[en] Creep and corrosion tests on four casts of Hastelloy alloy S have been conducted in simulated HTR helium and in air at temperatures in the range 750 to 1000°C. The creep data reviewed are from an accumulated test time of approximately 8 x 10⁴ specimen hours. The creep and rupture strengths are mainly within the - 20% scatter band seen on the manufacturers data. In air, the relative creep strength, compared with the manufacturer’s data, appears to be independent of the test time. In simulated HTR helium, there is a progressive loss of relative creep strength as test times are increased. The loss in helium is accompanied by extensive and progressive carburisation; penetration to a depth of 13 mm would occur at 900°C in 300000 hours. In its present form Hastelloy alloy S has excellent thermal stability and outstanding oxidation resistance, but despite these advantages its observed behaviour in simulated HTR helium demonstrates its unsuitability for use as a high temperature (≥ 800°C) primary circuit material for High Temperature Reactors. (author)