• Nuclear Engineering and Technology
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• v.15 no.1
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• pp.11-22
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• 1983

The reliability of the Chemical and Volume Control System has been analyzed in a pressurized water reactor. The boration failure was taken to be the top event for this reliability analysis. A detailed fault tree was constructed and the minimal cut sets were derived. It was computed that the unavailability of the Chemical and Volume Control System due to boration failure was 1.497$\times$10$^{-5}$ during plant operation. It was found that the reliability of boric acid transfer pumps were the most important factors in the availability of the Chemical and Volume Control System. As expected, human errors also introduce the high system unavilability.

• Journal of the Korean Society of Safety
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• v.26 no.4
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• pp.87-95
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• 2011

The urgent VAI(Vital Area Identification) method development is required since 'The Act of Physical Protection and Radiological Emergency' that is established in 2003 requires an evaluation of physical threats in nuclear facilities and an establishment of physical protection in Korea. The KAERI(Korea Atomic Energy Research Institute) has developed the VAI methodology and VAI software called as VIPEX(Vital area Identification Package EXpert) for identifying the vital areas. This study is to demonstrate the applicability of KAERI's VAI methodology to a hypothetical facility, and to identify the importance of information of cable and piping runs when identifying the vital areas. It is necessarily needed to consider cable and piping runs to determine the accurate and realistic TEPS(Top Event Prevention Set). If the information of cable and piping runs of a nuclear power plant is not considered when determining the TEPSs, it is absolutely impossible to acquire the complete TEPSs, and the results could be distorted by missing it. The VIPEX and FTREX(Fault Tree Reliability Evaluation eXpert) properly calculate MCSs and TEPSs using the fault tree model, and provide the most cost-effective method to save the VAI and physical protection costs.

• Nuclear Engineering and Technology
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• v.30 no.6
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• pp.531-540
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• 1998

The DYLAM-3 code which overcomes the limitation of event tree/fault tree was applied to LOOP (Loss of Off-site Power) in the mid-loop operation employing HEPs (Human Error Probabilities) supplied by the ASEP (Accident Sequence Evaluation Program) and the SEPLOT (Systematic Evaluation Procedure for Low power/shutdown Operation Task) procedure in this study. Thus the time history of core uncovery frequency during the mid-loop operation was obtained. The sensitivity calculations in the operator's actions to prevent core uncovery under LOOP in the mid-loop operation were carried out. The analysis using the time dependent HEP was performed on the primary feed & bleed which has the most significant effect on core uncovery frequency. As the result, the increment of frequency is shown after 200 minutes duration of simulation conditions. This signifies the possibility of increment in risk after 200 minutes. The primary feed & bleed showed the greatest impact on core uncovery frequency and the recovery of the SCS (Shutdown Cooling System) showed the least impact. Therefore the efforts should be taken on the primary feed & bleed to reduce the core uncovery frequency in the mid-loop operation. And the capability of DYLAM-3 in applying to the time dependent concerns could be demonstrated.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.134-134
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• 2022

Streamflow forecasting plays a crucial role in water resource control, especially in highly urbanized areas that are very vulnerable to flooding during heavy rainfall event. In addition to providing the accurate prediction, the evaluation of effects and importance of the input predictors can contribute to water manager. Recently, machine learning techniques have applied their advantages for modeling complex and nonlinear hydrological processes. However, the techniques have not considered properly the importance and uncertainty of the predictor variables. To address these concerns, we applied the GA-BART, that integrates a genetic algorithm (GA) with the Bayesian additive regression tree (BART) model for hourly streamflow forecasting and analyzing input predictors. The Jungrang urban basin was selected as a case study and a database was established based on 39 heavy rainfall events during 2003 and 2020 from the rain gauges and monitoring stations. For the goal of this study, we used a combination of inputs that included the areal rainfall of the subbasins at current time step and previous time steps and water level and streamflow of the stations at time step for multistep-ahead streamflow predictions. An analysis of multiple datasets including different input predictors was performed to define the optimal set for streamflow forecasting. In addition, the GA-BART model could reasonably determine the relative importance of the input variables. The assessment might help water resource managers improve the accuracy of forecasts and early flood warnings in the basin.

• Korean Chemical Engineering Research
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• v.52 no.6
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• pp.750-754
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• 2014

Quantitative risk analysis has been performed for a pervaporation process for production of high test peroxide. Potential main accidents are explosion and fire caused by a decomposition reaction. As the target process has a laboratory scale, the consequence is considered to belong to Category 3. An event tree has been developed as a model for occurrence of a decomposition reaction in the target process. The probability functions of the accident causes have been established based on the frequency data of similar events. Using the constructed model, the failure rate has been calculated. The result indicates that additional safety devices are required in order to achieve an acceptable risk level, i.e. an accident frequency less than $10^{-4}/yr$. Therefore, a layer of protection analysis has been applied. As a result, it is suggested to introduce inherently safer design to avoid catalytic reaction, a safety instrumented function to prevent overheating, and a relief system that prevents explosion even if a decomposition reaction occurs. The proposed method is expected to contribute to developing safety management systems for various chemical processes including concentration of hydrogen peroxide.

• Journal of the Korean Institute of Gas
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• v.9 no.4 s.29
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• pp.57-61
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• 2005

The objective of paper is to carry out a comparative Quantitative Risk Assessment (QRA) of two KOGAS tank designs using a fault tree methodology, a standard 'Full Containment' tank and a 'Membrane' tank. For the membrane tank, both the initial KOGAS design and 4 modified KOGAS designs have been assessed, giving six separate cases. In this paper, the frequencies of releases are quantified using a fault tree approach. For clarity in the analysis, and to ensure consistency, all cases have been quantified using the same fault tree. Logic within the fault tree is used to select each of the cases. Full quantification of risks is often difficult, owing to a lack of relevant failure data, but the aim of this study has been to be as quantitative as possible, with full transparency of failure information. The most significant general cause of external LNG leaks is predicted to be a seismic event, which has been quantified nominally. 4modified KOGAS desiens to Prevent damage of bottom membrane panels that was shown in preparatory estimation could quantitively confirm safety improvement. According to result, the predicted frequencies of an external LNG leak for the full containment and modified membrane tanks are very similar, failures due to dropped pumps are predicted to be significantly greater for the membrane tank with thickened plate than for the full containment tank.

• Radiation Oncology Journal
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• v.31 no.2
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• pp.88-96
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• 2013

Purpose: This study identifies accident sequences from the past accidents in order to help the risk analysis application to the external radiotherapy. Materials and Methods: This study reviews 59 accidental cases in two retrospective safety analyses that have collected the incidents in the external radiotherapy extensively. Two accident analysis reports that accumulated past incidents are investigated to identify accident sequences including initiating events, failure of safety measures, and consequences. This study classifies the accidents by the treatments stages and sources of errors for initiating events, types of failures in the safety measures, and types of undesirable consequences and the number of affected patients. Then, the accident sequences are grouped into several categories on the basis of similarity of progression. As a result, these cases can be categorized into 14 groups of accident sequence. Results: The result indicates that risk analysis needs to pay attention to not only the planning stage, but also the calibration stage that is committed prior to the main treatment process. It also shows that human error is the largest contributor to initiating events as well as to the failure of safety measures. This study also illustrates an event tree analysis for an accident sequence initiated in the calibration. Conclusion: This study is expected to provide sights into the accident sequences for the prospective risk analysis through the review of experiences.

• Nuclear Engineering and Technology
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• v.24 no.1
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• pp.14-29
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• 1992

COMCAF, a computer code for the common-cause failure analysis, is developed to treat the common-cause failures in nuclear power plants. In the treatment of common-cause failures, the minimal cut sets of the system are obtained first without changing the fault-tree structure. The occurrence probabilities of the minimal cut sets are then calculated accounting for the common-cause failures among components in the same minimal cut set or in different minimal cut sets. The basic parameter model is used to model the common-cause failures between similar or identical components. For dissimilar components, the assumption of symmetry used in the basic parameter model is applied to the basic events affecting two or more components. The top event probability is evaluated using the inclusion-exclusion method. In addition to the common-cause failures of components in the same minimal cut sets, failures of components in the different minimal cut sets are also easily accounted for by this method. This study applied this common-cause failure analysis to the PWR auxiliary feedwater system. The results in the top event probability for the system are compared with those of no common-cause failures.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.34 no.5
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• pp.319-325
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• 2021

The 2011 East Japan Earthquake caused accidents at a number of nuclear power plants in Fukushima, highlighting the need for a study on the seismic safety of multiple NPP (Nuclear Power Plant) units. In the case of nuclear power plants built on a site that shows a similar seismic response, there is at least a correlation between the seismic damage of structures, systems, and components (SSCs) of nuclear power plants. In this study, a probabilistic seismic safety assessment was performed for the loss of essential power events of twin units. To derive an appropriate seismic damage correlation coefficient, a probabilistic seismic response analysis was performed. Using the external event mensuration system program, we analyzed the seismic fragility and seismic risk by composing a failure tree of multiple loss of essential power events. Additionally, a comparative analysis was performed considering the seismic damage correlation between SSCs as completely independent and completely dependent.

• Journal of the Korean Geosynthetics Society
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• v.12 no.2
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• pp.67-74
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• 2013

In this study, the prediction of slope hazard probability was performed to the study area located in Hadae-ri, Woochun-myeon, Hoengsung-gun, Gangwon Province around Youngdong express way using the computer program SHAPP ver 1.0 developed by a decision tree model. The soil samples were collected at total 10 points, and soil tests were performed to measure soil properties. The thematic maps of soil properties such as coefficient of permeability and void ratio were made on the basis of soil test results. The slope angle analysis of topography was performed using a digital map. As the prediction result of slope hazard probability, 2,120 cells among total 27,776 cells were predicted to be in the event of slope hazards. Therefore, the predicted area of occurring slope hazards may be $53,000m^2$ because the analyzed cell size was $5m{\times}5m$.