Safety analysis of an integral small modular reactor under ATWS conditions

Abstract The Anticipated Transient Without Scram (ATWS) is classified as a Category II anticipated transient event in nuclear power plants and represents an important subject in reactor safety analysis. Due to the highly integrated configuration of small modular reactors (SMRs), their system responses under accident conditions differ significantly from those of conventional land-based pressurized water reactors. In this study, the thermal-hydraulic system analysis code Relap5 is applied to establish a nodalized model of a representative SMR. On the basis of verified steady-state conditions, a series of transient simulations under design basis accidents (DBAs) combined with ATWS scenarios are performed to evaluate the inherent safety features of the reactor and the accident mitigation capability of its passive safety systems. The results indicate that, for a reactivity insertion accident (RI), the reactor core exhibits strong self-regulating behavior, with power rapidly returning to the steady-state level. In the case of a loss-of-coolant flow accident (LOCF), the large negative feedback coefficients enable the core to reach hot shutdown within a short period, and the decay heat is effectively removed through natural circulation and the passive residual heat removal heat exchangers (PRHRs). For a small break loss of coolant accident (SBLOCA), the passive injection system continuously injects coolant into the core by gravity after the riser section is emptied, ensuring that the upper part of the fuel remains submerged throughout the transient.