According to the analysis of the Korean Radioactive Waste Society, saturation of nuclear power plant temporary storage is expected sequentially from 2031, and accordingly, the need for highlevel radioactive waste disposal facilities has emerged. In order to establish a repository for high-level radioactive waste, the performance and safety analysis of the repository must be conducted in compliance with regulatory requirements. For safety analysis, it needs a collection of arguments and evidence. and IAEA defined it as ‘Safety case’. The Systematic method, which derives scenarios by systematizing and combining possible phenomena around the repository, is widely used for developing Safety case. Systematic methods make use of the concept of Features, Events and Processes (FEP). FEP identifies features that affect repository performance, events that can affect a short period of time, and processes that can have an impact over a long period of time. Many countries, such as Finland, Sweden, Japan, United States, etc., are in process of licensing disposal facilities by using ‘Safety case’. And they then develop their own project-specified FEP lists and employ them for performing safety assessments. However, the systematic procedure for generating scenarios for safety evaluation is not clearly defined. According to the International Atomic Energy Agency (IAEA) Safety Standards Series (SSG- 23), the bottom-up method is an approach for conducting safety analysis using Features, Events, and Processes (FEPs). However, the process of how each FEP is utilized to establish a scenario for safety evaluation remains unclear. Additionally, there exists not only a bottom-up approach for generating scenarios using FEPs, but also a hybrid scenario development method that incorporates a top-down approach based on safety functions. Each country address scenario derivation in accordance with the adopted hybrid method. Nevertheless, a challenge arises in its application due to discrepancies between their approach and the hybrid approach specific which we are going on. Hence, this study introduces the FEP integration methodology for generating scenarios based on the hybrid scenario development method using the FEP list.

To prove the long-term safety of deep geological repository, the safety assessment is needed to ensure that the expected performance of repository satisfies the regulatory standards. Scenario development is process of analyzing events and evolutions that can directly or indirectly affect the performance of a disposal system and is a pre-step for quantitative safety assessment. Scenarios are used to identify and define cases to be assessed by numerical modeling, and cases are mainly divided into normal (also called the ‘reference’ and ‘expected evolution’) and abnormal scenarios. Mainly two approaches have been used to set up scenarios. One is a bottom-up approach that starts with features, events and processes (FEPs). This approach can analyze the evolution and events related to the performance of the disposal system in an inductive manner. The other is top-down approach that analyzes the events and evolution of disposal system, focusing on situations that may affect the safety function of the components. This approach starts with a set of intuitively predefined expected failures of safety function. Combining the two approaches is more effective in demonstrating comprehensiveness which is a main challenge of scenario analysis, and almost national radioactive waste management institutions combine top-down and bottom-up approaches for development of scenarios. An approach combining the two approaches is called a hybrid approach, and the detailed method differs from each institution and has not been determined. In this study, some work for constructing the scenario using hybrid approach was performed. Firstly, defining each component’s safety function and screening FEPs according to several rules were performed for a generic repository. Secondly, we extracted performance factors that are considered likely to affect safety functions. And lastly, we integrated FEPs correlated with performance factor to simplify the analysis. These results will be material to construct the scenario using hybrid approach.

Spent nuclear fuel temporary storage in South Korea is approximately 70% of total storage capacity as of the 4th quarter of 2022 amount is stored. In addition, according to the analysis of the Korean Radioactive Waste Society, saturation of nuclear power plant temporary storage is expected sequentially from 2031, and accordingly, the need for high-level radioactive waste disposal facilities has emerged. Globally, after the conclusion of the EU Taxonomy, for nuclear energy in order to become an ecofriendly energy, it is necessary to have a high-level radioactive waste disposal site and submit a detailed operation plan for high-level radioactive waste disposal site by 2050. Finland and Sweden have already received permission for the construction of high-level radioactive waste disposal facilities, and other countries, such as Switzerland, Japan, the United States, and Canada, are in the process of licensing disposal facilities. In order to establish a repository for high-level radioactive waste, the performance and safety analysis of the repository must be conducted in compliance with regulatory requirements. For safety analysis, it needs a collection of arguments and evidence. and IAEA defined it as ‘Safety case’. The Systematic method, which derives scenarios by systematizing and combining possible phenomena around the repository, is widely used for developing Safety case. Systematic methods make use of the concept of Features, Events and Processes (FEP). FEP identifies features that affect repository performance, events that can affect a short period of time, and processes that can have an impact over a long period of time. Since it is a characteristic of the Systematic method to compose a scenario by combining these FEP, the Systematic method is the basic premise for the development of FEP. Completeness is important for FEP, and comprehensiveness is important for scenarios. However, combining all the FEP into one scenario is time-consuming and difficult to ascertain the comprehensiveness of the scenario. Therefore, an Integrated FEP list is being developed to facilitate tracking between FEP and scenarios by integrating similar FEP. In this study, during the integrated FEP development process, a method for utilizing experts that can be used for difficult parts of quantitative evaluation and a quantitative evaluation process through the method were presented.

To obtain a license for a deep geological disposal repository for spent nuclear fuel, it is necessary to perform a safety assessment that quantifies the radiological impact on the environment and humans. One of the key steps in the safety assessment of a deep geological repository is the development of scenarios that describe how the repository evolves over the performance period and how events and processes affect performance. In the field of scenario development, demonstrating comprehensiveness is critical, which describes whether all factors that are expected to have a significant impact on the repository's performance have been considered. Mathematical proof of this is impossible. However, If the scenario development process is logical and systematic, it can support the claim that the scenario is comprehensive. Three primary approaches are being considered for scenario development: ‘Bottomup’, ‘Top-down’, and ‘Hybrid’. Hybrid approach provides a more systematic and structured process by considering both the FEPs (Features, Events, Processes) and safety functions utilized in the bottomup and top-down approaches. Many countries that develop recent scenarios prefer demonstrating scenario comprehensiveness using a hybrid approach. In this study, a systematic and structured scenario development process of a hybrid approach was formulated. Based on this, sub-scenarios were extracted that describe the phenomena occurring in the repository over the performance period, categorized by period. By integrating and screening the extracted sub-scenarios, a scenario describing the phenomena occurring over the entire period of disposal was developed.

Many countries have been developing their own FEP (Feature, Event, Process) lists to formulate radionuclide release scenarios in deep disposal repository of spent nuclear fuels and to assess the safety. The main issue in developing a FEP list is to ensure its completeness and comprehensiveness in examining all plausible scenarios of radionuclide release in a repository of interest. To this end, the NEA International FEP (IFEP) list as a generic reference have been developed and updated through long-term international collaborations. Leading countries advanced in the research field of deep geologic disposal of spent nuclear fuels have comparatively mapped their project-specific FEP (PFEP) lists with the IFEP list. Recently in 2019, NEA has published an updated version of IFEP list (ver. 3.0) which has a different classification system: the IFEP version 3.0 has the five main categories including the waste package, repository, geosphere, biosphere and external factors while the previous IFEP versions were mainly classified into the external, environmental, and contaminant factors. Most leading countries in this field, Finland and Sweden, recently succeeded to obtain the design and/or construction licenses for deep geologic disposal of spent nuclear fuel. Therefore, their PFEP lists should be good benchmark cases to the following countries. However, their PFEP lists have not comparatively mapped with the most recent version of IFEP and thus some gaps may exist in showing completeness and comprehensiveness in comparison to the IFEP version 3.0. In this study, we comparatively map the PFEP lists of Finland and Sweden to the IFEP version 3.0. The comparatively mapped PFEP list could be used as the basis for verifying the comprehensiveness and completeness of the domestic PFEP list currently under development in Korea.

Safety assessment is important for the radioactive waste repositories, and several methods are used to develop scenarios for the management of radioactive waste. The intent of the use of these scenarios is to show how the radio nuclides release can affect the safety of disposal system. It plays an essential role of providing scientific and technical information for performance assessment of safety functions. As important as scenario is, numerous studies for their own scenario development have been conducted in many countries. Scenario development methodology is basically divided into four categories: (1) judgmental, (2) fault/event-tree analysis, (3) simulation, and (4) systematic. Under numerous research, these methods have been developed in ways to strengthen the advantages and make up for the weakness. However, it was hard to find any judgmental or fault/event-tree analysis approach in recent safety assessments since they are not well-systemized and difficult to cover all scenarios. Simulation and systematic approaches are used broadly for their convenience of analyzing needed scenarios. Furthermore, several new methodologies, Process Influence Diagram (PID)/Rock Engineering System (RES)/Hybrid, were developed to reinforce the systematic approach in recent studies. Currently, a government project related to the disposal of spent nuclear fuel is in progress in Korea, and the scenario development for safety case is one of the important tasks. Therefore, it is necessary to identify the characteristics and strengths and weaknesses of the latest scenario development and analysis methods to create a unique methodology for Korea. In this paper, the existing methodologies and cases will be introduced, and the considerations for future scenario development will be summarized by considering those used in the nuclear field other than repository issues. Systematic approach, which is the mostly commonly used method, will be introduced in detail with its use in other countries at the subsequent companion paper entitled ‘Case Study for a Disposal Facility for the Spent Nuclear Fuel’.

In Korea, research on the development of safety case, including the safety assessment of disposal facility for the spent nuclear fuel, is being conducted for long-term management planning. The safety assessment procedure on disposal facility for the spent nuclear fuel heavily involves creating scenarios in which radioactive materials from the repository reach the human biosphere by combining Features, Events and Processes (FEP) that describe processes or events occurring around the disposal area. Meanwhile, the general guidelines provided by the IAEA or top-tier regulatory requirements addressed by each country do not mention detailed methods of ‘how to develop scenarios by combining individual FEPs’. For this reason, the overall frameworks of developing scenarios are almost similar, but their details are quite different depending on situation. Therefore, in order to follow up and clearly analyze the methods of how to develop scenarios, it is necessary to understand and compare case studies performed by each institution. In the previous companion paper entitled ‘Research Status and Trends’, the characteristics and advantages/disadvantages of representative scenario development methods were described. In this paper, which is a next series of the companion papers, we investigate and review with a focus on details of scenario development methods officially documented. In particular, we summarize some cases for the most commonly utilized methods, which are categorized as the ‘systematic method’, and this method is addressed by Process Influence Diagram (PID) and Rock Engineering System (RES). The lessons-learned and insight of these approaches can be used to develop the scenarios for enhanced Korean disposal facility for the spent nuclear fuel in the future.

Graphite is widely used as a reflector or moderator in nuclear reactors, and since it is exposed to high flux neutron irradiation, long-lived radioactive isotopes such as C-14 are formed. Therefore, quantitatively prediction of production amount is a very essential task for reliable radioactive waste management. In this study, considering nuclear reactions such as (􀝊, 􀟛) reaction by thermal neutrons and elastic scattering, various characteristics such as the rate of formation of C-14 and energy distribution of thermal neutrons according to the penetration depth from the graphite surface were numerically analyzed. The evaluation was carried out in consideration of the average energy of neutrons and reaction/collision cross-sectional area at given energy, and a comparative study was also performed when the thermal neutrons were in thermal equilibrium and when they were not. The numerically evaluated results were compared qualitatively with the experimental study, and methods to further increase the accuracy were also discussed.

After the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) was signed, Korea is undergoing nuclear inspection by the International Atomic Energy Agency (IAEA) as a non-nuclear-armed state. By the inspection, nuclear material measurement and management have been carried out according to safety measures. Uranium dioxide, a major component of nuclear fuel, is a material that naturally oxidizes at room temperature, yielding a volume change. In this case, it will have an impact on the management of nuclear material measurement, and a model for predicting this will be required. At room temperature, an oxide film is grown by oxygen diffusion on the surface of uranium dioxide, and if the thickness of the oxide film is predicted based on this, the volume change of uranium dioxide can also be predicted. In relation to this, Ghargozloo’s ionic diffusion oxidation model exists. Therefore, in this paper, an modified oxidation model based on Ghargozloo’s oxygen diffusion in uranium dioxide is presented and the volume change of uranium dioxide due to oxidation is predicted.

Safety evaluation of high-level radioactive waste disposal facilities including spent nuclear fuel is a very urgent and critical issue, and in order to do so, it is very important to develop a safety case that includes Feature, Event, Process (FEP) analysis, scenario development, and scenario uncertainty evaluation. In the case of Korea, the disposal of spent nuclear fuel is recognized as an unavoidable option, and in the end, Korea’s specific FEP (SFEP) development and safety evaluation according to the scenario should be conducted. Because each country’s situation and environment are different, it is necessary to develop an SFEP based on a generic FEP (International FEP). To this end, an understanding of IFEP is essential. In this study, about 1,000 major terms appearing in the OECD/NEA IFEP are classified to where each of them belongs among F, E, and P, and which FEP each word belongs to, and the correlation between the frequency of occurrence and each term is analyzed. This result will serve as a reference for the results of SFEP analysis such as POSIVA and SKB, which our research team will analyze later. In addition, each term belongs to which academic field, and the most appropriate translation for translating each term into Korean is also described.

Since July 2021, the Korea Radioactive Waste Agency has been conducting a safety case development study for the Korean deep geological repository program. The safety case includes generating scenarios in which radioactive materials from spent nuclear fuel repository reach the human biosphere by combining selective FEPs (Features, Events, and Processes). This safety case should be able to transparently explain the process in which conclusions have been drawn not only to stakeholders but also to the public by presenting safety arguments. The scenario development stage consisting of FEP screening, scenario generation, and uncertainty analysis procedures should have a database management system. Database management system was performed in countries such as Sweden, which obtained approval for the construction of spent nuclear fuel repositories, and the United States, where various preliminary research was carried out. Korea Atomic Energy Research Institute also has experience in designing and operating its own database, which has conducted preliminary research on disposal of the spent nuclear fuel. Currently, the safety assessment of the Korean spent nuclear fuel repository is in the early stages of research, but it is necessary to set up a basic framework for database design while the collection of FEP data from domestic and international preliminary studies is under development, and it is advantageous for efficient database construction and operation. Therefore, this paper presents the current status of database design considering completeness and transparency from the FEP screening stage to the scenario development stage in the safety assessment process of the Korean spent nuclear fuel repository. In this process, the functional requirements that the database should provide, the database schema capable of implementing them, and simple examples are presented together. The objectives of this database design are flexible FEPs management, high integrity and consistency, and expandability for linking with the safety case database. The FEP data to be inputted into the database includes a list of major opened FEPs, including International FEPs from Nuclear Energy Agency, which were referred for PFEPs (Project-specific FEPs), and PFEPs applied to POSIVA's Olkiluoto repository. As an additional function, queries from the database are used to visually express the process of deriving scenarios through Rock Engineering System, a widely known scenario generation methodology.

Comprehensive identification and systematic classification of all features, events and processes (FEP) that influence on the performance of a high-level radioactive waste disposal system is essential for safety assessment. Nuclear energy agency (NEA) has been developing and updating the standardized generic FEP list, so-called NEA international FEP list, which may be used as the basis to develop project-specific FEP lists to reflect diverse system and site characteristics in different countries. On the basis, Finland and Sweden have recently got licenses to construct spent nuclear fuel deep disposal facilities. Also in Korea, timely construction of a high-level radioactive waste disposal facility is an urgent issue for stable operation of nuclear power plants. For this end, a FEP list that properly considers for system and site characteristics of Korean high-level radioactive waste disposal facility needs to be developed. In this study, the most recent NEA international FEP list published in 2019 was comprehensively reviewed with focus on the structure of the classification system and the physicochemical mechanisms associated with the key elements. The obtained results will be used for the comparative analysis of domestic and oversea project-specific FEP lists and for the development of a generic FEP list relevant to Korean high-level radioactive waste disposal system.

Since the interaction between the second-phase particle and grain boundary was theoretically explained by Zener and Smith in the late 1940s, the interaction of the second-phase particle and grain boundary on the microstructure is commonly referred to as Zener pinning. It is known as one of the main mechanisms that can retard grain growth during heat treatment of metallic and ceramic polycrystalline systems. Computer simulation techniques have been applied to the study of microstructure changes since the 1980s, and accordingly, the second-phase particle–grain boundary interaction has been simulated by various simulation techniques, and further diverse developments have been made for more realistic and accurate simulations. In this study, we explore the existing development patterns and discuss future possible development directions.