ATTENUATION OF TEMPERATURE-FLUCTUATIONS IN THERMAL STRIPING

Thermal striping is random temperature fluctuation generated by the mi xing of flow streams at different temperatures. In a liquid-metal-cool ed fast breeder reactor (LMFBR), it occurs in the upper plenum as a re sult of the mixing of sodium coolant jets from fuel assemblies and con trol rods. It is necessary to consider the temperature cycling in the context of fatigue on the component. A simplified model has been devel oped to analyze the attenuation of temperature fluctuations from the f low streams to the structure surface. The analytical results were comp ared with experiments that simulated the upper core structure and impi nging jets from fuel sub-assemblies and control rod channels in an LMF BR. Sodium and water were used as simulated fluids to examine the diff erences in their physical properties. The surface attenuation ratio de creased with the jet flow velocity, The ratio of surface temperature f luctuation to fluid temperature fluctuation is 30-80% in water tests a nd is 20-50% in sodium tests. These experimental attenuation values al most agreed with the values calculated by the boundary layer theory.