Experimental investigations have shown that neutron radiation increases the strength and reduces the ductility of reactor vessel steels. The extent of steel embrittlement is a function of the steel, the irradiation environment (neutron energy and fluence), and the irradiation temperature. In some cases, as with the compact Army SM-1A reactor, the embrittlement may become severe enough to require modification of reactor operating procedures to avoid significant stress on the vessel when its temperature approaches the nil ductility transition temperature (NDT). While control of operating procedures met the changing NDT conditions of the SM-1A vessel for a time, continued embrittlement forced the development of another alternative, in-place heat treatment, annealing, for extending the projected operating life of the vessel. The SM-1A vessel was heat treated by raising the vessel temperature from the usual 430 to 572°F and holding it there for about one week using reactor heat from low power operation. In addition to operational procedures for minimizing the effects of neutron exposure to reactor vessel steels, design approaches used to meet this problem include specifying (based on experiment) a radiation-insensitive steel, shielding the vessel to reduce neutron exposure to a level consistent with the design lifetime of the plant, and providing for periodic in-place annealing using reactor heat.