The effect of interstitial impurities through dynamic strain-aging [DSA] on the mechanical and fracture characteristics of A533-B Class 1 steel was examined using tensile, subsize Charpy three-point bend and single specimen unloading compliance J-integral tests. The tensile and fracture properties were studied as a function of the test temperature and applied strain-rate on the as-received pressure vessel [PV] steel material. The effect of neutron radiation exposure on both the tensile and fracture behaviors was investigated. The effect of DSA was noted to result in decreased fracture energies in the upper-shelf region. Neutron irradiation suppressed DSA resulting in improved mechanical and fracture properties of this steel at some temperatures and strain-rates. However, at very high temperatures DSA exhibited embrittlement in addition to that caused by radiation. Temperatures corresponding to fracture energy minima and the applied strain-rates followed an Arrhenius equation with activation energy identifiable with that for diffusion of C and N in steels plus their binding energy with alloying elements such as Mn and V.