Nuclear thermal propulsion (NTP) demonstrated a reported technology readiness level of 5 during the work performed in the 1950s–1970s under the Rover program. This level of capability was achieved through the design, construction, and use of 22 experimental ground tests. These experiments served as testbeds for designs, materials, and instrumentation at prototypical NTP conditions. To continue the investigation into NTP system materials, components, and fuels, a modern experimental testbed has been designed and implemented. A steady-state, high-temperature, subscale, in-pile testbed has been developed to continue this investigation. The In-Pile Experiment Set Apparatus (INSET) has demonstrated that it can be used to test samples under two NTP prototypical environmental factors: temperature and neutron fluence. The demonstration using The Ohio State University Research Reactor is presented here. This demonstration required INSET to maintain a thermal environment below 1070 K for 15 min during a 5-h irradiation to achieve a neutron fluence around 1017 n/cm2.