The KIKO3DMG nodal reactor physics code and the ATHLET3.0 thermal-hydraulics system code have been extended in recent years for the analyses of fast reactors. This paper focuses on the capabilities of the coupled KIKO3DMG/ATHLET3.0 code in the case of an unprotected transient of the ALLEGRO demonstrator gas-cooled fast reactor. We show that the coupled code can simulate an asymmetrical control assembly withdrawal transient. The code provides adequate results by using 6-group constants instead of the widely used 24-group constants, and with this, the computation time of the transient is reduced by 95%. Moreover, we drove attention to the applicability of the point-kinetics methodology, which can be limited because of the approximate manner of the derivation of the corresponding parameters. Finally, we present a methodology to flatten the distribution of the outlet coolant temperature by applying flow restrictors.