Korea Institute Of Nuclear Safety Research Articles

Analysis of control rod driving mechanism nozzle rupture with loss of safety injection at the ATLAS experimental facility using MARS-KS and TRACE

Korea Atomic Energy Research Institute (KAERI) has operated an integral effect test facility, the Advanced Thermal-Hydraulic Test Loop for Accident Simulation (ATLAS), with reference to the APR1400 (Advanced Power Reactor 1400) for tests for transient and design basis accidents simulation. A test for a loss of coolant accident (LOCA) at the top of the reactor pressure vessel (RPV) had been conducted at ATLAS to address the impact of the loss of safety injections (LSI) and to evaluate accident management (AM) actions during the postulated accident. The experimental data has been utilized to validate system analysis codes within a framework of the domestic standard problem program organized by KAERI in collaboration with Korea Institute of Nuclear Safety. In this study, the test has been analyzed by using thermal-hydraulic system analysis codes, MARS-KS 1.5 and TRACE 5.0 Patch 6, and a comparative analysis with experimental and calculation results has been performed. The main objective of this study is the investigation of the thermal-hydraulic phenomena during a small break LOCA at the RPV upper head with the LSI as well as the predictability of the system analysis codes after the AM actions during the test. The results from both codes reveal that overall physical behaviors during the accident are predicted by the codes, appropriately, including the excursion of the peak cladding temperature because of the LSI. It is also confirmed that the core integrity is maintained with the proposed AM action. Considering the break location, a sensitivity analysis for the nodalization of the upper head has been conducted. The sensitivity analysis indicates that the nodalization gave a significant impact on the analysis result. The result emphasizes the importance of the nodalization which should be performed with a consideration of the physical phenomena occurs during the transient.

방사능 누출 상황 발생 시 과학기술정보통신부의 역할에 관한 연구

The Ministry of Science & ICT is a central agency in South Korea that oversees the Korea Atomic Energy Research Institute and the Korea Institute of Nuclear Safety as affiliated institutions. Furthermore, the ministry possesses a research reactor and is responsible for nuclear safety research. South Korea established a crisis management manual specifically for nuclear safety and nuclear safety field is coordinated by the Nuclear Safety and Security Commission, with relevant ministries holding operational manuals for nuclear safety crisis response. In this study, we analyze how central agencies with operational manuals for nuclear safety define the role of the Ministry of Science & ICT. Furthermore, through in-depth interviews with institutions related to nuclear safety, we extract the roles, tasks, and limitations that the Ministry of Science & ICT currently holds in the event of a radiological event. Finally, this study suggest the considerations such as establishing legal grounds, designating departments for roles specified in the standard manual, and exploring additional collaborative roles for the Ministry of Science & ICT’s crisis management manual.

Activity concentrations and radiological hazard assessments of 226Ra, 232Th, 40K, and 137Cs in soil samples obtained from the Dongnam Institute of Radiological & Medical Science, Korea

The radioactivity concentration of environmental radionuclides was analyzed for soil and sand at eight locations within a radius of 255 m centered on the Dongnam Institute of Radiological & Medical Science (DIRAMS), Korea. The average activity concentrations of 40K, 137Cs, 226Ra, and 232Th were 661.1 Bq/kg-dry, 0.9 Bq/kg-dry, 21.9 Bq/kg-dry, and 11.1 Bq/kg-dry, respectively. The activity of 40K and 137Cs was lower than the 3-year (2017–2019) average reported by the Korea Institute of Nuclear Safety, respectively. Due to the nature of granite-rich soil, the radioactivity of 40K was 0.6-fold higher than in other countries, while 137Cs was in the normal fluctuation range (15–30 Bq/kg-dry) of the concentration of radioactive fallout from nuclear tests. The activity of 226Ra and 232Th was lower than in Korean soils reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). The average activity concentrations of 232Th and 40K for the soil and sand samples from DIRAMS were within the range specified by UNSCEAR in 2000. The radium equivalent activity and internal and external hazard index values were below the recommended limits (1 mSv/y). These radionuclide concentration (226Ra, 232Th, 40K, and 137Cs) data can be used for regional environmental monitoring and ecological impact assessments of nuclear power plant accidents.

Analysis of 4-inch cold leg top-slot break LOCA in ATLAS experimental facility using MARS-KS

Abstract Korea Atomic Energy Research Institute (KAERI) has operated an integral effect test facility, the Advanced Thermal-Hydraulic Test Loop for Accident Simulation (ATLAS) with reference to the Advanced Power Reactor 1400 MW (APR1400) for transient and design basis accidents (DBAs) simulation. An experiment for a 4-inch cold leg top-slot break was performed at ATLAS to resolve a safety issue that the loop seal reformation (LSR) of APR1400 could lead to the increase of peak cladding temperature. In addition, the experimental data has been utilized to validate system codes within a framework of domestic standard problem (DSP) program organized by KAERI in collaboration with Korea Institute of Nuclear Safety (KINS). In this study, the experiment has been analyzed by thermal-hydraulic system analysis code, MARS-KS 1.5 and a comparison with experimental and calculation results has been performed. Since the top-slot break is not a typical break geometry for safety analyses, this study aims at examining the applicability of MARS-KS to the top-slot break accident where the LSR occurs repeatedly. The results revealed that overall physical behavior during the accident was predicted by the code, appropriately. MARS-KS showed the excursion of the peak cladding temperature because of the LSR as in experiment. It has been confirmed that the core integrity was maintained because the temperature excursion by the LSR was not large enough to alter the acceptance criteria. In addition, it is presented the results of the sensitivity analysis of parameter that affect the figure of merits.

Public Perception and Communication Patterns Pertaining to Nuclear Power in Korea: Focusing on the Transition Period from Pro-nuclear to De-nuclear Policy

Background: An effective communication strategy for reducing conflicts in South Korea has been designed through the analysis of public perception and communication variables on nuclear power under the conditions of rapidly changing nuclear power policies.Materials and Methods: This study conducted both qualitative research through group discussions based on social psychology and quantitative research through surveys.Results and Discussion: Nuclear power plant (NPP) area residents in favor of nuclear power indicated higher levels of communication, safety perception, and contribution than those against it. NPP area residents trusted the civilian expert groups (18.3%) and local government (17.3%) the most, while metropolitan city residents trusted the Nuclear Safety and Security Commission and the Korea Institute of Nuclear Safety (20.7%) the most. In determining nuclear power policy, both the NPP area residents (18.1%) and metropolitan city residents (17.1%) prioritized safety, health, and the environment. While metropolitan city residents thought that energy security and economic growth (16.4%) were important, NPP area residents thought the current issue of spent fuel rods (14.1%) to be important.Conclusion: It is necessary for the nuclear power industry to have and actively implement communication and conflict resolution strategies based on the patterns obtained in the study results.

Development of an Evaluation Methodology for Loss of Large Area induced from Extreme Events with malicious origin

Event of loss of large area (LOLA) induced from extreme external event at multi-units nuclear installation has been emerged a new challenges in the realm of nuclear safety and regulation after Fukushima Dai-Ichi accident. The relevant information and experience on evaluation methodology and regulatory requirements are rarely available and negative to share due to the security sensitivity. Most of countries has been prepared their own regulatory requirements and methodologies to evaluate impact of LOLA at nuclear power plant. In Korea, newly amended the Nuclear Safety Acts requires to assess LOLA in terms of EDMG (Extended Damage Mitigation Guideline). Korea Institute of Nuclear Safety (KINS) has performed a pilot research project to develop the methodology and regulatory review guidance on LOLA at multi-units nuclear power plant since 2014. Through this research, we proposed a methodology to identify the strategies for preventive and mitigation of the consequences of LOLA utilizing PSA techniques or its results. The proposed methodology is comprised of 8 steps including policy consideration, threat evaluation, identification of damage path sets, SSCs capacity evaluation and identification of mitigation measures and strategies. The consequence of LOLA due to malevolent aircraft crash may significantly susceptible with analysis assumptions including type of aircraft, amount of residual fuel, and hittable angle and so on, which cannot be shared overtly. This paper introduces a evaluation methodology for LOLA using PSA technique and its results. Also we provide a case study to evaluate hittable access angle using flight simulator for two types of aircrafts and to identify potential path sets leading to core damage by affected SSCs within damaged area.

A small-scale realistic inter-laboratory accident dosimetry comparison using the TL/OSL from mobile phone components

An inter-laboratory comparison of a small-scale field experiment was conducted in the framework of the Korea retrospective dosimetry network (KREDOS) to verify the reliability and compatibility of thermoluminescence and optically stimulated luminescence techniques for accident dosimetry. Three institutes participated in this comparison, including the Korea Atomic Energy Research Institute, the Korea Institute of Nuclear Safety and the University of Salzburg. Two adult CIRS torso phantoms were used as virtual subjects exposed to a high-activity Ir-192 source, developing both homogeneous and inhomogeneous exposures in a shielded enclosure. The target materials for dose estimation were mobile phone components, such as resistors, display glasses, and chip cards. LiF:Mg,Cu, Si thermoluminescent (TL) detectors were used as reference dosimeters. The mobile phones were attached to the chest, hip, and thighs of the anthropomorphic phantoms. The measured doses ranged from 100 mGy to 2 Gy, depending on the position of the mobile phone on the phantom. The proficiency of the dose assessment was tested at three laboratories. Using the applied protocols, the local dose distribution on the display glasses was unclear, but the distribution around the body was confirmed.

Statistical Indoor Exclusion Zone Analysis by Investigating Electromagnetic Fields inside a Nuclear Power Plant

This article investigates a statistical indoor exclusion zone (EZ) that can be efficiently applied to a nuclear power plant (NPP) by examining electromagnetic fields inside the actual NPP. To obtain the statistical indoor EZ, the indoor environment of the Korea Institute of Nuclear Safety (KINS) simulator room is modeled using the Wireless InSite commercial electromagnetic simulation software. The indoor space around the transmitting antenna is classified as multiple observation regions, and the EZ boundaries of each region are independently defined within each separate observation region. The EZ boundaries are then obtained using a margined regression model, which makes it possible to determine a reasonable boundary of the statistical indoor EZ. To validate the statistical indoor EZ, the received power inside the KINS simulator room is then measured, which agrees well with the simulated results. The results demonstrate that the proposed statistical indoor EZ can be properly obtained not only from the simulation data but also from the measurement data.

Preliminary study on the detection efficiency and estimation of minimum detectable activity for a NaI(Tl)-based seawater monitoring system

To monitor radioactivity levels in seawater Korea Institute of Nuclear Safety has installed and been operating 18 NaI(Tl)-based gamma detectors around the Korean peninsula. This study was conducted to estimate the detector efficiency and MDA of 137Cs in seawater for measurement situations. For this purpose, experiments in the air and a water tank, and Monte Carlo simulations were performed using a seawater radioactivity monitor system with 3 in. × 3 in. NaI(Tl) scintillation detector.In the geometry reliability assessment using certified reference materials in a disc source, the validity of simulations was obtained by comparing measurement and Monte Carlo simulation results. The FWHM of the seawater radioactivity monitor were obtained from the results of the water tank measurement for applying a Gaussian Energy Broadening (GEB) option to Monte Carlo N-Particle (MCNP) radiation transport code. In addition, the detection efficiency of 40K in the water tank was measured and compared with the Monte Carlo simulation results in order to estimate the MDA and the detection efficiency of the seawater radioactivity monitoring system. For the based condition of water tank, 40K concentration in water tank was controlled to 10.13±0.18 Bq/L, similar to that of real marine.In laboratory water tank experiments, the detection efficiency of the radioactivity monitor for 40K was measured at 0.184±0.005 cps/(Bq/L), the Monte Carlo simulations showed the similar result of 0.182±0.002 cps/(Bq/L), and the detection efficiency of 137Cs was estimated to be 0.224±0.009 cps/(Bq/L) from the simulations. For 3h measurement in the water tank based condition, the MDA of 137Cs was estimated to be 0.077±0.003 Bq/L. Future research will include detailed studies for detector sizes and seawater salinities.

Electromagnetic Field Propagation and Indoor Exclusion Zone Analysis in a Nuclear Power Plant

This article investigates the electromagnetic field strengths and the resulting indoor exclusion zones (EZs), taking into account the actual indoor nuclear power plant (NPP) environment. To obtain the accurate indoor EZ, a real indoor environment based on the simulator room of Korea Institute of Nuclear Safety was included in the numerical simulation. The maximum distance d of the indoor EZs depending on the position and polarization of the emitter is 1 to 2.1 times longer than the distances d of the free-space EZs. To validate the simulated indoor EZs, electromagnetic field strengths along the arc line with an arbitrary distance from the emitter are measured in the NPP, which are in good agreement with the simulation results. The result demonstrates that the indoor EZ should be investigated before applying the wireless system in the NPP because the indoor EZ and free space EZ can be significantly different.

Review of audit calculation activities on the applicability of CFD software to nuclear safety problems

The Korea Institute of Nuclear Safety (KINS) is tasked with the technical supporting for approving the safety of specific components or design modifications in nuclear power plant. When a licensee submits analysis to the Nuclear Safety & Security Commission (NSSC) for acceptance, KINS staff reviews this analysis and sometimes conducts independent audit calculations. Though recently licensing applications supported by using Computational Fluid Dynamics (CFD) software are increasing for nuclear safety problems, there is no CFD software which obtains a licensing from the domestic regulatory body until now. Additionally, there is no domestic regulatory guideline for the comprehensive evaluation of CFD software. Therefore, from the nuclear regulatory perspective, it is necessary to perform the systematic assessment and prepare the domestic regulatory guideline for checking whether valid CFD software is used for nuclear safety problems. In this paper, regulatory audit calculation activities on the applicability of CFD software to nuclear safety problems was briefly explained.

Korean Round-Robin Tests Result for New International Program to Assess the Reliability of Emerging Nondestructive Techniques

Abstract The Korea Institute of Nuclear Safety, as a representative organization of Korea, in February 2012 participated in an international Program to Assess the Reliability of Emerging Nondestructive Techniques initiated by the U.S. Nuclear Regulatory Commission. The goal of the Program to Assess the Reliability of Emerging Nondestructive Techniques is to investigate the performance of emerging and prospective novel nondestructive techniques to find flaws in nickel-alloy welds and base materials. In this article, Korean round-robin test results were evaluated with respect to the test blocks and various nondestructive examination techniques. The test blocks were prepared to simulate large-bore dissimilar metal welds, small-bore dissimilar metal welds, and bottom-mounted instrumentation penetration welds in nuclear power plants. Also, lessons learned from the Korean round-robin test were summarized and discussed.

History and Organizations for Radiological Protection.

International Commission on Radiological Protection (ICRP), an independent international organization established in 1925, develops, maintains, and elaborates radiological protection standards, legislation, and guidelines. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) provides scientific evidence. World Health Organization (WHO) and International Atomic Energy Agency (IAEA) utilise the ICRP recommendations to implement radiation protection in practice. Finally, radiation protection agencies in each country adopt the policies, and adapt them to each situation. In Korea, Nuclear Safety and Security Commission is the governmental body for nuclear safety regulation and Korea Institute of Nuclear Safety is a public organization for technical support and R&D in nuclear safety and radiation protection.

Raim – A model for iodine behavior in containment under severe accident condition

Following a severe accident in a nuclear power plant, iodine is a major contributor to the potential health risks for the public. Because the amount of iodine released largely depends on its volatility, iodine's behavior in containment has been extensively studied in international programs such as International Source Term Programme-Experimental Program on Iodine Chemistry under Radiation (EPICUR), Organization for Economic Co-operation and Development (OECD)-Behaviour of Iodine Project, and OECD-Source Term Evaluation and Mitigation. Korea Institute of Nuclear Safety (KINS) has joined these programs and is developing a simplified, stand-alone iodine chemistry model, RAIM (Radio-Active Iodine chemistry Model), based on the IMOD methodology and other previous studies. This model deals with chemical reactions associated with the formation and destruction of iodine species and surface reactions in the containment atmosphere and the sump in a simple manner. RAIM was applied to a simulation of four EPICUR tests and one Radioiodine Test Facility test, which were carried out in aqueous or gaseous phases. After analysis, the results show a trend of underestimation of organic and molecular iodine for the gas-phase experiments, the opposite of that for the aqueous-phase ones, whereas the total amount of volatile iodine species agrees well between the experiment and the analysis result.

SU‐E‐T‐427: Feasibility Study for Evaluation of IMRT Dose Distribution Using Geant4‐Based Automated Algorithms

Purpose:For intensity‐modulated radiation therapy (IMRT) treatment planning validation using Monte Carlo (MC) simulations, a precise and automated procedure is necessary to evaluate the patient dose distribution. The aim of this study is to develop an automated algorithm for IMRT simulations using DICOM files and to evaluate the patient dose based on 4D simulation using the Geant4 MC toolkit.Methods:The head of a clinical linac (Varian Clinac 2300 IX) was modeled in Geant4 along with particular components such as the flattening filter and the multi‐leaf collimator (MLC). Patient information and the position of the MLC were imported from the DICOM‐RT interface. For each position of the MLC, a step‐ and‐shoot technique was adopted. PDDs and lateral profiles were simulated in a water phantom (50×50×40 cm3) and compared to measurement data. We used a lung phantom and MC‐dose calculations were compared to the clinical treatment planning used at the Seoul National University Hospital.Results:In order to reproduce the measurement data, we tuned three free parameters: mean and standard deviation of the primary electron beam energy and the beam spot size. These parameters for 6 MV were found to be 5.6 MeV, 0.2378 MeV and 1 mm FWHM respectively. The average dose difference between measurements and simulations was less than 2% for PDDs and radial profiles. The lung phantom study showed fairly good agreement between MC and planning dose despite some unavoidable statistical fluctuation.Conclusion:The current feasibility study using the lung phantom shows the potential for IMRT dose validation using 4D MC simulations using Geant4 tool kits.This research was supported by Korea Institute of Nuclear safety and Development of Measurement Standards for Medical Radiation funded by Korea research Institute of Standards and Science. (KRISS‐2015‐15011032)

Numerical simulations of missile impacts on reinforced concrete plates: IRIS-2010/2012 benchmark project

This paper describes the procedures and results of the works done by the Korea Institute of Nuclear Safety (KINS) as a participant of the IRIS-2010 and 2012 benchmark projects, which have been organized by OECD/NEA IAGE working group. Within the scope of the IRIS-2012 project, uniaxial and tri-axial concrete tests were performed and the results were supplied. With these material test data, impact simulations of IRIS-2010 experiments (Punching P1 and Bending B1) were re-performed to improve the accuracy of the simulation results and to reduce the computation time.Numerical impact simulations were performed using an explicit dynamic code, LS-DYNA, and the initial FE model was updated through numerous parametric studies. Considering the symmetry of the structure, an updated model was constructed for ¼ of the structure, and the stiffness and mass of the supporting structure was considered. Loading function with a fictitious loading plate was applied instead of modeling projectiles to reduce the computation time. Three concrete models, Concrete damage rel. 3 (*MAT_072R3), Winfrith concrete (*MAT_084-085), and CSCM concrete (*MAT_159), were used for the impact simulations and the results were compared.

Status of and prospects for proton beam utilization at the KOMAC

The 1st proton beam utilization experiment using the 100-MeV proton accelerator at the Korea Multi-purpose Accelerator Complex (KOMAC) was successfully conducted on July 22, 2013. Forty-eight proposals for the second half year’s beam times were submitted, and 37 proposals were selected. The beam time was allocated by the PAC (Program Advisory Committee), which was composed of experts recommended by the KOPUA (Korea Proton Accelerator User Association). For proton beam utilization, the KOMAC constructed two target rooms, TR23 and TR103, for the 20-MeV and 100-MeV proton beam last year, and an operation license was issued by the KINS (Korea Institute of Nuclear Safety) in July, before the beam service started. Proton beams can be utilized in various application fields, such as nano-, bio-, space, semiconductor, and nuclear technologies, medical sciences, nuclear physics, and so on. Especially, the demands for high-dose irradiation with proton beams are increasing for nuclear- and fusion-material tests and radio-isotope production. In this paper, we review the achievements during last ten years and report the status of and the future prospects for beam utilization of the 100-MeV proton accelerator at the KOMAC.

원자로 내부유동 예측을 위한 상용 전산유체역학 소프트웨어 성능 비교 연구

전산유체역학 소프트웨어의 일부 개발자 및 사용자는 최신 전산유체역학 소프트웨어가 최소한 단상 원자로 안전문제는 타당하게 해석할 수 있을 것으로 생각하지만 계산 결과에는 여전히 제한성 및 불확실도가 존재한다. 현재 한국원자력안전기술원에서는 규제관점에서 원자로 안전문제에 대한 상용 전산유체역학 소프트웨어의 성능평가를 수행하고 있다. 본 연구에서는 축소 APR+ 원자로 내부유동 해석시 다공성 모델을 적용한 상용 전산유체역학 소프트웨어의 예측 성능을 평가하기 위해 ANSYS CFX R.14 및 FLUENT R.14 에 탑재된 수치모델을 이용하여 계산을 수행하였다. 결론적으로 전산유체역학 소프트웨어에 따라 축소 APR+ 원자로 내부유동 분포는 국부적으로 차이가 발생하였다. 비록 제한된 수의 측정치로 인해 상용 전산유체역학 소프트웨어간 예측성능을 평가하기에는 다소 한계가 있으나 CFX R.14 가 FLUENT R.14 에 비해 상대적으로 타당한 예측결과를 제시하였다. 한편 적용된 차분법의 차이로 인해 동일한 격자에 대해 FLUENT R.14 가 CFX R.14 에 비해 상대적으로 많은 계산 메모리를 필요로 하였다. 따라서 대용량 병렬 계산시 가용한 계산 자원에 적합한 전산유체역학 소프트웨어가 선정되어야 한다.

Is It Safe to Eat Fish?

After the Fukushima Daiichi's meltdown in 2011, it has been still plagued with the mishaps (1). Especially the recent leakages of highly radioactive water into the ocean from storage tanks lead to further increase of public frustration over the radiation risk and hazards (1). Nevertheless, frustration to radiation is mounting that the radiation leakage could lead to dangerous contamination of all seafood. The anxiety and distrust for Japanese marine fisheries produce several unbelievable rumors which were rampantly distributed through the borderless internet and social networking service (SNS). In response to the Japan Fukushima Daiichi nuclear accident, the Korea Institute of Nuclear Safety (KINS) is conducting the intensified environmental radioactivity monitoring program in addition to the national environmental radioactivity monitoring and marine radioactivity investigation tasks (2). The monitoring results reported that a concentration level of radioactive cesium-137 (Cs-137) from seawater was undetectable to 0.00187 Bq/L in first quarter of 2013, and is undetectable to 0.00235 Bq/L in the second quarter of 2013. Also, the level of Cs-137 in the fish was undetectable to 0.174 Bq/kg in 2013 monitoring results (2). The results of 3 times of sampling and analysis for marine environmental samples (seawater and marine organisms) are as low as that had normally been detected in previous tests before the Fukushima accident (undetectable to 0.00404 Bq/L of seawater, to 0.174 Bq/kg of fish) (3-5). Biological effects of radiation are typically divided into two categories. The first ones are acute or short term effects (called as acute radiation syndrome) when the body is exposed to high doses of radiation more than 1,000 mSv over short periods of time (6, 7). The second category represents exposure to low doses of radiation over an extended period of time producing chronic or long-term effects (8, 9). The associations between radiation exposure and the cancer development are mostly based on populations exposed to relatively high levels of ionizing radiation over 100 mSv (e.g., Japanese atomic bomb survivors, and recipients of selected diagnostic or therapeutic medical procedures). Even so, the International Commission on Radiological Protection (ICRP) conservatively assumes that any amount of radiation may pose some risks for causing cancer, and a linear no-threshold dose response relationship is used to describe the relationship between radiation dose and the occurrence of cancer (10). There is no environment with zero of radioactivity in the world. Radiation exists all around us. It comes from outer space (cosmic), the ground (terrestrial), and even from within our own bodies. The average of radiation exposure from natural source in the world is about 2.4 mSv/yr (range 1-10 mSv) (7, 8). In Korea, average exposure from natural radiation is about 3 mSv/yr, which is variable depending on the regions (2-5). Food can also be a common source of natural radioactivity, one of which is a component of potassium called potassium 40 (K-40). According to the national survey of natural radioactivity in the food conducted by KINS, the radioactive potassium exists in almost all food with 50-250 Bq/kg level, which is calculated as about 4,000 Bq in a man of 70 kg body (2-5). If Cs-137 enters the body, it is distributed fairly uniformly throughout the body's soft tissues. Unlike heavy metals, it is not accumulated in our body and eliminated through the urine. The biologic half-life of Cs-137 is about 70-120 days. The radiation dose from eating of 1 kg of frozen mackerel containing current dose limit 100 Bq of Cs-137 is calculated as 1.3 µSv. To reach the annually permissible dose of radiation to the public (1 mSv/yr), it needs eating as much as 1 ton of frozen mackerel (6). In conclusion, there is no impact on our marine water and marine products from the radiation leakage from the Fukushima power plant. However, the government should intensify to collaborate with the related society by diversified cooperation to quell the unrest and distrust of the public. And the government should make every effort to avoid public's exposure of radiation by various theoretical evaluation and analysis, and continuously watch the trend. All fishes and fishery products should be monitored in a more stringent manner including the place of origin, monitoring of radioactivity for all marine products. The scientific community and the government should have more proactive response to the international standards, and allay public fears and unnecessary misunderstandings by notifying the accurate facts to the public.

Optimum RCS depressurization strategy for effective severe accident management of station black out accident

Plant specific severe accident management guidelines (SAMG) for operating plants are developed and implemented in Korea as was required by government policy on severe accident. Korea Institute of Nuclear Safety (KINS) has recently reviewed feasibility of the developed SAMG for Ulchin unit 1 plant. Among the strategies referred in SAMG, we have intensively analyzed the reactor coolant system (RCS) depressurization strategy during station black out (SBO) accident scenario, which has a high probability of occurrence according to Ulchin unit 1 Probabilistic Safety Analysis (PSA). In depressurization strategy of the current SAMG, operators need to depressurize rapidly RCS pressure below 2.75 MPa using pressurizer (PZR) pilot operated safety relief valves (POSRVs) for high pressure accident like SBO. The rapid depressurization is effective in allowing the water of safety injection tank (SIT) to be injected into the core, but an excessive discharge of the SIT water is not desirable for an economical use of SIT inventory. Lack of SIT water accelerates the core damage in case the failed electric power do not recover in due to time. The SIT inventory economy means here that we should not waste the water inventory of SIT and use it in the most efficient way to cool the core. In case we do not use it in an economical way, the SIT might be depleted too rapidly, thus leaving an insufficient reservoir for post-depressurization cooling. The quantification of this SIT inventory economy for plant specific situation is of interest to develop an optimum depressurization strategy. In this study we have analyzed an effectiveness of current depressurization strategy for SBO accident with the severe accident analysis code MELCOR 1.8.5 which has been used for regulatory purpose in KINS. The entry time of severe accident management, a grace time gained by the current strategy, and the economy of the discharge mass flow rate for Ulchin plant were evaluated. Moreover, through a simple energy balance equation we could find an optimum strategy for RCS depressurization. The proposed strategy is based on finding an optimum discharge rate for an efficient use of the SIT inventory and it allows us to handle an SBO accident with higher confidence. The proposed strategy is yet a theoretical one, but possibilities of how to incorporate this strategy into engineered safety features are also discussed.