CRITICAL POWER EVALUATION WITH CONSIDERATION OF DIFFERENCES IN DROPLET BEHAVIOR DUE TO SPACER SHAPE INFLUENCE

An evaluation method for droplet deposition coefficients, correspondin g to spacer shapes, was developed in order to predict nuclear fuel bun dle critical power. The method is based on a three dimensional numeric al analysis of dispersed flow around the spacer. The analysis showed t hat coolant droplet deposition characteristics onto fuel rods differed between grid type and ferrule type spacers, and also between fuel rod s due to flow tabs on the upper end of the spacer band. The droplet de position coefficient was modeled from the analysis results and adopted into a critical power prediction code based on subchannel analysis. C omparisons between critical powers calculated by the subchannel analys is code and those measured with full-scale test assemblies showed good agreement and improved prediction accuracy of the code.