Results 1 - 10 of 57
Results 1 - 10 of 57. Search took: 0.016 seconds
|Sort by: date | relevance|
[en] A percolation model is proposed to determine the interlinkage fraction of intergranular pores in the restructured and cracked fuel following irradiation at high temperature under steady-state condition. A fuel rod is simulated as a large hexagon which consists of many small hexagonal grains. The fuel rod is divided into four zones having different morphological and thermal properties, of which boundaries are calculated by critical temperatures. Grain size grown during irradiation is calculated using the FASTGRASS code and is used to calculate the number of hexagonal rings in zones. It is assumed that there is a circumferential crack at each zone boundary and radial cracks dividing zones radially. The algorithm for calculation of the pore interlinkage fraction (PIF) includes several steps; checking the site-occupancy, labeling the occupied sites, checking the site's connectivity to the nearest occupied sites, and counting the number of sites in the cluster connected to the free space. The Monte Carlo method is used for checking site-occupancy and the Hoshen-Kopelman method is applied to labeling. The site occupation probability is assumed to be the ratio of the current swelling to the maximum fractional swelling of pores in the grain edge, which is approximately 8.7%. The proposed model provides zone-dependent PIFs as a function of the site occupation probability. Comparisons of the calculated results with experimental data including the AECL-2230,CBX fuel rod of fractional gas release are done by replacing the PIF function in the FASTGRASS code with the calculated zone-dependent PIFs. Restructuring with cracks is found to affect fission gas release significantly. The calculated fission gas release as a function of linear heat rate shows better fitting to the experimental data than the simple model in the FASTGRASS code. The fission gas release is also sensitive to the maximum fractional volume swelling. The main advantage of this model is to treat the interlinkage phenomena in the grain boundaries more realistically than the single PIF correlation and to take into account grain growth and cracks parametrically
[en] For many years important programs have been undertaken in the field of severe accidents and their results have been shared through international 'networks.' The Committee on the Safety of Nuclear Installations (CSNI) of the OECD Nuclear Energy Agency (NEA) has played a major role in organizing and administering cooperative research programs in this area of severe accidents. This paper deals with the NEA's working method on the severe accident and related activities. All this information was achieved while the author was working for the NEA as a cost-free expert from 2006 to 2008
[en] We report density functional theory calculations for the bulk ZnO divacancy in hexagonal ZnO and compare the results with the monatomic bulk vacancies. We find that the ZnO divacancy could be viewed as a tightly-bound state of a doubly-negative Zn vacancy and the doubly-positive O vacancy from the structural and the electronic points of view. The singly-positive ZnO divacancy is found to carry a magnetic moment of 1μB, but it can only be stabilized in very high p-type samples. Otherwise, the non-magnetic neutral divacancy is stable. These results suggest that the ZnO divacancies in the bulk region cannot play a significant role in stabilizing the recently-observed ferromagnetism in ZnO nanoparticles.
[en] We investigated the structural and the binding properties of tetrahedral P4 molecular pairs by employing density functional theory (DFT) calculations using both the local density approximation (LDA) and the generalized gradient approximation (GGA). To consider the van der Waals interaction, we employed the GGA with dispersion correction, specifically Grimme's approach (DFT-D2). This DFT-D2 in conjunction with the GGA (GGA-D2) well fixes the failure of the GGA in reproducing a reasonable binding energy curve. The binding energy and the equilibrium intermolecular distance are found to be in close correlation with the number of adjacent atomic pairs between two molecules. Application of the LDA and the GGA-D2 to a molecular crystal β-P4 and comparison with experiments confirms that the GGA-D2 should be used for a reliable description of P4-based molecular systems.
[en] We present density functional theory calculations on the structural, energetic, electronic, magnetic, and elastic properties of κ-carbides, (Fe,Mn)3AlC and their parent structure Fe3Al-L12. All the crystals have ferromagnetic ground state, and Fe3AlC is energetically most favored. The inclusion of C enhances the energetic stability, reduces the magnetization, and makes the crystal rigid. The Mn substitution has opposite effects for the energetics and magnetism, but corroborates that C addition hardens the crystals. The κ-carbides show an almost rigid band shift, and the electronic structure around the Fermi level is necessary to understand the property changes upon Mn substitution. We speculate that the configurational entropy may play an important role in stabilizing κ-carbides with finite Mn content.
[en] In order to analyze severe accident phenomena, Korea Institute of Nuclear Safety (KINS) made a MELCOR model for APR1400 to examine natural circulation and creep rupture failure in the Reactor Coolant System (RCS) under station blackout (SBO). In this study, we are trying to advance the former model to describe natural circulation more accurately. After Fukushima accident, the concerns of severe accident management, assuring the heat removal capability, has risen for the case when the SBO is happened and there are no more electric powers to cool down decay heat. Under SBO there are three kinds of natural circulation which can delay the core heatup. One is in vessel natural circulation in the upper plenum of reactor vessel and the second is countercurrent natural circulation in hot leg through steam generator tubes and the last is full loop natural circulation when the reactor coolant pump loop seal is cleared and reactor coolant pump sealing is damaged by high temperature and high pressure. Among them this study focuses on the countercurrent natural circulation model using MELCOR1.8.6
[en] 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
[en] The first approach is a mechanistic approach which is used in LIRIC in which more than 200 reactions are modeled in detail. This approach enables to perform the detailed analysis. However, it requires huge computation burden. The other approach is a simplified model approach which is used in the IMOD, ASTEC/IODE, and etc. Recently, KINS has developed RAIM (Radio-Active Iodine chemistry Model) based on the simplified model approach. Since the numerical analysis module in RAIM is based on the explicit Euler method, there are major issues on the stability of the module. Therefore, implementation of a stable numerical method becomes essential. In this study, RAIM is refined via implementation of implicit Euler method in which the Newton method is used to find the solutions at each time step. The refined RAIM is tested by comparing to RAIM based on the explicit Euler method. In this paper, RAIM was refined by implementing the implicit Euler method. At each time step of the method in the refined RAIM, the reaction kinetics equations are solved by the Newton method in which elements of the Jacobian matrix are expressed analytically. With the results of OECD-BIP P10T2 test, the refined RAIM was compared to RAIM with the explicit Euler method. The refined RAIM shows better agreement with the experimental data than those from the explicit Euler method. For the rapid change of pH during the experiment, the refined RAIM gives more realistic changes in the concentrations of chemical species than those from the explicit Euler method. In addition, in terms of computing time, the refined RAIM shows comparable computing time to that with explicit Euler method. These comparisons are attributed to ⁓10 times larger time step size used in the implicit Euler method, even though computation burden at each time step in the refined RAIM is much higher than that of the explicit Euler method. Compared to the experimental data, the refined RAIM still shows discrepancy, which are attributed to the coefficients used in the iodine chemistry model
[en] OECD/NEA THAI (Thermal hydraulics, Hydrogen, Aerosol and Iodine) Project was conducted from 2007 for three years to address open questions concerning the behavior of hydrogen, iodine and aerosols in the containment of water cooled reactors during severe accidents. In the project nine OECD member countries joined including Korea (KAERI and KINS). Recently the final report of the project was published to provide information on research results relevant to the open hydrogen and fission product issues. In this paper, major experimental results of HR (hydrogen recombiner) test series for passive autocatalytic recombiners (PARs) that can be referred to or considered in domestic R and D projects, are introduced. We quote the OECD report to obviously transfer the facts found in test series. Since new or refurbished nuclear power plants (NPPs) in Korea are considering the installation of PARs, the results obtained from HR test series may also give useful information to the licensees as well as the regulators. Therefore this paper aims for sharing the fruitful PAR related findings with Korean Nuclear Society members
[en] The aim of this study is to present the status of post-Fukushima actions with respect to accident management and also provides the current status of developing EDMGs and applicability of a FLEX strategy in Korea. As part of the post-Fukushima actions in Korea, SAMGs will be revised to improve the effectiveness of accident management. For this purpose, it is recommended to revise the EOPs and SAMGs and establish the EDMGs with consideration of prolonged SBO, spent fuel pool cooling, using mobile equipment for accident control, feedback of the implementation of the action items of the special safety inspection, multiple severe accidents for all reactors at a site. It is considered that the FLEX strategy may be useful to mitigate the accidents like Fukushima. Therefore, it is recommended to adopt this strategy including provision of the equipment with protection from external events. The Fukushima accident revealed that EOPs and SAMGs were not effectively coping with and mitigating the severe accident caused by extreme natural hazards such as earthquake and tsunami. The accident indicated needs for strengthening the existing accident management procedures such as emergency operating procedures (EOPs) and severe accident management guidelines (SAMGs). In particular, these procedures should address the possibility of extreme natural hazards causing a prolonged SBO condition, which affects multiple-units and Spent Fuel Pools (SFPs) (NTTF Recommendation 9). In addition, in order to prevent and mitigate the potential damage in an extensive scale at a multi-unit site due to external events, fire, various kinds of countermeasures are required by the Regulatory Body. These are the follow-up actions to the special safety inspection carried out just after the Fukushima accident and the stress tests for old plants. Especially, the Extensive Damage Mitigation Guidelines (EDMGs) are being provided by the utility in conjunction with adoption of the FLEX strategy (diverse and flexible mitigation capability) proposed by the utility in the U. S.