Ydinreaktorin koon ja tyypin vaikutus jälkilämmönpoistoon liittyvien turvallisuustoimintojen passiivisuuteen

This thesis investigated how reactor size and type affect the feasibility of passive decay heat removal under SBO conditions. The study focused on PWRs, LFRs, and HTGRs, and included a conduction cooldown case study for a 15 MW HTGR representative of the Micro Modular Reactor (MMR) design by USNC. Higher power levels were also investigated. PWRs rely on natural circulation with time-limited coolant inventories for passive decay heat removal, placing the practical limit for inherent passive decay heat removal in the range of tens of MWs. LFRs also depend on natural circulation and require heat rejection through the vessel or cavity, limiting the range to a few hundred MW. HTGRs can rely on conduction and radiation without coolant motion, remaining inherently passive also on the order of a few hundred MWs. The case study demonstrated that the 15 MW configuration maintained thermal margins during protected station blackout and retained a finite margin even when the power was increased by an order of magnitude. Overall, inherent and indefinite passive decay heat removal was feasible only at small scale, while higher power levels remained possible but required engineered safety features to maintain equivalent margins. Passive measures can still contribute to decay heat removal above these ranges, but their operation becomes time-limited and they cannot be relied upon as the sole means of decay heat removal.