Considering the non-linear behavior of structure and soil when evaluating a nuclear power plant's seismic safety under a beyond-design basis earthquake is essential. In order to obtain the nonlinear response of a nuclear power plant structure, a time-domain SSI analysis method that considers the nonlinearity of soil and structure and the nonlinear Soil-Structure Interaction (SSI) effect is necessary. The Boundary Reaction Method (BRM) is a time-domain SSI analysis method. The BRM can be applied effectively with a Perfectly Matched Layer (PML), which is an effective energy absorbing boundary condition. The BRM has a characteristic that the magnitude of the response in far-field soil increases as the boundary interface of the effective seismic load moves outward. In addition, the PML has poor absorption performance of low-frequency waves. For this reason, the accuracy of the low-frequency response may be degraded when analyzing the combination of the BRM and the PML. In this study, the accuracy of the analysis response was improved by adjusting the PML input parameters to improve this problem. The accuracy of the response was evaluated by using the analysis response using KIESSI-3D, a frequency domain SSI analysis program, as a reference solution. As a result of the analysis applying the optimal PML parameter, the average error rate of the acceleration response spectrum for 9 degrees of freedom of the structure was 3.40%, which was highly similar to the reference result. In addition, time-domain nonlinear SSI analysis was performed with the soil's nonlinearity to show this study's applicability. As a result of nonlinear SSI analysis, plastic deformation was concentrated in the soil around the foundation. The analysis results found that the analysis method combining BRM and PML can be effectively applied to the seismic response analysis of nuclear power plant structures.

Nuclear power plants, which are important national facilities, require special attention against the threat of terrorism using various methods. Among the terrorist threats, as structural damage and human casualties due to explosions continue to occur, interest in the blast load is increasing. However, domestic nuclear power plants do not have sufficient design requirements for protection against the threat of explosives. To prepare for the threat of terrorism using explosives, it is necessary to evaluate the physical protection performance of nuclear power plants against blast load, and to use this to improve protection performance and establish regulatory standards. Most of the explosion-proof designs used abroad use the empirical chart presented by UFC 3-340- 02 (DoD 2008), which does not take into account the effect of near-field explosions. When explosions occur inside nuclear power plants, near-field explosions occur in most cases. In this study, it was assumed that explosives were installed in the corridor inside nuclear power plants. A spherical TNT was placed in the middle of the corridor floor to simulate near-field explosions, and the structure response according to the weight of the TNT was evaluated. The corridor was modeled with a reinforced concrete material and the LS-DYNA program was used for analysis. For the explosion model, the Arbitrary-Lagrangian-Eulerian (ALE) analysis technique applying the advantages of the Lagrangian and Eulerian methods were used. By analyzing the pressure history and the degree of deformation of the structure according to the explosion, the degree of threat caused by the explosion was analyzed. Based on the analysis of this study, physical barriers performance database (DB) using Modeling & Simulation (M&S) will be constructed by performing sensitive analysis such as representative structure shape setting, boundary conditions, material of structures, etc. The constructed DB is expected to be used to establish regulatory standards for the physical barriers of nuclear power plants related to explosives.

Spent nuclear fuel is a very complex material because various elements such as fission products, transuranium elements and activation products are produced from initial fresh UO2 fuel after irradiation. These elements exist in UO2 with various forms and can change the structure and of physicochemical properties of UO2. These changes could provide the surface activation site that could enhance chemical reactions and corrosion processes, and would significantly affect the storage environment for long-term disposal of spent nuclear fuel. Therefore, it can be important to understand the characteristics of spent nuclear fuel to design reliable and safe geological repositories. However, it is too hard to study the characteristics of spent nuclear fuel, because it is a very complex material by itself and not easy to handle due to its radioactivity, and it is also difficult to independently understand the effects of each element. Therefore, a simulated spent nuclear fuel containing an element that forms a solid solution and epsilon particle was manufactured to understand the change in characteristics of each element. Most of the elements that form solid solutions are lanthanides or actinides and can change the structure of the UO2 lattice itself. The epsilon particles exist as metals at the grain boundaries of UO2. In this study, structural changes were measured using XRD, SEM, and Raman spectroscopy, and physical and chemical properties were also identified by measuring electrical conductivity and electrochemical properties. The results were summarized, and the effects of solid solution elements and epsilon particles on the structure and properties of UO2 matrix were compared and discussed.

원자력발전소 기기 내진설계 및 지진해석은 비연계모델을 대상으로 수행된다. 그러나 이러한 비연계해석은 실제 구조물-기기 간 상호작용 등의 실제 현상을 모사할 수 없기 때문에 연계해석에 비하여 정확하지 못한 결과를 발생시키게 된다는 한계를 가진다. 이러 한 배경 아래 이 연구는 실제 원전 격납건물 구조물 및 관련 부계통을 대상으로 질량비와 고유진동수비를 고려하여 지진 연계해석과 비연계해석을 수행하고, 이를 바탕으로 부계통에서의 응답을 비교 분석하였다. 결과적으로 지진 연계해석 결과가 비연계해석 결과보 다 대다수 작은 값을 주는 것을 확인하였다. 이러한 결과는 기존 연구인 단순한 연계모델에 대한 해석 결과와 유사하지만, 부계통 응답 차이는 훨씬 더 두드러지게 나타나는 것을 확인하였다. 또한, 이는 지진파의 입력 주파수의 영향보다는 부계통의 설치위치에 영향을 받는 것으로 확인되었다. 마지막으로 비연계 및 연계 지진해석의 차이가 부계통의 질량비가 크고, 고유진동수가 거의 일치하는 영역 에서 발생하는 이유는 이 영역에서 주계통과 부계통 동적 상호작용이 크게 나타나기 때문인 것으로 보인다.

In decommissioning a nuclear power plant, numerous concrete structures need to be demolished and decontaminated. Although concrete decontamination technologies have been developed globally, concrete cutting remains problematic due to the secondary waste production and dispersion risk from concrete scabbling. To minimize workers’ radiation exposure and secondary waste in dismantling and decontaminating concrete structures, the following conceptual designs were developed. A micro-blast type scabbling technology using explosive materials and a multi-dimensional contamination measurement and artificial intelligence (AI) mapping technology capable of identifying the contamination status of concrete surfaces. Trials revealed that this technology has several merits, including nuclide identification of more than 5 nuclides, radioactivity measurement capability of 0.1–107 Bq·g−1, 1.5 kg robot weight for easy handling, 10 cm robot self-running capability, 100% detonator performance, decontamination factor (DF) of 100 and 8,000 cm2·hr−1 decontamination speed, better than that of TWI (7,500 cm2·hr−1). Hence, the micro-blast type scabbling technology is a suitable method for concrete decontamination. As the Korean explosives industry is well developed and robot and mapping systems are supported by government research and development, this scabbling technology can efficiently aid the Korean decommissioning industry.

In this study, the well-known non-destructive acoustic emission (AE) and electrical resistivity methods were employed to predict quantitative damage in the silo structure of the Wolsong Low and Intermediate Level Radioactive Waste Disposal Center (WLDC), Gyeongju, South Korea. Brazilian tensile test was conducted with a fully saturated specimen with a composition identical to that of the WLDC silo concrete. Bi-axial strain gauges, AE sensors, and electrodes were attached to the surface of the specimen to monitor changes. Both the AE hit and electrical resistance values helped in the anticipation of imminent specimen failure, which was further confirmed using a strain gauge. The quantitative damage (or damage variable) was defined according to the AE hits and electrical resistance and analyzed with stress ratio variations. Approximately 75% of the damage occurred when the stress ratio exceeded 0.5. Quantitative damage from AE hits and electrical resistance showed a good correlation (R = 0.988, RMSE = 0.044). This implies that AE and electrical resistivity can be complementarily used for damage assessment of the structure. In future, damage to dry and heated specimens will be examined using AE hits and electrical resistance, and the results will be compared with those from this study.

콘크리트 구조물 절단에 사용되고 있는 다이아몬드 와이어 쏘가 장착된 당김형 절단 장치의 단점을 개선하여 밀기형 절단장치를 개발하였다. 개발된 밀기형 절단장치에는 먼지 집진 커버가 부착되며 마찰열을 냉각하기 위한 건식이나 습식방법을 선택할 수 있다. 개발된 절단장치의 동작특성과 집진 먼지의 누설률 측정을 실험하였다. 시험결과 원활한 동작특성을 보였으며, 먼지의 누설률은 1.7%인 것으로 나타났다. 개발된 절단장비를 사용하여 생물학적 차폐 콘크리트 절단 시 작업자의 내부 피폭선량을 평가하였다. 보수적 평가를 위해 노심 중심부분을 절단하는 경우를 가정하였다. 비방사능이 99.5 Bq·g-1인 누설 먼지로 인해 반면마스크를 착용한 작업자의 예탁유효선량은 0.25 mSv로 평가되었다. 개발된 밀기형 절단장비 사용 시 미량의 먼지 누설률로 인해 작업자의 방사선 피폭이 저감되며, 사용의 편리성으로 세부 절단 계획을 수립할 수 있어 방사성 콘크 리트 폐기물 감량에도 기여할 수 있다. 따라서 원전의 방사화된 생물학적 차폐 콘크리트를 비롯하여 철근 콘크리트 구조물 해체 작업 시 절단 장비로서 사용될 수 있을 것이다.

In this paper, we study the existing results of the structure-soil-structure interaction (SSSI) effect on seismic responses of structures and summarize important parameters. The parameters considered in this study are a combination of buildings in the power block of a nuclear power plant, the characteristics of earthquake ground motions and its direction, and the characteristics embedded under the ground. Based on these parameters, the seismic analysis model of the structures in the power block of the nuclear power plant is developed and the structure-soil-structure interaction analyses are performed to analyze the influence of the parameters on the seismic response. For all analyses, the soil-structure interaction (SSI) analysis program CNU-KIESSI, which was developed to enable large-sized seismic analysis, is used. In addition, the SSI analyses is performed on individual structures and the results are compared with the SSSI analysis results. Finally, the influence of the parameters on the seismic response of the structure due to the SSSI effect is reviewed through comparison of the analysis results.

The Gyeong-Ju earthquake in the magnitude of 5.8 on the Richter scaleoccurred in September 12, 2016. Because there are many nuclear power plants (NPP) near the epicenter of the Gyeong-Ju earthquake, the seismic stability of nuclear power plants is becoming a social problem. In order to evaluate the safety of seismically isolated NPP, the seismic response of a NPP subjected to the Gyeong-Ju earthquake was compared with those of 30 sets of artificial earthquakes corresponding to the nuclear standard design spectrum (NSDS). A 2-node model and a simple beam-stick model were used for the seismic analysis of seismically isolated NPP structures. Using 2-node model, the effect of internal temperature rise, decrease of shear stiffness, increase of lateral displacement and decrease of vertical stiffness according to nonlinear behavior of lead-rubber bearing (LRB) were evaluated. The displacement response, the acceleration response, and the shear force response of the seismically isolated nuclear containment structure were evaluated using the simple beam-stick model. It can be observed that the seismic responses of the isolated nuclear structure subjected to Gyeong-Ju earthquake is significantly less than those to the artificial earthquakes corresponding to NSDS.

This study intends to evaluate the conservativeness of the fixed-base analysis as compared to the soil-structure interaction (SSI) analysis for the seismically isolated model of a nuclear power plant in Korea. To that goal, the boundary reaction method (BRM), combining frequency-domain and time-domain analyses in a twofold process, is adopted for the SSI analysis considering the nonlinearity of the seismic base isolation. The program KIESSI-3D is used for computing the reaction forces in the frequency domain and the program MIDAS/Civil is applied for the nonlinear time-domain analysis. The BRM numerical model is verified by comparing the results of the frequency-domain analysis and time-domain analysis for the soil-structure system with an equivalent linear base isolation model. Moreover, the displacement response of the base isolation and the horizontal response at the top of the structure obtained by the nonlinear SSI analysis using BRM are compared with those obtained by the fixed-base analysis. The comparison reveals that the fixed-base analysis provides conservative peak deformation for the base isolation but is not particularly conservative in term of the floor response spectrum of the superstructure.

The Eradi Quake System (EQS) is a seismic isolation bearing system designed to minimize forces and displacements experienced by structures subjected to ground motion. The EQS dissipates seismic energy through friction of Poly Tetra Fluoro Ethylene (PTFE) disk pad. In general, a force-displacement relationship of EQS has post yield stiffness hardening during large inelastic displacement. In this study, seismic responses of seismically isolated nuclear power plant (NPP) subjected to design basis earthquake (DBE) and beyond design basis earthquakes (150% DBE and 167% DBE) are compared considering the post yield stiffness hardening effect of EQS. From the results, it can be observed that if the post-yield stiffness hardening effect of EQS is increased, the displacement response of EQS is reduced, and the acceleration and shear responses of containment structures of NPP is increased.

ASCE 4 requires that a hard stop be built around the seismic isolation system in nuclear power plants. In order to maintain the function of the isolation system, this hard stop is required to have clearance-to-stop, which should be no less than the 90th-percentile displacements for 150% Design Basis Earthquake (DBE) shaking. Huang et al. calculated clearance-to-stop by using a Latin Hypercube Sampling technique, without considering the rocking behavior of the isolated structure. This paper investigates the effects on estimation of clearance-to-stop due to 1) rocking behavior of the isolated structure and 2) sampling technique for considering the uncertainties of isolation system. This paper explains the simplified analysis model to consider the rocking behavior of the isolated structure, and the input earthquakes recorded at Diablo Canyon in the western United States. In order to more accurately approximate the distribution tail of the horizontal displacement in the isolated structure, a modified Latin Hypercube Sampling technique is proposed, and then this technique was applied to consider the uncertainty of the isolation system. Through the use of this technique, it was found that rocking behavior has no significant effect on horizontal displacement (and thus clearance-to-stop) of the isolated structure, and the modified Latin Hypercube Sampling technique more accurately approximates the distribution tail of the horizontal displacement than the existing Latin Hypercube Sampling technique.

This study investigated the influence of probabilistic variability in stiffness and nonlinearity of soil on response of nuclear power plant (NPP) structure subjected to seismic loads considering the soil-structure interaction (SSI). Both deterministic and probabilistic methods have been employed to evaluate the dynamic responses of the structure. For the deterministic method, SRPmin method given in USNRC SRP 3.7.2(2013) (envelope of responses using three shear modulus profiles of lower bound(GLB), best estimate(GBE) and upper bound(GUB)) and SRPmax method (envelope of responses by more than three ground profiles within range of GLB≤G≤GUB) have been considered. The probabilistic method uses the Latin Hypercube Sampling (LHS) that can capture probabilistic feature of soil stiffness defined by the median and the standard deviation. These analysis results indicated that 1) number of samples shall be larger than 60 to apply the probabilistic approach in SSI analysis and 2) in-structure response spectra using equivalent linear soil profiles considering the nonlinear behavior of soil medium can be larger than those based on low-strain soil profiles.

This paper presents a detailed procedure for a nonlinear soil-structure interaction of a seismically isolated NPP(Nuclear Power Plant) structure using the boundary reaction method (BRM). The BRM offers a two-step method as follows: (1) the calculation of boundary reaction forces in the frequency domain on an interface of linear and nonlinear regions, (2) solving the wave radiation problem subjected to the boundary reaction forces in the time domain. For the purpose of calculating the boundary reaction forces at the base of the isolator, the KIESSI-3D program is employed in this study to solve soil-foundation interaction problem subjected to vertically incident seismic waves. Wave radiation analysis is also employed, in which the nonlinear structure and the linear soil region are modeled by finite elements and energy absorbing elements on the outer model boundary using a general purpose nonlinear FE program. In this study, the MIDAS/Civil program is employed for modeling the wave radiation problem. In order to absorb the outgoing elastic waves to the unbounded soil region, spring and viscous-damper elements are used at the outer FE boundary. The BRM technique utilizing KIESSI-3D and MIDAS/Civil programs is verified using a linear soil-structure analysis problem. Finally the method is applied to nonlinear seismic analysis of a base-isolated NPP structure. The results show that BRM can effectively be applied to nonlinear soil-structure interaction problems.

The nuclear accident due to the recent earthquake in Japan has triggered awareness of the importance of safety with regard to nuclear power plants (NPPs). An earthquake is one of the most important parameters which governs the safety of NPPs among external events. Application of a base isolation system for NPPs can reduce the risk for earthquakes. At present, a soil-structure interaction (SSI) analysis is essential in the seismic design of NPPs in consideration of the ground structure interaction. In the seismic analysis of the base-isolated NPP, it is restrictive to consider the nonlinear properties of seismic isolation devices due to the linear analysis of the SSI analysis programs, such as SASSI. Thus, in this study, SSI analyses are performed using an iterative approach considering the material nonlinearity of the isolators. By performing the SSI analysis using an iterative approach, the nonlinear properties of isolators can be considered. The difference between the SSI analysis results without iteration and SSI with iteration using SASSI is noticeable. The results of the SSI analysis using an effective linear (non-iterative) approach underestimate the spectral acceleration because the effective linear model cannot consider the nonlinear properties of isolators. The results of the SSI analysis show that the horizontal response of the base-isolated NPP is significantly reduced.

The bumblebee, Bombus ignitus (Hymenoptera: Apidae), is a valuable natural resource that is widely utilized for greenhouse pollination in South Korea. Understanding the magnitude of genetic diversity and geographic relationships is of fundamental importance for long term preservation and utilization. As a first step, we sequenced a partial COI gene of mitochondrial DNA (mtDNA) corresponding to the “DNA barcode” region and the complete internal transcribed spacer 2 (ITS2) of nuclear ribosomal DNA from 88 individuals collected in nine South Korean localities. The complete ITS2 sequences were longest among known insects, ranging in size from 2,034 bp ~ 2,052 bp, harboring two duplicated 112-bp long repeats. The 658-bp long mtDNA sequences provided only six haplotypes with a maximum sequence divergence of 0.61% (4 bp), whereas the ITS sequences provided 84 sequence types with a maximum sequence divergence of 1.02% (21 sites). The combination of the current COI data with those of published data suggest that the B. ignitus in South Korea and China are genetically a large group, but those in Japan can be roughly separated into another group. Overall, a very high per generation migration ratio, a very low level of genetic fixation, and no discernable hierarchical population were found to exist among the South Korean populations of B. ignitus, which suggests panmixia. This finding is consistent with our understanding of the dispersal capability of the species.