**2.4 Needs for nuclear thermal propulsion material testing**

The tests described in the previous section are being performed to collect information on the performance limits of fuel forms and cooling configurations. To meet mission requirements, it is desirable to maximize fuel temperatures, but higher temperatures introduce other issues: expansion, stresses, Doppler broadening, and chemical interactions. For the latter, early graphite fuel experiments under Rover highlighted the need to use coatings on the fuel grains. It is also known that fuel hydration from direct contact between fuel and hydrogen coolant has a deleterious effect on fuel performance [36]. Test specimens often include cladding materials on flow surfaces, which requires an additional evaluation in terms of clad/fuel interactions. Cladding is also an additional challenge in fuel fabrication. Cycling of the fuel from zero power to high power, operation at steady state for tens of minutes, and the return to zero power results in the potential buildup of temperature-driven stresses, which could ultimately lead to failure. Hence, material testing must address all of these physics, either in integral or separate effects testing. Both TREAT and NTREES provide capabilities for such tests. NTREES allows for larger specimen sizes and (until the SIRIUS-4 experiment is fabricated) is the only facility that provides for high temperature hydrogen flow. TREAT allows for direct nuclear testing with energy distributions that would be more typical of an NTP configuration. However, hydrogen flow within fuel specimens will be introduced within TREAT with the first Prototypic Reactor Irradiation for Multicomponent Examination (PRIME) experiments. PRIME-1 (also known as SIRIUS-4) will use CERMET fuel, while PRIME-2 will repeat the experiment with a CERCER fuel sample. Both are shown on the timeline in **Figure 2**. After PRIME-2, further experiments will focus on the evaluation of CERCER fuel specimens.
