Single-phase turbulent cross-exchange in very tight-packed fuel rod bundles

For the design of fuel-rod bundles of light-water reactors the distribution of temperature during operating conditions is one of the most important criterion in safety and economical perspectives. One influence factor on temperature distribution is the lateral exchange of mass, momentum and energy between the subchannels of fuel-rod bundles. For many decades researches performed experimental investigations to find a model that describes the cross exchange for implementation in subchannel-analysis codes. In tight-packed rod bundle geometries the effect of flow pulsation was discovered. An overview over this phenomenon is given by Meyer [2010]. Many important experimental findings and model derivations were developed by Rehme [1992], Moeller [1992] and Krauss [1996]. In single-phase flow in a bare rod bundle without spacers, there are a Diversion Cross Flow due pressure differences and the Turbulent Mixing effect that contributes to the lateral cross-exchange of mass, momentum or energy between subchannels. The cross-exchange can be described with the coefficient β that is defined in equation 1. For mass-exchange it represents the exchanged mass flux Gij' divided through the average mass flux anti G of two connected channels i and j. β = (Gij')/(anti G). This compact shows the influence of flow pulsations that occur in tight packed fuel rod bundles on the lateral cross exchange of mass. The results of CFD-Simulations over a large number of geometry- and flow-conditions are presented that completely capture the pulsation phenomenon. With this data the model for β is being improved for rod configurations were flow pulsations occur. (orig.)