Several engineered safety features are provided in the reactor to safeguard the nuclear power plant's containment during a postulated accident. The suppression pool is one such system provided for removing decay heat and scrubbing the fission products released during the accident. During the Fukushima Daiichi nuclear accident (2011), unit-3 had a steeper containment pressure rise than unit-2, and one of the reasons attributed to this higher pressure was the thermal stratification of the suppression pool. Therefore, the thermal stratification aspects of the suppression pool are of safety interest to the nuclear industry and are the focus of the present study.

To this end, thermal stratification characteristics of a reduced-scale toroidal suppression pool are brought out in this study. Firstly, the tunnelling effect observed in the early development of the thermal stratification process is discussed. Secondly, the temporal average temperature distribution with respect to elevation and temporal variation of the volume fraction of the pool for a given temperature range are estimated. In addition, the dead volume/stagnation zone not participating in the heat evacuation process is estimated to be 34.54%. Further, the temperatures of pool surface and bulk fluid are evaluated and it is observed that the surface temperature is consistently higher than the bulk temperature. Finally, the consequence of thermal stratification on containment pressurization is discussed.