The goal of the present work is to compare the TRACE code, which is part of the PSI fast reactor code system FAST, and the French CATHARE code system for a range of different transients for the 2400MWth GFR (Gas-cooled Fast Reactor) indirect-cycle reactor concept. For the purpose, CATHARE and TRACE models for the 2400MWth GFR have been established having the same level of modelling. Additionally to the steady-state conditions, transient behaviour has been analysed. Two protected transients have been selected for the analysis. The first is a protected loss of flow accident (PLOF), to investigate the natural convection capability of the decay heat removal (DHR) loops to remove the decay heat under pressurized conditions without the help of the blowers. The other transient is a protected loss of coolant accident (PLOCA), to evaluate the capability of the DHR loops to remove the decay heat under depressurized conditions in the main loop, with the help of the battery-driven DHR blowers. In addition, to investigate the core response for unprotected accidents, a ULOF and a ULOCA have been analysed using a point kinetics model for the 2400 MWth core. In general, the comparison can be considered as a study of the sensitivity of results to the use of different codes and assumptions by independent teams analysing the same advanced fast reactor system.