A mathematical model was developed to describe the effect of ferrite grain size on the nil ductility transition temperature increase observed in irradiated pressure vessel steels. Experimental data illustrating the grain size effect are presented. The engineering significance of both the theoretical model and the experimental data are discussed, and it is shown that the neutron embrittlement problem may be greatly minimized by use of fine-grain steels. For Types A302B and A212B steels, shifts of less than 65 F in the 30 ft-1b Charpy transition temperature at an exposure of 3 × 10¹⁹ nvt (>1 Mev) can be obtained with a ferrite grain size smaller than ASTM 9.