[en] In resolving the safety issue of sump clogging due to debris generated by the type of high-energy line break known as a GSI-191 event, determination of the debris transport fraction is very important in the sizing of the sump screen area. In general method for evaluation of debris transport fraction, the mean fluid velocity distribution within the containment floor on recirculation transport mode during LOCA by CFD analysis is combined fundamental transport properties, such as tumbling velocities, of various types of debris by experimental research to identify the debris transport fraction. In the determination of the debris floor transport, it is also advised that the turbulent kinetic energy effect (TKE) as well as mean flow velocity on turbulent flooding flow is considered by previous experimental researches. However, the quantification results pertaining to the debris floor transport by TKE were not published in the literature. In the present study, the debris floor transport on flooding flow is evaluated with and without consideration of turbulence effect. To do this, experiments involving tumbling velocities measurements of the surrogate debris and supplementary CFD analyses are performed to verify the turbulence effect on debris transport. From these findings, the turbulence effect on the degree of debris floor tumbling augmentation was found to be represented by the algebraic sum of the mean horizontal velocity and the horizontal fluctuating velocity deduced from the turbulent kinetic energy

[en] The Picard scheme involves successive updating of the coefficient on the previously calculated values. The outer-iteration is terminated at that time being satisfied with boundary condition on which a lateral pressure difference between subchannels is even at exit plane. Diversion cross flow is generated to reduce the lateral pressure difference at each axial node. The physics can be numerically implemented with using approximation to force the lateral pressure difference to be the zero. The idea is firstly realized by prediction-correction method by C. Chiu. In this code, two-step method is adopted to approximate the lateral pressure difference term using diversion cross flow. The approximation allows the outer-iteration free scheme. The present study describes the implementation of outer-iteration free scheme, called non-iterative prediction-correction method into MATRA code. Outer-iteration free algorithm is implemented into the subchannel code MATRA. Original prediction-correction method applied only two channel is successfully expanded into the multichannel application. In comparison with the convectional outer-iteration numerical scheme, the present algorithm showed the more efficient and compatible accuracy on the verification problems, such as SMT- 5x5 problem and KSNP single assembly problem. In addition, outer-iteration free algorithm can be calculated in lower mass flow condition in which conventional scheme is breakdown

[en] Krylov subspace method was implemented to perform the efficient whole core calculation of SMART with pin by pin subchannel model without lumping channel. The SMART core consisted of 57 fuel assemblies of 17 by 17 arrays with 264 fuel rods and 25 guide tubes and there are total 15,048 fuel rods and 16,780 subchannels. Restarted GMRES and BiCGStab methods are selected among Krylov subspace methods. For the purpose of verifying the implementation of Krylov method, whole core problem is considered under the normal operating condition. In this problem, solving a linear system Aχ = b is considered when A is nearly symmetric and when the system is preconditioned with incomplete LU factorization(ILU). The preconditioner using incomplete LU factorization are among the most effective preconditioners for solving general large, sparse linear systems arising from practical engineering problem. The Krylov subspace method is expected to improve the calculation effectiveness of MATRA code rather than direct method and stationary iteration method such as Gauss elimination and SOR. The present study describes the implementation of Krylov subspace methods with ILU into MATRA code. In this paper, we explore an improved performance of MATRA code for the SMART whole core problems by of Krylov subspace method. For this purpose, two preconditioned Krylov subspace methods, GMRES and BiCGStab, are implemented into the subchannel code MATRA. A typical ILU method is used as the preconditioner. Numerical problems examined in this study indicate that the Krylov subspace method shows the outstanding improvements in the calculation speed and easy convergence

[en] Nuclear vendors and utilities perform lots of simulations and analyses in order to ensure the safe operation of nuclear power plants (NPPs). In general, the simulations are carried out using vendor-specific design codes and best-estimate system analysis codes and most of them were developed based on 1-dimensional lumped parameter models. These thermal-hydraulic system analysis codes require user input for pressure loss coefficient, k-factor; since they numerically solve Euler-equation. In spite of its high impact on the safety analysis results, there has not been good validation method for the selection of loss coefficient. During the past decade, however; computers, parallel computation methods, and 3-dimensional computational fluid dynamics (CFD) codes have been dramatically enhanced. It is believed to be beneficial to take advantage of advanced commercial CFD codes in safety analysis and design of NPP5. The present work aims to validate pressure loss coefficient evaluation for simple geometries and k-factor calculation for PWR based on CFD. The performances of standard k-ε model, RNG k-ε model, Reynolds stress model (RSM) on the simulation of pressure drop for simple geometry such as, or sudden-expansion, and sudden-contraction are evaluated. The calculated value was compared with pressure loss coefficient in handbook of hydraulic resistance. Then the present work carried out analysis for flow distribution in downcomer and lower plenum of Korean standard nuclear power plants (KSNPs) using STAR-CD. The lower plenum geometry of a PWR is very complicated since there are so many reactor internals, which hinders in CFD analysis for real reactor geometry up to now. The present work takes advantage of 3D CAD model so that real geometry of lower plenum is used. The results give a clear figure about flow fields in the reactor vessel, which is one of major safety concerns. The calculated pressure drop across downcomer and lower plenum appears to be in good agreement with the data in engineering calculation note. The present 3-dimensional calculation domain was split into sub-domains such that each domain corresponds to a node of the KSNP for an event analysis using RELAP5/MOD3. Volume average pressure of each sub-domain, area and area average velocity at each interface between sub- domains was calculated. Based on these information k-factors were evaluated for each junction. (authors)

[en] The results show that the averaging BDFT is a promising flow meter for the accurate measurement of flow rates in the fouling condition of the NPPs. A new instrumentation, an averaging BDFT, was proposed to measure the accurate flow rate under corrosion environment. In this study, to validate the applicability of the averaging BDFT on the fouling conditions, flow analyses using the CFD code were performed. Analyses results show that this averaging BDFT does not lose the measuring performance even under the corrosion environment. Therefore, it is expected that the averaging BDFT can replace the type flow meters for the feedwater pipe of steam generator of NPPs. Most of the NPPs adopt pressure difference type flow meters such as venturi and orifice meters for the measurement of feedwater flow rates to calculate reactor thermal power. However, corrosion products in the feedwater deposits on the flow meter as operating time goes. These effects lead to severe errors in the flow indication and then determination of reactor thermal power. The averaging BDFT has a potentiality to minimize this problem. Therefore, it is expected that the averaging BDFT can replace the type venturi meters for the feedwater pipe of steam generator of NPPs. The present work compares the amplification factor, K, based on CFD calculation against the K obtained from experiments in order to confirm whether a CFD code can be applicable to the evaluation of characteristic for the averaging BDFT. In addition to this, the simulations to take into account of fouling effect are also carried out by rough wall option

[en] A fully ceramic micro-encapsulated(FCM) fuel based on the dispersed particle fuel concept was considered on the one of accident tolerant fuel(ATF). A mixed core is established using the FCM fuel in the existing LWR core where UO2 fuel pellet was loaded. In order to demonstrate the thermal-hydraulic compatibility of the FCM fuel in the existing LWR core, pin by pin analysis is performed for transition period from mixed core with FCM fuel and UO2 fuel to the FCM fuel only. Parallelized MATRA code using MPI is developed for pin by pin calculation. Pin by pin analysis on mixed transition core for FCM fuel and reference UO2 fuel was performed on a quarter core with 13310 subchannels in assistance with the parallel algorithm with MPI with 20 cores. The thermal margin for the pin by pin model was evaluated by employing a quarter-core power distribution data provided by MASTER code that is nodal code. The pin by pin model shows the feasibility of mixed transition core of FCM fuel assembly based on the MDNBR results

[en] An operator should provide the correct and fast actions on a cause of alarms and failure for reducing the effect of failure. There are a lot of study. But most of those studies may use a physical knowledges or causal relationships. Most of those studies impose on high level information like the physical knowledges or causal relationships of failure rather than the logical states or process signals as the detail causes of failure. It is very difficult that the physical knowledges or causal relationships are to be implemented and verified. This paper proposes a methodology for tracking alarm by logic of alarms. This methodology uses the proper logical knowledges on the proven logic and alarm diagram or electrical alarm relay logic than the uncertain physical knowledges or causal relationships. This system is to display the highlighted alarm procedure related to the causes. The system can be used for operator to identify the detail causes of alarm without checking all candidates for causes in alarm response procedure and the logical states of alarm with alarm logic disgrams provided on CRT dynamically