• Nuclear Engineering and Technology
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• v.49 no.4
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• pp.817-824
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• 2017

Residual stress is a critical element in determining the integrity of parts and the lifetime of welded structures. It is necessary to estimate the residual stress of a welding zone because residual stress is a major reason for the generation of primary water stress corrosion cracking in nuclear power plants. That is, it is necessary to estimate the distribution of the residual stress in welding of dissimilar metals under manifold welding conditions. In this study, a cascaded support vector regression (CSVR) model was presented to estimate the residual stress of a welding zone. The CSVR model was serially and consecutively structured in terms of SVR modules. Using numerical data obtained from finite element analysis by a subtractive clustering method, learning data that explained the characteristic behavior of the residual stress of a welding zone were selected to optimize the proposed model. The results suggest that the CSVR model yielded a better estimation performance when compared with a classic SVR model.

• Nuclear Engineering and Technology
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• v.41 no.6
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• pp.765-774
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• 2009

For over 30 years, investigations of the thermohydraulic behavior of pressurized-water reactors under accident conditions have been carried out in the PKL test facility at AREVA NP in Erlangen, Germany. The PKL facility models the entire primary side and significant parts of the secondary side of a of pressurized water reactor at a height scale of 1:1. Volumes, power ratings and mass flows are scaled with a ratio of 1:145. The experimental facility consists of four primary loops with circulation pumps and steam generators (SGs) arranged symmetrically around the reactor pressure vessel (RPV). The investigations carried out encompass a very broad spectrum from accident scenario simulations with large, medium, and small breaks, over the investigation of shutdown procedures after a wide variety of accidents, to the systematic investigation of complex thermohydraulic phenomena. The PKL tests began in the mid 1970s with the support of the German Research Ministry. Since the mid 1980s, the project has also been significantly supported by the German PWR operators. Since 2001, 25 partner organizations from 15 countries have taken part in the PKL investigations with the support and mediation of the OECD/ NEA (Nuclear Energy Agency). After an overview of PKL history and a short description of the facility, this paper focuses on the investigations carried out since the beginning of the international cooperation, and shows, by means of some examples, what insights can be derived from the tests.

• Nuclear Engineering and Technology
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• v.53 no.1
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• pp.351-356
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• 2021

Korean nuclear power plants (NPPs) have various radiation protection programs to attain radiation exposure as low as reasonably achievable (ALARA). In terms of ALARA, this paper provides a comprehensive overview of administrative dose control for occupationally-exposed workers in Korean NPPs. In addition to dose limits, administrative dose constraints are implemented to resolve an inequity of radiation exposure in which some individuals in NPPs receive relatively higher doses than others. Occupational dose constraints in Korean NPPs are presented in this paper with the background of how those values were determined. For pressurized water reactors, 80% and 90% of the annual average limit for an effective dose, 20 mSv/y, are set as the primary and secondary dose constraints, respectively. Pressurized heavy water reactors (PHWRs) have also established the primary and secondary dose constraints corresponding to 70% and 80% of the effective dose limit, and additional constraints for tritium concentration are provided to control internal exposure in PHWRs. Follow-up measures for exceeding these administrative dose constraints are also introduced compared to exceeding the dose limits. Finally, analysis results of dose distributions show how the implementation of administrative dose constraints impacted the occupational dose distributions in Korean NPPs during the years 2009-2018.

• Nuclear Engineering and Technology
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• v.54 no.9
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• pp.3225-3234
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• 2022

Alloy 690 tubing has been shown to be highly resistant to primary water stress corrosion cracking (PWSCC). Nevertheless, predicting the failure by PWSCC in Alloy 690 SG tubes is indispensable. In this work, a Bayesian-based statistical approach is proposed to predict the occurrence of failure by PWSCC in Alloy 690 SG tubing. The prior distributions of the model parameters are developed based on the prior knowledge or information regarding the parameters. Since Alloy 690 is a replacement for Alloy 600, the parameter distributions of Alloy 600 tubing are used to gain prior information about the parameters of Alloy 690 tubing. In addition to estimating the model parameters, analysis of tubing reliability is also performed. Since no PWSCC has been observed in Alloy 690 tubing, only right-censored free-failure life of the tubing are available. Apparently the inference is sensitive to the choice of prior distribution when only right-censored data exist. Thus, one must be careful in choosing the prior distributions for the model parameters. It is found that the use of non-informative prior distribution yields unsatisfactory results, and strongly informative prior distribution will greatly influence the inference, especially when it is considerably optimistic relative to the observed data.

• Proceedings of the Korea Water Resources Association Conference
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• 2023.05a
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• pp.175-175
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• 2023

Detection and monitoring of wildfires are essential for limiting their harmful effects on ecosystems, human lives, and property. In this research, we propose a novel method running in the Google Earth Engine platform for identifying and characterizing burnt regions using a hybrid of Sentinel-1 (C-band synthetic aperture radar) and Sentinel-2 (multispectral photography) images. The 2022 Uljin wildfire, the severest event in South Korean history, is the primary area of our investigation. Given its documented success in remote sensing and land cover categorization applications, we select the Random Forest (RF) method as our primary classifier. Next, we evaluate the performance of our model using multiple accuracy measures, including overall accuracy (OA), Kappa coefficient, and area under the curve (AUC). The proposed method shows the accuracy and resilience of wildfire identification compared to traditional methods that depend on survey data. These results have significant implications for the development of efficient and dependable wildfire monitoring systems and add to our knowledge of how machine learning and remote sensing-based approaches may be combined to improve environmental monitoring and management applications.

• Journal of Korean Society on Water Environment
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• v.34 no.1
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• pp.121-131
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• 2018

Domestic sewage treatment plants (STPs) consume about 0.5 % of total electric energy produced annually, which is equivalent to 207.7 billion Korean won per year. To minimize the energy consumption and as a way of mitigating the depletion of energy sources, the sewage treatment strategy should be improved to the level of "energy positive". The core processes for the energy positive sewage treatment include A-stage for energy recovery and B-stage for energy-efficient nitrogen removal. The integrated process is known as the A/B-process. In A-stage, chemically enhanced primary treatment (CEPT) or high rate activated sludge (HRAS) processes can be utilized by modifying the primary settling in the first stage of sewage treatment. CEPT utilizes chemical coagulation and flocculation, while HRAS applies returned activated sludge for the efficient recovery of organic contents. The two processes showed organic recovery efficiencies ranging from 60 to 70 %. At a given recovery efficiency of 80 %, 17.3 % of energy potential ($1,398kJ/m^3$) is recovered through the anaerobic digestion and combustion of methane. Besides, anaerobic membrane bioreactor (AnMBR) can recover 85% of organic contents and generate $1,580kJ/m^3$ from the sewage. The recovered energy is equal to the amount of energy consumption by sewage treatment equipped with anaerobic ammonium oxidation (ANAMMOX)-based B-stage, $810{\sim}1,620kJ/m^3$. Therefore, it is possible to upgrade STPs as efficient as energy neutral. However, additional novel technologies, such as, fuel cell and co-digestion, should be applied to achieve "energy positive" sewage treatment.

• Journal of Korean Society on Water Environment
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• v.36 no.4
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• pp.322-331
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• 2020

By introducing chemically enhanced primary treatment (CEPT) in the first stage of sewage treatment, organic matter in sewage can be effectively recovered. Because CEPT sludge contains a high biodegradable organic matter in volatile solids (VS), it is feasible to convert the collected CEPT sludge into energy through anaerobic digestion. This study examined the properties and biochemical methane potential (BMP) of the CEPT sludge obtained from a sewage treatment plant located in an ocean area. The CEPT sludge contains a VS content of 37,597 mg/L, which is higher than that of excessive sludge (ES), i.e., 33,352 mg-VS/L. In the methane generation reaction, the lag period was as short as 1 to 2 days. The BMP for the CEPT sludge was 0.57 ㎥-CH₄/kg-VS_removed which is better than that of ES, i.e., 0.36 ㎥-CH₄/kg-VS_removed. Unfortunately, the CEPT sludge showed a high salinity as 0.56~0.75% probably due to the saline sewage. Due to the salinity, repeated BMP testing in a sequencing batch reactor showed significantly low methane production rates and BMPs. Also, the ES showed a strongly reduced BMP when the salinity was adjusted from 0.20 to 0.70% by NaCl. The ES mixture with higher CEPT content showed a better BMP, which is suitable for co-digestion. Besides, anaerobic digestion for 100% CEPT sludge can be a considerable option instead of co-digestion.

• Journal of Korean Society on Water Environment
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• v.23 no.1
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• pp.19-26
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• 2007

The lab-scale BNR processes fed with Municipal Wastewater Before or After Primary Clarifier (MWBPC or MWAPC) were operated to observe the behavior of particle organic matter in terms of nitrification and denitrification efficiency. As a result of the fractionation of the COD from MWBPC or MWAPC using an aerobic respirometric serum bottle reactor, the total mass of biodegradable organic matter from MWBPC is about 52% greater than the mass from MWAPC. Batch reactors were operated to observe the effect of the Particulate Organic Matter (POM) on substrate utilization for denitrification. Although the consumption of POM for denitrification was observed, the increment of the Specific Denitrification Rate (SDNR) was not great. In terms of the effect of POM on nitrification at different HRTs, activate sludge reactors were operated to determine the optimal HRT when MWBPC and MWAPC were fed relatively. All reactors showed a great organic matter removal efficiency. Reactors fed with MWAPC had obtained the nitrification efficiency above 90% when the HRT of 4 hr, at least, was maintained, while reactors fed with MWBPC had same efficiency when the HRT longer than 5 hr was kept. Three parallel $A^2/O$ systems fed with MWBPC or MWAPC relatively were operated to investigate the effects of POM on BNR processes with varying the HRT of an anoxic reactor. For all systems, the efficiency of organic matter removal and denitrification, respectively, was great and about the same. In case of denitrification efficiency, system with MWAPC had 1.5% lower than system with MWBPC at the same HRT of anoxic reactor of 2 hr, and the increasing the HRT of the anoxic reactor by 1 hr in systems fed with MWBPC resulted in a 3.5% increment. The denitrification rate was similar while the consumption of organic matter in systems fed with MWBPC was higher than system fed with MWBPC. It suggests that POM in MWBPC was not be used significantly as a substrate for denitrification in system with the HRT of 3 hr of an anoxic reactor.

• Journal of Korean Society on Water Environment
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• v.20 no.3
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• pp.290-295
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• 2004

In this study, ozone was adopted for the reduction and stabilization of waste sludge from the municipal sewage treatment plant. The waste sludge used in the experiments was primary sludge, excess sludge and a mixture of the two. The sludge cells and flocs were disrupted by ozonation resulting in the reduction of TSS and VSS concentrations. In the case of the primary sludge with a concentration of 20 gTS/L, the TSS and VSS concentrations were reduced 28%, 33% and the TCOD concentration was reduced 20% respectively. The consumption of ozone was $0.18gO_3/gSS$. In the case of the excess sludge with a concentration of 7.5 gTS/L, the TSS and VSS concentrations were reduced 37%, 41%, and the TCOD concentration was reduced 19% respectively. The consumption of ozone was $0.33gO_3/gSS$. In the case of the mixed sludge of 9.3 gTS/L, the TSS and VSS concentrations were reduced by 45%, 53%, and the TCOD was reduced 26% respectively. The desirable consumption of ozone was $0.27gO_3/gSS$. As ozonation proceeded, the level of SCOD increased due to the release of intracellular materials. However TCOD was reduced due to mineralization resulting from the transformation of organic materials to inorganic materials.

• Journal of Korean Society on Water Environment
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• v.23 no.4
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• pp.498-503
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• 2007

The performance of elutriated acid fermentation to evaluate the effects of microwave irradiation and pH control as pretreatment was investigated. The MW pH 7 reactor which was used the pretreated primary sludge as microwave irradiation was operated at pH 7 and $35^{\circ}C$. The EAF pH 9 reactor was operated at pH 9 and $35^{\circ}C$ without pretreatment. The SCOD and VFAs production rate were 0.17 gSeOD/gVSrem. and 0.27 gVFAs as COD/gVSrem. in MW pH 7 reactor, 0.16 gSCOD/gVSrem. and 0.24 gVFAs as COD/gVSrem. in EAF pH 9 reactor, respectively. VS and Volume reduction were 54% and 48% in MW pH 7 reactor, 54.6% and 36% in EAF pH 9 reactor, respectively. A comparison of the microwave irradiation and controlled pH in elutriated acid fermentation showed that the former is more efficient in SCOD and VFAs production and it rises to slightly higher reduction in the volume of the sludge. In addition, E. coli. was not detected in the wasting sludge of MW pH 7 reactor. Based on the results, microwave irradiation appeared to be one of the viable options for generating class A sludge. According to the batch tests, sequencing batch test which was used the pretreated primary sludge as microwave was performed at pH 7 and $35^{\circ}C$, SCOD production was 0.16 gSCOD/gVSrem., VS reduction and volume reduction were 64% and 63%, respectively.