Reactor experimentation plays a critical role in advancing capabilities in many disciplines, such as deep space exploration and advanced reactor instrumentation. There is no substitute for the knowledge gained or the experimental data produced from in-core experiments in material test reactors. The design of these experiments poses unique engineering challenges due to neutron-induced material degradation, extreme temperatures and/or pressures, corrosive environments, and activation concerns. Oak Ridge National Laboratory is at the leading edge of in-core experimental research, conducting a multitude of tests at the High Flux Isotope Reactor and university research reactors. This two-part talk will discuss the methodologies and experimental designs for in-core experiments developed at ORNL. The first part of this talk will present the development of a test target for Pu-238 production to provide source material in NASA’s deep space Radioisotope Power Systems. Part 2 of this talk will cover development and testing of a high temperature fission chamber to be used in next generation molten salt reactors.