On the Role of Helium in High Temperature Embrittlement of Irradiated Austenitic Steels

Abstract

Helium that is produced by nuclear transmutations can cause severe degradation of the mechanical properties in structural components of fusion and fast fission reactors. For studying the mechanisms of irradiation embrittlement, the helium effects have been simulated by helium implantation from cyclotrons or by making use of (n,α) reactions from thermal neutron fluxes. As is discussed in the present paper, the results from neutron irradiations and helium implantations are contradictory with respect to the helium levels at which helium embrittlement appears. To study the combined effect of helium and neutron irradiation on embrittlement, two titanium stabilized austenitic steels of identical composition, with the exception of their boron content of 40 and 2 ppm, respectively, have been neutron-irradiated at 845 K to thermal neutron fluences of 2.6 × 1025 n/m2. The precipitate and helium bubble microstructure has been quantitatively characterized by transmission electron microscopy (TEM) and has been correlated with the embrittlement behavior, studied in post-irradiation creep experiments at 973 K. Contrary to the expectation the material with the lower helium content was found to exhibit the higher reduction in creep strength after irradiation. This result and the differences between low-dose neutron and alpha irradiations can only be explained if a new assessment of the role of helium in high temperature irradiation embrittlement is made. Helium embrittlement and a proposal for an additional embrittling mechanism operative under neutron irradiation are discussed.