The Superphénix (SPX) Sodium Fast Reactor (SFR) core behaviour in a hypothetical Unprotected Loss Of Flow (ULOF) transient scenario is simulated and the impact of the core modelling options on the results are assessed. The analysis has been performed using the TRACE system code modified for the fast reactor applications using a point kinetics neutronics model and 1D multichannel thermal-hydraulic model. The SPX start-up core benchmark specification was used for calculation of neutron physics data employed as input in the TRACE simulations. The analysis covered transient simulation before sodium boiling onset. The influence of control rod drive lines (CRDL) expansion reactivity effect on the grace time before boiling was found to be significant. Series of calculations have been performed in order to assess sensitivity of the results to the core specification data and modelling details in both thermal hydraulics and neutronics inputs. In particular, two different models representing fuel subassemblies (three-channel and multichannel) have been employed. In addition, the influence of reactivity effects uncertainty on transient behaviour has been assessed, as well as sensitivity of the results to account for the spatial distribution of reactivity effects in the core. The results of this study serve further as a basis for the transient benchmark exercise proposed by Ponomarev, et al., 2018 within the framework of the Horizon-2020 ESFR-SMART project.