A new capsule design was prepared and tested at the High Flux Advanced Neutron Application Reactor (HANARO) for neutron irradiation of the core materials of research reactors at a low temperature (of <100°C). The capsule was first designed at HANARO to have the coolant flow through the capsule to cool down the irradiation temperature of the specimens. The safety of the newly designed capsule should be fully checked before irradiation testing. Out-pile performance and endurance testing before HANARO irradiation testing was performed using a capsule in the HANARO out-pile test facilities. The new capsule had a much higher coolant flow-induced vibration than a standard capsule, resulting in fatigue failure at the rod tip of the capsule. The lifetime of the rod tip was greatly improved by changing the material from Type 304 stainless steel to Type 316L stainless steel and by changing the welding method from tungsten inert gas welding to electron beam welding. With the optimized design, the capsule was successfully irradiated at low temperatures of <100°C for up to eight cycles (6075 MWd) at HANARO.