Studies of high-temperature materials behaviour irradiated by neutrons are required to develop advanced fission and fusion power technologies. A novel irradiation device, ISHTAR, has been developed in the MARIA Research Reactor’s Nuclear Facilities Operations Department in response to scientific interest. The irradiation conditions in the device reflect those occurring in the core of the advanced helium-cooled nuclear reactor at very high temperatures reaching 1000°C. Additionally, the highly flexible core of MARIA reactor enables studies in various neutron conditions, including fusion-related neutron spectrum. This work shows the design of the ISHTAR capsule, together with computational studies in thermalhydraulics and neutronics. Comparing computational data with the experimental data gathered during ex-core and in-core test campaigns proves that the device is reliable, inherently safe, and provides required irradiation conditions.