The relative importance of energy redistribution by thermal conduction and sweep flow mixing in a wire-wrapped fuel assembly are quantitatively described at various Reynolds numbers. For a given bundle geometry, a critical Reynolds number exists below which thermal conduction appears to govern the temperature distribution within the bundle. As the thermal conduction effects become progressively important at low Reynolds numbers, the transverse temperature gradient in the bundle decreases. This result would have an important effect on incoherency in assembly voiding. If one were to develop a model of a full-size liquid-metal fast breeder reactor bundle to study incoherency in voiding, an important parameter is the maximum temperature difference at the bundle exit. Whereas this parameter is the same for a 19- and 217-pin bundle at design operating conditions, it is significantly different at low Reynolds numbers. This low Reynolds number bundle-size effect was determined by analysis of steady-state data and is valid for very slow transients where the thermal inertia of the structure is unimportant. Inclusion of the structure thermal inertia would tend to diminish this bundle-size effect.