[en] The effect of exposure to simulated HTGR primary circuit helium (400 μatm H₂/2 μatm H₂O/40 μatm CO/0.2 μ atm CO₂/20 μatm CH₄ in He at 2 atm total pressure) on the properties of structural alloys is studied over the temperature range of 750 to 1050⁰C (1382 to 1922⁰F). Creep-rupture testing is being performed using both single specimen and multispecimen equipment. Specimens are also being exposed unstressed for post-exposure evaluation of oxidation, thermal aging effects and carburization and for evaluating changes in tensile, impact and fatigue properties due to exposure. Some air tests are being performed for comparison. The results to date (exposure times to approximately 10,000 hours) are discussed for a variety of wrought iron- and nickel-based austentic alloys, cast nickel-based alloys, and oxide dispersion strengthened alloys. It generally appears that the environmental effect is significant only in the highest temperature range: thermal aging appears to be the cause of most of the changes seen during lower temperature exposures accomplished to date