The next generation of nuclear reactors (Gen IV), is expected to provide answers to many of the problems limiting public acceptability of earlier commercial nuclear power plants. Amongst the proposed designs, the sodium-cooled fast reactor (SFR) is the most mature, and has been extensively studied at CEA, the French Atomic Energy and Alternative Energy Commission. One of the most dangerous accidental transient in a SFR is the Loss Of Flow WithOut Scram (LOFWOS). To study how to limit its consequences, several experiments were carried out in the 1980s on an early research fast reactor in the USA, named FFTF. A benchmark based on one of these tests (LOFWOS Test #13) has been lately proposed by the International Atomic Energy Agency (IAEA) to improve the simulation capabilities and member states, and CEA is among the partecipating organizations.