The lead based reactor concept is considered to be the most promising reactor option for the Accelerator Driver Subcritical (ADS) system. Lead based reactor engineering validation facility CLEAR-S is a large scale integrated non-nuclear pool type facility which has been built to provide a unique experimental platform for full-scale prototype components test of lead based reactors. In this article, three-dimensional steady state analysis of flov in the hot and cold pools of CLEAR-S was carried out by using computational fluid dynamics (CFD) code FLUENT during forced and natural circulation conditions. The results showed that the simulation had a good agreemen with the design parameters of CLEAR-S and the temperature profile was uniform in the whole domain of the ho and cold pools without considering any conduction through the coupled walls. The temperature distribution it the hot and cold pools became non-uniform with the coupled wall conduction and thermal stratification wa: observed in the cold and hot pools near the bottom wall of the hot pool. This non-uniform distribution o temperature could cause thermal stresses on equipment, which is dangerous for the integrity of the system During natural circulation conditions, the coupled wall conduction enhanced the natural circulation and kept the hot pool at a relatively low temperature because the cold pool cooled the hot pool through heat transfer betweer the walls of the hot and cold pools. Therefore, wall conduction has some advantages and disadvantages as i enhances the natural circulation on one hand but disturbs temperature uniformity on the other hand.