Stability of the Defect State in Neutron-Irradiated Molybdenum

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Abstract
The thermal stability of neutron-induced defects in molybdenum irradiated in Experimental Breeder Reactor II (EBR-II) to a fast-neutron fluence of ∼1 × 1022 n/cm2 (E >1 MeV) clearly suggests that there are critical temperature regimes that should be avoided by reactor design engineers. These regions are manifested by a rapid change in the micro structure within a small temperature interval, a circumstance that can significantly influence the strength and corresponding ductility of the material. One critical temperature occurs at ∼800°C, where the irradiation-induced modulus-corrected strength could vary significantly compared to unirradiated molybdenum for a small temperature variation around 800°C. Voids have been shown to occur in specimens irradiated at 430, 580, 700, 800, 900, and 1000°C; these voids are stable at temperatures up to ∼0.60 Tm, rather than the 0.55 Tm value reported earlier for low fluence irradiations. The increase in the complete void removal temperature is suggested to exist due to the presence of a larger void size and the ordered void lattice structure in EBR-II samples.