To study the mechanisms of deformation hardening and flow localization of radioactive materials, a noncontact “digital marker extensometry” technique has been employed. It allows researchers to easily define plasticity parameters and true stresses in experiments where highly radioactive miniature specimens are used. The engineering and “true stress-true local strain” relationships of irradiated metal polycrystals during plastic flow and hardening have been investigated experimentally after irradiation in two reactors in Kazakhstan. The true curves were obtained for copper, nickel, iron, molybdenum, as well as for the Russian stainless steels 08Cr16Ni11Mo3 and 12Cr18Ni10Ti. Describing these curves using the equation σⁱ = σ⁰ + kεⁱ, demonstrates that the concept of ultimate stress in highly irradiated materials is an artifact arising from flow localization and is not fully informative of the operating hardening mechanisms.