Investigation of decay heat removal systems in the ALLEGRO helium-cooled fast reactor

• CATHARE model development of the ALLEGRO Decay Heat Removal system. • Investigation of helium, nitrogen and CO2 as a Decay Heat Removal working fluid. • Higher gas pressure leads to better cooling performance. • Denser coolant ensures better cooling performance. • CO2 is a promising candidate as a Decay Heat Removal secondary fluid. ALLEGRO is a demonstrator reactor of the GFR2400 helium-cooled gas fast reactor selected for future research by the Generation IV International Forum (GIF) in the early 2000s. This 75 MW thermal power demonstrator reactor has been developed in several European projects over the past two decades. The current ALLEGRO design consists of pressurized water on the secondary side of both the main heat exchangers (MHX) and the decay heat removal systems (DHR). Nevertheless, in case of a primary-secondary break at the MHXs or the DHRs, the water coolant may ingress into the gas-cooled reactor core which may cause reactivity and corrosion issues. For this reason, replacing the secondary water in the MHXs and the DHRs is an important topic during the design phase of ALLEGRO. In this paper, the gases of helium, nitrogen and CO 2 are investigated as possible secondary coolants of the DHRs at different pressures using the CATHARE thermal hydraulics system code. The results show that from a cooling performance point of view, CO 2 gas is a promising candidate and by using this gas, a relatively small design change is needed concerning the size of the DHR heat exchangers.