For 540 MWe PHWRs onwards, two independent, fast acting and diverse reactor shut down systems are provided. Shut Down System#2 (SDS#2), on actuation, injects liquid poison (Gadolinium Nitrate solution in heavy water) into bulk moderator inside the Calandria through six horizontal perforated Liquid Poison injection (LPI) tubes using high pressure helium to rapidly shut down the reactor. The poison emerges in the form of jets through an array of holes along the length of the LPI tube. These poison jets mix with the moderator and induce negative reactivity in the Calandria. 3D Numerical calculation is carried out to predict the poison jet growth in the moderator inside Calandria for initial 3 sec of the poison injection in order to estimate the reactivity worth, using CFD technique. The study is carried out using commercial CFD software PHOENICS. It can be seen from the results of the analysis that poison jets emerge from the LPI tube after commencement of injection and rise nearly uniformly and penetrate deep inside the Calandria domain within the period of 3.0 seconds. CFD study has provided valuable information regarding the spatial distribution of poison in the moderator inside Calandria. The results of the CFD study can further be used for the estimation of reactivity worth.