Comprehensive safety analysis of anticipated transient without scram in an integrated marine reactor

• SMR model with passive safety systems built for ATWS under ocean motion. • SMR achieves rapid shutdown through negative reactivity feedback. • Primary-loop circulation mode governs the intensity of ocean effects. Anticipated Transient Without Scram (ATWS) accident represents a severe transient in which the normal reactor shutdown function fails and the study of its consequences is crucial for reactor safety analysis. For the Integrated Small Modular Reactor (SMR), due to the compact and integrated arrangement scheme and the coupling effect of multi-degree-of-freedom ocean motions, the accident progression mechanisms differ from those in traditional land-based nuclear power plant. Therefore, it is necessary to reassess the safety margins for ATWS accidents under dynamic marine environmental conditions. In this study, a systematic safety analysis of ATWS accidents in integrated marine reactor is carried out focusing on four initiating events in Category II conditions which are reactivity insertion (RI), loss of coolant flow (LOCF), station black out (SBO), and loss of main feedwater (LOMFW). Sensitivity analysis is also conducted to investigate the impact of ocean conditions on safety. The results show that during the ATWS process, the core self-regulation mechanism (including Doppler feedback and moderator density effect) can drive the negative feedback regulation to automatically introduce the fission power down to the decay heat dominant level within 300 s. Sensitivity analyses demonstrate that the time-varying marine acceleration field significantly affects natural circulation-dominated ATWS scenarios, particularly LOCF and SBO accidents. In contrast, forced circulation-governed ATWS events, notably RI and LOMFW, exhibit considerably lower sensitivity.