Challenges in reactor core thermal hydraulics: subchannel analysis, CFD modeling and rod bundle CHF

Avital area impacting the safety and economics of most nuclear power plants is the reactor core thermal hydraulics for all types of reactors. Several issues of concern include: understanding of fundamental physical phenomena, various thermal hydraulic limits (such as critical heat flux (CHF) flow instability (FI) and natural circulation cooling (NC) strong multi physics coupling, severe accidents, flow structure interactions (FSI) as well as the influence of mixing vane spacer grids in fuel bundles. Understanding these complex physical phenomena of multiscale, multidimensional and multiphase flows is limited. The challenges in subchannel thermal hydraulics continue to draw major focus from academic and industrial researches. Various investigations in advanced simulation, modeling and measurement techniques are carried out to enhance the understanding of the thermal hydraulic phenomena under complex conditions. Advancements in subchannel codes are being pursued to overcome massive computational limitations for reactor simulation and to account for the complicated mixing vane grid effects on local thermal hydraulic calculations. Recently, the activities in computational fluid dynamics (CFD) with the advancement in the computational algorithms, fast computers, turbulence modeling, boiling heat transfer, two phase flow mixing and flow structure interactions are attracting a lot of attention in nuclear applications. In this issue 15 articles were selected from over 80 papers submitted to the 2 International Seminar on Subchannel Analysis, CFD Modeling and Verification, CHF Experiment and Benchmarking (ISACC 2015) With continued focus on CFD mixing vane grid modeling and rod bundle CHF evaluation, renewed interests are also geared towards subchannel flow instability, GPU MPS modeling of fuel coolant interaction (FCI) as well as mixing vane grid impacts on post accident fuel bundle cooling. These papers cover the broad field of subchannel thermal hydraulics: from subchannel code development, experimental study and subchannel measurements, CFD modeling flow structure coupling, post accident cooling, correlation developments, subchannel flow instability and to other subchannel applications. Riley et al. analyzed steam cooling behavior in the PWR subchannels using RBHT data to examine the effect of spacer grids on heat transfer after accident events. Yang et al. examined separate effects of key components in typical spacer grids using experimentally verified CFD modeling techniques. Park et al. presented experimental investigations examining the impact on heat transfer characteristics and peak cladding temperature through studying the dynamics of a single droplet impacting a hot surface above the Leidenfrost point temperature. Mao et al. presented an approach to improving the subchannel code local condition predictability by adapting mixing vane grid effects through different source terms with mixing parameters derived. Moon et al. investigated the coolability of partially blocked cores during a loss of coolant accident in 2 2 and 5 5 bundles. Han et al. analyzed the influence of key components in mixing vane grids, such as the dimple location, distance of mixing vane to the nearest strip and the effect of inter subchannel mixing among subchannels on pressure drop and downstream flow fields. Ding et al. designed a new algorithm to decompose the penta diagonal matrix and improve computational efficiency based on Stone s incomplete LU (ILU) decomposition method. Chen et al. carried out a CFD study to analyze the effects of spacer grid with mixing vanes under single and adiabatic two phase flow. Han et al. adapted a new CFD modeling approach to enhance the predictability of CHF location in an axially non uniformly heated bundle. Zhang et al. presented the CFD modeling results for steam generators under various single and two phase flow conditions to examine the effects of tube support plates (TSP) on thermal hydraulics. Liu et al. presented analysis of experimental results from high heat flux pool boiling and liquid film boiling experiments (BETA) and used to support modeling of micro layer hydrodynamics under high heat flux. Mao et al. delivered a comprehensive review of the grid enhanced heat transfer correlations. A predictive model was also presented to account for mixing mechanisms, such as swirl flow and crossflow, associated with the mixing vane. Yang et al. evaluated the performance of various turbulence models for the application of CFD modeling in rod bundles fuel assemblies. Gou et al. applied graphical processing unit (GPU) implementation of the moving particle semi implicit (MPS) method to simulate the phenomena of fuel coolant interaction (FCI) Wang et al. applied RELAP code to examine the stability in single, 2 2, and 3 3 channel systems for both closed and opened subchannels. The editors of this special issue would like to acknowledge their most sincere gratitude to all authors that submitted papers for their contribution and efforts.