• Generation IV
• LMFBRs
• nuclear reactors
• RELAP code
• SIMMER code

Among the Gen IV systems, Lead-cooled Fast Reactors (LFRs) represent one of the most promising technologies, offering advantageous features related to the use of molten lead as reactor coolant. These include the employment of a fast neutron spectrum, with the associated potential for spent fuel inventory reduction and generation of more fissile material than they consume, the possibility to operate at atmospheric pressure, the radiation shielding, the very high boiling point of the coolant which eliminates the risk of voiding, and the chemical inertness.

The aim of this work is to investigate thermal-hydraulic safety aspects of the lead-cooled fast reactors, employing nuclear thermal-hydraulic codes such as SIMMER-III, RELAP5/mod3.3, and ASYST-LM. The study begins by assessing the capability of these codes to accurately simulate some fundamental phenomena critical for reactor operation, including natural circulation and the interaction between the coolant and non-condensable gases. Subsequently, some reference transient scenarios were considered and simulated using the previously mentioned codes. The progression of these accidents was then analysed to draw conclusions about the design choices for these reactors, thereby contributing to the assessment of their feasibility, reliability and safety.