ALLEGRO is a helium cooled fast reactor which is being developed by the Czech Republic, France, Hungary, Slovakia and Poland. It will be the demonstrator of the GFR2400MWth Generation IV gas cooled fast reactors. In ALLEGRO a three loop safety system is designed to remove decay heat during accident conditions. The nonrotating blower blades may represent a huge pressure loss in the DHR (Decay Heat Removal) loop, which hinders natural circulation. The lower the pressure loss coefficient of the DHR blower blades is, the better cooling is available during natural circulation. On the other hand, a large core bypass develops if a DHR valve is opened inadvertently during normal operation. In this case the higher DHR blower pressure loss is better from core cooling point of view. Consequently, the low pressure loss of the DHR blower is advantageous for core cooling in SBO (Station Blackout) event but disadvantageous for inadvertent DHR valve opening event. Both the above mentioned cases may lead to insufficient core cooling in accident conditions, which threatens the integrity of the reactor core. In this study we present CATHARE thermohydraulic calculations to assess the sensitivity of the DHR blower pressure loss coefficient for the above mentioned two cases.