPRI), uses a geometry code to calculate geometry effects. Such a relatively simple geometry code is limited in acquiring the accuracy of FAC prediction. Recently, EPRI revisited the geometry code with the intention of updating it. In this study, numerical simulations were performed for two adjacent elbows and the results were analysed in terms of the proximity effect between the two adjacent elbows.

Tearing resistance of heterogeneous interface region of a dissimilar metal weld characterised with sub-sized single edge bend specimens

Determination of tearing resistance of heterogeneous dissimilar metal welds (DMW) that are critical structures of pipes in nuclear power plants (NPPs) is important in assessment of the structural integrity. Currently, tearing resistance of heterogeneous materials is determined by standards developed for homogeneous materials. The fracture behaviour of heterogeneous materials differs from fracture behaviour of homogeneous materials. Thus standards developed for homogeneous materials are not necessary directly applicable to heterogeneous materials. To develop a standard for heterogeneous materials the current level of knowledge of fracture behaviour in heterogeneous specimens needs to be increased. To enhance the knowledge of fracture in heterogeneous materials tearing resistance of 10×10×55 mm3 and 10×20×100 mm3 single edge bend (SE(B)) specimens extracted from a NPP ferrite-austenite DMW were measured. The specimens were extracted at three different crack locations in a region close to the fusion line between ferritic steel and weld metal of the DMW. As tearing resistance curves of the two specimen geometries are compared toughness and scatter varies, which can be explained by differing crack location with respect to the fusion line. However, the effect of crack location on tearing resistance was not microstructurally quantified, but the qualitative analysis of crack location done in this work implies that a standard developed for heterogeneous materials shall contain guidelines on characterisation of crack location.

Interpretation of creep crack growth data for ½ CMV steel weldments

½Cr½ Mo¼ V (½ CMV) steel has been used in high temperature power plant piping due to its enhanced weld properties. Creep crack growth testing has been performed on compact tension C(T) specimens of ½CMV (low alloy ferritic) steel at 540◦C on both parent metal specimens as well as fine and coarse grained heat affected zone (HAZ) specimens, where the initial crack is located within the HAZ. The data has been interpreted using the fracture mechanics parameter C* against the crack growth rate. The creep toughness parameter, Kcmat, is also evaluated for the material. It was seen that, for a given C* value, the fine grained HAZ material generally exhibits higher crack growth rates than the post weld heat treated coarse grained HAZ.

Vibration Control of Nuclear Power Plant Piping System Using Stockbridge Damper under Earthquakes

Generally the piping system of a nuclear power plant (NPP) has to be designed for normal loads such as dead weight, internal pressure, temperature, and accidental loads such as earthquake. In the proposed paper, effect of Stockbridge damper to mitigate the response of piping system of NPP subjected to earthquake is studied. Finite element analysis of piping system with and without Stockbridge damper using commercial software SAP2000 is performed. Vertical and horizontal components of earthquakes such as El Centro, California, and Northridge are used in the piping analysis. A sine sweep wave is also used to investigate the control effects on the piping system under wide frequency range. It is found that the proposed Stockbridge damper can reduce the seismic response of piping system subjected to earthquake loading.

Analysis of Fluid Structure Interaction in High Pressure Elbow Pipe Connections

Pipes in power plants generally subjected to high pressures and temperatures. These are connected by elbow, T-joints to get the continuity between different stages. Due to excessive joints the outlet velocity and pressure will drops by considerable amount. Stresses will be produced due to high pressure and temperature of fluid flow, which in turn creates the failure of the pipes. The turbulence of the fluid passing through the pipes will also plays a vital role to decide the outlet pressure and velocity. In this present study pipes are connected by the elbow joint are considered and observed the effect of pipe thickness, turbulence intensity and length of elbow on outlet pressure, velocity, von mises stress and turbulence kinetic energy. It results that with increase in pipe thickness and length of elbow, the velocity, von mises stress and turbulence kinetic energy are decreases but with increase in turbulence intensity, the velocity and turbulence kinetic energy are increases.

Cavitation erosion of low-density polyethylene coatings for pipe liners

The relationship between mechanical properties and the erosion rate was examined for chloroprene rubber and a number of polyethylene materials produced by different methods. As electric power plants are in operation over long periods of time, the effect of aging was also examined by testing material intended for use in pipes in electric power plants. Cavitation erosion tests were carried out by using a flowing apparatus as specified in the American Society for Testing Materials G134-95 standard. A flow velocity of 150 m/s and a test time of 24hours, were the experimental conditions used for a cavitating liquid jet test on polyethylene. The maximum depth of erosion rate (MaxDER) of polyethylene was found to decrease with the increase in hardness. Among all the tested materials, the relatively high molecular weight polyethylene with low density (m-LLDPE-H), showed the best resistance to cavitation erosion in terms of MaxDER. The effect of aging on the erosion rate of polyethylene was limited.

An Itô model of pit corrosion in pipelines for evaluating risk-informed decision making

The purpose of this paper is to present a stochastic model based on the Itô differential equation to the time evolution of pit corrosion depth in nuclear power plant piping systems. The stochastic solution obtained by analytical and numerical methods (for comparison purposes) is compared to the deterministic solution, which is obtained by setting the stochastic term of the Itô equation equal to zero. The study of extended qualified useful life of nuclear power plants has two components, one deterministic and another probabilistic and one complements the other. The evaluation based on deterministic methods defines the difference between the current state and condition of the item in the qualifying phase, but does not define its probability of continuing performing its function adequately for a period longer than the one defined by its qualified life. The corrosion process is modeled by the Itô stochastic equation, which describes the time evolution of pit corrosion depth among different states. The Itô equation is determined by the knowledge of two functions known as the drift and diffusion coefficients. This work shows that these functions can be used to estimate pit corrosion depths from plant inspection data. An Itô modeling of pit corrosion can also be developed for any component of a nuclear power plant for which corrosion depth data is available. The above proposed approach provides a precise and easy way in which pit corrosion depth can be monitored over time, which is critical for developing reliability and risk-informed models for inspection and maintenance planning of corroded pipelines. The stochastic model presented is developed considering the influence of the Gaussian white noise on corrosion. The equations of the stochastic model are solved analytically based on the Itô integral. The maximum operating time of a piping before its wall reaches the minimum allowable thickness is calculated. This study uses pit corrosion depth data from the Angra I nuclear power plant.

Determination of high-temperature elastoplastic properties of welded joints by indentation test

The stress and strain fields formed in the heat-affected zone (HAZ) of welds in pipes in thermal power plants has recently attracted attention as a key plant management issue. The microstructure in the HAZ changes locally as a result of the thermal history imposed by the welding procedure; these changes are manifested in the form of a stress–strain response different from that of the base metal. In the present study, a method based on indentation tests is proposed to estimate the variation of yield stress over the HAZ. The well-known indentation tests to infer the stress–strain curve from indentation load and penetration depth measurements are familiar. However, there are difficulties in applying this approach at high temperatures. An alternative procedure based on impression size is proposed and applied to evaluate the variation of mechanical properties of Grade 122 steel over the HAZ in a welded part at 873 K.

EFFECTS OF FORGING AND HEAT TREATMENTS ON STRESS CORROSION BEHAVIOR OF 316LN STAINLESS STEEL IN HIGH TEMPERATURE CAUSTIC SOLUTION

The reactor coolant piping in the third generation nuclear power plants of AP1000 is manufactured by integrally forging. Therefore, it is of vital importance to investigate the effects of forging and heat treatments on the stress corrosion cracking(SCC) resistance of 316 LN stainless steel(316LNSS), which is the candidate material for the reactor coolant piping in AP1000 nuclear power plants. In this work, electron back scattering diffraction(EBSD) and microhardness measurements(HV) were used to characterize the microstructure and residual strain of the as-received 316 LNSS, the forged and solution anneal treated 316 LNSS and the forged and stress relief treated 316 LNSS, respectively. The average grain size of the as- received 316 LNSS was the largest, and the forged 316 LNSS followed by solution anneal treatment and stress relief treatment showed no obvious differences on grain size. The as-received 316 LNSS exhibited the highest residual strain followed by the forged and stress relief treated 316 LNSS and then solution anneal treated 316 LNSS. Besides, the residual strain in the as- received316 LNSS concentrated on grain boundaries, while the residual strain in the forged and stress relief treated316 LNSS was characterized by a band-like distribution. The U-bend specimens were utilized to investigate the SCC behavior of the 3 kinds of 316 LNSS specimens in high temperature caustic solution. After SCC experiments, the crack morphologies of the 3 kinds of 316 LNSS specimens were examined by SEM. Then the macro and micro fracture morphologies were examined by OM and SEM, respectively. Grain morphology, residual strain and grain boundary character distribution near the SCC crack tip of the forged and stress relief treated 316 LNSS were investigated using EBSD. The results showed that the forged and solution anneal treated 316 LNSS exhibited the lowest SCC sensibility, while the as-received the highest, with the most cracks and the highest growth rate. The as-received and the forged and solution anneal treated 316 LNSS showed obvious intergranular cracking, while the forged and stress relief treated 316 LNSS showed a mixed cracking mode. The larger average grain size and higher residual strain, especially concentrating on the grain boundaries, were considered to be responsible for the highest SCC sensibility of the as-received 316 LNSS. Compared with the forged and stress relief treated 316 LNSS, the higher content of coincidence site lattice boundary(CSLB) and lower residual strain contributed to the lower SCC sensibility of forged and solution anneal treated 316 LNSS. The stress relief treatment failed to eliminate the band-like microstructure effectively, which disadvantaged the SCC resistance.

Development and validation of a leak before break criterion for cylindrical pressure vessels

Leak before break is a fail–safe design concept for application in pressure vessels and piping of power and process plants. A quantitative maximum allowable flaw size is required to establish to set acceptance/rejection limit to predict whether the specific cracked pipe will leak or break. A new modification and its boundary based on Modified Two Parameter Fracture Criterion is capable of separating the leak and break cases distinctly in order to predict the behavior of cracked cylinders, pipelines and pressure vessels in advance for taking necessary precautions by the plant operator and also very much handy for the designers. For the given operating pressure under the observed crack dimensions, whether the crack will leak or break can be assessed from the boundary generated for the material concerned using Modified Two Parameter fracture assessment procedure.

Ethanolamine degradation and energy recovery using a single air-cathode microbial fuel cell with various separators

Ethanolamine (ETA) is commonly used for alkalinization to prevent corrosion of pipes in nuclear power plants. ETA, however, causes water to deteriorate, increasing the concentration of organic matter or nutrient salts in water systems. To generate power and degrade ETA at the same time, ETA was used as carbon source for microorganisms in a single air-cathode microbial fuel cell (MFC). Using a membrane as a separator in an MFC makes them costly to set-up. To reduce the expense and evaluate MFC performance, the experiments were conducted with three different separators: a proton exchange membrane (PEM), a cation exchange membrane (CEM), and polypropylene (PP) felt. The PP felt-MFC resulted in the most efficient COD (94%) and ammonium removal (52%). The CEM-MFC produced the highest power density of 583.7 mW m−2 at a current density of 0.15 mA cm−2. The Coulombic efficiencies (CEs) were 25.1, 23.7, and 10.5% for PEM-, CEM-, and PP felt-MFCs, respectively. Although using PP felt decreased the power generation compared with membrane MFCs in terms of energy recovery, it increased ETA degradation and reduced the cost of initial set-up.