ABSTRACT Experiments were carried out to evaluate the thermal behavior of nuclear spent fuel stored in the auxiliary pool of Egypt Test Research Reactor No.2 (ETRR-2) during natural convection cooling condition (pump failure). An experimental test rig was designed and constructed to simulate this case. The test rig was equipped with two vertical and parallel electrically heated aluminum plates with a uniform heat flux. The test rig was designed to investigate the effect of the extension ratio (L* = Lt/Lh), the aspect ratio (B* = W/b) and the inlet coolant temperature (Ti) on the natural convection cooling behavior. Four different extension ratios (1.44, 1.94, 2.44 and 2.94) and four different channel aspect ratios of 2, 2.67, 4 and 8 were studied. The inlet coolant temperature was varied from 30 up to 45 oC. The present experiments were conducted within a range of modified Rayleigh number Ra* from 1.2 x105 to 2.4 x108. The results showed that the heat transfer coefficient increases with the increase in either the extension ratio or the inlet coolant temperature, while it decreases with the increase in the aspect ratio. New correlation was obtained for the average Nusselt number as a function of the channel extension ratio L*, the channel aspect ratio B* and the modified Rayleigh number Ra*.