Research on the natural circulation capability and heat transfer capacity of the new separated water tank for the passive containment cooling system and secondary passive residual heat removal system

. Passive Containment Cooling System (PCS) and the Secondary Passive Residual Heat Removal System (PRS) are two major passive systems that ensure the safety of nuclear power plants under accident conditions. Aiming at the engineering pain points of the existing integrated concrete water tank in HPR1000, including high construction difficulty, long construction period, and low utilization rate of lower tube bundles caused by uneven pool boiling heat transfer, this paper proposes a new separated PCS/PRS steel water tank design. A numerical model of the natural circulation heat transfer system for the separated water tank was established using the RELAP5 system code. The transient thermal-hydraulic characteristics of the separated water tank under accident conditions were systematically investigated. The heat transfer performance difference between the separated water tank and the original HPR1000 integrated water tank was quantitatively compared. The results show that the separated water tank significantly enhances the natural circulation driving force by utilizing the height difference between the upper and lower tanks, increasing the steady-state heat transfer power compared with the original scheme, and effectively improves the uneven heat transfer defect of pool boiling. The system can stably remove core residual heat within the 40-hour design period, meeting the safety requirements of design extension conditions. The proposed separated steel water tank realizes factory prefabrication and integral hoisting, shortening the on-site construction period by about 30%, and provides a new technical approach for the optimization of passive safety systems in nuclear power plants.