• Nuclear Engineering and Technology
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• v.56 no.6
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• pp.2092-2098
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• 2024

Probabilistic safety assessment (PSA) plays a critical role in ensuring the safe operation of nuclear power plants. In PSA, event trees are developed to identify accident sequences that could lead to core damage. These event trees are then transformed into a core-damage fault tree, wherein the accident sequences are represented by usual and complemented logic gates representing failed and successful operations of safety systems, respectively. The core damage frequency (CDF) is estimated by calculating the minimal cut sets (MCSs) of the core-damage fault tree. Delete-term approximation (DTA) is commonly employed to approximately solve MCSs representing accident sequence logics from noncoherent core-damage fault trees. However, DTA can lead to an overestimation of CDF, particularly when fault trees contain many nonrare events. To address this issue, the present study introduces a new zero-suppressed ternary decision diagram (ZTDD) algorithm that averts the CDF overestimation caused by DTA. This ZTDD algorithm can optionally calculate MCSs with DTA or prime implicants (PIs) without any approximation from the core-damage fault tree. By calculating PIs, accurate CDF can be calculated. The present study provides a comprehensive explanation of the ZTDD structure, formula of the ZTDD algorithm, ZTDD minimization, probability calculation from ZTDD, strength of the ZTDD algorithm, and ZTDD application results. Results reveal that the ZTDD algorithm is a powerful tool that can quickly and accurately calculate CDF and drastically improve the safety of nuclear power plants.

• Journal of the Korea Institute of Building Construction
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• v.10 no.3
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• pp.99-111
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• 2010

In recent years, the number of high-rise building construction projects has grown, and the number of construction safety accidents has also been increasing. Therefore, the objective of this study is to propose plans to prevent accidents by systematically organizing accident principles and developing a tree diagram for the process of safety accidents that occur in the construction industry. This study aims to show the diverse characteristics of construction accidents based on KOSHA's annual reports on safety accidents(burial, conflagration, explosion, burn) from 1993 to 2009. To achieve these objectives, in this study we first examined the risk factors for burial, conflagration, explosion, and burn. We then systematically organized the classification viewpoint of accident causes, and suggested a methodology for the rationalization of safety management through an analysis of the primary causes of accidents by work type. The results of this study based on this methodology can be divided into three areas: 1)the types of facilities were divided into 43 categories by analyzing the information of KOSHA's annual reports; 2)the causes of burial, conflagration, explosion, and burn were divided into 63types; 3)the types of work were divided into 29 categories.

• Journal of the Korean Institute of Gas
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• v.19 no.4
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• pp.49-54
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• 2015

The purpose of this study is to assess the risk of depending on the presence or absence of safety device of domestic heating and drying furnaces, by derivation and analysis of accident frequency of safety devices through FTA (Fault Tree Analysis). Installation standards are lacking in Korean for the safety device of LPG heating and drying furnace, which have a risk of explosion due to structure to trap the leaked gas. Four different safety devices were selected on the basis of NFPA and national standards for combustors of other equipment. Effects of frequency reduction in accidents were analyzed before and after installing the safety devices respectively. As a result, a minimal leakage safety device was presented for preventing damages from gas leak of domestic LPG heating and drying furnace.

• International Journal of High-Rise Buildings
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• v.3 no.4
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• pp.297-304
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• 2014

Recently, there have been some reported examples of structural collapse due to gravity, subsequent to damage from accident or an excitation that was not prepared for in the design process. A close view of new concepts, such as a redundancy and key elements, has been taken with the aim of ensuring the robustness of a structure, even in the event of an unexpected disturbance. The author previously proposed a sensitivity index of the vertical load carrying capacity to member disappearance for framed structures. The index is defined as the ratio of the load carrying capacity after a member or a set of an adjacent member disappears, to the original load carrying capacity. The member with the highest index may be regarded as a key element. The concept of bio-mimicry is being applied to various fields of engineering, and tree-shaped structures are sometimes used for the design of building structures. In this study a sensitivity analysis is applied to the irregular-framed structures such as tree-shaped structures.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.41 no.4
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• pp.171-178
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• 2018

Recently, the continuing operation of nuclear power plants has become a major controversial issue in Korea. Whether to continue to operate nuclear power plants is a matter to be determined considering many factors including social and political factors as well as economic factors. But in this paper we concentrate only on the economic factors to make an optimum decision on operating nuclear power plants. Decisions should be based on forecasts of plant accident risks and large and small accident data from power plants. We outline the structure of a decision model that incorporate accident risks. We formulate to decide whether to shutdown permanently, shutdown temporarily for maintenance, or to operate one period of time and then periodically repeat the analysis and decision process with additional information about new costs and risks. The forecasting model to predict nuclear power plant accidents is incorporated for an improved decision making. First, we build a one-period decision model and extend this theory to a multi-period model. In this paper we utilize influence diagrams as well as decision trees for modeling. And bayesian statistical approach is utilized. Many of the parameter values in this model may be set fairly subjective by decision makers. Once the parameter values have been determined, the model will be able to present the optimal decision according to that value.

• Journal of Korean Society on Water Environment
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• v.33 no.6
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• pp.689-695
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• 2017

This study was carried out to investigate the physicochemical and adsorptive characteristics of black carbon (BC) materials for cesium in case of severe nuclear accidents. The BC was prepared with a xylem of oak and pine trees incompletely combusted with different ramp rate and final temperature. Carbon (C), hydrogen (H) and oxygen (O) atomic ratios, BET, pore structure, and zeta potential were characterized for the produced BC. A low cesium concentration ($C_w{\approx}10^{-7}M$) was used for sorption batch experiments. The H/C and O/C ratios of BC decreased with the increase of final temperature, which indicates a carbonization of the wood materials regardless of ramp rate and tree species. However, SEM images showed different pore structures depending on tree species such as steric and plate-like for oak-BC and pine-BC, respectively. The greatest sorption distribution coefficients of $K_{d,Cs}{\approx}1,200{\sim}1,800L\;kg^{-1}$ were observed for the oak-BC produced at $400^{\circ}C$, while comparatively low $K_{d,Cs}$ < $100L\;kg^{-1}$ for pine-BC. In addition, the sorption capabilities of BC declined with the increase of combustion temperature up to $600^{\circ}C$, because high temperature destroyed surface functionalities with the rise of ash components in the BC. Therefore, the sorption processes of BC for radioactive cesium are predominantly controlled by final production temperature of BC as well as raw materials (e.g., tree species).

• Journal of the Korean Institute of Landscape Architecture
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• v.51 no.5
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• pp.1-12
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• 2023

This study evaluated the overturning stability of portable tripod ladders used for high-altitude work such as tree management and pruning work in landscaping construction and management. Portable tripod ladders, which are included in general mobile or portable ladders frequently used in industrial sites, are supported in a triangular support structure, not a 4-point support like common A-type Ladders. In addition, since the working height is more than twice that of a mobile or portable ladder, the possibility of an overturning accident that threatens the safety of workers with a fall accident is high. Therefore, based on the overturning stability test specified in ANSI-ASC A14.7 and EN 131-Part 7, which are related standards for about 130 types of portable tripod ladders sold and used in Korea. An equation to calculate each moment according to working height was derived. Then, each calculated moment was compared to evaluate the safety factor for overturning and stability. As a result of the overturning stability evaluation according to each standard, when the provisions of EN 131-Part 7 were applied, portable tripod ladders with 8 steps in the rear direction and 6 steps or more in the side direction were evaluated as unstable against overturning, but according to ANSI-ASC A14.7 regulations. It was evaluated that the stability against overturning was secured in all directions and number of steps.