[en] A new, advanced thermohydraulic simulation code for LMFBRs is described. This code, designated as the Super System Code (SSC), simulates operational and safety transients that may be initiated anywhere in the system. The SSC code provides for the simulation of all essential components in the primary and intermediate heat transport systems. The steam generating circuit along with the turbine and condenser are also included. The usefulness of this code in future LMFBR plant designs is demonstrated by its application to the Clinch River Breeder Reactor-type plant. Several design and operational transients such as normal scram, control rod withdrawal, after-heat removal by natural convection and load follow-up are analyzed. A pipe rupture accident in the primary heat transport system is also discussed