Chi Bum Bahn

Evaluation of Thermal Liquid Junction Potential of Water-Filled External Ag/AgCl Reference Electrodes

Pressure-balanced external Ag/AgCl reference electrodes have been extensively used for corrosion monitoring in both pressurized water reactor and boiling water reactor environments. In order to prolong the electrode lifetime, pure water is often employed as the electrode filling solution. Characterization of the potential of the water-filled external Ag/AgCl reference electrode was performed by estimating a thermal liquid junction potential (TLJP) originating from the thermal diffusion of ionic species in the tilling solution. The potential of the thermoelectrochemical cell, Ag/AgCl vs. that of the standard hydrogen electrode at temperature T, was expressed as the sum of the isothermal potentials and TLJP. The TLJP was analyzed for the Soret steady state based on irreversible thermodynamics by calculating the heat of transport after Agar et als theory and estimating the limiting ionic conductance from Quist et als work. Calculated potential of the water-filled external reference electrode was compared with experimental data, showing a qualitative agreement.

CHEMICAL EFFECTS ON PWR SUMP STRAINER BLOCKAGE AFTER A LOSS-OF-COOLANT ACCIDENT: REVIEW ON U.S. RESEARCH EFFORTS

Industry- or regulatory-sponsored research activities on the resolution of Generic Safety Issue (GSI)-191 were reviewed, especially on the chemical effects. Potential chemical effects on the head loss across the debris-loaded sump strainer under a post-accident condition were experimentally evidenced by small-scale bench tests, integrated chemical effects test (ICET), and vertical loop head loss tests. Three main chemical precipitates were identified by WCAP-16530-NP: calcium phosphate, aluminum oxyhydroxide, and sodium aluminum silicate. The former two precipitates were also identified as major chemical precipitates by the ICETs. The assumption that all released calcium would form precipitates is reasonable. CalSil insulation needs to be minimized especially in a plant using trisodium phosphate buffer. The assumption that all released aluminum would form precipitates appears highly conservative because ICETs and other studies suggest substantial solubility of aluminum at high temperature and inhibition of aluminum corrosion by silicate or phosphate. The industry-proposed chemical surrogates are quite effective in increasing the head loss across the debris-loaded bed and more effective than the prototypical aluminum hydroxide precipitates generated by in-situ aluminum corrosion. There appears to be some unresolved potential issues related to GSI-191 chemical effects as identified in NUREG/CR-6988. The United States Nuclear Regulatory Commission, however, concluded that the implications of these issues are either not generically significant or are appropriately addressed, although several issues associated with downstream in-vessel effects remain.

Effects of Cracking Test Conditions on Estimation Uncertainty for Weibull Parameters Considering Time-Dependent Censoring Interval

It is extremely difficult to predict the initiation time of cracking due to a large time spread in most cracking experiments. Thus, probabilistic models, such as the Weibull distribution, are usually employed to model the initiation time of cracking. Therefore, the parameters of the Weibull distribution are estimated from data collected from a cracking test. However, although the development of a reliable cracking model under ideal experimental conditions (e.g., a large number of specimens and narrow censoring intervals) could be achieved in principle, it is not straightforward to quantitatively assess the effects of the ideal experimental conditions on model estimation uncertainty. The present study investigated the effects of key experimental conditions, including the time-dependent effect of the censoring interval length, on the estimation uncertainties of the Weibull parameters through Monte Carlo simulations. The simulation results provided quantified estimation uncertainties of Weibull parameters in various cracking test conditions. Hence, it is expected that the results of this study can offer some insight for experimenters developing a probabilistic crack initiation model by performing experiments.

The Hydration Structure at Yttria-Stabilized Cubic Zirconia (110)-Water Interface with Sub-Ångström Resolution

The interfacial hydration structure of yttria-stabilized cubic zirconia (110) surface in contact with water was determined with ~0.5 Å resolution by high-resolution X-ray reflectivity measurement. The terminal layer shows a reduced electron density compared to the following substrate lattice layers, which indicates there are additional defects generated by metal depletion as well as intrinsic oxygen vacancies, both of which are apparently filled by water species. Above this top surface layer, two additional adsorbed layers are observed forming a characteristic interfacial hydration structure. The first adsorbed layer shows abnormally high density as pure water and likely includes metal species, whereas the second layer consists of pure water. The observed interfacial hydration structure seems responsible for local equilibration of the defective surface in water and eventually regulating the long-term degradation processes. The multitude of water interactions with the zirconia surface results in the complex but highly ordered interfacial structure constituting the reaction front.

Uncertainty Evaluation of Weibull Estimators through Monte Carlo Simulation: Applications for Crack Initiation Testing

The typical experimental procedure for testing stress corrosion cracking initiation involves an interval-censored reliability test. Based on these test results, the parameters of a Weibull distribution, which is a widely accepted crack initiation model, can be estimated using maximum likelihood estimation or median rank regression. However, it is difficult to determine the appropriate number of test specimens and censoring intervals required to obtain sufficiently accurate Weibull estimators. In this study, we compare maximum likelihood estimation and median rank regression using a Monte Carlo simulation to examine the effects of the total number of specimens, test duration, censoring interval, and shape parameters of the true Weibull distribution on the estimator uncertainty. Finally, we provide the quantitative uncertainties of both Weibull estimators, compare them with the true Weibull parameters, and suggest proper experimental conditions for developing a probabilistic crack initiation model through crack initiation tests.

Development of an On-Line Ultrasonic System to Monitor Flow-Accelerated Corrosion of Piping in Nuclear Power Plants

Designs of contemporary nuclear power plants (NPPs) are concentrated on improving plant life as well as safety. As the nuclear industry prepares for continued operation beyond the design lifetime of existing NPP, aging management through advanced monitoring is called for. Therefore, we suggested two approaches to develop the on-line piping monitoring system. Piping located in some position is reported to go through Flow Accelerated Corrosion (FAC). One is to monitor electrochemical parameters, ECP and pH, which can show occurrence of corrosion. The other is to monitor mechanical parameters, displacement and acceleration. These parameters are shown to change with thickness. Both measured parameters will be combined to quantify the amount of FAC of a target piping. In this paper, we report the progress of a multidisciplinary effort on monitoring of flow-induced vibration, which changes with reducing thickness. Vibration characteristics are measured using accelerometers, capacitive sensor and fiber optic sensors. To theoretically support the measurement, we analyzed the vibration mode change in a given thickness with the aid of finite element analysis assuming FAC phenomenon is represented only as thickness change. A high temperature flow loop has been developed to simulate the NPP secondary condition to show the applicability of new sensors. Ultrasonic transducer is introduced as validation purpose by directly measuring thickness. By this process, we identify performance and applicability of chosen sensors and also obtain base data for analyzing measured value in unknown conditions. Introduction Since NPPs started their operation, plant condition monitoring for safe operation is always a hot issue. For the extended lifetime operation of the plant, development of condition monitoring technology becomes more important these days. Ageing of the material is one of the most severe problems against the extended lifetime operation of NPPs. After the pipe rupture accident at Surry Unit 2, Flow Accelerated Corrosion (FAC) becomes one of the most problematic phenomena. Therefore, researches were concentrated on figuring out this problem. [1,2,3] FAC invokes the reduction of pipe thickness, which makes it susceptible to pipe rupture. The extent of pipe thickness reduction can be monitored by Non-Destructive Evaluation (NDE) techniques. During the plant overhaul, monitoring using Ultrasonic Technique (UT) is performed to inspect defects, in specific, localized area. When the plant is operated more than designed age, material deterioration becomes severe. Then, it might not be enough to monitor system integrity during overhaul only. Therefore, we suggest an advanced system for the FAC monitoring. Here, we introduce new monitoring parameters based on the recent achievement of the research on FAC mechanism, and sensors available to detect each parameter. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 2232-2238 doi:10.4028/www.scientific.net/KEM.270-273.2232

Theoretical Analysis of the Electrode Potential of the Newly Designed KCl Buffered External Ag/AgCl Electrode

Pressure-balanced external Ag/AgCl electrodes have been extensively used for electrochemical measurements in high-temperature aqueous experiments. A simple design is suggested that can be available in temperatures where polymer-based materials cannot be applied to, as an electrode body. In this design, the poly(tetrafluoroethylene) tube-ceramic tube junction at the low-temperature region and ceramic tube-ceramic plug junction jointed by brazing in the hot-temperature region play crucial roles. The potential of the KCl-buffered external Ag/AgCl electrode was characterized by estimating the thermal liquid junction potential (TLJP) occurring from the thermal diffusion of buffered ionic species existing in the filling solution. The potential of the thermoelectrochemical cell, Ag(T 0 )/AgCl(T 0 )/KCl(T 0 )/KCl(T)/SHE(T), was divided into the sum of the isothermal potentials and TLJP. The TLJP for the Soret steady state was estimated by adopting Macdonald et al.s work [J. Electrochem. Soc., 126, 1618 (1979)] and by calculating the heat of transport after Agar et al.s theory [J. Phys. Chem., 93, 2079 (1989)], and Kirkwood et al.s linear response theory [J. Chem. Phys., 18, 857 (1960)]. The potential depression at the low-temperature region, which resulted from the thermal diffusion, was also calculated. The calculated potential of the KCl-buffered external Ag/AgCl electrode together with the experimental data was presented.

Effect of defects on reaction of NiO surface with Pb-contained solution

In order to understand the role of defects in chemical reactions, we used two types of samples, which are molecular beam epitaxy (MBE) grown NiO(001) film on Mg(001) substrate as the defect free NiO prototype and NiO grown on Ni(110) single crystal as the one with defects. In-situ observations for oxide-liquid interfacial structure and surface morphology were performed for both samples in water and Pb-contained solution using high-resolution X-ray reflectivity and atomic force microscopy. For the MBE grown NiO, no significant changes were detected in the high-resolution X-ray reflectivity data with monotonic increase in roughness. Meanwhile, in the case of native grown NiO on Ni(110), significant changes in both the morphology and atomistic structure at the interface were observed when immersed in water and Pb-contained solution. Our results provide simple and direct experimental evidence of the role of the defects in chemical reaction of oxide surfaces with both water and Pb-contained solution.

Manufacturing Stress Corrosion-Cracking Tube Specimens for Eddy Current Technique Evaluation

To detect degradation in steam generator (SG) tubes, periodic inspection using nondestructive examination techniques, such as an eddy current testing, is a common practice. Therefore, it is critical to evaluate and validate the reliability of the eddy current technique for ensuring the structural integrity of the SG tubes. The eddy current technique could be evaluated by comparing the data estimated by the eddy current with the destructive examination data of field cracks, which would be both costly and labor intensive. A viable alternative to pulled tube data is to manufacture crack specimens that closely represent actual field cracks in laboratory environments. A crack manufacturing method that can be conducted at room temperature and atmospheric pressure conditions is proposed. The method was applied to manufacture different types of stress corrosion cracking (SCC) specimens: axial outer-diameter (OD) SCC for straight tubes, circumferential ODSCC and primary water SCC (PWSCC) at hydraulic expansion transition regions, and axial PWSCC at the apex and tangential regions of U-bend tubes. To help the growth of SCC into the tube, corrosive chemicals (sodium tetrathionate) and tensile stress were applied. Eddy current and destructive examination data for SCC specimens were compared with the available field crack data to determine whether those SCC specimens are representative. It was determined that the proposed method could manufacture the representative crack specimens.

Structural integrity assessment of nuclear steam generator

The problem of evaluating structural integrity of primary water reactor steam generator tubes during normal operation, main steam line break, and severe accidents is addressed. Failure models for intact tubes as well as tubes with axial and circumferential machined flaws are discussed. Two types of tube failure are identified—ligament rupture and unstable burst. The models are generalized for application to complex stress corrosion cracks that are observed in the field. Results from rupture tests of flawed steam generator tubes are discussed and used to validate the failure models.