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• ALFRED is the Lead Fast Reactor European concept currently developed by FALCON Consortium. • ALFRED reactor design underwent a wide design review tackling several thermal–hydraulic challenges. • FALCON Consortium and related supporting organizations performed several experimental and numerical investigations to provide confidence on reactor coolant system design claims. Research and development on Generation IV concepts cooled by heavy liquid metals have been supported through several national and international funds. Looking at the European community, the technology benefited from approximately 145 M€ of investments, increasing the technology readiness level through various areas including safety and resilience, neutronics and thermal hydraulics. One of the projects currently pursued in Europe and supported by numerous research centers, private companies and member states is ALFRED (Advanced Lead Cooled Fast Reactor European Demonstrator), born around 10 years ago within the LEADER project. Following the FP7 project, Romania declared its support in ALFRED development by proposing as hosting country and the FALCON consortium (Fostering ALFRED Construction) was set out, consisting of Ansaldo Nucleare, ENEA and RATEN-ICN. The consortium has recently revised the reactor concept to solve some thermal hydraulic issues with an increase of technology know-how. The changes were supported by a new deployment strategy based on different power stages, appointing ALFRED as the demonstrator for the future fleet of commercial SMRs cooled by liquid lead. Several experimental campaigns and numerical analyses developed by the consortium and its supporting organizations have been of extreme importance. The areas that benefited mostly from this effort were the thermal hydraulics of the reactor pool, of the core and of the safety systems, thanks to numerical studies made with system codes, computational fluid dynamic tools supported by ENEA technological infrastructure. The paper presents the new plant concept, reporting some of the most important results that have made it possible to solve complex problems such as thermal striping, thermal stratification and freezing of lead by overcooling. Lastly, the paper highlights the next steps for ALFRED development, such as the construction of various experimental facilities that will develop the test campaigns for the safety justification and licensing of the plant. [ABSTRACT FROM AUTHOR]

The safe operation of nuclear reactors cooled by liquid metal is an important issue to be addressed for the development of nuclear technology and presents specific aspects related to the properties of the coolant. While the main parameters influencing safety for water-cooled reactors are reasonably well known, DHR systems for reactors cooled by liquid metal present additional challenges related to coolant freezing, since solidification temperature is higher than the temperature of the final heat sink. If the primary coolant solidifies obstructions of the primary flow path can occur, inhibiting natural circulation and core cooling. This paper presents an innovative passive safety system for decay heat removal which passively delays the coolant freezing. The system adopts noncondensable gases to passively control the power removed from the primary system and to delay freezing in the long term while keeping primary system temperatures below an acceptable limit. The system is simulated by means of the Relap5-3D computer code for a loss of offsite power of the innovative lead-cooled reactor ALFRED. A sensitivity analysis has been carried out in order to study the effect of noncondensable gas pressure on the performance of the passive decay heat removal system, and in particular on the primary coolant temperatures. [ABSTRACT FROM AUTHOR]