Optimization Strategies to Improve the Safety Behaviour of a Soluble-Boron-Free SMR Core During a Rod Ejection Accident

Soluble-boron-free designs for water-cooled small modular reactors offer advantages such as reduced corrosion and simplified systems. However, the absence of soluble boron necessitates higher total control rod worth for reactivity control and the shutdown margin, leading to excessive individual control rod worth, which can lead to severe power excursions during a rod ejection accident (REA), potentially threatening the fuel integrity and core-cooling capability. The analysis of a hypothetical REA for an equilibrium core design showed that the fuel rod cladding failed due to the high reactivity worth of the ejected control rod. To enlarge the safety margins of this design under accidental conditions, two strategies were adopted: implementing a hybrid control rod configuration to decrease the local reactivity worth within single fuel assembly and re-arranging the refuelling loading pattern to prevent fresh fuel clustering. Using an in-house CoreOptimizer tool, the CASMO5 and SIMULATE5 simulations were automatized to find out an optimized equilibrium core design. The results demonstrated that all safety parameters of the optimized equilibrium core designs are within regulatory limits during normal operation and under REA conditions. By reducing the individual control rod worth, power spikes are considerably mitigated, thereby ensuring fuel integrity during an REA.