[en] The influence of the helium environment on superalloys, expected in the very high temperature nuclear reactor under development, is described with emphases placed on the test methodology and the development of alloys with enhanced compatibility. For accurate prediction of long time performance, importance of controlling chemical potentials of the gaseous impurities in the material-environment interface is pointed out. Examination of the performance of the alloys modified for such specific purpose showed substantial enhancement of the degradation resistance. (author)