Estimated duration of effective operation of autonomous passive residual heat removal system (PRHRS) at comparing heat removal to cooling water vs ambient air

The article presents the layout and operation principles of an autonomous thermosiphon-based passive residual heat removal system (PRHRS) from the primary circuit of the nuclear power plant with VVER reactor under beyond-design accident conditions with complete long-term blackout, loss of feed water and primary LOCAs. Peculiarities of residual heat removal to water or ambient air by autonomous thermosiphon PRHRS are presented. The results of the analytical evaluation and estimated duration of effective operation of the autonomous PRHRS removing residual heat to water and air and providing a safe stable reactor condition are shown. The results were received based on numerical simulation using RELAP5 / MOD3.4 thermal-hydraulic code. The comparative evaluation of the results is made. The advantages and disadvantages of the use of water vs air as the final heat absorber are analyzed.