IRON, steel, and other body-centred cubic metals show an increase in ductile-brittle transition temperature as a result of neutron irradiation. In an attempt to elucidate the mechanism of this embrittlement for EN2 steel, Hull and Mogford1 studied the effect of neutron irradiation on the parameters σi and k y in the Petch relation: where σy is the yield stress and 2d the grain diameter. They found that σi, the lattice friction stress, increased, but k y which is a measure of the strength of dislocation locking, remained constant. They interpreted this to mean that the embrittlement was caused by point defects or groups of defects which impeded the movement of slip dislocations in their glide planes. This simple phenomenological description of irradiation embrittlement enables changes in ductile-brittle transition temperature caused by irradiation to be predicted from Cottrell's brittle fracture criterion2: once σi has been measured as a function of integrated neutron flux.