PRESSURE LOSS COEFFICIENT EVALUATION BASED ON CFD ANALYSIS FOR SIMPLE GEOMETRIES AND PWR REACTOR VESSEL WITHOUT GEOMETRY SIMPLIFICATION

摘要

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 NPPs. 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-s model, RNG k-ε model, Reynolds stress model (RSM) on the simulation of pressure drop for simple geometry such as, orifice, 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.