The helium cooled Gas Fast Reactor (GFR) is one of the six reactor concepts selected in the frame of the Generation IV International Forum. Since no gas cooled fast reactor has ever been built, a medium power demonstrator reactor — named ALLEGRO — is necessary on the road towards the 2400MWth GFR power reactor. The French CEA completed a wide range of studies on the early stage of development of ALLEGRO, and later the ALLEGRO reactor have been developed in several European Union projects in parallel with the GFR2400. The 75 MW thermal power ALLEGRO is recently being developed in the frame of European ALLIANCE project. As a result of the collaboration between CEA and MTA EK new improvements were done in the CATHARE modeling of ALLEGRO. In particular, the capability of simulation of breaks located in the crossduct (concentrically arranged pipes with the hotduct located inside the colduct) has been developed. A first scenario of hotduct break has been simulated, that does not lead to the depressurization of the system because of the crossduct technology. Nevertheless this transient leads to a high bypass of the core. Then a scenario of full rupture of the hotduct and the colduct has been tested, leading to beyond design state with depressurized situation combined with a large bypass of the core. However this study shows that the peak cladding temperature can be kept below the cladding melting point using nitrogen injection. In this paper the CATHARE model implemented for the crossduct rupture scenario and the results of the simulation are presented and discussed.

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