Abstract: Heat pipes are passive heat transfer devices, with very long lives. Material and testing reactors (MTR) have residual heat after shutdown. Also they have an auxiliary spent fuel tank that also have heat generated need to removed, also heat should be removed in case of emergency to keep fuel temperature within safe limit. A gravity assisted two-phase closed heat-pipe loop (GTPHL) with evaporator and condenser lengths each 100m helical coil shape with outer diameter 15 cm and 3 mm thickness as a passive cooling system for a nuclear spent fuel storage pool is used to remove this heat. This paper presents and proposes a completely passive cooling system using loop thermosyphon for cooling and dissipation the residual heat of wet spent fuel storage by running as main or alternative cooling system. The design is focus on heat removal from the auxiliary spent fuel tank of the research reactor to be in safe mode. The model considers natural convection by air for the condenser part of the heat-pipe loop to confine the residual heat. A numerical simulation using special design of gravity assisted two-Phase closed heat-pipe loops were used to investigate the (GTPHL) thermal performance. The effects of heat loads were analyzed. Demineralized water was used as the (GTPHL) working fluid. The atmospheric air was circulated around the condenser as a cooling system. The effect of heat input (25kW≤Q≤150kW) working fluid filling ratio (100%) The results show that the best thermal performance was obtained at high evaporator heat load.