The effect of interstitial impurities on the upper shelf fracture characteristics of A533B Class 1 steel was examined using single specimen unloading compliance J-integral tests. Three-point bend tests were performed on fatigue precracked side-grooved Charpy-size specimens using a specially designed compression jig that minimized misalignments usually encountered in direct compression tests. Experimental results obtained as a function of the test temperature and strain rate revealed the effect of dynamic strain aging (DSA) as a drop in the critical crack initiation fracture toughness, J_q. As predicted from DSA models, the minimum in J_q shifted to higher temperatures at higher test speeds (strain rates). The kinetics of the toughness minima agreed with the diffusion of carbon or nitrogen, or both in steels. The effect of neutron irradiation was noted to result in a shift of the J-integral minimum to higher temperatures, resulting in an apparent increase in toughness at lower temperatures following neutron radiation exposure. The usual radiation embrittlement was noted at higher temperatures where DSA was absent.