Development of New Earth System Model and Investigation of the Impacts of Vegetation on Hydrological Cycle Using Developed ESM

Dongmin Kim Graduate School of UNIST 2017 국내박사

This thesis is to develop a version of the earth system model (ESM) with the interactive vegetation parameterization scheme and investigate the influences and feedback processes driven by terrestrial vegetation on climate and global hydrological cycle. Previous observation and modeling studies suggested that the temporal variation of atmospheric CO2 concentration depends largely on the carbon uptake by terrestrial biosphere, compared with that by oceanic counterpart. In the state-of-the-art ESMs, the parameterization uncertainty in the carbon cycle between land and atmosphere is still large, which tend to produce a significant spread in the model simulations for the ambient CO2 amount and its secular trend. The huge spread of CO2 in turn causes a wide spread in the simulated climate through vegetation-climate feedbacks, which make their future climate projection less accurate and less reliable. One aspect of the parameterization uncertainties lies in the nitrogen process, a critical process coupled with the carbon cycle by limiting supplement of nutrients both to subsurface soil organic matter (SOM) and plants at the surface, which is either absent or represented poorly in current ESMs. A few ESMs with a primitive version of the carbon-nitrogen (C-N) coupling exhibit a significant model deficiency such as the underestimation in the carbon pools or storages in the vegetation and sub-surface soil systems and substantially weak carbon uptake, whereas the other models without C-N coupling tend to overestimate them. The interaction and feedbacks between vegetation and climate should be, therefore, understood based on a more improved understanding of the C-N cycle and its representation in the ESMs. Before developing new ESM, the model intercomparision using 11 CMIP5 ESMs has been done first to evaluate the overall representation of the terrestrial biogeochemical cycle and the model dependences. Using the MODIS satellite estimates, the study validates the simulation of gross primary production (GPP), net primary production (NPP), and the carbon use efficiency (CUE) depending on plant function types (PFTs). The models show noticeable deficiencies from MODIS in the simulation of horizontal patterns of GPP and NPP as well as large simulation differences, although their multi model ensemble (MME) mean represents realistic global mean value and the spatial distributions. Larger model spreads in GPP and NPP than those in surface temperature and precipitation suggest that the simulation difference in terrestrial carbon cycle is largely attributed to the uncertainties in the dynamic vegetation model parameterizations. The models also exhibit large differences in the simulation for CUE geographically and at the change of dominant PFTs, primarily due to the differences in parameterization. While the MME of CUE shows strong dependence on surface temperature, the observed CUE from MODIS shows more complex and non-linear sensitivity. To developing new ESM, this study uses the Geophysical Fluid Dynamics Laboratory Earth System Model version 2 (GFDL-ESM2M) as a base model for the further development. GFDL-ESM2M has a full capability of interactive vegetation parameterizations, and with decent simulations in the long-term climatology and climate variability compared with other ESMs in the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5). The model has comprehensive carbon cycle both for the terrestrial and aquatic environments, yet with no C-N coupling. In order to implement the C-N cycle in the model, this study replaces the Land Model version 3 of GFDL-ESM2M with the Community Land Model version 4 (CLM4). The latter land model is the most recent version in the development suits by NCAR, with more detailed vegetation types and biogeochemistry as well as the interactive C-N parameterizations. However, the Community Earth System Model (CESM) with CLM4 has a well-known bias of the pronounced underestimation in the terrestrial carbon uptake. This is one of the major motivations in this research. The developed ESM named as UNIST-ESM shows the reasonable simulation skill for climate conditions and terrestrial carbon fluxes. The features of distribution of carbon cycle is depended onto land surface models and the features of spatial distribution of climate conditions is dominant to characteristics for atmospheric model in UNIST-ESM. In terrestrial carbon cycle, UNIST-ESM still has systematic bias for simulating GPP in the globe. The underestimation (overestimation) of GPP over high latitude region (tropics) in UNIST-ESM is major deficiencies of simulation of terrestrial carbon cycle. For improvement of deficiencies of terrestrial carbon cycle in UNIST-ESM, this study develops a new parameterization method for determining Q10 by considering the soil respiration dependence on soil temperature and moisture obtained by multiple regression. This study further investigates the impacts of the new parameterization on the global carbon cycle budget. Our results show that non-uniform spatial distribution of Q10 tends to enhance heterogeneous anomaly of soil respiration comparing with the control simulation with uniform Q10. Moreover, it tends to improve the simulation of observed relationship between soil respiration and soil temperature and moisture, particularly over cold and dry regions. The new parameterization improves the simulation of gross primary production (GPP) by reducing bias in the global mean comparing with the FLUXNET-MTE observation data. Besides, GPP over high latitudes is significant increased by about two times from the control simulation. The realistic Rs and GPP simulation induced to represent carbon balance between release at the subsurface and uptake at the surface over terrestrial biosphere reasonably. Overall, enhanced heterogeneous temperature sensitivity in the soil decomposition process in the model showed the improvement of production and respiration. For evaluation of vegetation feedback on the climate, this study investigated the vegetation-climate feedback and impacts of vegetation change on the hydrological cycle in the East Asian monsoon region using comprehensive and developed UNIST-ESM. The intensity of East Asian summer monsoon (EASM) in the future increase due to enhance moisture flux at the surface by increased vegetation in EA. However, low-level relative humidity in the future decreases due to relatively increasing temperature comparing with enhanced moisture increase over EA. This process induces to suppress the formation of low-level cloud in the future. Therefore, enhanced incoming solar radiation over China regions is occurred by suppressed low-level cloud in the future climate. On the other hand, the exception of additional anthropogenic heating in future climate, the role of increase vegetation on the variation of EASM is opposite signal from the future climate scenarios. In terms of local hydrological process, the increased vegetation induces incareasing evapotranspiration and surface moisture flux over vegetated regions. This increment of moisture source from increased vegetated area tends to increase formation of low-level cloud due to small perturbation of temperature increase in this simulation. Enhanced amount of low-level cloud tends to decrease incoming solar radiation at the surface due to reflecting by clouds. It emphasized surface cooling in the increased vegetation regions. This surface cooling also affects to depress local vertical updraft over China where has large plants. Suppressed vertical updraft makes atmosphere stabilization over land area. The convective precipitation is affected to decrease by depressed vertical updraft associated with cooling temperature. In the focus of the large-scale circulation, the cooling temperature over vegetated area tends to decrease the land-sea contrast. Thermally more uniformed land and ocean surface make weak low-level circulation and meridional moisture transport over EA regions. This study suggested that the roles of vegetation to climate variation and hydrological cycle are homogenous not only location but also environmental climate conditions.

Effect of Gd-alloyed Neutron Absorber on Thermal Performance of a Spent Fuel Cask

Hee-Jae Lee Graduate School of UNIST 2017 국내석사

A spent fuel cask must be designed to provide safety functions. In particular, in order to maintain the sub-criticality safety, different neutron absorbers have been used for the spent nuclear fuel management system. Although BORAL is the most used neutron absorber, several problems have been reported and it has some potential problems for long term storage of spent nuclear fuels. Recently, a Gd-alloyed duplex stainless steel demonstrated the possibility of fabrication and under development for an advanced neutron absorber and structural material as well. In this study, the effect of Gd alloyed neutron absorber on thermal performance of a spent fuel cask has been studied. The thermal properties of Gd-alloyed DSS was measured using the specimens provided by KITEC. The effect of Gd-alloyed duplex stainless steel adoption was analyzed for the reference casks, KSC-1 and KORAD-21. The analysis method was verified by comparing the analysis results with the reported values. Their system with Gd-alloyed DSS should remove decay heat with passive cooling. The KSC-1 and KORAD-21 casks were modeled with a 2D axis-symmetry condition and 3D symmetry condition, respectively, using ANSYS FLUENT v17.0. Based on the verified method, thermal performance of KORAD-21 which adopts Gd-alloyed DSS was analyzed. The maximum fuel cladding temperature with Gd-alloyed DSS exceeded allowable temperature of 400 ℃ and it could affect the fuel integrity. Therefore, basket wall thickness and disk thickness were optimized to enhance thermal performance. When the basket wall thickness was reduced, the gap between the basket surface and disk square holes was consequently increased. The increased gap enhanced the upward flow of helium and it improved the decay heat removal. Additionally, disk thickness was optimized to 60mm from 20mm. The increased heat-conducting surface enhanced the conduction heat transfer. As a result, UNIST-version design of the KORAD-21 cask was developed with the 5.0mm basket wall thickness and 60mm disk thickness. Thermal performance of UNIST design cask satisfied thermal requirements in normal operation.

Development of a Second Harmonic Dual Mode Gyrotron for Generation of Orbital Angular Momentum Beam

Ashwini Sawant Graduate School of UNIST 2019 국내박사

This thesis presents the idea of the existence of the orbital angular momentum in gyrotron modes and includes the design process of a gyrotron source to demonstrate the generation of the OAM modes. The cavity modes of gyrotrons are rotating modes with no electrical field intensity along the axis. These modes are analogous to an energy packet revolving around an axis and generating an orbital angular momentum of order (l). We have numerically derived the existence of the orbital angular momentum in the gyrotron modes, which is proportional to the azimuthal index of the mode. We have reported the existence of the orbital angular momentum in gyrotron through a low power test carried out by a quasi-optical mode converter to generate a TE6,2 mode. We found out that the phase pattern of the emitted modes is having spiral patterns, which is the easiest way to identify the order of the low power OAM modes. We also presented a new modified phase-retrieval technique to determine the topological charges of the high-power OAM beams. The OAM property of the high-power gyrotron modes can be useful in wireless communication for enhancing the channel capacity and transmission distance. Development of a 190 GHz, dual mode gyrotron has been carried out for this work in UNIST. This gyrotron can operate on TE8,3 and TE11,2 mode to resonate at 188.5 GHz and 190.5 GHz respectively. The gyrotron is having an axial output, where the cavity modes are directly emitted into the free space through an overmoded tapered waveguide section to reduce its divergence. The gyrotron modes are second harmonic modes and generates around 30 kW of power with an efficiency of around 18%. To carry out the design simulations of the gyrotron, a time-dependent numerical code named as UNIST Gyrotron Design Tool (UGDT) has been developed. Second harmonic gyrotron modes are always susceptible to be suppressed by fundamental mode excitation. To avoid this fundamental mode excitation, the cavity opted for the gyrotron is a perturbed cavity with two sinusoidal perturbation at the straight section of the cavity to excite stable higher order axial modes. After a rigorous analysis of the perturbed cavity, a cavity with two sinusoidal perturbations and a raised cosine up-taper has been manufactured. Developed 190 GHz OAM gyrotron has been tested and its performance analysis has been carried out. The generated modes have been recognized based on the frequency measurements using heterodyne frequency measurement system. However, the preliminary results concluded in this thesis suggests the excitation of the desired TE8,3 mode have been observed along with the spurious fundamental mode excitation due to the alignment problem of the vacuum tube in the superconducting magnet. We confirmed this issue with the simulation results and will resolve it in our future work.

Evaluation of deposition samplers for polycyclic aromatic hydrocarbon monitoring

정근식 Graduate School of UNIST 2014 국내석사

Polycyclic aromatic hydrocarbons (PAHs) are an important environmental concern due to their carcinogenic and toxic properties. Moreover, several PAHs have been classified into mutagenic compounds. The sources of PAHs are classified into anthropogenic and natural sources such as incomplete combustion of fossil fuel, biomass burning, industrial boilers, and forest fires etc. Emitted PAHs can distribute both gaseous phase and particulate phase. As a large amount of PAHs emitted to the atmosphere is deposited to the land or sea, atmospheric deposition of PAHs is a significant phenomenon. There have been many studies of atmospheric deposition of PAHs, and several kinds of deposition samplers have been used. However, the results with different deposition samplers cannot be directly compared, because their performance was not fully compared or calibrated. Besides, the deposition fluxes and deposition velocities measured in the previous studies showed large variations even though the same samplers were used. Therefore, a through evaluation of different types of deposition samplers are required to clearly understand the process of atmospheric deposition of PAHs. In this study, various information on widely used deposition samplers (basic theories, structures, and advantages/disadvantages) was collected. Then, the performance of four types of deposition samplers was evaluated. In addition, a high volume air sampler was used to investigate the ambient levels of PAHs and deposition velocities. The samplers were deployed on the roof of the engineering building #2 in the Ulsan National Institute of Science and Technology (UNIST) from May 2013 to October 2013. Four types of deposition samplers (dry deposition sampler (DDP), velcro deposition sampler, resin deposition sampler, and bulk deposition sampler) were used. Particulate and gaseous PAHs were indivisually collected by a high volume air sampler once a week. The target compounds in this study were thirteen US-EPA priority PAHs except naphthalene, acenaphthene, and acynaphthylene. After sample extraction and clean up using silica gel columns, a gas chromatograph/mass spectrometer (GC/MS) was used for PAH qualification and quantification. In order to further interpretate the source-recepor relationship of PAHs, houly data of criteria air pollutants (CO, SO2, NO2, O3, and PM10) measured nearby UNIST was acquired from the Ulsan Institute of Health and Environment (UIHE). The ranges of SO2 were 1.3–9.9 ppb (Mean: 3.8 ppb) and 3.4–15.0 ppb (Mean: 6.8 ppb) in Samnam and Mugeo air pollution monitoring stations. The SO2 level showed seasonal variations related with fossil fuel consumption and wind direction. The level of SO2 in Mugeo was higher than that of Samnam because Mugeo is more influenced by vehicles and has more urban characteristics. The ranges of NO2 were 6.7–21.9 ppb (Mean: 12.6 ppb) in Samnam and 4.7–43.1 ppb (Mean: 24.0 ppb) in Mugeo. The ranges of O3 were 14.1–63.9 ppb (Mean: 34.0 ppb) in Samnam and 13.5–60.5 ppb (Mean: 27.4 ppb) in Mugeo. The ranges of PM10 were 15.1–57.1 μg/m3 (Mean: 30.9 μg/m3) in Samnam and 14.6–74.8 μg/m3 (Mean: 38.4 μg/m3) in Mugeo. The sampling site at UNIST was confirmed to be seasonally influenced by air pollution sources in Samnam and Mugeo according to geographical positions and major wind directions. The levels of gaseous PAHs were 1.10–7.02 ng/m3 (Mean: 4.23 ng/m3), and those of particulate PAHs were 0.85–2.82 ng/m3 (Mean: 1.67 ng/m3). Namely, the total PAH concentrations were 1.95–9.84 ng/m3 (Mean: 5.91 ng/m3) during the sampling period. The variation of PAH concentrations was not large, but they increased in spring and fall. The sampling site in this study is located in a rural area. Therefore, it was assumed that the sampling site was affected by criteria air pollutants and PAHs emitted from urban sources. Deposition fluxes and velocities of PAHs were calculated based on the amount of deposited PAHs and ambient air concentrations. The ranges of deposition fluxes of PAHs collected by DDP, velcro, resin, and bulk samplers were 4.55–15.13 μg/m2/d (Mean: 8.89 μg/m2/d), 13.14–30.92 μg/m2/d (Mean: 22.04 μg/m2/d), 7.72–55.41 μg/m2/d (Mean: 28.43 μg/m2/d), and 32.72–49.44 μg/m2/d (Mean: 40.13 μg/m2/d), respectively. This result indicates that deposition fluxes derived from different types of deposition samplers do not coincide and they should not be directly compared. DDP could not collect high molecular weight-PAHs mostly associated with fine particles. Therefore, the performance of the velcro sampler as a dry deposition sampler was better than that of DDP. As a bulk (dry/wet deposition) sampler, the performance of the Resin and bulk samplers was similar. On the basis of this result, further studies were suggested to improve the calculation of deposition fluxes and velocities.

Evaluation of deposition sampler for polycyclic aromatic hydrocarbon monitoring

Kuen-Sik Jung Graduate School of UNIST 2014 국내석사

Polycyclic aromatic hydrocarbons (PAHs) are an important environmental concern due to their carcinogenic and toxic properties. Moreover, several PAHs have been classified into mutagenic compounds. The sources of PAHs are classified into anthropogenic and natural sources such as incomplete combustion of fossil fuel, biomass burning, industrial boilers, and forest fires etc. Emitted PAHs can distribute both gaseous phase and particulate phase. As a large amount of PAHs emitted to the atmosphere is deposited to the land or sea, atmospheric deposition of PAHs is a significant phenomenon. There have been many studies of atmospheric deposition of PAHs, and several kinds of deposition samplers have been used. However, the results with different deposition samplers cannot be directly compared, because their performance was not fully compared or calibrated. Besides, the deposition fluxes and deposition velocities measured in the previous studies showed large variations even though the same samplers were used. Therefore, a through evaluation of different types of deposition samplers are required to clearly understand the process of atmospheric deposition of PAHs. In this study, various information on widely used deposition samplers (basic theories, structures, and advantages/disadvantages) was collected. Then, the performance of four types of deposition samplers was evaluated. In addition, a high volume air sampler was used to investigate the ambient levels of PAHs and deposition velocities. The samplers were deployed on the roof of the engineering building #2 in the Ulsan National Institute of Science and Technology (UNIST) from May 2013 to October 2013. Four types of deposition samplers (dry deposition sampler (DDP), velcro deposition sampler, resin deposition sampler, and bulk deposition sampler) were used. Particulate and gaseous PAHs were indivisually collected by a high volume air sampler once a week. The target compounds in this study were thirteen US-EPA priority PAHs except naphthalene, acenaphthene, and acynaphthylene. After sample extraction and clean up using silica gel columns, a gas chromatograph/mass spectrometer (GC/MS) was used for PAH qualification and quantification. In order to further interpretate the source-recepor relationship of PAHs, houly data of criteria air pollutants (CO, SO2, NO2, O3, and PM10) measured nearby UNIST was acquired from the Ulsan Institute of Health and Environment (UIHE). The ranges of SO2 were 1.3–9.9 ppb (Mean: 3.8 ppb) and 3.4–15.0 ppb (Mean: 6.8 ppb) in Samnam and Mugeo air pollution monitoring stations. The SO2 level showed seasonal variations related with fossil fuel consumption and wind direction. The level of SO2 in Mugeo was higher than that of Samnam because Mugeo is more influenced by vehicles and has more urban characteristics. The ranges of NO2 were 6.7–21.9 ppb (Mean: 12.6 ppb) in Samnam and 4.7–43.1 ppb (Mean: 24.0 ppb) in Mugeo. The ranges of O3 were 14.1–63.9 ppb (Mean: 34.0 ppb) in Samnam and 13.5–60.5 ppb (Mean: 27.4 ppb) in Mugeo. The ranges of PM10 were 15.1–57.1 μg/m3 (Mean: 30.9 μg/m3) in Samnam and 14.6–74.8 μg/m3 (Mean: 38.4 μg/m3) in Mugeo. The sampling site at UNIST was confirmed to be seasonally influenced by air pollution sources in Samnam and Mugeo according to geographical positions and major wind directions. The levels of gaseous PAHs were 1.10–7.02 ng/m3 (Mean: 4.23 ng/m3), and those of particulate PAHs were 0.85–2.82 ng/m3 (Mean: 1.67 ng/m3). Namely, the total PAH concentrations were 1.95–9.84 ng/m3 (Mean: 5.91 ng/m3) during the sampling period. The variation of PAH concentrations was not large, but they increased in spring and fall. The sampling site in this study is located in a rural area. Therefore, it was assumed that the sampling site was affected by criteria air pollutants and PAHs emitted from urban sources. Deposition fluxes and velocities of PAHs were calculated based on the amount of deposited PAHs and ambient air concentrations. The ranges of deposition fluxes of PAHs collected by DDP, velcro, resin, and bulk samplers were 4.55–15.13 μg/m2/d (Mean: 8.89 μg/m2/d), 13.14–30.92 μg/m2/d (Mean: 22.04 μg/m2/d), 7.72–55.41 μg/m2/d (Mean: 28.43 μg/m2/d), and 32.72–49.44 μg/m2/d (Mean: 40.13 μg/m2/d), respectively. This result indicates that deposition fluxes derived from different types of deposition samplers do not coincide and they should not be directly compared. DDP could not collect high molecular weight-PAHs mostly associated with fine particles. Therefore, the performance of the velcro sampler as a dry deposition sampler was better than that of DDP. As a bulk (dry/wet deposition) sampler, the performance of the Resin and bulk samplers was similar. On the basis of this result, further studies were suggested to improve the calculation of deposition fluxes and velocities.

Generation of Windowed Multipole Library for on-the-fly Doppler Broadening in UNIST In-house Monte Carlo Code

Azamat Khassenov Graduate School of UNIST 2018 국내석사

This thesis presents the generation and application results of the windowed multipole (WMP) library in UNIST in-house Monte Carlo code (MCS). The given library is used in the on-the-fly Doppler broadening of microscopic cross section at the temperature of the interest during a Monte Carlo simulation. This feature is one of the major requirements for multi-physics simulations; in other words, coupled calculations of neutron transport, thermal-hydraulics, and fuel performance codes. A windowed multipole library significantly reduces the memory necessary for cross section storage with a reasonable increase in the total Monte Carlo simulation time. The ENDF/B-VII.1 neutron induced library contains information on 430 isotopes, where 331 contain resonance parameters for the resolved resonance region. In the first step, the parameters from the evaluated nuclear data file were converted into corresponding rigorous multipole parameters, which can be directly used in temperature dependent cross section reconstruction. Further, in order to reduce the cross section generation time the energy window concept was applied with an accuracy threshold of 0.1 % relative difference in comparison to the reference cross section. The accuracy and effect of the generated windowed multipole library on the MCS simulation was evaluated using fresh and burned PWR fuel pins, and assemblies.

SOCIAL SUPPORT FOR INTERNATIONAL STUDENTS : A STUDY ON A KOREAN COLLEGE CAMPUS

Munkhgerel Enkhjargal Graduate School of UNIST 2018 국내석사

This study examined UNIST international students’ satisfaction rate in both academic and social life on campus. International students face many challenges such as culture shock, language difficulty, homesickness and difficulty in academic life. In order to overcome these challenges, students have adopted resources that mainly are derived from the university to overcome these challenges. Thus, as demonstrated in this study, having a better understanding of these students’ academic challenges, university faculty and staff can recognize students’ needs and effectively offer supportive campus resources and services. This study also suggests that some preparations need to be made by the university that will embrace international students upon their arrival. I have two hypotheses to explain the international students’ overall experience and perception about their college life. Results demonstrate that assistance in transition, student support, and professor support have significant positive effects on university’s service for international students and overall experience.

LEARNING HOW STUDENTS ARE LEARNING IN PROGRAMMING LAB SESSIONS

SeyedMoein Mirhosseini Graduate School of UNIST 2018 국내석사

Programming lab sessions help students learn to program in a practical way. Although these sessions are typically valuable to students, it is not uncommon for some participants to fall behind throughout the sessions and leave without fully grasping the concepts covered during the session. In my thesis, I will be presenting LabEX, a system for instructors to understand students' progress and learning experience during programming lab sessions. LabEX utilizes statistical techniques that help distinguishing struggling students and understand their degree of struggle. LabEX also helps instructors to provide in-situ feedback to students with its real-time code review. LabEX was evaluated in an entry-level programming course taken by more than two hundred students in UNIST, establishing that it increases the quality of programming lab sessions.

Improving online mental health screening with a customized interim report to overcome mental health literacy and privacy concerns

Yoon Heo Graduate School of UNIST 2020 국내석사

Introduction University student’s mental health is one of the important problems because students with severe mental illness are increasing as well as many students are suffering from mental disease such as depression, anxiety disorder and eating disorder. The World Health Organization World Mental Health Surveys showed that only 6.7-23.1% of students took treatment for their mental disorders. Students are reluctant to visit the healthcare center due to barriers such as stigma, lack of awareness, lack of time, and perceived need. Awareness and perceived needs are related to Mental Health Literacy (MHL). MHL has been highlighted as a solution to the main factors of barriers to mental health services especially attitude or stigma, and help-seeking efficacies. The service to be introduced in this study is intended to be developed with a focus on the following four aspects. First, students do not need to check one symptom, but rather to examine the comprehensive disorders. Second, it should help to enable students to self-profile and increase awareness because it visualizes the results with short comments after finishing the screening. Third, it requests to evaluate students’ own mental health and compare the perceived mental health to the screening results. Lastly, students can access the screening without any personal information, which will reduce public stigma. Methods The screening was open January 15, 2020 for two weeks, and it was sent via Healthcare center e-mail asking UNIST students to use the screening in both Korean and English. There are four steps. The first two steps (introduction and first overall screening) are mandatory and the other steps (second advanced screening and service registration) are optional. Once the first overall screening is complete, users can apply for service or proceed with the second screening. If the user goes through the second screening process, the user can also apply for the service on the results page when the second screening is completed. Results A total of 189 students participated in the screening. Of 38 (20%) were excluded for two reasons. First, I excluded those who used the English version screening (27 participants) due to between the Korean and English version screening. In the English screening, there are only simple comments, but no second screening and no application link for service. Second, an item for validation is placed at the end question of each screening e.g., PHQ-9 originally consists of 9 questions, and the last one included the question such as “Are you a man?” for validation. So, PHQ-9 was changed into 10 questions. There were 11 people who were inconsistent compared with validation question and intro page questionnaires, and they were excluded. Therefore, the analysis was conducted on 151 (79%) people who finished the basic screening. Of the 151 people, four applied for service directly and 65 used the second screening. Discussion The screening service checks overall mental health as a first step. Traditional online screening usually checks only one symptom, but it checks overall because lots of students have a barrier to mental health service by awareness. I assume that it is better to check overall for students who have low mental health literacy. In addition, it also can download results. The service suggests the second advanced screening and offline service registration. It also helps to resolve stigma, which is often experienced in mental health, because the screening does not request any information about privacy when using it. When each step is complete, offline services are also linked, so students will get enough information about mental health just by using the service. Forty percent of participants followed the proposed service and the reason for finishing the first overall screening was also not negative reason (83%). Furthermore, the service has even increased interest in mental health by employing ideas of similar concepts in other fields. So, the study showed that this service is valuable enough because it has created a well-founded service by adopting similar ideas from other fields.

Phenomenological Study of Fuel Relocation Behavior and Formation of Debris Bed in Metal-fueled Sodium-cooled Fast Reactor

Hyo Heo Graduate School of UNIST 2019 국내박사

Metallic fuel has been proposed as one of fuel types in sodium-cooled fast reactor (SFR). The metallic fuel has advantages in terms of safety issues. Especially, it has been known that severe accident for metal-fueled SFR might be terminated earlier due to its safety characteristics. Hypothetical core disruptive accidents (HCDA) are considered as the accident which causes fuel melting and cladding failure in SFR. When cladding bleach occurs in initiating phase of HCDA, molten fuel is relocated debris is formed in fuel assembly. Severe accident consequences are determined depending on these phenomena. Thus, it is necessary to study fuel behavior and solidification phenomena in severe accident. However, the studies related to the metallic fuel in severe accident condition were partly performed. There is still a lack of fundamental knowledge about the severe accident phenomena and related physics. Therefore, the present study focuses on fuel relocation behavior and debris formation phenomena, which is in specific accident scenario, with experimental approaches. The fuel relocation behavior has been not studied clearly, especially in initiating phase of HCDA. To understand the phenomena and provide physical insights, there were visual studies for various experimental conditions. The studies were performed as a parametric study so key parameters affecting the fuel relocation behavior were selected. The parameter discussed in the experiment are coolant boiling, channel condition, initial melt temperature, and initial melt ejection pressure etc. In the fuel relocation experiments, simulants were used instead of metallic fuel. Wood’s metal and gallium were used as the simulant for the metallic fuel in most of experiment. Since the fuel behaviors are driven by force balance, Froude and Weber numbers were compared to investigate similarity. High speed video camera was used to observe the fuel relocation behavior and visual analysis methods were applied on the parameter study. There were two steps to conduct the visualization experiments. Firstly, possible fuel relocation behaviors were observed in small-scaled experimental facility. The experimental facility is called UNIST molten core and coolant interaction experimental facility (UNICORN)-B (baby). From the experiments, it was found that sodium boiling could be powerful driving force for fuel dispersal regardless of channel condition. Then, there were additional visual studies using UNICORN-C (child) which was established considering actual scale of fuel assembly in metal-fueled SFR. As a result, it was clarified that debris bed formation was highly dependent on the fuel relocation behaviors. Although fuel is ejected into coolant channel, core could be cooled from hydrodynamic point of view. If high porous debris bed is formed, decay heat would be removed using natural circulation flow. UNICORN-A (adult) was established to simulate actual fuel ejection condition in HCDA. Simultaneous occurrence of unprotected transient over power (UTOP) and unprotected loss-of-flow (ULOF) event was selected as a target scenario. A severe accident code SAS4A was used to calculate initial experimental conditions. Radiographic images were obtained to analyze melt relocation behavior. The experimental results show that melt was not swept out from active core region. Mass fraction of frozen melt was investigated along axial distance. It showed melt was radially dispersed rather than the axial melt dispersal. In addition, most of melt was frozen near cladding failure point where the melt was directly ejected out. Debris bed seemed like agglomerated shape leading to local flow blockage. Since debris bed porosity affects to pressure drop and flow rate of fuel assembly, the porosity was evaluated after the experiment. The porosity was measured with two methods; classical method and post-processing method of radiograph image. The classical method was performed using volume fraction of test section before and after the fuel relocation experiment. Debris bed was measured 0.89 of its porosity with these methods. It could ensure that core has coolability by natural circulation. Thus, it was verified that there was a possibility of early termination of severe accident. The characteristics of debris bed are mainly determined from physical form of individual debris. Especially, debris bed porosity is affected by debris morphology. In severe accident condition in SFR, metallic fuel had ligament-like shape of debris. This morphology made characteristics of debris bed porosity. The metal fuel had relatively high debris bed porosity. In previous research group, quenching experiments were conducted using molten metal droplet. The debris morphology was analyzed quantitatively. It was insufficient to qualitatively investigate the debris morphology. The present study was performed with experimental works based on theoretical model regarding rapid solidification. It is suggested that debris morphology is attributed to freezing point and instantaneous contact interface temperature between melt and coolant. From rapid solidification experiments, it was shown that high porous debris bed would be formed above porosity of 0.83 based on the reactor accident conditions in ULOF.