The ULTRAFLOW spacer—an advanced feature of ATRIUM fuel assemblies for boiling water reactors

Abstract

Critical power performance of reload fuel assemblies has a significant influence on economy in terms of fuel cycle costs and on operational flexibility of boiling water reactors. Well-known advantages in critical power performance attributed to the spacer design have been a stimulus for the development of a new spacer concept. The development was aimed at maximizing the effectiveness of phase separation in the subchannels of the fuel assemblies while minimizing the pressure drop across the spacers. Air-water tests at virtually atmospheric pressure and room temperature were used for a comparative qualitative study of the typical effects on subchannel and film flow downstream of various types of spacer. The most promising design concept proved to be the ULTRAFLOW TM (Siemens trademark) spacer, which is an egg-crate-type spacer with swirl vanes. Critical power and pressure drop performance of the ULTRAFLOW spacer have been measured at the Karlstein test facility of Siemens. The results show that the critical power performance of this spacer is on an average 8% higher than that of the ring spacer and 16% higher than that of the egg-crate spacer without swirl vanes while its pressure drop is lower compared with the above types.