Passive decay heat removal and sodium fire are two major key issues of nuclear safety in sodium-cooled fast reactor (SFR). Several decay heat removal systems (DHR) were suggested for SFR around the world so far. Those DHRS mainly classified into two concepts: Direct reactor cooling system and ex-vessel cooling system. Direct reactor cooling method represented by PDHRS from PGSFR has disadvantages on its additional in-vessel structure and potential sodium fire risk due to the sodium-filled heat exchanger exposed to air. Contrastively, ex-vessel cooling method represented by RVACS from PRISM has low decay heat removal performance, which cannot be applicable to large scale reactors, generally over 1000MWth. No passive DHRSs which can solve both side of disadvantages has been suggested yet. The goal of this study was to propose ex-vessel cooling system using two-phase closed thermosyphon to compensate the disadvantages of the past DHRSs. Reference reactor was Innovative SFR (iSFR), a pool-type SFR designed by KAIST and featured by extended core lifetime and increased thermal efficiency. Proposed ex-vessel cooling system consisted of 4 trains of thermosyphons and designed to remove 1% of thermal power with 10% of margin. The scopes of this study were design of proposed passive DHRS, validation of system analysis and optimization of system design. Mercury was selected as working fluid to design ex-vessel thermosiphon in consideration of system geometry, operating temperature and required heat flux. SUS 316 with chrome coated liner was selected as case material to resist against high corrosivity of mercury. Thermosyphon evaporator was covered on the surface of reactor vessel as the geometry of hollow shell filled with mercury. Condenser was consisted of finned tube bundles and was located in isolated water pool, the ultimate heat sink. Operation limits and thermal resistance was estimated to guarantee whether the design was adequate. System analysis was conducted by in-house code and both axial and radial direction of heat transfer was considered. In-house code was validated by the high temperature thermosyphon experiment using liquid metal conducted by other researchers. Thermosyphon was designed based on cold pool temperature and heat flux from reactor vessel in consideration of structural constraints of reference reactor. Design parameters, such as filling ratio, evaporator length, condenser tube length and number, were optimized. Designed ex-vessel cooling thermosyphon showed 270% enhanced heat removal performance compared to conventional RVACS design. In conclusion, proposed DHRS design compensates the disadvantages of conventional DHRS for SFR. Proposed DHRS allows simplified in-vessel structure by the elimination of in-vessel DHRS. Sodium fire risk was excluded by using mercury as intermediate fluid. Moreover, enhanced heat removal performance allows the application to larger reactors.
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