The Gas Fast Reactor (GFR) is one of the six concepts selected for further studies in the frame of the GEN IV forum and it is of high priority in the French Atomic Commission (CEA) R&D program on the future Nuclear Energy Systems. The conceptual design is shared between European partners through the GCFR 6th Framework Program. The ETDR (Experimental Technology Demonstrator Reactor) will be the first GFR ever built. The main characteristics of the ETDR ([1]) are a 50 MW concept with a starting core design with existing or close-to-existing technology (MOX pin S/A with metallic cladding) to qualify the whole sub-assembly concept, with an inlet temperature of 260°C and an outlet temperature of 560°C. Under accidental design operating conditions the maximum clad temperature should not exceed 850°C. The system design is largely influenced by the Decay Heat Removal strategy (DHR). In case of depressurised transients, DHR helium flow always relies on forced convection (the back up pressure is assumed equal to 3 bar) whereas in case of pressurised transients, DHR helium flow relies on forced convection but in case of failure of the blower, natural convection should be efficient enough to ensure the core cooling. Therefore a specific system has been designed. It consists of three loops in extension to the vessel including a heat-exchanger, located at a certain elevation in order to ensure natural convection for pressured situation, and a blower in order to start and/or maintain DHR flow especially when low pressures are reached. A CATHARE modelling based on the actual design has been developed and is actually used to carry on first accidental transient analysis on the starting core. The paper first describes the CATHARE modelling. Then the paper analysed two different families of protected transients (pressurised and depressurised transients) and some aggravating events are considered. Finally perspectives and future work are discussed.

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