This paper deals with the numerical investigation on thermal stratification phenomena in the Gravity Driven Water Pool (GDWP) of the Advanced Heavy Water Reactor (AHWR). Study has been performed at decay power level to simulate thermal stratification in GDWP for Station Black-Out (SBO) conditions. Thermal stratification in GDWP has been simulated using a three-volume pool nodalization approach. The study points to characteristic nature of progress of thermal stratification in the GDWP. Full potential of the pool water can be realised only if the entire inventory of water rises to saturation temperature. This can be achieved only by mixing of water. Thermal stratification prevents realisation of the full heat removal potential of the pool water volume particularly below the Isolation Condenser (IC). The single vertical shroud deployed in the pool is the simplest passive internal, which ensures participation of whole pool inventory. A parametric study on single and three-shroud configurations has been performed by varying shroud location or spacing and shroud height. The study indicates limited suppression of thermal stratification by single-shroud configuration compared to three-shroud configuration. The performance of both single and three-shroud configurations was found to be similar for the first  three days. However, during a prolonged SBO, the three-shroud configuration was found to be superior due to enhanced mixing. Parametric study was conducted to arrive at an optimum spacing and shroud height for three-shroud configuration. Studies were also conducted to understand the influence of conductivity of shroud material and leakage through shroud on its performance.