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The high temperature and high neutron flux environment of a thermionic space power reactor presents a challenge in the design of the sheath insulator within a thermionic fuel element. The present alumina insulator design is suspect to degradation due to the neutron flux. The alumina insulator also requires a barrier coating to isolate it from the liquid alkali metal coolant. Although the alumina sheath development is progressing, the alumina insulator remains a potential point of significant performance loss in the thermionic fuel element. The recent successes in depositing polycrystalline diamond film onto cylindrical refractory metal substrates has led to the consideration of diamond as a potentially ideal sheath insulator. Investigations have been conducted into the durability of diamond thin film under exposure to simulated thermionic reactor conditions.