In the frame of the international forum GenIV, the Gas Fast Reactor is considered as a promising concept, combining the benefits of fast spectrum and high temperature, using helium as coolant. In the Gas Fast Reactor development plan, ALLEGRO is the first necessary step towards the electricity generating prototype GFR. ALLEGRO is a low power Gas Cooled Fast Reactor studied in the European R&D framework. It is a loop type, non electricity generating reactor. In the current preliminary viability studies ALLEGRO safety system relies on centrifugal blowers in case of depressurised conditions. The present study concerns the use of blowers in case of nitrogen injection. It is a well-known fact that heavier gases (than helium) enhance natural circulation. The use of such gases (nitrogen is considered) can also enhance forced convection. A specific work on the impact of the use of alternative gas on blower behaviour is presented. Transient behaviours in depressurised situations are computed with the CATHARE2 code and analyzed. Applications to GFR concept are also described.

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