[en] The Korean Next Generation Reactor (KNGR) has been evolved to adopt an advanced design feature, a digital Plant Protection System (PPS) as an effort of enhancing reliability and safety of the plant. Although the digital PPS can be designed with high reliability, it is considered to be vulnerable to the Common Mode Failure (CMF) in the system software resulting in a total loss of the built-in hardware redundancy. Therefore, a comprehensive evaluation has been performed to demonstrate the intrinsic capability of the KNGR design in coping with the design basis events concurrent with CMF in the digital PPS. Instead of the conservative bounding analysis methodology, a best-estimate analysis methodology has been developed and utilized since the design basis events accompanied by CMF in the digital PPS are categorized as beyond design bases events. A variety of diverse means such as Alternate Protection System (APS), process control systems, and timely operator actions have been verified to be effective in mitigating the design basis events with CMF in the digital PPS

[en] At the outset of the development of the thermal-hydraulic code (THC), efforts have been made to utilize the recent technology of the computational fluid dynamics. Among many of them, the unstructured mesh approach was adopted to alleviate the restriction of the grid handling system. As a natural consequence, a mesh handler (MH) has been developed to manipulate the complex mesh data from the mesh generator. The mesh generator, Gambit, was chosen at the beginning of the development of the code. But a new mesh generator, Pointwise, was introduced to get more flexible mesh generation capability. An open source code, Paraview, was chosen as a post processor, which can handle unstructured as well as structured mesh data. Overall data processing system for THC is shown in Figure-1. There are various file formats to save the mesh data in the permanent storage media. A couple of dozen of file formats are found even in the above mentioned programs. A competent mesh handler should have the capability to import or export mesh data as many as possible formats. But, in reality, there are two aspects that make it difficult to achieve the competence. The first aspect to consider is the time and efforts to program the interface code. And the second aspect, which is even more difficult one, is the fact that many mesh data file formats are proprietary information. In this paper, some experience of the development of the format conversion programs will be presented. File formats involved are Gambit neutral format, Ansys-CFX grid file format, VTK legacy file format, Nastran format and CGNS

[en] In order to assess applicability of SPACE as a system analysis code, various calculations for the phenomenological problems, the separate effects tests (SETs) and the integral effect tests (IETs) have been performed so far. The phenomenological problems are used to check whether the code shows quantitatively and qualitatively good agreement with the physics of each problem. Intensive study including comparison of SPACE predictions on wide variety of events with the results from SETs and IETs has been performed. In this paper, simulation results for the evaluation of special process model, such as GE Level Swell test, Dukler's air/water flooding test and HDU experiments, are presented

[en] Polyhedral mesh has been known to have some benefits over the tetrahedral mesh. Efforts have been made to set up a polyhedral mesh generation system with open source programs SALOME and TetGen. The evaluation has shown that the polyhedral mesh generation system is promising. But it is necessary to extend the capability of the system to handle the viscous layers to be a generalized mesh generator. A brief review to the previous works on the mesh generation for the viscous layers will be made in section 2. Several challenging issues for the polygonal prism mesh generation will be discussed as well. The procedure to generate a polygonal prism mesh will be discussed in detail in section 3. Conclusion will be followed in section 4. A procedure to generate meshes in the viscous layers with PolyGen has been successfully designed. But more efforts have to be exercised to find the best way for the generating meshes for viscous layers. Using the extrusion direction of the STL data will the first of the trials in the near future

[en] The local instantaneous balance equation reads as, ∂/∂t(ρ_kψ_k)+∇·(ρ_kψ_ku_k)=-∇·J_k+ρ_kψ_k (1) where ρ_k,ψ_k,u_k,J_k and φ_k are the density, property of extensive characteristics, velocity, flux and source of k-phase, respectively. Nomenclatures for other variables are found in. Table-1 shows the field variables. e_k,q_k,ρ_k,γ_k,g_k,q_k,M_k,E_k are internal energy, heat flux, pressure, vaporization, gravity and internal heat rate, interfacial momentum and energy sources respectively. is the stress tensor and is decomposed into pressure and shear. The time averaged balance equation for any property ψ_k of k-phase can be presented as, ∂/∂t(ρ_kψ_k)+∇·(ρ_kψ_kU_k)=-∇·J_k+ρ_kφ_k+I_k. I_k≡-I/ΔtΣ_j{I/u_ni[(ρ_kψ_k)n_k·(u_k-u_i)-(n_k·J_k)]} (2) where the bar over a quantity indicates the time-averaging operation.is the surface normal vector in this paper. Considering the time-fluctuating terms, the time-averaged balance equation can be represented by using weighted mean variables as, (J_k^T: turbulent effectsTkJ): ∂/∂t(ρ_kψ_k)+∇·(ρ_kψ_ku_k)=-∇·(J_k+J_k^T)+ρ_kφ_k+I_k

[en] KOPEC has been developing a system thermal hydraulics solver of SPACE, which is a safety analysis code of nuclear power plants, using two-fluid, three field governing equations for two phase flow. Several numerical schemes, such as collocated, staggered, semi-implicit, and implicit schemes, have been tried so far and a lot of effort has been made to verify the applicability of the SPACE code. In this paper, boron transport model was developed. And the boron precipitation and dilution test was performed using the SPACE code. The test results showed that the SPACE is applicable to the post LOCA long term cooling evaluation, especially on the USNRC safety issues addressed in the USNRC letter

[en] SPACE is a nuclear power plant safety analysis code, which is under development through a joint research between Korean nuclear industry and research institute. It adopts two-phase three field governing equations. Several mesh systems and numerical schemes, such as structured, unstructured, staggered, collocated meshes, semi-implicit, and nearly implicit numerical schemes, have been tried so far. In this paper, focus is placed on the evaluation of its nodalization capability as a system analysis code. Especially, the flexible feature of the staggered mesh system of SPACE will be described, focusing on how to model the complex geometries and various types of their connections typically encountered in nuclear power plant systems. The semi-implicit numerical solution scheme using the staggered mesh system and some of the application results will be also presented

[en] The governing equations for the thermal-hydraulic solver can be obtained through several steps of modeling and approximations from the basic material transport principles. The volume averaging process gives rise to the concept of the volume porosity. Considering the structural complexity of the reactor internal design, one can imagine how much efforts should be put to get the proper porosity data. Personal experience tells that over one man-year of efforts are not enough to develop a reasonable input preparation report for a reactor because of the notorious error-prone tedious calculations. To overcome this problem, any one can imagine that the utilization of a CAD system will be very much helpful. But, some efforts have to be exercised to evaluate the capability of the present day CAD system for this type of application. Even if the evaluation is affirmative, some efforts have to be devoted to develop the necessary procedure for the porosity calculation. In this paper, the CAD system, Pro/Engineer is utilized for this purpose. Detailed review tells that, sole Cad system is not enough but a mesh generator is also necessary. A post-data processor may be combined to further enhance the capability

[en] The MEDUSA is a system thermal hydraulics code developed by Korea Power Engineering Company (KOPEC) for Non-LOCA and LOCA analysis, using two fluid, three-field governing equations for two phase flow. The detailed descriptions for the MEDUSA code are given in Reference. A lot of effort is now being made to investigate the applicability of the MEDUSA code especially to Non-LOCA analysis, by comparing the analysis results with those from the current licensing code, CESEC-III: The comparative simulations of Pressurizer Level Control System(PLCS) Malfunction and Feedwater Line Break(FLB), which have been accomplished by C.E.Park and M.T.Oh, respectively, already showed that the MEDUSA code is applicable to the analysis of Non-LOCA events. In this paper, detailed thermal hydraulic analyses for Steam Line Break(SLB) without loss of off-site power were performed using the MEDUSA code. The calculation results were also compared with the CESEC-III, 1000(OPR1000), for the purpose of the code verification

[en] Recently developed porous body approach codes such as SPACE and CUPID require a CAD system to estimate the porosity. Since they use the unstructured mesh and they also require reliable mesh generation system. The combination of CAD system and mesh generation system is necessary to cope with a large number of cells and the complex fluid system with structural materials inside. In the past, a CAD system Pro/Engineer and mesh generator Pointwise were evaluated for this application. But, the cost of those commercial CAD and mesh generator is sometimes a great burden. Therefore, efforts have been made to set up a mesh generation system with open source programs. The evaluation of the TetGen has been made in focusing the application for the polyhedral mesh generation. In this paper, SALOME will be described for the efforts to combine TetGen with it. In section 2, brief introduction will be made on the CAD and mesh generation capability of SALOME and Tetgen. SALOME and TetGen codes are being integrated to construct robust polyhedral mesh generator. Procedures to merge boundary faces and to cut concave cells are developed to remove concave cells to get final convex polyhedral mesh. Treating the internal boundary face, i.e. non-manifold face will be the next task in the future investigation