[en] A corrosion programme is being carried out to establish the long term corrosion behaviour of candidate alloys for the PNP reactor primary circuit. Results are presented for corrosion tests performed in impure helium over the temperature range of 700 to 900°C for times up to 10000 h. In the helium atmosphere containing 0.5-1.5 pbar H₂O, 15 pbar CO, 20 μbar CH₄ and 500 pbar the three main PNP reference alloys - Inconel 617, Nimonic 86 and Hastelloy X - exhibit excellent corrosion resistance with carbon uptakes of less than 0.02% in 10000 h at temperatures up to 900°C. Differences in behaviour are related to the composition and morphology of protective surface scales and minor variations in gas impurity levels during test start up. The influence of variations in alloy composition on long term stability of these oxide scales is also discussed. (author)