Irradiation effects and consequences on in - service properties under proton and neutron mixed spectra: an overview

Spallation reactions induced by neutrons in the GeV-range in high-Z targets are considered for the transmutation of nuclear waste through an accelerator driven system. The use of liquid metal targets minimizes radiation damage essentially restricting it to container and window materials. This article presents a systematic of irradiation damage: 3 classes of irradiation defects can be distinguished, namely structural, microchemical and changes in bulk composition, with corresponding consequences for microstructure and macroscopic properties. The cross sections for atomic displacement as well as those for production of light elements become virtually equal for protons and neutrons above 20 MeV. But in comparison to fast fission or fusion reactors, the energies of the irradiation particles (neutrons and protons) are much higher with the following consequences: 1) harder spectrum of atomic recoils, causing larger average sizes of displacement cascades and the emergence of sub-cascades of constant size; 2) higher production rates of light atoms (mainly hydrogen and helium) that cause severe embrittlement; and 3) production of other atomic species from spallation debris that might form precipitates in the structural materials and put their mechanical integrity at risk. (A.C.)