ATTENUATION OF TEMPERATURE-FLUCTUATIONS IN THERMAL STRIPING

Thermal striping is random temperature fluctuation generated by the mixing of flow streams at different temperatures. In a liquid-metal-cooled fast breeder reactor (LMFBR), it occurs in the upper plenum as a result of the mixing of sodium coolant jets from fuel assemblies and control rods. It is necessary to consider the temperature cycling in the context of fatigue on the component. A simplified model has been developed to analyze the attenuation of temperature fluctuations from the flow streams to the structure surface. The analytical results were compared with experiments that simulated the upper core structure and impinging jets from fuel sub-assemblies and control rod channels in an LMFBR. Sodium and water were used as simulated fluids to examine the differences in their physical properties. The surface attenuation ratio decreased with the jet flow velocity, The ratio of surface temperature fluctuation to fluid temperature fluctuation is 30-80% in water tests and is 20-50% in sodium tests. These experimental attenuation values almost agreed with the values calculated by the boundary layer theory.