Through-Thickness Neutron Fluence and Embrittlement Gradients in Reactor Pressure Vessels

Decreases in neutron fluence and the related alteration in transition temperature increase (ΔTT) across the 2.4-in. thickness of the A350-LF3 steel of the PM-2A reactor vessel wall and to a depth of -in. in both A212-B and A350-LF1 (modified) steel inside a simulated vessel wall were obtained in support of research on Army reactor vessel integrity. The Charpy V notch ductility specimens used showed a decrease in ΔTT from the inner vessel surfaces that correlated with microfracture mechanisms which changed from predominately cleavage at the inner surfaces to increasing amounts of dimpled rupture (ductile behavior) at locations nearer the outer vessel surface. These data follow the slope of a reference fluence decrease, derived from measurements and calculations of a number of reactors, that shows a 95% decrease in flux across an 8-in.-thick vessel wall.The 60°F (33°C) gradient in ΔTT across the <3-in. PM-2A vessel wall suggested that while the inner vessel edge was at the nil-ductility transition (NDT) temperature, the outer edge would be at Fracture Transition Elastic (FTE) temperature, (NDT plus 60°F), wherein stresses in excess of yield are required to propagate a flaw. The pattern provided by the reference fluence decrease indicates that a heavy-section, >6-in. irradiated vessel wall could attain FTE characteristics under the NDT + 130°F criterion imposed by the mechanical constraint effect in thick-plate steel sections. This inherent, superior ductility at positions progressively farther from the vessel inner surface is projected to suggest a considerable margin against fracture and deserves recognition in vessel embrittlement analyses.