Yongsun Yi | Khalifa University (original) (raw)

Papers by Yongsun Yi

Identification of the mechanism responsible for primary water stress corrosion cracking (PWSCC) i... more Identification of the mechanism responsible for primary water stress corrosion cracking (PWSCC) in nickel-base alloys is a controversial topic. Numerous mechanisms, including the film-rupture/oxidatjon (i.e., slip-oxidation or slip-dissolution) mechanism, have been proposed to explain PWSCC. According to this mechanism, the observed sensitivity of PWSCC to material and environmental factors may be explained by the combined effects of repassivation kinetics, oxide rupture strain, and crack tip strain rate (which includes creep). Previqus research has shown that increasing the Cr content of Ni-9%Fe-Cr from 16 to 30 wt?lo strongly decreases PWSCC susceptibility. Consequently, measurements of these three fundamental parameters (repassivation, oxide rupture, and creep) were performed as a function of Cr content, and SCC crack growth rates were predicted on the basis of the resulting data. This paper illustrates that considering these three parameters concurrently may contribute to the understanding of Cr effects on PWSCC of Ni-base alloys. However, it is not clear whether the film-rupture/oxidation mechanism can adequately predict the observed crack growth rates for Alloy 600 at 338°C.

Progress in Nuclear Energy

Journal of Pressure Vessel Technology, 2001

Oxidation kinetics of recently developed ferritic heat-resistant steels, HCM12A, NF616, and HCM2S... more Oxidation kinetics of recently developed ferritic heat-resistant steels, HCM12A, NF616, and HCM2S, were investigated in a superheated steam to evaluate the effects of chemical composition of the steels, testing temperature (560–700°C), steam pressure (1–10 MPa), and degrees of microstructural evolution by aging on oxidation. The contribution of alloyed Cr to oxidation resistance was pronounced above 600°C, while no material dependency was found at 600°C or lower. The apparent activation energy of the oxidation rate clearly changed at around 600°C for NF616 and HCM12A. In contrast, HCM2S showed single activation energy over the range of temperatures. Although temperature and chemical composition were the major factors, steam pressure also showed a clear negative effect on the oxidation rate in the lower temperature range, 570–600°C.

Materials Science Forum, 2005

High Cr alloys were corrosion tested in supercritical water and the oxide scale was analyzed. Com... more High Cr alloys were corrosion tested in supercritical water and the oxide scale was analyzed. Commercial grade two steel specimens; 9CrMoVNb steels, one 9CrMoVNbW steel, one 12Cr-MoVNbWCu steel and one 20Cr Fe-based O.D.S (Oxide Dispersion Strengthened) alloy specimen were investigated. Corrosion tests were conducted within non-deaerated pure supercritical water at 627, 550, and 500oC with 25 MPa. Corrosion rate was estimated by the weight change per unit surface area and the oxide layer was analyzed using a grazing incidence X.R.D (x-ray diffractometer), S.E.M (scanning electron microscope) and T.E.M (transmission electron microscope) equipped with an E.D.S (energy dispersive spectroscope). Corrosion rates of the 9Cr steel specimens were observed to follow the parabolic growth rate law, while those of the specimens with a 12 per cent or higher Cr content showed significantly lower rates. Oxide scale on the 9Cr steel specimen after a corrosion test in a supercritical water was found...

Energies

Chloride diffusion through concrete is influenced by harsh environmental conditions such as high ... more Chloride diffusion through concrete is influenced by harsh environmental conditions such as high ambient temperature and relative humidity. This paper examined the influence of temperature gradient on chloride diffusion in concrete under high ambient temperature conditions. Chloride diffusion tests using cylindrical concrete samples were performed in constant temperature and temperature gradient conditions. In a temperature gradient condition, a much higher chloride concentration was measured than at constant temperatures, which could not be explained only by the mass diffusion driven by the concentration gradient. A new analytical model of chloride diffusion with the mass diffusion term including the temperature effect and the thermo-diffusion term including the temperature gradient effect was applied to the results, which showed that the thermo-diffusion contribution was significant. Using the analytical model with the mass diffusion (DCl) and thermo-diffusion (DT) coefficients, t...

Ninth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors, 2013

Creep tests were performed to compare the creep behavior of commercial nickel-base alloys as a fu... more Creep tests were performed to compare the creep behavior of commercial nickel-base alloys as a function of s!ress, temperature, and the environment. Alloy 600 (nominal and low carbon) and the precipitation strengthened Alloys 625 and X-750 in the AH and HTH conditions were tested at constant load in &aerated primary water containing 40-60cc/kg hydrogen. The stress dependence of Alloy 600 in the mill-annealed (MA) condition was obtained at 337°C and 360"C. 'Ihe stress exponent was determined to be between 3 and 9. The activation energy of creep was 64 kcal/mole. Both results support a dislocation climb controlled mechanism of creep in commercial Alloy 600. The creep twults were compared to the known stress corrosion cracking (SCC) performance of these alloys. The creep rates of the low carbon (LC) alloy are higher on average than those with nominal carbon level. Intergranular (IG) cracking occurred only in the X-750 AH alloy and the precipitation treated LC A600 alloy. These results support earlier work that showed that low carbon alloys are more susceptible to creep and IG cxacking than are high carbon alloys. However, these results also show a smaller influence of a water environment on the creep rate of commercial, creep-resistant alloys compared to high purity alloys.

Electrochemicalbehavior ofcarbon steel for nuclear reactor containment buildings was investigated... more Electrochemicalbehavior ofcarbon steel for nuclear reactor containment buildings was investigated using potentiodynamic, potentiostatic, and galvanostatic polarization techniques in Ca(OH)2 solutions with NaCl. From potentiodynamic and potentiostatic polarization tests, it was found that the occurrence of pitting on the carbon steel in the solutionswas strongly dependent on the applied potential and chloride concentration. Metastable pitting that has usually been observed on stainless steels in chloride containing solutions was not observed on the carbon steel used in this study.Using galvanostatic polarization technique, the chloride threshold conditions of the carbon steel were evaluated. First, samples were pre-passivated in Ca(OH)2 solutions without NaCl for 2 days by immersion. Then, galvanostatic current of 5μA/cm 2 was applied to the prepassivated samples. The occurrence of depassivation was evaluated as a function of pH and NaCl concentrations of solutions. Being expressed i...

In the Nuclear Hydrogen Development and Demonstration (NHDD) project, two types of VHTRs (Very Hi... more In the Nuclear Hydrogen Development and Demonstration (NHDD) project, two types of VHTRs (Very High Temperature Reactors), prismatic or pebble bed, are under investigation as the nuclear heat source for a hydrogen production. In general, the targeted coolant outlet temperature of the VHTR is 950~1000°C and the maximum allowable fuel temperature is 1250°C during a normal operation. In the case of the prismatic reactor (PMR), conventional fuel designs result in a small margin in the maximum fuel temperature [1]. This is one of the biggest demerits of the prismatic type. In this study, a technique for lowering the maximum fuel temperature is suggested. The PMR fuel assembly is comprised of many coolant holes and fuel channels as shown in Fig. 1. Cylindrical fuel compacts are stacked inside the fuel channel. Consequently, there should be a physical gap between the fuel compact and graphite block, which is filled with He gas in the conventional design. The heat transfer coefficient of th...

Primary water stress corrosion cracking (PWSCC) of nickel-base alloys has become one of the serio... more Primary water stress corrosion cracking (PWSCC) of nickel-base alloys has become one of the serious problems in the primary water loops of pressurized water reactors (PWRs). Several mechanisms have been proposed to account for the PWSCC behavior of nickelbase alloys in the high temperature water (~300C). In this study, we consider the slip oxidation model as a potential PWSCC mechanism, and apply this model to the estimation of the crack growth rates quantitatively. A film-rupture / slip-oxidation (SO) process has been proposed to account for SCC of Alloy 600 as a PWSCC mechanism. The rate controlling factors affecting the crack propagation in the SO model are the rupture rate of the oxide film, the rate of repassivation, and the rate of the diffusion of the dissolved metal ions away from the crack tip. The SO model, as proposed by Ford and Andresen [1], has been successful in predicting the crack propagation rate in austenitic stainless steels and lowalloy steels in 288C water, sym...

Annals of Nuclear Energy, 2022

Sensors, 2021

There has been considerable interest in inorganic scintillators based on lutetium due to their fa... more There has been considerable interest in inorganic scintillators based on lutetium due to their favorable physical properties. Despite their advantages, lutetium-based scintillators could face issues because of the natural occurring radioisotope of 176Lu that is contained in natural lutetium. In order to mitigate its potential shortcomings, previous works have studied to understand the energy spectrum of the intrinsic radiation of 176Lu (IRL). However, few studies have focused on the various principal types of photon interactions with matter; in other words, only the full-energy peak according to the photoelectric effect or internal conversion have been considered for understanding the energy spectrum of IRL. Thus, the approach we have used in this study considers other principal types of photon interactions by convoluting each energy spectrum with combinations for generating the spectrum of the intrinsic radiation of 176Lu. From the results, we confirm that the method provides good ...

Nuclear Engineering and Design, 2018

Using ABAQUS, a non-linear finite element analysis (FEA) was performed to evaluate and compare th... more Using ABAQUS, a non-linear finite element analysis (FEA) was performed to evaluate and compare the effects of degradation mechanisms by aging on the ultimate pressure capacity (UPC) of APR1400 reactor containment building (RCB). As the primary degradation mechanisms, prestress loss, concrete aging, rebar corrosion, and liner corrosion were simulated in the modelling and calculations. The results for unaged reactor containment buildings showed that the failure sequence by the internal pressure build-up consisted of 4 stages and reinforcement yielding strain of reinforced concrete would be the most important factor governing the UPC of RCBs. Among the four degradation mechanisms, corrosion occurring on the outer and inner rebar layers was identified as the main degradation mechanism affecting most significantly the ultimate pressure capacity of the APR1400 reactor containment building.

Nuclear Engineering and Technology, 2018

Low-cycle fatigue (LCF) tests were performed for Alloy 690 and 316 SS in a simulated pressurized ... more Low-cycle fatigue (LCF) tests were performed for Alloy 690 and 316 SS in a simulated pressurized water reactor (PWR) environment. Alloy 690 showed about twice longer LCF life than 316 SS at the test condition of 0.4% amplitude at strain rate of 0.004%/s. Observation of the oxide layers formed on the fatigue crack surface showed that Cr and Ni rich oxide was formed for Alloy 690, while Fe and Cr rich oxide for 316 SS as an inner layer. Electrochemical analysis revealed that the oxide layers formed on the LCF crack surface of Alloy 690 had higher impedance and less defect density than those of 316 SS, which resulted in longer LCF life of Alloy 690 than 316 SS in a simulated PWR environment.

Transactions of the Japan Society of Mechanical Engineers Series A, 1997

Scripta Materialia, 2004

ABSTRACT Two grain boundary characterization methods, including and excluding twins were compared... more ABSTRACT Two grain boundary characterization methods, including and excluding twins were compared. They appeared to be equivalent in determining crack arrest probability within one grain size using a probabilistic approach. However, in determining the probability of crack arrest within a length, twins should be excluded.